Rieppel, 1998 by Felipe Elias

Zoological Journal of the Linnean Society (1998), 124: 1–41. With 8 figures Article ID: zj970131

Corosaurus alcovensis Case and the phylogenetic interrelationships of Triassic stem-group Sauropterygia OLIVIER RIEPPEL Department of Geology, The Field Museum, Roosevelt Rd. at Lake Shore Dr. Chicago, IL 60605-2496, U.S.A. Received January 1997; accepted for publication July 1997

The holotype and other material of Corosaurus alcovensis Case from the Alcova Limestone of Casper, Wyoming, was further prepared using a combination of chemical techniques. Anatomical revision resulted in the definition of new characters for the analysis of the phylogenetic interrelationships of Triassic stem-group Sauropterygia that document paraphyly of the Eusauropterygia. Corosaurus is the sister-taxon to a clade comprising Cymatosaurus, Pistosaurus, and by extension, plesiosaurs and pliosaurs.  1998 The Linnean Society of London

CONTENTS

Introduction . . . . . . . . . . . Material and methods . . . . . . . The geological provenance of Corosaurus Systematic palaeontology . . . . . . Character definitions . . . . . . . . Cladistic analysis . . . . . . . . . Classification of the Sauropterygia . . . Discussion and conclusions . . . . . . Acknowledgements . . . . . . . . References . . . . . . . . . . .

. . . . . . . . . .

1 2 3 3 5 24 33 35 38 38

INTRODUCTION

Corosaurus alcovensis Case, 1936, from the Alcova Limestone of Casper, Wyoming, was until recently the only Triassic stem-group sauropterygian known from the New World. Case (1936) placed the genus in the suborder Nothosauroidea sensu Peyer (1933–34), noting its intermediate position between the Pachypleurosauridae and Nothosauridae as defined by Peyer (1933–34). F.v. Huene (1948) placed Corosaurus, together with Simosaurus and Conchiosaurus (in fact a senior synonym of Nothosaurus: Rieppel & Wild, 1996; see also Rieppel & Brinkman, 1996), in his Simosauridae, a E-mail: rieppel@fmppr.fmnh.org 0024–4082/98/090001+41 $30.00/0

 1998 The Linnean Society of London

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family he believed to be related to the Pistosauridae and, by extension, to plesiosaurs and pliosaurs. Zangerl (1963: 120) considered Corosaurus to be “the most advanced member of the Nothosauria”, a conclusion he believed to be in accordance with his interpretation of the Alcova Limestone as Late Triassic in age. The holotype (Case, 1936) and further material pertaining to the taxon (Zangerl, 1963) was the subject of a recent monographic treatment by Storrs (1991). Storrs (1991) placed the Alcova Limestone in the uppermost Lower Triassic (Spathian); his cladistic analysis placed Corosaurus as sister-taxon of the Eusauropterygia sensu Tschanz (1989). In view of our present knowledge of geographic occurrence of Triassic stemgroup Sauropterygia in China, Europe and in the western United States, the phylogenetic relationships of Corosaurus are of crucial importance for the understanding of the early paleobiogeographic history of the clade. The importance of the relative phylogenetic position of Corosaurus is emphasized by the fact that the earliest stem-group sauropterygians to occur in Europe are the pachypleurosaurs Dactylosaurus (Rieppel & Lin, 1995) and the eosauropterygians Cymatosaurus (E.v. Huene, 1944) and Nothosaurus (Meyer, 1847–55), all from the uppermost Lower Triassic (Ro¨t, so2; Spathian), and hence of equivalent age to Corosaurus. Unfortunately, the stratigraphic control over the occurrence of stem-group sauropterygians in China is less precise. Since the monographic treatment of Corosaurus by Storrs (1991), the European stem-group sauropterygians have been subject to extensive reviews (e.g. Rieppel, 1993a,b, 1994a, 1995, 1996; Rieppel & Lin, 1995; Rieppel & Wild, 1996), which resulted in an expansion of the data matrix, and some changed definitions of characters for the analysis of sauropterygian interrelationships as presented by Rieppel (1994a). It is for this reason that the phylogenetic interrelationships of Corosaurus will here be tested again against this expanded data set.

MATERIAL AND METHODS

The material of this analysis includes the holotype of Corosaurus alcovensis Case, 1936, kept at the University of Wyoming in Laramie (UW 5485). The specimen consists of several blocks of matrix which fit together and allow the restoration of the skeleton as preserved (Case, 1936, fig. 1; Storrs, 1991, fig. 3). This specimen has been further prepared, and some of the skeletal elements have been completely removed from the matrix. Additional specimens included in this review is the material first described by Zangerl (1963; FMNH PR480, PR135), some of which was also further prepared, as well as the specimen collected in 1996 (FMNH 2018). Mechanical preparation of Corosaurus embedded in the Alcova Limestone is extremely difficult and time consuming due to the hardness of the matrix. For this reason, further preparation of Corosaurus involved a combination of chemical methods. After filling in cracks and sealing the bone surface with acryloid resin, the specimen was exposed to the Waller solution, following the Waller method described by Blum, Maisey and Rutzky (1989). This method is designed to remove ferruginous matter from the matrix. After suitable exposure to the Waller solution, a thin layer of powdery matrix was removed by exposing the specimen to a solution of 5% buffered formic acid. When the matrix ceases to react with the acid, the cycle begins again with exposure of the specimen to Waller solution. The details of this procedure will be published elsewhere (Passaglia & McCarroll, 1996).

TRIASSIC STEM-GROUP SAUROPTERYGIA

The geological provenance of Corosaurus The holotype of Corosaurus alcovensis was collected in 1935 on a quarry dump derived from an excavation in the Alcova Limestone in Jackson Canyon 9 miles southwest of Casper, Natrona County, Wyoming (Case, 1936). Today, large parts of the Jackson Canyon, and the adjacent Goose Egg area, featuring massive outcrops of Alcova Limestone on top of Chugwater red shales and sandstones, are private land and constitute the Jackson Canyon Wildlife Viewing Area, rendering access difficult. Southwest of the Jackson Canyon area is Bessemer Mountain, with vast exposures of Alcova Limestone which, again, are difficult to access today since they are surrounded by private land. A Field Museum field party visited the Alcova Limestone in 1948, and collected FMNH PR480 and PR135 in outcrops 3 miles north-east of Freeland, Natrona County, Wyoming (Zangerl, 1963). In his monograph on Corosaurus and the Alcova Limestone, Storrs (1991) added a third locality yielding Corosaurus on the south slope of Muddy Mountain, south of Casper (outcrops located on the Milne Ranch; Storrs, 1991, fig. 45). A field party from the Field Museum returned to the outcrops of the Alcova Limestone in the general neighbourhood of Casper in the summer of 1996 (Fig. 1). Vast outcrops of Alcova Limestone on federal land were intensively searched (under permit # P96-WY-018), including an extensive ridge 4 miles north of Freeland extending northwards, the massive outcrops 3 miles northeast of Freeland first visited by Zangerl and his crew in 1948, outcrops on the north slope of Muddy Mountain, as well as the exposures of the Alcova on the south slope of Muddy Mountain searched by Storrs (1991). Corosaurus proved to be widespread throughout the Alcova Limestone in the Casper area, but is very scarce and represented by very fragmentary material only (Fig. 1). Only one partially articulated skeleton was found and collected, on the north slope of Muddy Mountain (FMNH 2018). The stratigraphic correlation and hence determination of the relative geological age of the Alcova Limestone is rendered difficult by the general paucity of fossils, vertebrates (Corosaurus) and invertebrates alike (for a review see Storrs, 1991). The presence of the nothosauriform reptile Corosaurus has led some workers (Colbert, 1957; Zangerl, 1963) to assign the Alcova to the Upper Triassic. Storrs (1991: 101) accepted a late Lower Triassic (Scythian, Spathian) or, perhaps, an early Middle Triassic (Anisian) age for the Alcova Limestone.

SYSTEMATIC PALEONTOLOGY

Sauropterygia Owen, 1860 Eosauropterygia Rieppel, 1994a Corosaurus Case, 1936 Corosaurus alcovensis Case, 1936 1936 Corosaurus alcovensis, Case, p. 1. 1948 Corosaurus, Huene, p. 41f 1963 Corosaurus alcovensis, Zangerl, p. 117. 1991 Corosaurus alcovensis, Storrs, p. 1.

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CASPER N 220

251

Goose Egg

Jackson Canyon

Bessemer Mountain

Casper

Mountain

505 * *

487

* Muddy Mountain Freeland 402 *

Figure 1. Localities (â&#x2C6;&#x2014;) in the Alcova limestone surveyed in summer 1996, yielding Corosaurus bone.

Holotype. UW 5485, skull and partial skeleton (Figs 2â&#x20AC;&#x201C;6). Horizon and Distribution. Alcova (Limestone) Member, Crow Mountain Formation, Chugwater Group, Triassic. Vicinity of Casper, east-central Wyoming. Diagnosis. An eosauropterygian with a relatively short and unconstricted snout; postorbital region of skull subequal in length to preorbital region of skull; nasals elongated, only slightly shorter than frontals; upper temporal fossa subequal in size to orbit; frontals closely approaching the upper temporal fenestra; parietal skull table weakly constricted; mandibular symphysis weakly enforced; maxillary fang(s) in anterior position; distinct coronoid process on lower jaw present; the posterior aspect of the dorsal tip of the neural spines distinctly broadened; sacral ribs fused to respective vertebrae; transverse processes on caudal vertebrae distinctly elongated; dorsal part of medial surface of ilium ornamented by densely packed tubercles; pubis of distinctive shape, with convex anterior and concave posterior margin; deltopectoral crest on humerus weakly developed; medial gastral rib element frequently with two-pronged lateral process.

TRIASSIC STEM-GROUP SAUROPTERYGIA

Figure 2. Skull of Corosaurus alcovensis Case (holotype, UW 5485), in dorsal view. Scale bar=5cm.

CHARACTER DEFINITIONS

The characters listed below are based on the data used previously in the analysis of the phylogenetic interrelationships of Simosaurus (Rieppel, 1994a). The data matrix

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m n

f prf

pof po

ju?

pof

sq so

op-eo q

Figure 3. Skull of Corosaurus alcovensis Case (holotype, UW 5485), in dorsal view. Scale bar=20mm. Abbreviations: f, frontal; ju, jugal; m, maxilla; n, nasal; op-eo, opisthotic-exoccipital (paroccipital process); p, parietal; pm, premaxilla; po, postorbital; pof, postfrontal, prf, prefrontal; q, quadrate; so, supraoccipital; sq, squamosal.

relies heavily on the work of Gauthier, Kluge and Rowe (1988), further augmented by the addition of characters taken from Evans (1988) and Storrs (1991, 1993). Additional references pertaining to the coding of non-sauropterygian taxa can be found in Rieppel (1994a). Codings for sauropterygians other than Corosaurus are based on recent revisionary work (Rieppel, 1993a,b,c, 1994a,b,c, 1995, 1996, 1997; Rieppel & Lin, 1995; Rieppel & Wild, 1994, 1996). Coding for the postcranium of Cymatosaurus is based on the hypothesis that Proneusticosaurus Volz,1902, is a subjective junior synonym of Cymatosaurus Fritsch, 1894 (Rieppel & Hagdorn, 1997). 1. Premaxillae small (0) or large (1), forming most of snout in front of external nares. The premaxillae cover the entire snout region of the skull in front of the external nares in Corosaurus (1) (Figs 2, 3). 2. Premaxilla without (0) or with (1) postnarial process, excluding maxilla from posterior margin of external naris.

TRIASSIC STEM-GROUP SAUROPTERYGIA

Figure 4. Dorsal vertebrae 27 through 31 of Corosaurus alcovensis Case (holotype, UW 5485). A, left lateral view; B, dorsal view. Scale bar=2cm.

Corosaurus, as all Sauropterygia, lacks this process. 3. Snout unconstricted (0) or constricted (1). In the ‘eusauropterygian’ genera Cymatosaurus, Germanosaurus, Nothosaurus and (most) Lariosaurus, the snout is distinctly constricted at the level of the anterior margin of the external naris. A constriction of the snout is absent in pachypleurosaurs and in Simosaurus; in Pistosaurus, the snout gradually tapers to a blunt anterior tip. The snout is constricted in Placodus, but tapering in all cyamodontoids except Henodus. A snout constriction is absent in Corosaurus. Indeed, the contours of the snout of Corosaurus (Figs 2, 3) resemble exactly those of the specimen of Lariosaurus described by Mazin (1985). 4. Temporal region of skull relatively high (0) or strongly depressed (1). Placodus, pachypleurosaurs, Corosaurus, Cymatosaurus, Simosaurus and Pistosaurus show a distinctly lesser degree of depression of the temporal region of the skull than is observed in the Germanosaurus–Nothosaurus–Lariosaurus clade.

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Figure 5. Postcranial elements of Corosaurus alcovensis Case. A, dorsal vertebra 26 in posterior view (UW 5485); B, dorsal vertebra in posterior view (FMNH PR245); C, left scapula in medial view (UW 5485); D, right ilium in dorsomedial view (UW 5485). Scale bars: A=1cm; C, D=2cm.

5. Nasals shorter (0) or longer (1) than frontal(s). As in other â&#x20AC;&#x2DC;eusauropterygiansâ&#x20AC;&#x2122;, the premaxillae form a combined posteromedian process which extends far between the nasals, without reaching the anteromedial process of the frontals. The nasals taper anteriorly, forming a slender process which lines the medial (dorsal) margin of the external naris up to a level somewhat in front of the midpoint of the external naris. Taking their whole length into account, the nasals are only slightly shorter than the frontals (the right nasal is somewhat longer than the left nasal). At the level of the Eosauropterygia (Rieppel, 1994a), the relatively long nasals are autapomorphic in Corosaurus. The posterior tips of the nasals lie behind the level of the anterior margin of the orbit, as is the case in Eosauropterygia generally with the exception of Simosaurus (reduced nasals) and, possibly, early pachypleurosaurs. 6. Nasals not reduced (0), somewhat reduced (1), or strongly reduced or absent (2). The nasals of Corosaurus are unusually large in comparison to other Sauropterygia (Figs 2, 3). Cymatosaurus and Pistosaurus share strongly reduced or perhaps absent nasals (Rieppel, 1994a). The nasals are also reduced, although to a lesser degree, in Germanosaurus and Simosaurus.

TRIASSIC STEM-GROUP SAUROPTERYGIA

Figure 6. Humerus of Corosaurus alcovensis Case (holotype, UW 5485).

7. Nasals do (0) or do not (1) enter external naris. The nasal variably enters the external naris or remains excluded therefrom in Cymatosaurus; the nasal is excluded from the external naris in Pistosaurus. In all other stem-group Sauropterygia, the nasal enters the external naris. 8. Nasals meet in dorsomedial suture (0), or are separated from one another by nasal processes of the premaxillae extending back to the frontal bone(s) (1). This character is highly variable in Eosauropterygia (Sander, 1989; Rieppel & Wild, 1996), and variational studies in the pachypleurosaur genus Neusticosaurus have shown that the nasals may or may not grow so as to cover the premaxillary-frontal contact in dorsal view (Sander, 1989). The nasals always remain separated in the genera Cymatosaurus, Germanosaurus, Pistosaurus, and Simosaurus with reduced nasals. The large nasals meet in Corosaurus (Figs 2, 3). The character is polymorphic in Nothosaurus and Lariosaurus among â&#x20AC;&#x2DC;Eusauropterygiaâ&#x20AC;&#x2122;. 9. The lacrimal is present and enters the external naris (0) or remains excluded from the external naris by a contact of maxilla and nasal (1), or the lacrimal is absent (2). The lacrimal is universally absent in Sauropterygia, including Corosaurus. 10. The prefrontal and postfrontal are separated by the frontal along the dorsal margin of the orbit (0), or a contact of prefrontal and postfrontal excludes the frontal from the dorsal margin of the orbit (1). In Corosaurus, the prefrontal and postfrontal are separated by a distinct gap along

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the dorsal margin of the orbit (Figs 2, 3). The two bones approach each other more closely, however, than is the case in Germanosaurus, Lariosaurus, and Nothosaurus, which show relatively smaller pre-and postfrontals. A contact of prefrontal and postfrontal is variably present in Cymatosaurus, and is always present in Placodus. 11. Dorsal exposure of prefrontal large (0) or reduced (1). In the holotype of Corosaurus alcovensis, the right prefrontal is missing, but the frontal shows a discrete facet which received its posteromedial (posterodorsal) tip. The left prefrontal is incompletely preserved, but the preserved parts indicate a large dorsal exposure of the bone, and a broad contact with the adjacent nasal. The nasal–prefrontal contact is a very variable character among sauropterygians with reduced (Nothosaurus: Rieppel & Wild, 1996) or not reduced (Simosaurus: Rieppel, 1994a) prefrontals. This particular character is therefore not included in the present analysis. However, the dorsal exposure of the prefrontal is distinctly reduced in Nothosaurus, Lariosaurus and Germanosaurus as compared to other sauropterygians. 12. Preorbital and postorbital region of skull: of subequal length (0), preorbital region distinctly longer than postorbital region (1), postorbital region distinctly longer (2). In Corosaurus, the distance from the posterior margin of the orbit to the mandibular condyle of the quadrate (postorbital skull) closely approximates the distance from the anterior margin of the orbit to the tip of the snout (preorbital skull), or is even slightly shorter (note that the postorbital skull is separated from the preorbital skull in the holotype). In pachypleurosaurs, the preorbital skull is generally longer than the postorbital skull. In the ‘eusauropterygian’ genera Germanosaurus, Lariosaurus, Nothosaurus, and Simosaurus, the postorbital skull is distinctly elongated. In Cymatosaurus, the postorbital skull is relatively longer, but not as distinctly as in the Nothosauria. In Pistosaurus, the situation is less easily assessed, because the only skull still available for study lacks the tip of the snout. Illustrations of the lost specimen indicate, however, that the preorbital skull is relatively longer. 13. Upper temporal fossa absent (0), present and subequal in size or slightly larger than the orbit (1), present and distinctly larger than orbit (2), or present and distinctly smaller than orbit (3). As the postorbital region of the skull is not distinctly elongated in Corosaurus, the upper temporal fenestra is relatively smaller (as compared to other ‘eusauropterygians’), and while still larger than the orbit, the discrepancy is not as striking as in sauropterygians with an elongated postorbital skull. This is certainly a relatively plesiomorphic trait of Corosaurus, indicating a lesser development of the dual jaw adductor muscle system as compared to ‘eusauropterygians’ (Rieppel, 1994a). The longitudinal diameter of the (left) orbit is 26.5mm in the holotype of Corosaurus alcovensis, the longitudinal diameter of the (left) upper temporal fossa is approximately 35mm. Dividing the longitudinal diameter of the upper temporal fossa by the longitudinal diameter of the orbit yields a ratio of 1.32; corresponding values are 1.6-2.0 for Cymatosaurus (1.3 for the juvenile specimen designated ‘specimen II’ of C. gracilis by Schrammen, 1899, with relatively large orbits), 1.87 for Germanosaurus, 1.7-2.0 for Lariosaurus, 2.1-3.9 for Nothosaurus, and 1.5-2.45 for Simosaurus. 14. Frontal(s) paired (0) or fused (1) in the adult. The frontals are paired in Corosaurus (Figs 2, 3). Among other Sauropterygia, the frontals remain separate in Dactylosaurus, Cymatosaurus and Germanosaurus; they are incompletely fused in some specimens of the Serpianosaurus–Neusticosaurus clade (Carroll & Gaskill, 1985; Rieppel, 1989; Sander, 1989), and in Pistosaurus (Rieppel, 1994a,

TRIASSIC STEM-GROUP SAUROPTERYGIA

fig. 38). The frontals are fused in Simosaurus, Nothosaurus, and Lariosaurus. Fusion of frontals is variable in Placodus (Rieppel, 1995). 15. Frontal(s) without (0) or with (1) distinct posterolateral processes. The frontal of Corosaurus shows a distinct posterolateral process which, together with a posteromedial process, embraces an anterior process of the parietal. This arrangement results in a deeply interdigitating fronto-parietal suture (Fig. 3). 16. Frontal widely separated from the upper temporal fossa (0), narrowly approaches the upper temporal fossa (1), or enters the anteromedial margin of the upper temporal fossa (2). Corosaurus resembles Cymatosaurus and Germanosaurus by the presence of distinct posterolateral processes of the frontals which very narrowly approach the anteromedial margin of the upper temporal fossa (Figs 2, 3), and which enter the margin of the upper temporal fossa in some Cymatosaurus, as well as in Pistosaurus. The frontals do not approach the upper temporal fenestra as closely, and never enter its anteromedial margin in pachypleurosaurs, Simosaurus, and the Nothosaurus–Lariosaurus clade. 17. Parietal(s) paired (0), fused in their posterior part only (1), or fully fused (2) in adult. A distinct suture separates the parietals in front of the pineal foramen in the skull of Corosaurus. Behind the pineal foramen, the suture can be followed up to the posterior margin of the parietal table (Figs. 2, 3). The parietals are also paired along their entire length in pachypleurosaurs. In Cymatosaurus, the parietals are either completely fused, or they remain separate at least in front of the pineal foramen; the parietals also remain separate in front of the pineal foramen in Germanosaurus and Pistosaurus (as seen in the lost skull of ‘P. II’: Meyer, 1847–55). The parietals are fully fused in Placodus, Simosaurus, Nothosaurus and Lariosaurus. 18. Pineal foramen close to the middle of the skull table (0), is displaced posteriorly (1), is displaced anteriorly (2), or is absent (3). The relatively large pineal foramen of Corosaurus may lie somewhat in front of the midpoint of the parietal, but it is located well behind the fronto-parietal suture (Figs. 2, 3). In sauropterygians with an anteriorly displaced pineal foramen, the latter borders on the fronto-parietal suture (Placodus, Pistosaurus). 19. Parietal skull table broad (0), weakly constricted (1), strongly constricted (2), or forming a sagittal crest (3). The parietal skull table in Corosaurus is broad in front and immediately behind the pineal foramen (Figs. 2, 3). More posteriorly, the parietal narrows somewhat before bifurcating and extending posterolaterally to meet the squamosal along the posterior margin of the broad upper temporal fossa. The parietal skull table is strongly constricted, at least in its posterior part behind the pineal foramen, in Cymatosaurus, and in the Nothosaurus–Lariosaurus clade. Among stem-group Sauropterygia, a sagittal crest is formed in one species of Nothosaurus (Rieppel & Wild, 1994), in one species of Cymatosaurus (Rieppel & Werneburg, 1998), and in Pistosaurus. 20. Postparietals present (0) or absent (1). Postparietals are universally absent in Sauropterygia. 21. Tabulars present (0) or absent (1). Tabulars are universally absent in Sauropterygia. 22. Supratemporals present (0) or absent (1). Supratemporals are universally absent in Sauropterygia. 23. The jugal extends anteriorly along the ventral margin of the orbit (0), is

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restricted to a position behind the orbit but enters the latter’s posterior margin (1), or is restricted to a position behind the orbit without reaching the latter’s posterior margin (2). Storrs (1991) reconstructed a splint-like jugal in Corosaurus, wedged in between the postorbital and the posterior tip of the maxilla but without reaching the posterior margin of the orbit. In fact, the posteroventral corner of the orbit is not preserved on the right side of the skull, and is very difficult to interpret on the left side due to dorsoventral flattening of the skull which pushed the pterygoid–ectopterygoid flange up into the posteroventral corner of the orbit. Also, the lower end of the left postorbital is incompletely preserved. There is a triangular splint of bone lying above the posterior tip of the maxilla without reaching the posterior margin of the orbit (fig. 3), but it is impossible to determine whether this represents the jugal or a fragment of the ectopterygoid. Above that splint lies a larger element which an anterior process following the lower margin of the orbit anteriorly up to the level of its mid-point. Again, this bone could be the jugal, or the lower end of the postorbital. If this bone is interpreted as part of the postorbital, Corosaurus would be autapomorphic with respect to that character, because no other sauropterygian is known in which the postorbital extends anteriorly to the midpoint of the lower margin of the orbit (except for an undescribed species of Nothosaurus from the Muschelkalk of Nahal Ramon, Israel). If the element is considered the jugal, it would be represented in its plesiomorphic condition. I am inclined to consider the evidence inconclusive, and code the position of the jugal as unknown for Corosaurus. 24. The jugal extends backwards no farther than to the middle of the cheek region (0), or nearly to the posterior end of the skull (1). Due to the wide open ventral cheek region in Sauropterygia, the jugal does not extend far posteriorly. Where the cheek is closed (Placodus), the jugal does not extend beyond the middle of the cheek region. 25. The jugal remains excluded from (0) or enters (1) the upper temporal arch.In some stem-group Sauropterygia (Simosaurus, the Nothosaurus–Lariosaurus clade, Germanosaurus, Pistosaurus, and Placodus), the jugal takes an integral part in the formation of the upper temporal arch. 26. Postfrontal large and plate-like (0), with a distinct lateral process overlapping the dorsal tip of the postorbital (1), or postfrontal with a reduced lateral process and hence more of a narrow and elongate shape (2). The postfrontal in Corosaurus has a distinctly triangular shape, with a strong lateral process overlapping the proximodorsal tip of the postorbital. A distinct lateral process on the postfrontal is also present in Placodus, Cyamodus, Germanosaurus, Simosaurus and Pistosaurus, but it is reduced in pachypleurosaurs, in Nothosaurus (with the exception of Nothosaurus juvenilis: Rieppel, 1994c) and in Lariosaurus. 27. Lower temporal fossa absent (0), present and closed ventrally (1), present but open ventrally (2). Since the presence of a ventrally open lower temporal fenestra logically implies the loss of the lower temporal bar, and hence the previous presence of a ventrally closed temporal fenestra, this character is the only one of all multistate characters included in this analysis which was coded as ordered (as well as unordered) in the cladistic analysis (see discussion below). This character assumes on an a priori basis that sauropterygians are derived from diapsids by the loss of the lower temporal arch (Kuhn-Schnyder, 1967), rather that having undergone ventral emargination of the cheek region.

TRIASSIC STEM-GROUP SAUROPTERYGIA

28. Squamosal descends to (0), or remains broadly separated from (1) ventral margin of skull. The left part of the occiput is well preserved in Corosaurus, and shows the squamosal to descend far down towards the ventral margin of the skull laterally, leaving only the mandibular condyle of the quadrate exposed. 29. Quadratojugal present (0) or absent (1). As noted by Storrs (1991), the lateral edge of the laterally descending flange of the squamosal, covering the quadrate in lateral view, is finely broken, but the fragments appear all to be in place. Although the presence of a quadratojugal was hypothesized by Case (1936), the character cannot be assessed unequivocally. 30. Quadratojugal with (0) or without (1) anterior process. In Sauropterygians with a widely open cheek region, the quadratojugal never bears a distinct anterior process. This character is coded as unknown for those taxa which lack the quadratojugal. 31. Occiput with paroccipital process forming the lower margin of the posttemporal fossa and extending laterally (0), paroccipital processes trending posteriorly (1), or occiput plate-like with no distinct paroccipital process and with strongly reduced posttemporal fossae (2). The renewed preparation of the holotype of Corosaurus alcovensis further exposed the left paroccipital process, which extends posterolaterally, and which seems to have loosely articulated in a distinct notch in the lower margin of the occipital exposure of the squamosal with its distal tip (fig. 3). 32. Squamosal without (0) or with (1) distinct notch to receive distal tip of paroccipital process. The notch in the squamosal receiving the distal tip of the paroccipital process is also observed in Cymatosaurus (Rieppel, 1994a, fig. 39; Rieppel & Werneburg, 1997), but is otherwise currently unknown in other sauropterygians. 33. Mandibular articulations approximately at level with occipital condyle (0) or displaced to a level distinctly behind occipital condyle (1), or positioned anterior to the occipital condyle (2). If placed in situ, the left mandibular condyle of the quadrate lies distinctly behind the level of the occipital condyle in the holotype of Corosaurus alcovensis. 34. Exoccipitals do (0) or do not (1) meet dorsal to the basioccipital condyle. The left exoccipital is well exposed on the left side of the occipital condyle in the holotype of Corosaurus alcovensis, and indicates that the exoccipitals were not in contact dorsal to the basioccipital, as is also the case in all other adequately preserved Sauropterygia. 35. Supraoccipital exposed more or less vertically on occiput (0), or exposed more or less horizontally at posterior end of parietal skull table (1). The supraoccipital is not exposed in articulation with the skull table in Corosaurus. The bone slants sharply in a posteroventral direction in Placodus, Simosaurus, Cymatosaurus and Pistosaurus, as was most probably also the case in Corosaurus. In pachypleurosaurs, and in the Nothosaurusâ&#x20AC;&#x201C;Lariosaurus clade, the supraoccipital is broadly exposed between the posteriorly diverging temporal processes of the parietal, indicating a more horizontal orientation of the bone. 36. Occipital crest absent (0) or present (1). In Corosaurus, the parietal and squamosal form a distinct occipital crest, which is also present in Nothosaurus, Lariosaurus, and Pistosaurus. This occipital crest is absent

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in Placodus, pachypleurosaurs, Simosaurus, Germanosaurus, and in some, but not all, Cymatosaurus. 37. Quadrate with straight posterior margin (0) or quadrate shaft deeply excavated (concave) posteriorly (1). Corosaurus shares with ‘eusauropterygians’ sensu Tschanz (1989) the straight posterior margin of the quadrate. 38. Quadrate covered by squamosal and quadratojugal in lateral view (0), or quadrate exposed in lateral view (1). As in other sauropterygians, the descending process of the squamosal does not completely cover the quadrate in lateral view in Corosaurus. 39. Dorsal wing of epipterygoid broad (0) or narrow (1). The base of the epipterygoid is well preserved in Corosaurus, but its dorsal wing is broken (Case, 1936). Still, what is preserved of the bone indicates a narrow dorsal process. The dorsal wing of the epipterygoid is also narrow in Placodus (Rieppel, 1995), perhaps in pachypleurosaurs (Carroll & Gaskill, 1985, although the unequivocal identification of the epipterygoid is very difficult in this material), in Cymatosaurus (Rieppel & Werneburg, 1997), and in Pistosaurus (Rieppel, 1994a, fig. 38). The dorsal wing of the epipterygoid is broad in Simosaurus, Nothosaurus and most probably in Lariosaurus (Rieppel, 1994b). The epipterygoid is unknown in Germanosaurus. 40. Lateral conch on quadrate absent (0) or present (1). 41. Palatobasal articulation present (0) or absent (1). An immovable palatobasal articulation is characteristic of Sauropterygia in general (Storrs, 1991; Taylor, 1992; Taylor & Cruickshank, 1993; Rieppel, 1994a). The palate of Cymatosaurus shows the absence of interpterygoid vacuities and no indication of a movable palatobasal articulation, although the squamosal shows a distinct notch on the ventral margin of its occipital flange, which must have received the distal tip of the paroccipital process in a loose articulation. This latter feature, together with the fact that the otico-occipital segment of the braincase (prootic, opisthotic, exoccipital) has dropped out of the dermatocranium in all Cymatosaurus skulls available, might indicate a metakinetic skull in that genus. However, a beautifully preserved specimen of Cymatosaurus (Rieppel & Werneburg, 1997) shows fusion of the basicranium with the dermal palate, indicating an immovable palatobasal articulation. The same condition is assumed for Corosaurus, although neither the basicranium, nor the dermal palate are exposed. Interpterygoid vacuities are known in Pistosaurus (Edinger, 1935) and the plesiosaur–pliosaur clade, but in no other sauropterygian. Again, their presence in Corosaurus remains unknown (Storrs, 1991). 42. Basioccipital tubera free (0) or in complex relation to the pterygoid, as they extend ventrally (1) or laterally (2). This character was discussed extensively by Rieppel (1994b, 1995; Nosotti & Pinna, 1993). For lack of adequate preparation, this character cannot be assessed for Corosaurus. 43. Suborbital fenestra absent (0) or present (1). Renewed preparation of the holotype of Corosaurus alcovensis did expose bone in the floor of the right orbit, indicating that the suborbital fenestra is absent, as in all other Triassic stem-group Sauropterygia. 44. Pterygoid flanges well developed (0) or strongly reduced (1). Pterygoid-ectopterygoid flanges are well developed in Placodus (Rieppel, 1995), Nothosaurus giganteus (Rieppel & Wild, 1996), Cymatosaurus (Rieppel, 1997) and Pistosaurus (Rieppel, 1994a, fig. 38), but reduced in other Eosauropterygia. Storrs

TRIASSIC STEM-GROUP SAUROPTERYGIA

(1991) described a strongly developed pterygoid-ectopterygoid flange in Corosaurus, as may, indeed, be indicated by breakage of the bone at the posterior end of the left maxilla. 45. Premaxillae enter internal naris (0) or are excluded (1). This character cannot be assessed for Corosaurus. 46. Ectopterygoid present (0) or absent (1). Fragments of the ectopterygoid are exposed at the posterior tip of the left maxilla in Corosaurus (Case, 1936). The exposed left transverse process of the pterygoid retains parts of the ectopterygoid attached at the pterygoid-ectopterygoid suture. Among Sauropterygia, the ectopterygoid appears to be absent in pachypleurosaurs. 47. Internal carotid passage enters basicranium directly (0) or through quadrate ramus of pterygoid (1). Simosaurus and Nothosaurus show a peculiar passage of the internal carotid artery, which coming up from the neck enters the skull through a foramen on the posterior aspect of the quadrate ramus of the pterygoid (Rieppel, 1994b). The same derived course of the internal carotid is observed in a specimen of Cymatosaurus (Rieppel & Werneburg, 1997). For lack of adequate preservation, this character remains unknown for pachypleurosaurs, Germanosaurus, Lariosaurus, Pistosaurus, and Corosaurus. 48. Retroarticular process of lower jaw absent (0) or present (1). The lower jaw of Corosaurus (FMNH PR 246) shows a very prominent retroarticular process. The chorda tympani foramen is located posteroventromedial to the mandibular articular facet. 49. Distinct coronoid process of lower jaw absent (0) or present (1). Corosaurus (FMNH PR 246) is unusual among stem-group Eosauropterygia as it shows not only a well developed coronoid process on the lower jaw, but also a prominent lateral exposure of the coronoid bone. In other eosauropterygians (except for some plesiosaurs), the coronoid process is weakly developed (pachypleurosaurs) or virtually absent, and the coronoid bone located predominantly on the medial side of the mandible at the anterior margin of the adductor fossa (Rieppel, 1994a). Unfortunately, the lower jaw of Cymatosaurus remains very incompletely known. 50. Surangular without (0) or with (1) strongly projecting lateral ridge defining the insertion area for superficial adductor muscle fibres on the lateral surface of the lower jaw. The external jaw adductor muscle inserts into the dorsal surface of the surangular. In most reptiles, particularly those with an incomplete ventral margin of the dermal covering of the cheek region, the insertion of superficial fibers extends downwards on to the lateral surface of the lower jaw. A usually weakly expressed ridge on the surangular commonly delineates the ventral margin of the insertional area (Rieppel & Gronowski, 1981). Corosaurus shows an unusually well developed lateral ridge on the surangular, forming an almost horizontal shelve for the insertion of superficial jaw adductor muscle fibres. In that respect, Corosaurus resembles Simosaurus, Nothosaurus and Lariosaurus among sauropterygians (Rieppel, 1994a). Unfortunately, the lower jaw of Cymatosaurus remains incompletely known. 51. Mandibular symphysis short (0), somewhat enforced (1), or elongated and ‘scoop’-like (2). In contrast to pachypleurosaurs, which lack enlargement of the mandibular symphysis, Placodus and the ‘Eusauropterygia’ (with the exception of Simosaurus) share an elongated, ‘scoop’-like symphysis. The mandibular symphysis of Corosaurus is

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slightly enforced and ‘scoop’-shaped, but less so than in Cymatosaurus, Nothosaurus and Lariosaurus (Rieppel, 1994a). 52. Splenial enters the mandibular symphysis (0), or remains excluded therefrom (1). The splenial may be excluded from the mandibular symphysis in Corosaurus (Storrs, 1991, fig. 9C), but the character cannot be determined unequivocally on the basis of available material. 53. Teeth set in shallow or deep sockets (0) or superficially attached to bone (1). As in all Sauropterygia, the dentition of Corosaurus is thecodont. 54. Anterior (premaxillary and dentary) teeth upright (0) or strongly procumbent (1). Corosaurus resembles the ‘Eusauropterygia’ sensu Tschanz (1989) with strongly procumbent anterior dentary and maxillary teeth. 55. Premaxillary and anterior dentary fangs absent (0) or present (1). In the holotype of Corosaurus alcovensis, five premaxillary teeth are preserved, and they gradually decreased in size from front to back. The anterior dentary teeth even exceed the anterior premaxillary teeth in size (Case, 1936; Storrs, 1991). The presence of premaxillary fangs in Pistosaurus is coded on the basis of alveolar size in the figure of the (now lost) skull provided by Schrammen (1899). 56. One or two caniniform teeth present (0) or absent (1) on maxilla. Typically in ‘Eusauropterygia’ sensu Tschanz (1989), the maxillary bears one or two enlarged fangs located between the external naris and the orbit. This part of the maxilla shows no enlarged teeth in Corosaurus. However, the (preserved) anteriormost tooth on the maxilla is distinctly larger than the third tooth (the second alveolus is empty but large again). The succeeding maxillary teeth (a minimum of 10 is preserved) are all distinctly smaller than the anteriormost (preserved) tooth, and decrease in size from front to back. The conclusion, therefore, is that Corosaurus shares the presence of one, perhaps two enlarged fangs, only these are located in a more anterior position, lateral to the external naris, in an unconstricted snout. 57. The maxillary tooth row is restricted to a level in front of the posterior margin of the orbit (0), or it extends backwards to a level below the posterior corner of the orbit and/or the anterior corner of the upper temporal fossa (1), or it extends backwards to a level below the anterior one third to one half of the upper temporal fossa (2). Among stem-group Sauropterygia, Simosaurus, Nothosaurus and Lariosaurus show the posterior extension of the maxillary tooth row to a level well behind the anterior margin of the upper temporal fossa. In Corosaurus, the teeth extend to the posterior corner of the orbit. 58. Teeth on pterygoid flange present (0) or absent (1). No tooth fragments can be detected in the area of the crushed ectopterygoid– pterygoid flange in the holotype of Corosaurus alcovensis, and pterygoid teeth are therefore considered to be absent as in all other Sauropterygia. 59. Vertebrae notochordal (0) or non-notochordal (1). Non-notochordal vertebrae are generally characteristic of Eosauropterygia (Rieppel, 1994a); the non-notochordal nature of the vertebrae of Corosaurus was ascertained by complete separation of the 26th and 27th dorsal elements through a combination of chemical methods of preparation. 60. Vertebrae amphicoelous (0), platycoelous (1) or procoelous/opisthocoelous (2).

TRIASSIC STEM-GROUP SAUROPTERYGIA

The vertebrae of ‘Eusauropterygia’ are generally platycoelous; in Corosaurus, the dorsal vertebrae a deeply amphicoelous. 61. Dorsal intercentra present (0) or absent (1). 62. Cervical intercentra present (0) or absent (1). 63. Ventral surface of cervical centra rounded (0) or keeled (1). The holotype of Corosaurus shows the cervical vertebrae in lateral view only, their lower edge concealed by cervical ribs. FMNH PR 135 includes a transversally sectioned cervical element with somewhat irregular contours. It can be interpreted, however, as showing a keeled ventral surface of the centrum, as is the case in all other adequately preserved Sauropterygia. 64. Zygosphene-zygantrum articulation absent (0) or present (1). Storrs (1991) identified a zygosphene-zygantrum articulation between the 29th and 30th dorsal vertebrae of the holotype of Corosaurus alcovensis. The string of dorsal vertebrae 26 through 31 has been completely removed from the surrounding matrix (Fig. 4). These vertebrae are very tightly articulated with one another, which makes it exceedingly difficult to ascertain the presence of an accessory intervertebral articulation. Separation of the 26th (fig. 5A) from the 27th element exposed a badly eroded base of the neural spine in both elements, again rendering assessment of the character equivocal. The posterior surface of a separated neural arch (FMNH PR 245, fig. 5B) is relatively well preserved. It shows the base of the neural spine carrying a distinct posterior projection above the postzygapophyses. Below this posteriorly projecting spine, and above the postzygapophyses, is situated a small yet somewhat compressed pit. FMNH PR2018 allowed the preparation of an even better preserved posterior surface of an isolated dorsal neural arch. The neural spine carries a distinct ridge on its posterior surface, protruding into a posteriorly projecting tip at its base. Again a small yet distinct and deep pit is located below this spine, and between the postzygapophyses. If this pit is, indeed the zygantrum (as is assumed here on grounds of parsimony), it is undivided by a vertical septum, as is also the case in Cymatosaurus (Rieppel & Hagdorn, 1997). The zygantrum of Simosaurus and Nothosaurus is distinctly different, however, in that it is both broader and wider, internally divided by a vertical septum, and not recessed below a posterior basal projection of the neural spine. 65. Sutural facets receiving the pedicels of the neural arch on the dorsal surface of the centrum in the dorsal region are narrow (0) or expanded into a cruciform or ‘butterfly-shaped’ platform (1). The partially preserved neural arch referred to above (FMNH PR 245; Fig. 5B) is distorted so as to show the larger part of the base of the pedicels. These are somewhat expanded and would have met the dorsal surface of the centrum on a broadened ‘butterfly-shaped’ platform. In contrast to other sauropterygians, the neural arches seem to separate less easily form their respective centra in Corosaurus. The neural suture remains distinct throughout the vertebral column, however. 66. Transverse processes of neural arches of the dorsal region relatively short (0) or distinctly elongated (1). Corosaurus shows an autapomorphic elongation of the transverse processes on the proximal caudal vertebrae (Case, 1936; Storrs, 1991), but although distinct, the transverse processes of the dorsal vertebrae are not distinctly elongated to an extent that would be similar to that seen in Placodus or archosauromorph reptiles. 67. Vertebral centrum distinctly constricted in ventral view (0) or with parallel lateral edges (1).

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The vertebral centra are not constricted in ventral view in Dactylosaurus, in the Serpianosaurus–Neusticosaurus clade (Rieppel & Lin, 1995), and they are not or only slightly constricted in Nothosaurus and Lariosaurus. The centra are distinctly constricted in Corosaurus as well as in Placodus and Simosaurus. 68. Distal end of transverse processes of dorsal vertebrae not increasing in diameter (0) or distinctly thickened (1). The transverse processes on the dorsal vertebrae of Corosaurus show a distinct expansion into a vertically oriented, oblong or oval articular head. This character is shared by Pistosaurus (Sues, 1987) among the sauropterygians included in this analysis. A relatively high, vertically oriented and oval articular surface is also observed on the transverse processes of dorsal vertebrae of pachypleurosaurs, Simosaurus, Nothosaurus (Rieppel & Wild, 1996), Lariosaurus, and some isolated vertebrae referred to Cymatosaurus (Rieppel & Hagdorn, 1997), but in all these taxa the transverse processes are distinctly shorter and overall stouter than in Corosaurus and Pistosaurus, and a distal expansion is, if at all, only very weekly expressed. 69. Zygapophyseal pachyostosis absent (0) or present (1). Zygapophyseal pachyostosis, resulting in a ‘swollen’ appearance of pre- and postzygapophyses, is present in all pachypleurosaurs, as well as in some species of Nothosaurus and all species of Lariosaurus. It is absent in all other Sauropterygia, including Corosaurus. 70. Pre-and postzygapophyses do not (0) or do (1) show an anteroposterior trend of increasing inclination within the dorsal and sacral region. The pre-and postzygapophyses of Corosaurus show a more or less horizontal orientation in the anterior dorsal region; in the three sacral vertebrae, the exposed zygapophyses show an inclined articular surface (Storrs, 1991). A similar change of zygapophyseal orientation within the vertebral column is also known in Placodus (Rieppel, 1995; the trend is reversed in the sacral region in Placodus) and Simosaurus Rieppel, 1994a), but not in pachypleurosaurs, nor in the Nothosaurus–Lariosaurus clade. 71. Cervical ribs without (0) or with (1) a distinct free anterior process. Cervical ribs are well exposed in the holotype of Corosaurus alcovensis, and they show a distinct free anterior process (Storrs, 1991), as is generally the case in stemgroup Sauropterygia. 72. Pachyostosis of dorsal ribs absent (0) or present (1). Pachyostosis of the dorsal ribs is absent in Dactylosaurus, present in the Serpianosaurus– Neusticosaurus clade, and it is also present in some Nothosaurus and some Lariosaurus species. It is absent in all other Sauropterygia, including Corosaurus. 73. The number of sacral ribs is two (0); three (1); four or more (2). The three sacral ribs of Corosaurus are firmly fused to their respective vertebrae, and they show a marked distal expansion. This distinguishes them from the first caudal rib (transverse process), which is turned anteriorly, but lacks the distal expansion and probably did not contact the ilium. 74. Sacral ribs with (0) or without (1) distinct expansion of distal head. The sacral ribs of Corosaurus show a distinct distal expansion, as is also the case in Simosaurus and Placodus among stem-group Sauropterygia. 75. Sacral (and caudal) ribs or transverse processes sutured (0) or fused (1) to their respective centrum. Corosaurus is unusual among Sauropterygia by showing sacral ribs (or transverse processes: see Rieppel, 1993d) fused to their respective centrum. This condition is

TRIASSIC STEM-GROUP SAUROPTERYGIA

not known to occur in Placodus, pachypleurosaurs, or in any of the ‘eusauropterygians’ included in this study with the possible exception of Nothosaurus giganteus (Rieppel & Wild, 1996, fig. 55). 76. Cleithrum present (0) or absent (1). The cleithrum is universally lacking in Sauropterygia. 77. Clavicles broad (0) or narrow (1) medially. The best preserved and fully exposed clavicle of Corosaurus shows the element to taper to a blunt tip medially (Storrs, 1991, fig. 11B), as is also observed in Placodus and some (as yet undescribed; kept in private collections) specimens of Nothosaurus (Rieppel & Wild, 1996). In other eosauropterygians, the clavicles are broad medially. 78. Clavicles positioned dorsally (0) or anteroventrally (1) to the interclavicle. The anteroventral position of the clavicles with respect to the interclavicle was recognized as a sauropterygian synapomorphy by Carroll and Gaskill (1985). 79. Clavicles do not meet in front of the interclavicle (0) or meet in an interdigitating anteromedial suture (1). This character cannot be assessed unequivocally in the holotype of Corosaurus alcovensis. Further preparation of the right clavicle has revealed a greater width of the anteromedial margin than figured by Storrs (1991, fig. 11A), resulting in greater similarity of the element with the clavicle shown in figure 11B by Storrs (1991). Unfortunately, the medial tip of the right clavicle of the holotype remains obscured by an overlapping rib. Based on the specimen figured by Storrs (1991, fig. 11B), the absence of a medial interdigitating suture is assumed (as by Storrs, 1991, in his reconstruction of the pectoral girdle). 80. Clavicles without (0) or with (1) anterolaterally expanded corners. The right clavicle of the holotype of Corosaurus alcovensis shows a distinctly expanded anterolateral corner, which is also seen in Dactylosaurus among pachypleurosaurs, and in Simosaurus, Nothosaurus and Lariosaurus among stem-group ‘eusauropterygians’. 81. Clavicle applied to the anterior (lateral) (0) or to the medial (1) surface of scapula. The position of the clavicle medial rather than anterior (lateral) to the scapula was recognized as a sauropterygian synapomorphy by Carroll and Gaskill (1985). The left scapula of the holotype of Corosaurus alcovensis has been completely freed from the surrounding matrix (Fig. 5C), and reveals a mediolaterally compressed state of preservation. The anterior slope of the glenoidal portion of the scapula is complete and smooth, however, and it delineates the lateral margin of an articular facet which is located on the anteromedial aspect of the scapula, much as in other Eosauropterygia (Rieppel, 1994a). 82. Interclavicle rhomboidal (0) or T-shaped (1). As indicated by Case (1936) and Storrs (1991), the interclavicle is a triradiate element approaching a T-shaped structure modified by some reduction of the posterior stem. 83. Posterior process on (T-shaped) interclavicle elongate (0), short (1), or rudimentary or absent (2). The posterior stem of the interclavicle of Corosaurus is still about as long as either anterior lateral process, which corresponds to similar proportions of the interclavicle in Simosaurus. The posterior stem is much more reduced in Placodus or in the Nothosaurus–Lariosaurus clade. 84. Scapula represented by a broad blade of bone (0), or with a constriction separating a ventral glenoidal portion from a posteriorly directed dorsal blade (1).

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The scapula of Corosaurus is strongly constricted at the transition from the triangular (and laterally compressed) glenoidal portion and the posterodorsal blade or process. With respect to this character, Corosaurus resembles other Eosauropterygia more closely than Placodus, where the scapula is a broad plate of bone as in plesiomorphic amniotes. 85. The dorsal wing or process of the eosauropterygian scapula tapers to a blunt tip (0) or is ventrally expanded at its posterior end (1). The posterodorsal process on the scapula of the holotype of Corosaurus alcovensis is broken at its posteroventral edge, but the curved margin of the remaining bone clearly indicates some distal expansion of the scapular blade (Fig. 5C; see also Storrs, 1991, fig. 12A). This is a character which Corosaurus shares with Pistosaurus (Sues, 1987) among the ‘Eusauropterygia’ included in this analysis. In pachypleurosaurs, Simosaurus, Nothosaurus and Lariosaurus, the posterodorsal process of the scapula never expands distally, but is either parallel edged (Simosaurus), or tapers to a blunt tip. 86. Supraglenoid buttress present (0) or absent (1). The supraglenoid buttress is uniformly absent in Sauropterygia. 87. One (0) or two (1) coracoid ossifications. Corosaurus shares a single coracoid ossification with all other Sauropterygia. 88. Coracoid of rounded contours (0), slightly waisted (1), strongly waisted (2), or with expanded medial symphysis (3). The coracoid of Corosaurus is an almost rectangular plate with only slightly convex anterior and posterior margins. This contrasts with the shape of the coracoid in other stem-group Eosauropterygia, which has strongly concave anterior and posterior margins, resulting in a distinctly waisted shape. Pistosaurus approaches plesiosaurs with an expanded medial symphysis between the coracoids (Sues, 1987). 89. Coracoid foramen enclosed by coracoid ossification (0), or between coracoid and scapula (1). The scapula and coracoid of Corosaurus are not well enough known to allow the unequivocal assessment of this character. It appears, however, that a slight indentation on the ventral margin of the posterior part of the glenoidal portion of the scapula might indicate the position of the coracoid foramen between scapula and coracoid, as is characteristic of other stem-group Sauropterygia. 90. Pectoral fenestration absent (0) or present (1). See Storrs (1991) for further comments on this character. 91. Limbs short and stout (0) or long and slender (1). 92. Humerus rather straight (0) or ‘curved’ (1). The ‘curved’ humerus has been used as a ‘nothosaurian’ character for a long time, and was used by Storrs (1991) in support of his hypothesis that Placodontia is the sister-group of ‘Eusauropterygia’, coding a straight and slender humerus for pachypleurosaurs. Assessment of the character is somewhat complicated by sexual dimorphism of the humerus in pachypleurosaurs. It is, however, rather difficult to distinguish any significant difference in shape of the humerus of adult Dactylosaurus (sex y: Rieppel, 1993b; 1994a; Rieppel & Lin, 1995) and adult representatives of the Serpianosaurus–Neusticosaurus–clade (Carroll & Gaskill, 1985; Rieppel, 1989, Sander, 1989) from a typical humerus of Nothosaurus (see Rieppel, 1994a, fig. 58). The curved humerus is therefore coded present for pachypleurosaurs (as in Rieppel, 1994a). However, sauropterygian humeri differ in the relative development of the deltopectoral crest. 93. Deltopectoral crest well developed (0) or reduced (1).

TRIASSIC STEM-GROUP SAUROPTERYGIA

The humerus of Corosaurus appears more distinctly curved than that of most other Sauropterygia due to a lesser development of the deltopectoral crest (fig. 6). A humerus with a well developed deltopectoral crest (pachypleurosaurs sex y, with the exception of Keichousaurus; Simosaurus, Nothosaurus, Cymatosaurus) shows an angulation of the proximal part of the preaxial margin of the humerus. Below the deltopectoral crest, the anterior margin of the humerus may be straight or even somewhat concave, resulting in a slightly waisted mid-diaphyseal region. In Corosaurus, the deltopectoral crest is not prominent, and the preaxial margin of the humerus evenly convex, resulting in a more strongly curved appearance of the humerus. This humeral morphology is shared by some Lariosaurus, and by Placodus among the Sauropterygia included in this analysis. 94. Insertional crest for latissimus dorsi muscle prominent (0) or reduced (1). The postaxial margin of the humerus of Corosaurus is evenly concave, with no indication of a projecting crest for the insertion of the latissimus dorsi muscle. This contrasts with the condition observed in Nothosaurus (Rieppel, 1994a, fig. 58F), particularly in large specimens (Rieppel, 1994a, fig. 59B), where the crest for the insertion of the latissimus dorsi muscle projects into the concavity of the posterior margin of the humerus. This degree of development of the crest for the insertion of the latissimus dorsi muscle is not known in Placodus (Rieppel, 1994a, fig. 60), pachypleurosaurs (e.g. Rieppel & Lin, 1995, fig. 14), Simosaurus (Rieppel, 1994a), Lariosaurus and Pistosaurus (Sues, 1987), but it does occur in humeri attributed to Cymatosaurus (Rieppel, 1994a, fig. 57). 95. Humerus with prominent (0) or reduced (1) epicondyles. The humerus of Corosaurus shows strongly reduced epicondyles as is typical of most Sauropterygia with the exception of humeri attributed to Cymatosaurus (Rieppel, 1994a, fig. 57), Dactylosaurus (Rieppel & Lin, 1995, fig. 14; see also Rieppel, 1993b, fig. 8), and some Neusticosaurus (Rieppel & Lin, 1995). 96. The ectepicondylar groove is open and notched anteriorly (0), open without anterior notch (1), or closed (2) (i.e. ectepicondylar foramen present). The ectepicondylar groove is distinct in Corosaurus, and deeply notched distally. This character is variable among other Sauropterygia. 97. Entepicondylar foramen present (0) or absent (1). The entepicondylar foramen is present in Corosaurus, as in most stem-group Sauropterygia with the exception of Placodus, Simosaurus, and Pistosaurus. 98. Radius shorter than ulna (0), or longer than ulna (1) or approximately of the same length (2). The left radius and left ulna of the holotype of Corosaurus alcovensis have been completely removed from the matrix. Both elements are broken at their proximal end, such that their individual length cannot be established. The radius appears generally as a more robust bone with a triangular cross-section compared to the flattened ulna. The ulna, on the other hand, shows a greater degree of terminal expansion, and a greater degree of concavity of its preaxial margin (compared to the postaxial margin of the radius). If the two bone fragments are placed in a natural position, it appears from the degree of their curvature that the two elements would be of similar length (see also Storrs, 1991). 99. Iliac blade well developed (0), reduced but projecting beyond level of posterior margin of acetabular portion of ilium (1), reduced and no longer projecting beyond posterior margin of acetabular portion of ilium (2), or absent, i.e. reduced to simple dorsal stub (3).

O. RIEPPEL

The right ilium of Corosaurus (FMNH PR243; Storrs, 1991, fig. 15C) has been completely removed from the surrounding matrix (fig. 5D). It closely resembles the ilium of Placodus, Simosaurus and Nothosaurus in being partitioned into a roughly triangular or pyramidal ventral (acetabular) portion, and a reduced dorsal blade carrying a small anterior and a distinct posterior projection. The posterior process of the dorsal blade of the ilium projects distinctly beyond the posterior margin of the bone in Corosaurus, as is also the case in Placodus, but not in Simosaurus and Nothosaurus. The medial surface of the ilium, facing dorsomedially in the articulated skeleton (as indicated by articulated material of Lariosaurus (Ceresiosaurus) calcagnii: H. Lanz, pers. comm.; see also Nothosaurus (Paranothosaurus) amsleri: Peyer, 1939, pl. 67), is finished perichondral bone with an interesting ornamentation consisting of numerous, densely packed tubercles in the dorsal region, between the broad ventral (acetabular) portion and the dorsal wing. A similarly distinct ornamentation, probably related to muscle insertions, has until now not been observed in any other stem-group sauropterygian. The acetabular surface of the ilium is unfinished, and appears somewhat eroded. 100. Pubis with convex (0) or with concave (1) ventral (medial) margin. The pubis of Corosaurus is autapomorphic (at the level of the Sauropterygia) by having a convex anterior and a concave posterior margin. It therefore lacks the â&#x20AC;&#x2DC;waistedâ&#x20AC;&#x2122; appearance (concave anterior and posterior margins) typical for the pubis of other stem-group Sauropterygia other than Placodus. In contrast to Corosaurus (as well as Placodus and Cymatosaurus [syn. Proneusticosaurus]), which shows a rounded (convex) ventral (medial) margin of the pubis, Simosaurus and Nothosaurus have developed a distinct concavity in the ossified ventral margin of the pubis. A similar, if shallower concavity is observed in Serpianosaurus, and in some Lariosaurus. 101. Obturator foramen closed (0) or open (1) in adult. The obturator foramen in the pubis of Corosaurus is open, i.e. not completely enclosed within the pubis. The same condition is observed in Cymatosaurus (syn. Proneusticosaurus), in some adults of the Serpianosaurusâ&#x20AC;&#x201C;Neusticosaurus clade, and in Lariosaurus. The obturator foramen is usually closed in adults of Simosaurus and Nothosaurus (Rieppel, 1994a), and in Placodus. 102. Thyroid fenestra absent (0) or present (1). The thyroid foramen is present in Corosaurus as a consequence of the concave posterior margin of the pubis and the concave anterior margin of the ilium. 103. Acetabulum oval (0) or circular (1). 104. Femoral shaft stout and straight (0) or slender and sigmoidally curved (1). The femur of Corosaurus is distinctly longer than the humerus, and of a more gracile appearance. It is a slender, sigmoidally curved bone with a cylindrical diapophysis, as is generally the case in stem-group Sauropterygia. 105. Internal trochanter well developed (0) or reduced (1). The femur of Corosaurus shows a distinct internal trochanter (Storrs, 1991), similar to the internal trochanter of Placodus (Rieppel, 1994a, figs 62, 63) and Cymatosaurus (syn. Proneusticosaurus). The internal trochanter is more strongly reduced in other Sauropterygia. 106. Intertrochanteric fossa deep (0), distinct but reduced (1), or rudimentary or absent (2). In Corosaurus, the internal trochanter is set off from the proximal articular

TRIASSIC STEM-GROUP SAUROPTERYGIA

head by a very shallow intertrochanteric fossa only, as is also the case in other sauropterygians with the exception of Placodus (Rieppel, 1994a, figs 62, 63). 107. Distal femoral condyles prominent (0) or not projecting markedly beyond shaft (1). The distal articular head of the femur shows weakly developed articular condyles vaguely separated from one another but not projecting beyond the femoral shaft. 108. Anterior femoral condyle relative to posterior condyle larger and extending further distally (0) or smaller/equisized and of subequal extent distally (1). 109. The perforating artery passes between astragalus and calcaneum (0), or between the distal heads of tibia and fibula proximal to the astragalus (1). FMNH PR 480 shows a partially preserved pes with two large, circular bones in the proximal tarsal row. The larger of these must be the astragalus, the smaller one the calcaneum. The astragalus of Nothosaurus and Simosaurus (Rieppel 1994a, figs 34, 64) shows a distinct concavity on the proximal edge, which may articulate with the distal end of the tibia and/or indicate the passage of the perforating artery proximal to the astragalus (Rieppel, 1994a, fig. 64). Such a concavity is absent on the astragalus of Placodus and Dactylosaurus, but it is present in the Serpianosaurusâ&#x20AC;&#x201C;Neusticosaurus clade, and in Lariosaurus. However, in no sauropterygian is there evidence of even an ill defined foramen located between astragalus and calcaneum for the passage of the perforating artery. 110. Astragalus without (0) or with (1) a proximal concavity. The astragalus may show a well defined articular facet for the tibia on its proximal margin in a variety of amniotes. In sauropterygians, the astragalus generally shows a reduced degree of ossification, and in articulated tarsi is often located in an intermedium position distal to the spatium interosseum. The concavity on its proximal margin may therefore indicate the passage of the perforating artery (Rieppel, 1994a, fig. 64). A concavity is absent on the proximal margin of the astragalus in Corosaurus. 111. Calcaneal tuber absent (0) or present (1). 112. Foot short and broad (0) or long and slender (1). 113. Distal tarsal 1 present (0) or absent (1). 114. Distal tarsal 5 present (0) or absent (1). 115. Total number of tarsal ossifications four or more (0), three (1) or two (2). The partially preserved tarsus of FMNH PR 480 shows the fourth distal tarsal in articulation with the calcaneum. There is no evidence of any other distal tarsal bone, and considering the state of preservation, three tarsal ossifications is considered the natural number. 116. Metatarsal 5 long and slender (0) or distinctly shorter than the other metatarsals and with a broad base (1). In the partially preserved tarsus of FMNH PR 480, three metatarsals and one phalanx are preserved. There is, however, no way to identify the exact homology of the metatarsals, although the fifth would be expected to be shorter than the ones present. 117. Metatarsal 5 straight (0) or â&#x20AC;&#x2DC;hookedâ&#x20AC;&#x2122; (1). 118. Mineralized sternum absent (0) or present (1). See deBraga and Rieppel (1997) for further discussion of this character. 119. The medial gastral rib element always only has a single (0) lateral process, or may have a two-pronged lateral process (1). Isolated median gastral rib elements of Corosaurus may show a two-pronged lateral

O. RIEPPEL

process. The same character is observed in Simosaurus and Nothosaurus (Rieppel, 1994a), but it has never been observed in pachypleurosaurs, and it also does not occur in Cymatosaurus (syn. Proneusticosaurus). Two-pronged lateral processes also occur on medial gastral elements of Microleptosaurus schlosseri Skuphos, 1893, but this taxon is too incompletely known to be entered in this phylogenetic analysis (Rieppel, 1996).

CLADISTIC ANALYSIS

The data matrix shown in Table 1 was analyzed using the software package PAUP version 3.1.1 developed by David L. Swofford (Swofford, 1990; Swofford & Begle, 1993). The heuristic search option implemented invariably employed random stepwise addition (10 replications unless noted otherwise), and branch swapping (on minimal trees only) was effected by tree bisection an reconnection. All searches were run with all multistate characters unordered unless noted otherwise. An initial analysis included sauropterygian taxa only in search for an unrooted network. Excluding all non-sauropterygian taxa from the analysis rendered a large number of characters uninformative (1, 2, 5, 9, 15, 20, 21, 22, 24, 27, 28, 30, 34, 38, 40, 41, 42, 43, 47, 48, 52, 53, 58, 59, 61, 62, 63, 64, 65, 66, 71, 73, 76, 78, 81, 84, 86, 87, 88, 89, 90, 91, 92, 102, 103, 104, 107, 108, 109, 111, 112, 113, 114, 116, 117, 118), and yielded one single most parsimonious network (Fig. 7A) which would not support a monophyletic Eusauropterygia (Tschanz, 1989), Nothosauriformes (Storrs, 1991) or Eosauropterygia (Rieppel, 1994a), no matter where the root is placed. Including all 23 terminal taxa in a search for an unrooted network rendered character 63 the only uninformative one, and yielded three most parsimonious networks with lack of resolution among archosauromorph taxa only. The segment of that network including Sauropterygia (Fig. 7B) potentially supports a monophyletic Eosauropterygia, but would not support a monophyletic Eusauropterygia or Nothosauriformes no matter where the root is placed. Rooting the tree always assumed monophyly of the ingroup and paraphyly of the outgroup. Rooting the Sauropterygia (ingroup) on all other taxa included in the analysis (character 63 uninformative) yielded three most parsimonious trees (MPTs) with a tree length (TL) of 446 steps, a Consistency Index (CI) of 0.655, and a Retention Index (RI) of 0.697. Relationships within Sauropterygia are fully resolved. Tree topology reads as follows (see also fig. 8): (Placodus ((Corosaurus (Cymatosaurus, Pistosaurus)) ((Dactylosaurus, Serpianosaurusâ&#x20AC;&#x201C;Neusticosaurus) (Simosaurus (Germanosaurus (Nothosaurus, Lariosaurus)))))). The strict consensus tree of the three MPTs reveals lack of resolution to be restricted to archosauromorph taxa. Identical (and fully resolved) ingroup interrelationships were found by rooting the Sauropterygia (ingroup) on the outgroup taxa used by Storrs (1991), i.e. Captorhinidae, Araeoscelidia, Younginiformes and Claudiosaurus. The analysis yielded one single MPT (TL=251; CI=0.697; RI=0.726), but many of the characters were rendered uninformative by the deletion of all other terminal taxa from the analysis (2, 5, 24, 28, 34, 40, 53, 63, 66, 111, 116, 117). Treating Diapsida (including Testudines: Rieppel & deBraga, 1996) as ingroup, and rooting the analysis on Captorhinidae (character 63 uninformative) yielded three MPTs (TL=446; CI=0.655; RI=0.697), with lack of resolution restricted to the archosauromorph clade. Sauropterygian interrelationships are fully resolved and identical to those indicated above.

TRIASSIC STEM-GROUP SAUROPTERYGIA

T 1.

Data matrix for the analysis of the phylogenetic relationships of Triassic stem-group Sauropterygia.

Corosaurus-Data-141

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

0 0 0 0 0 0 0 0 1 1 1 1 1 0 1 1 1 1 1 1 1 1 1 1

0 0 0 0 0 0 0 0 1 1 ? 1 1 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 1 1 0 1

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 1 0 0

0 0 0 0 0 0 0 0 0 1 0 1 1 0 0 0 0 0 0 0 0 0 0 1

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 2 1 2 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 &1 0 1 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0&1 1 0&1 0&1 0 1 1 1 0

0 0 1 0 1 1 1&2 1&2 1 1 ? 1 1 1 2 2 2 2 2 2 2 2 2 2

0 0 0&1 0 0 0 0 0&1 0 0 1 0 0 0 0 0 0 0 0 0 0&1 0 0 1

Corosaurus-data-142

1 2 3 4 5 6 7 8 9 10 11 12 13

Ancestor Captorhinidae Testudines Araeoscelidia Younginiformes Kuehneosauride Rhynchocephalia Squamata Rhynchosauria Prolacertiformes Trilophosaurus Choristodera Archosauriformes

0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0&2 1 1 0 0 0&1&2 0 1 1 2 0&1

0 0 0 1 1 1 1 1 1 1 1 1 1

0 0 0 0 0 0 & & 1 & 0 0 &

0 0&2 3 0 0 2 1 0&2 0 1 0&2&3 0 3 1 2&3 3 3 1 0&3 0

0 0 0 0 0 0 &2 &2 3 0 3 1 &2

0 0 1 0 0 1 1 1 1 1 1 1 1

14 15 16 17 18 19 20 21 22 23 24

Claudiosaurus Dactylosaurus Serpiano-Neustico Simosaurus Nothosaurus Lariosaurus Corosaurus Cymatosaurus Germanosaurus Pistosaurus Placodus

0 0 0 0 1 1 0 0 1 0 0

1 1 1 2 2 2 0 2 2 1 0

1 3 3 2 2 2 1 2 2 2 2

0 0 0 1 2&3 2 1 2&3 1 3 0

1 1 1 1 1 1 1 1 1 1 1

Ancestor Captorhinidae Testudines Araeoscelidia Younginiformes Kuehneosauridae Rhynchocephalia Squamata Rhynchosauria Prolacertiformes Trilophosaurus Choristodera Archosauriformes Claudiosaurus Dactylosaurus Serpiano-Neustico Simosaurus Nothosaurus Lariosaurus Corosaurus Cymatosaurus Germanosaurus Pistosaurus Placodus

0 0 0 0 0 ? 0 0 ? 0 1 0 0 1 0 0 1 0 0&1 0&1 0 0&1 0 0 0 0 0 0 0&1 0 0 0 1 0 0 0 0&1 0&1 0&1 &2 0 1 0 0 1 0 0 1 0 1 0 0 1 1 0 1 1 0 0 1 1 0 1 1&2 0 1 1 0 1 2 0&1 1 0

0 0 0

0 0 0 2 2 2 0 1&2 1 1 0&2

0 0 0 1 1 1 0 0 1 2 2

O. RIEPPEL

T 1—continued Corosaurus-Data-143 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Corosaurus-Data-144 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

0 1 1 0 0&1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

0 0 & 0 0 1 & & 1 & ? 1 & 1 1 1 1 1 1 1 1 1 1 1

0 0 0 0 0 0 & & 0 0 0 0 0 0 ? 0 0 & & ? 1 2 ? 0

0 0 0 0 0 0 1 0 1 0&1 ? 0 1 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 1 1 1

0 0 0

0 0 0 0 0 0 0&1 0 1 1 1 1 0&1 0 2 2 2 2 2 1 1 ? 1 1

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 ? ? 0

1 1

0 0

1 1

2 2

0 0 1 0 0 1 0 1 1 1 ? 1 1 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 1 0 1 0 0&1 0 0&1 0 0 0 0 0 0&1 ? ? 1 ? 1 0

0 0 0 0 0 1 1 ? 0 1 ? 0 0 1 1 1 1 1 ? ? ? ? ? 0

0 0 1 0 1 1 1 1 1 1 1 1 1 0 1 1 0 0 0 0 0 0 0 1

0 0 1 0 1 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 1 1

0 0 0 ? ? ? 1 1 1 ? 0 ? ? ? 1 1 0 0 0 1 ? ? 1 1

0 0 0 0 0 1 0&1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 ? 0 1 0&1 1 1 1 2 1 1&2 1 2 1 1 1 2 1 0 1 1 1 1 1 2 2 2 2 2 1 2 1&2 2 2 2 1 2 1 2 1 2 1 2 1 2

0 0 0 0 0 ? 0 0 0&2 0 0 0 0 1 0 0 0 1 0 0 0 1 0 1 0 1 0 0&1 0&1&2 0&1 0&1 0&1 0 0 0 0 0 1 0 0 2 ? 0 ? 1 1 0 1 0&1&2 1 0 0&1 0 ? 0 0 0 ? 1 0 0 ? 1 0 1 1 0 0 0&1 1 1 1 0 ? 1 1 1 1 0 1 1 ? 0 0&1 ? ? ? 0 1 ? 0 1 0 1 0 0

TRIASSIC STEM-GROUP SAUROPTERYGIA

T 1—continued Corosaurus-Data-145

0 0 0&1 0 0 0 0 0 0 0 0 ? 0&1 0 1 1 1 1 1 1 1 1 1 1

0 0 0 0 0 0 0 0 0 0 0 0 0 0 ? ? 2 2 ? ? ? ? ? 1

0 0 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 ? 0 0

Corosaurus-Data-146

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 2 1 2 ? ? 2

0 0 1 1 ? ? 1 1 0 1 ? 1 1 1 ? ? 1 1 ? ? 1 ? ? 0

0 0 ? 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 ? 0 0 0 0 0 ? 0 ? 0 0 0 0 0 1 1 1 1 1 1 1 1

0 ? ? 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

0 0 0 0 0 0 0 0 0 0 0 0 0 0 ? ? 1 1 ? ? 1 ? ? 0

0 0 0&1 0 1 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 ? ? ? 1

0 0 0 0 1 0 1 1 1 0&1 1 0 0&1 0 0 0 0 0 0 1 0 ? ? 1

0 0 0 0 0 0 0 0 0 0 0 0 0 ? 0 0 1 1 1 1 ? ? ? 0

0 0 ? 0 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 1 0 1

0 0 ? 0 0 0 0 0 0 0 0 0 0 0 0 0 2 2 2 1 1 1 1 0

0 0 0 0 0 1 1 1 1 0&1 1 0 0&1 0 1 1 1 1 1 1 1 ? 1 1

0 0 0 0 0 1 0 0&1 1 0 0 0 0 0 0 0 ? ? 0 0 0 0 0&1 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0&1 0&1 1 0 0 1 0 1 1 0 1 1 0 0 0&1 1 0 1 ? 0 0 ? 0 0 1 0 ? ? ? 0 ? 0 0 1 0

0 0 0&1 0 0 1 0&1 1 1 1 1 1 1 0 ? 1 1 1 1 1 1 1 ? 1 0

0 0 0&2 0 0 0 0 0&2 0 0&2 1&2 1 &1&2 0 0 0 1 1 1 0 1 ? 1 0

O. RIEPPEL

T 1—continued Corosaurus-Data-147

0 0 1 0 0 1 0&1 1 0 0&1 0 1 0&1 0 1 1 1 1 1 1 1 ? ? 1

0 0 0&1 0 0 1 0 0 0 0&1 0 0 0&1 0 1 1 1 1 1 1 1 ? ? 1

0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ? ? ? 1

0 0 0 0 0 0 1 0&1 0 0 0 0 0 0 1 1 1 1 ? 1 1 ? 1 0

0 ? 0 ? 0 ? 0 0 ? ? ? 1 0 ? 1 1 1 1 2 1 1 ? 1 0

0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 ? 0 1

0 0 0 0 0 0 0 0 0 0 ? 0 0 0 1 1 0 1 1 0 0 ? 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 ? 1 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0&1 1 0 ? ? 0 0

0 0 0 0 0 1 0 0&1 0&1 0 0 0 0&1 0 0 0 1 0 0 1 ? ? ? 1

Corosaurus-Data-148

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

0 0 0 0 0 ? 0 0 0 1 1 1 1 1 1 1 1 1 1 1 ? ? 1 1

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0&1 0&1 0 0 ? 0 0

0 0 0 0 0 0 0 0 0 0 0 1 0&2 0 1 1 1 1 2 1 ? ? ? 1

0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 1 1 0 ? ? ? 0

0 0 0 0 0 ? 0 0 0 0 0 1 0 0 1 1 1 1 1 1 ? ? ? 1

0 0 0 0 0 ? 0 0 0 0 0 0 0 0 1 1 1 1 1 0 ? ? ? 0

0 0 0 0 0 ? 0 0 0 0 0 0 0 0 1 0 1 1 1 1 ? ? ? 0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Ancestor Captorhinidae Testudines Araeoscelidia Younginiformes Kuehneosauridae Rhynchocephalia Squamata Rhynchosauria Prolacertiformes Trilophosaurus Choristedera Archosauriformes Claudiosaurus Dactylosaurus Serpiano0Neustico Simosaurus Nothosaurus Lariosaurus Corosaurus Cymatosaurus Germanosaurus Pistosaurus Placodus

0 0 0 0 ? 0 0 1 0 0 0 0 0&1 0&1 1 1 ? ? 1 1 1 1 1 0&1 0 1 1 1 1 1 ? 1 1 0 1 1 0&1 1 1 1 1 1 0 1 0 0 1 0 0 1 0 0&1 1 0&1 0 1 0 1 1 1 0 1 ? ? 1 ? ? 1 ? 0 1 1

TRIASSIC STEM-GROUP SAUROPTERYGIA

T 1—continued Corosaurus-Data-149

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

0 0 0 0 0 ? 0 0 0 0 0 0 0 0 1 1 1 1 1 1 ? ? ? 1

0 0 1 1 0&1 ? 1 1 1 0 1 1 1 1 ? 0&1 1 0&1 ? 1 ? ? ? 1

0 ? 0 0 0 ? 0 0&1 0 ? 0 0 0 0 ? 2 1 2 ? 1 ? ? ? 1

0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 ? ? 1 0

0 ? ? ? ? ? ? ? ? ? ? ? ? ? 0 0 0 0 0 1 ? ? 1 ?

0 0 ? 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ? ? 1 1

0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 2 2 2 2 1 ? ? 3 0

0 0 1 0 0 0 0 0 0&1 0 0 0 0&1 0 1 1 1 1 1 1 ? ? ? 1

0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 ? ? ? 1

Corosaurus-Data-140

100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

0 0 0 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 ? 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 0 ? 1 1

0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0&1 1 1 0 ? ? 1

0 0 1 0 0 1 0 1 1 1 1 1 1 0 0 0 1 0 0 0 0 ? 1 1

0 0 0 0 0&1 2 0 0 0 1&2 0 2 0 0 1 1&2 2 2 2 2 ? ? 2 2

0 0 1&2 0 0 0 0 0 0 0 0 0 0 0 3 3 2 2 3 1 ? ? 0 1

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0&1 1 1 0&1 0 0 ? ? 0

0 0 0 0 0 0 0 0&1 0&2 0 0 0 0 0 0&2 0 0 2 0 0 2 0 0 0&2 0 0 0 0 0 0 ? 0 0 0 0 0&2 0 0&1 0 1 1 1 1 0 0 1 0&1 0&1 1 1 1 0&1 1 0&1 1 1 1 1 1 0 0 0 0 ? ? ? 1 1 1 1 1 0

O. RIEPPEL

T 1—continued Corosaurus-Data-141 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Corosaurus-Data-142 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

101

102

103

104

105

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0&1 0 0 1 1 1 ? ? 0

0 0 1 0 0 1 1 1 0 0&1 0 0 0 0 1 1 1 1 1 1 1 ? ? 1

0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ? ? 1

0 0 0 0 1 1 1 1 1 1 1 1 1 1 ? 1 1 1 1 1 1 ? 1 1

0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 0 0 ? 1 0

111

112

113

114

115

116

0 0 0 0 0 ? 0 0 1 0&1 1 1 1 0 ? 0 0 0 0 0 ? ? ? 0

0 0 0 1 1 ? 1 1 1 1 1 1 1 1 ? 0 0 0 0 0 ? ? ? 0

0 0 & 0 0 ? & 1 0 & 0 0 & 0 ? 1 1 1 1 1 ? ? ? 1

0 0 1 0 0 ? 1 1 1 1 1 1 1 0 ? 1 1 1 1 1 ? ? ? 1

0 0 0 0 0 ? 0 0 0 0 0 0 0 0 ? 2 1 1 0 1 ? ? ? 2

0 0 1 0 1 ? 1 1 1 1 1 1 1 0 ? 0 0 0 0 ? ? ? ? 0

106

107

108

109

110

0 0 1 0 1 1 1 1 1 1 1 1 1 1 ? 1 1 1 1 1 1 ? ? 1

0 0 1 0 0 ? 1 1 1 0 0 ? 0 0 ? 1 1 1 1 1 ? ? ? 1

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 0 ? ? ? 0

117

118

119

0 0 1 0 0 ? 1 1 1 0&1 1 1 1 0 ? 0 0 0 0 ? ? ? ? 0

0 0 0 1 1 ? 1 1 ? 1 ? ? 1 ? 0 0 0 0 0 0 0 0 0 0

0 0 ? 0 0 ? 0 ? 0 0 ? ? 0 0 0 0 1 1 0 1 0 ? ? 0

0 0 0 0 0 1 0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 1 1&2 0&1 1 1 ? ? 2 1 2 1 2 1 2 1 1 1 1 1 ? ? ? ? 1 1

TRIASSIC STEM-GROUP SAUROPTERYGIA

EOSAUROPTERYGIA EUSAUROPTERYGIA Simosaurus Germanosaurus

Nothosaurus

Dactylosaurus

Larisaurus Pistosaurus Serpiano-Nesuticosaurus Placodus

Corosaurus Cymatosaurus

EOSAUROPTERYGIA Dactylosaurus

SerpianoNesuticosaurus Simosaurus Germanosaurus

All other Diapsida

Nothosaurus

Larisaurus Pistosaurus Placodus B

Corosaurus Cymatosaurus

Figure 7. A, unrooted network for ingroup taxa only; B, unrooted network for ingroup and outgroup taxa. For further discussion see text.

Treating all terminal taxa as ingroup (Reptilia), and rooting the search on an all0-ancestor (all characters informative) yielded three MPTs (TL=450; CI=0.651; RI=0.713), again with fully resolved sauropterygian interrelationships, and with lack of resolution restricted to the archosauromorph clade. In the final analysis (search options as indicated above, 100 replications), character 27 was treated as ordered (for reasons discussed in the definition of that character), and all terminal taxa (Reptilia, monophyletic ingroup) were rooted on the all-0ancestor. (Treating character 27 as unordered decreased tree length by one step, but increased the number of most parsimonious trees by one, decreasing resolution within the archosauromorph clade.) The analysis yielded two MPTs (TL=451; CI=0.650; RI=0.713), with fully resolved sauropterygian interrelationships as shown in the strict consensus tree (fig. 8). Bootstrap values (1000 replications) and decay indices (the number of steps beyond the most parsimonious tree it takes to break a node) are shown in Figure 8. Placodus comes out as sister-taxon to a monophyletic Eosauropterygia. However, the Eusauropterygia sensu Tschanz (1989) become paraphyletic due to altered

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ty lo sa

s D

od u Pl ac

ur S us N erp eu ia st n ic oos Si au m ru os s au ru G s er m an os au N ru ot ho s sa ur us La ri sa u C or rus os au ru s C ym at os au ru s Pi st os au ru s

92% DI : 3 53% DI : 4 56% DI : 3

94% DI : 3

53% DI : 3 51% DI : 4

<50% DI : 2

60% DI : 2 100% DI : >4

Figure 8. Strict consensus tree of two equally parsimonious trees (TL=451; CI=0.650; RI=0.713), showing fully resolved sauropterygian interrelationships. For further discussion see text.

relationships of the Pachypleurosauria. Indeed, the Eosauropterygia divide into two major lineages. One clade includes Corosaurus as sister-taxon to Cymatosaurus and Pistosaurus [(Corosaurus (Cymatosaurus, Pistosaurus))]. Pachypleurosaurs, on the other hand, turn out to be the sister-group of the Nothosauria, a clade including Simosaurus and the Nothosauridae [(Pachypleurosauria (Simosaurus (Germanosaurus (Nothosaurus, Lariosaurus))))]. DELTRAN character optimization will tend to push synapomorphies up the tree, with the consequence that fewer characters will diagnose more inclusive clades, but subclades will tend to preserve diagnostic characters as reversal is less likely to occur than with ACCTRAN character optimization. For this reason, the listing of diagnostic characters below will be based on DELTRAN character optimization (unless otherwise noted). Unequivocal synapomorphies (with a consistency index of 1.0) are indicated with an asterisk. Sauropterygia: 1(1), 9(2)∗, 13(2), 27(2), 28(0), 31(1), 39(1), 41(1)∗, 43(0), 54(1), 58(1), 62(1), 71(1), 73(1), 78(1), 81(1)∗, 83(1), 90(1)∗, 92(1), 94(1), 95(1), 98(2), 116(0), 117(0). Eosauropterygia: 47(1)∗, 64(1), 65(1), 80(1), 84(1), 97(0), 115(1). Nothosauroidea (Pachypleurosauria plus Nothosauria): 30(1), 31(2), 44(1), 77(0), 79(1)∗, 88(2)∗, 105(1), 106(2). Nothosauria (Simosaurus plus Nothosauridae): 8(1), 12(2), 17(2), 18(1), 19(1), 25(1), 37(0), 39(0), 42(2)∗, 50(1), 57(2), 60(1), 96(1), 99(2), 100(1)∗, 110(1). Nothosauridae (Germanosaurus plus Nothosaurinae): 3(1), 4(1)∗, 11(1)∗, 23(2)∗, 55(1). Nothosaurinae (Nothosaurus plus Lariosaurus): 14(1), 19(2), 35(1), 36(1), 51(2), 56(0), 67(1), 74(1). Pistosauroidea (Corosaurus plus Pistosauria): 32(1)∗, 33(1), 36(1), 37(0), 55(1), 56(0), 57(1), 85(1)∗, 101(1). Pistosauria (Cymatosaurus plus Pistosaurus): 6(2), 8(1), 16(2), 17(1), 19(3), 29(1), 60(1).

TRIASSIC STEM-GROUP SAUROPTERYGIA

CLASSIFICATION OF THE SAUROPTERYGIA

The increased data set for the analysis of sauropterygian interrelationships results in a new inclusive hierarchy of ingroup taxa with some important paleobiological consequences. The formal classification retrieved form the hierarchy of sauropterygian taxa reads as follows: 1. Sauropterygia Owen, 1860 Definition. A monophyletic taxon including the Placodontia and the Eosauropterygia. Diagnosis. Premaxilla large, forming most of snout in front of external nares; lacrimal absent; upper temporal fossa distinctly larger than orbit (reversed in pachypleurosaurs); lower temporal fossa open ventrally; squamosal descends towards ventral margin of skull; paroccipital process trending posterolaterally; dorsal wing of epipterygoid narrow; palate akinetic; suborbital fenestra absent; mandibular symphysis enforced and ‘scoop’-like (51[2], ACCTRAN; DELTRAN character optimization renders this a synapomorphy of Nothosaurinae, convergent in Placodus and Cymatosaurus); anterior premaxillary and dentary teeth procumbent; teeth on pterygoid flange absent; cervical intercentra absent; cervical ribs with free anterior process; three or more sacral ribs; clavicles positioned anteroventrally to interclavicle; clavicles applied to medial surface of scapula; posterior stem of interclavicle short; pectoral fenestration present; humerus curved; insertional crest for latissimus dorsi muscle reduced; humerus with reduced epicondyles; radius and ulna of equal length; fifth metatarsal long and slender; straight fifth metatarsal. 1.1. Eosauropterygia Rieppel, 1994a Definition. A monophyletic taxon including the Pachypleurosauria, Nothosauria, Corosaurus and the Pistosauria. Diagnosis. Basioccipital tubera in complex relation to pterygoid as they extend laterally (42[2]; ACCTRAN); internal carotid enters quadrate ramus of pterygoid; zygosphene-zygantrum articulation present (64[1]); pedicels of neural arch received on ‘butterfly’-shaped platform on centrum; clavicles with anterolaterally expanded corners; scapula constricted to separate a ventral glenoid portion from posterodorsal blade or process; entepicondylar foramen present; three tarsal ossifications. 1.1.1. Nothosauroidea Definition. A monophyletic taxon including the Pachypleurosauria and the Nothosauria. Diagnosis. Quadratojugal without anterior process; occiput plate-like, with strongly reduced posttemporal fossae (unknown in Germanosaurus); supraoccipital horizontally exposed (35[1], ACCTRAN; unknown in Germanosaurus); pterygoid-ectopterygoid flanges strongly reduced (unknown in Germanosaurus); clavicles broad medially (unknown in Germanosaurus); clavicles meet in anteromedial suture (unknown in Germanosaurus); coracoid strongly waisted (unknown in Germanosaurus); internal trochanter reduced (unknown in Germanosaurus); intertrochanteric fossa rudimentary or absent (unknown in Germanosaurus).

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1.1.1.1. Nothosauria Definition. A monophyletic taxon including Simosaurus and the Nothosauridae. Diagnosis. Nasals separated by premaxillaâ&#x20AC;&#x201C;frontal contact; postorbital skull distinctly longer than preorbital skull; parietal fully fused in adult; pineal foramen displaced posteriorly; parietal skull table weakly constricted; jugal enters upper temporal arch; quadrate with straight posterior margin; dorsal blade of epipterygoid broad; surangular with distinctly projecting lateral flange for the insertion of superficial jaw adductor muscle fibres; maxillary tooth-row extending backwards to level beyond the anterior corner of the upper temporal fossa; vertebrae platycoelous; ectepicondylar groove open without anterior notch; posterior dorsal process does not project beyond posterior margin of acetabular portion of ilium; pubis with concave ventral (medial) margin; astragalus with proximal concavity; medial gastral rib element may have two-pronged lateral process (119[1], ACCTRAN). 1.1.1.2. Nothosauridae Definition. A monophyletic taxon including Germanosaurus and the Nothosaurinae. Diagnosis. snout constricted; temporal region of skull strongly depressed; reduced dorsal exposure of prefrontal; jugal restricted to a position behind the orbit without entering the latterâ&#x20AC;&#x2122;s posterior margin; premaxillary and anterior dentary fangs present. 1.1.1.3. Nothosaurinae Definition. A monophyletic taxon including Nothosaurus and Lariosaurus. Diagnosis. Frontals fused in adult; parietal skull table strongly constricted (at least posteriorly); occipital crest present; maxillary canines present; dorsal centra not constricted in ventral view; sacral ribs without distal expansion. 1.1.2. Pistosauroidea Definition. A monophyletic taxon including Corosaurus and the Pistosauria. Diagnosis. Frontal closely approaches upper temporal fossa (16[1], ACCTRAN); jugal restricted to position behind orbit, but enters the latterâ&#x20AC;&#x2122;s posterior margin (23[1], ACCTRAN); quadratojugal absent (29[1], ACCTRAN); occipital exposure of squamosal with distinct notch receiving the distal end of the paroccipital process; mandibular articulation displaced to a level distinctly behind the occipital condyle; occipital crest present; quadrate with straight posterior margin; premaxillary and anterior dentary fangs present; maxillary canines present; maxillary tooth row extends posteriorly to a level below the posterior margin of orbit or anterior margin of upper temporal fossa; posterodorsal wing of scapula ventrally expanded at its posterior end; obturator foramen open in adult. 1.1.2.1. Pistosauria Definition. A monophyletic taxon including Cymatosaurus, Pistosaurus, and plesiosaurs and pliosaurs.

TRIASSIC STEM-GROUP SAUROPTERYGIA

Diagnosis. Nasals strongly reduced (or absent); nasals may fail to enter external naris (7[1]. ACCTRAN; reversal in some Cymatosaurus), nasals separated by premaxillaprefrontal contact; frontal enters anteromedial margin of upper temporal fossa (reversal in some Cymatosaurus); parietal incompletely fused in adult (completely fused in some Cymatosaurus); vertebrae platycoelous.

DISCUSSION AND CONCLUSIONS

The revision of the phylogenetic interrelationships of the Sauropterygia as detailed above partially falsifies the elegant evolutionary scenario of their adaptation to a secondary aquatic existence first proposed by Sues (1987; see also Rieppel, 1989, 1994a; Tschanz, 1989). The concept of the monophyletic Eusauropterygia with pachypleurosaurs as their sister-group, with Corosaurus and Simosaurus as their most basal clades, and with plesiosaurs and pliosaurs as their crown group clades, permitted the hypothesis of a progressive morphological adaptation of Sauropterygia as their successively more derived clades moved away from lagoonal or shallow water nearshore habitats towards the open sea. The new phylogeny, indicating paraphyly of the Eusauropterygia, a basal dichotomy within the Eosauropterygia, and showing the Pachypleurosauria as sister-group to the Nothosauria, may turn a number of characters formerly believed to be plesiomorphic in pachypleurosaurs and in Simosaurus into reversals. These characters involve snout constriction, the elongation of the postorbital skull and the enlargement of the upper temporal fossa correlated with the development of a dual jaw adductor system (Rieppel, 1989, 1994a), closure of the occiput, the loss of the impedance-matching middle ear (as inferred from the loss of the posterior concavity of the quadrate: Rieppel, 1989), the enforcement of the mandibular symphysis, and the development of a piscivorous dentition. The study of character evolution will, of course, change with the application of different character optimization techniques. Implementing both ACCTRAN and DELTRAN character optimization results in the same interpretation of the following functionally important characters: rostral constriction (3[1]) is a synapomorphy of the Nothosauridae, convergent in Placodus and Cymatosaurus; elongation of the postorbital region of the skull (12[2]) is a synapomorphy of the Nothosauria, convergent in Cymatosaurus; enlargement of the upper temporal fossa (13[2]) is a synapomorphy of the Sauropterygia, reversed in Corosaurus—the small upper temporal fossa (13[3]) is an autapomorphy of pachypleurosaurs; closure of the occiput and reduction of the posttemporal fossae (31[2]) is a synapomorphy of Nothosauroidea; reduction of the ectopterygoid-pterygoid flange (44[1]) likewise is a synapomorphy of Nothosauroidea; the presence of anterior dentary and premaxillary fangs (55[1]) is a synapomorphy of the Nothosauridae convergent in the Pistosauroidea ; the presence of maxillary fangs (56[0]) is a synapomorphy of the Nothosaurinae convergent in the Pistosauroidea. The only reversal occurring in pachypleurosaurs using both ACCTRAN and DELTRAN character optimization concerns the strongly procumbent anterior dentary and premaxillary teeth (54[1]) which are synapomorphic at the level of the Sauropterygia. Using DELTRAN character optimization, the strongly enforced and ‘scoop’-shaped mandibular symphysis (51[2]) is synapomorphic in the Nothosaurinae (unknown in Germanosaurus), convergent in Cymatosaurus (or Pistosauria respectively). Using ACCTRAN optimization, the same

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character (51[2]) is synapomorphic at the level of Sauropterygia, reversed at the level of the Nothosauroidea, and re-developed again in the Nothosauridae (also reversed in Corosaurus). Again using DELTRAN character optimization, the straight posterior border of the quadrate (37[0], assumed to indicate the loss of the impedance matching middle ear) is synapomorphic at the level of the Nothosauria, and convergent in the Pistosauroidea. Using ACCTRAN character optimization, the same character (37[0]) is synapomorphic at the level of the Eosauropterygia, and reversed in pachypleurosaurs. In summary, DELTRAN character optimization continues to allow the earlier conclusion (Sues, 1987; see also Rieppel, 1989, 1994a; Tschanz, 1989) that pachypleurosaurs and Simosaurus retain a plesiomorphic anatomy relative to other Eosauropterygia. It does so, however, at the cost of extensive convergence in the Nothosauria (and subclades) on the one hand, and in the Pistosauroidea (and subclades) on the other. At a more general level, the invasion of the Mesozoic seas by the Sauropterygia becomes a more complex phenomenon than had previously been assumed. One major clade of the Eosauropterygia, the Nothosauroidea (including pachypleurosaurs and Nothosauria) seems to have remained restricted to shallow warm-water epicontinental seas or near shore habitats, whereas the other major clade, the Pistosauroidea (including Corosaurus and Pistosauria) appears to have rapidly invaded the open sea (plesiosaurs and pliosaurs). Placodonts are restricted to the western Tethyan province (Europe and Mediterranean), where they first occur at the transition from the Upper Buntsandstein (RoÂ¨t, Scythian) to the Muschelkalk (Anisian) (Pinna, 1990; Rieppel, 1995). The group diversified throughout the Anisian and Ladinian in the epicontinental Germanic Basin, and in peritethyan intraplatform basins (see Pinna, 1990, and Rieppel, 1995, for a review and further references), and as a whole seems to have shared the same paleoecological constraints as are evident in the pachypleurosaurâ&#x20AC;&#x201C;nothosaurian clade within the Eosauropterygia. Pachypleurosaurs, simosaurids and nothosaurs occur both in the western Pacific (China: Young, 1958, 1959, 1960, 1965, 1978) and western Tethyan (Europe and Mediterranean: Brotzen, 1957; Gorce, 1960; Haas, 1959, 1963, 1969, 1975, 1980, 1981; Halstead & Stewart, 1970; Lehman, 1965; Rieppel & Lin, 1995; Rieppel & Wild, 1996) provinces. The earliest occurrence of the Nothosauroidea in China is the lower Middle Triassic (Young, 1965). In the western Tethyan province the clade appears first at the transition from the Upper Buntsandstein (RoÂ¨t, Scythian) to the Muschelkalk (Anisian) in the Germanic Triassic (Rieppel & Lin, 1995; Rieppel & Wild, 1996), as well as in the basal Muschelkalk layers of Makhtesh Ramon in the Negev (Brotzen, 1957; Parnes, 1975), and of Djebel Rehach in southern Tunisia (Gorce, 1960). The clade diversified throughout Anisian and Ladinian times within the epicontinental Germanic Basin, the Alpine intraplatform basin facies (which it invaded at the Anisian-Ladinian boundary: Rieppel & Hagdorn, 1997), and the shallow carbonate platform extending along the northern Gondwanan shelf. The first appearance of the clade in the western Tethyan faunal province coincides with the onset of cyclic marine transgressions lasting from the early Anisian through the early Carnian. All occurrences of the Nothosauroidea are in shallow epicontinental warm-water sea deposits, or in the near shore lagoonal setting of a carbonate platform. Even within the Germanic Muschelkalk sea, the Nothosauroidea are found predominantly in nearshore areas (Hagdorn, 1993), indicating a distinct facies interdependence which is in accordance with their overall patchy geographical

TRIASSIC STEM-GROUP SAUROPTERYGIA

distribution. The high degree of taxonomic, and morphological, diversification would appear to have reduced ecological competition between the representatives of this clade within each basin. At the same time, the clade preserves some of the progressive morphological trends identified to characterize sauropterygian evolution (Sues, 1987), such as: loss of the tympanic membrane (Nothosauria as opposed to pachypleurosaurs), development of an elongated snout and constricted rostrum furnished with procumbent premaxillary fangs (Nothosauridae as opposed to Simosaurus), development of maxillary fangs (Nothosaurinae as opposed to Germanosaurus), and a progressive depression of the skull correlated with a progressive elongation of the postorbital skull region (upper temporal fenestra) to accommodate an increasingly differentiated dual jaw adductor muscle complex (Rieppel, 1994a). The clade goes extinct during the Upper Triassic. With its diversity peaking during Ladinian times, the evolution of the Nothosauroidea appears to be directly correlated with “the progressive spread of sea from the early Triassic to a Ladinian–early Carnian maximum, followed by a retreat . . . and corresponding shallowing of the sea in the Tethyan zone” which has been attributed to a “fundamental eustatic effect” (Hallam, 1981: 33; see also Haq, Hardenbol & Vail, 1987). Until recently, Corosaurus was a singular sauropterygian occurrence in the uppermost Lower Triassic (early Middle Triassic?) Alcova Limestone of the western United States. The genus documents an early widespread geographic distribution of the Pistosauroidea, further supported by the recent finding of a new pistosaur in the Western United States, i.e. Augustasaurus from the Upper Anisian of northwestern Nevada (Sander, Rieppel & Bucher, 1997). Corosaurus in particular shares with the Chinese representatives of the pistosaur clade (Chinchenia sungi Young, 1965; Sanchiaosaurus dengi Young, 1965; Kwangsisaurus orientalis Young, 1959) a derived character which is absent in the western Tethyan representatives of that clade, i.e. a very distinct groove on the posterior surface of the proximal region of the dorsal ribs. Although the analysis of phylogenetic interrelationships among all known Triassic Pistosauroidea lacks detail, and is seriously hampered by the incompleteness of the Chinese material, this character provides at least some indication that the phylogenetic relationship of Corosaurus from the eastern Pacific province might be with pistosaurs from the western Pacific province, rather than with the western Tethyan taxa. This makes sense in view of the fact that there were no direct marine connections or seaways between the western Tethyan province and the eastern Pacific province during the Triassic (Ricou, 1996; Dore´, 1991; Ziegler, 1988). TransPacific relationships of east Asiatic and western North American floral and faunal elements have been recognized for several groups of organisms from late Paleozoic through Tertiary times (MacGinitie, 1969; Ross and Ross, 1981, 1985; Grande, 1994), and pistosaurs might provide just another example. Among western Tethyan stem-group pistosauroids, Cymatosaurus is only known from the Lower Muschelkalk of the Germanic Triassic (Lower Anisian), and from a singular occurrence in the Alpine Triassic (Rieppel & Hagdorn, 1997). Analysis of detailed geographic and stratigraphic distribution shows an almost complete mutual exclusion of the morphologically very similar genera Cymatosaurus and Nothosaurus in the Lower Muschelkalk (Rieppel & Werneburg, 1997). Pistosaurus appeared in the Germanic Basin with the Upper Muschelkalk transgression (first appearance: atavus biozone, Upper Illyrian), and persisted until the beginning of the regression (last appearance: postspinosus biozone). Bones of Pistosaurus are found most frequently—but still very rarely—in the Tonplatten facies of the spinosus to

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postspinosus biozones which were deposited during the maximum flooding period in earliest Ladinian times (H. Hagdorn, pers. comm.). By the Jurassic, pelagic plesiosaurs and their fossil relatives had become more frequent and diverse, and achieved a global distribution. The clade persisted to the Upper Cretaceous. In summary, the evolutionary history of the two clades of Eosauropterygia reflects the two major extinction events affecting marine reptiles that were identified by Bardet (1994).

ACKNOWLEDGEMENTS

I thank Brent H. Breithaupt and Jean-Pierre Cavigelli from the University of Wyoming Geological Museum for the permission to further prepare the holotype of Corosaurus. Robert Masek, Paul Brinkman, Jessica Scott, and my family, were good company in the field. Laurie Bryant from the Casper District Office of the Bureau of Land management was very helpful and receptive to our needs. John and Jo-Ann Milnes generously granted access to their ranch land on the south slope of Muddy Mountain. The photographic work for this paper was done by John S. Weinstein, the careful preparation of Corosaurus was performed by Kathryn Passaglia, both on staff at the Field Museum. I am particularly greatful to John Merck and Hans Hagdorn for the many opportunities they provided to discuss the material presented in this paper, and to Hans-Dieter Sues and Glenn Storrs, who read the paper, offering much helpful advice and criticism. This study was supported by NSF-grants DEB-9220540 and DEB-9419675.

REFERENCES

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