Rieppel, 2000

Zoological Journal of the Linnean Society (2000), 130: 635–659. With 10 figures doi:10.1006/zjls.2000.0232, available online at http://www.idealibrary.com on

Paraplacodus and the phylogeny of the Placodontia (Reptilia: Sauropterygia) OLIVIER RIEPPEL FMLS Department of Geology, The Field Museum, 1400 S. Lake Shore Drive, Chicago, IL 60605–2496, U.S.A. Received October 1999; accepted for publication February 2000

The skeletal anatomy of Paraplacodus broilii Peyer from the Grenzbitumen-horizon (Anisian– Ladinian boundary) of Monte San Giorgio (Switzerland) is described and compared with that of other placodonts. Paraplacodus is found to share a number of potential synapomorphies with Placodus which could potentially corroborate the monophyly of the Placodontoidea, but Placodus also shares an number of potential synapomorphies with the armored placodonts (Cyamodontoidea) which are absent in Paraplacodus. Parsimony analysis rejects the monophyly of the Placodontoidea, and places Paraplacodus at the root of the placodont tree, as the sistertaxon of all the other representatives of the clade. This correlates with a configuration of the temporal region of the skull that suggest the loss of the lower temporal arch in a diapsid skull. The loss of the lower temporal arch is therefore recognized as a sauropterygian synapomorphy, and might even be a lepidosauromorph synapomorphy.  2000 The Linnean Society of London

CONTENTS

Introduction . . . . .

Material . . . . . .

Systematic palaeontology Paraplacodus broilii Peyer Cladistic analysis . . .

Discussion . . . . .

Acknowledgements . .

References . . . . .

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INTRODUCTION

The genus Paraplacodus from the Grenzbitumen-horizon (Anisian–Ladinian boundary) of Monte San Giorgio, Switzerland (southern Alpine Triassic), has been claimed to be of special importance for the understanding of placodont phylogeny, as it was thought to represent the most ‘primitive’ representative of the group (Peyer & KuhnSchnyder, 1955). First described by Peyer (1931a, b, 1935), the taxon was known from incomplete and disarticulated material only. A complete and articulated skeleton, collected in 1936, was cursorily described by Kuhn-Schnyder (1942). An 0024–4082/00/120635+25 $35.00

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Figure 1. The skull of Paraplacodus broilii Peyer (BSP 1953 XV 5). Scale bar=20 mm.

isolated skull, and indeed the best preserved skull available for the genus (Fig. 1) was commented upon by Zanon (1989), and discussed, as well as figured, by Pinna (1989) and Rieppel (1995). An isolated tooth plate from the middle to upper Anisian (Formazione di Braies) of the Val Pusteria (southern Alps, northern Italy) was referred to Paraplacodus by Nosotti (1986), although the specimen is not diagnostic.

An isolated dorsal rib with indications of a broadened uncinate process, from the upper Ladinian of Henarejos (Cuenca, Spain), was referred to Paraplacodus by Pinna (1990). Isolated teeth very similar to those of Paraplacodus have been reported from the lower Muschelkalk (Anisian) of southwest Germany (Hagdorn & Rieppel, 1999).

Very fragmentary material from the Anisian of Transylvania, Romania ( Jurczak, 1976), and from the upper Anisian and/or lower Ladinian of Israel (Haas, 1975) has also been referred to Paraplacodus, but all this material is not diagnostic, even at the genus level.

In his study of the dentition of placodonts, Mazin (1989) recognized a basal dichotomy among placodonts, reflecting the Placodontoidea and Cyamodontoidea respectively of Peyer & Kuhn-Schnyder (1955; see also Mazin & Pinna, 1993). The Placodontoidea include the genera Paraplacodus with no osteoderms, and Placodus with a single row of osteoderms along the dorsal midline of the body. The Cyamodontoidea include a variety of placodonts characterized by the development of a turtle-like dermal armor composed of interdigitating or fused osteoderms. This phylogeny of placodonts was recently confirmed by cladistic analysis (Rieppel & Zanon, 1997), yet it removes Paraplacodus from its position as sister-taxon of all other placodonts, as would be expected if Paraplacodus were, indeed, the most ‘primitive’ member of the clade (Zanon, 1989). It is the purpose of this paper to summarize all available anatomical information on the anatomy of Paraplacodus, and to analyse its interrelationships within the Placodontia.

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Figure 2. The skull of Paraplacodus broilii Peyer (PIMUZ T4775). Scale bar=20 mm.

MATERIAL

The first fragments of the skull of Paraplacodus to be found were a disarticulated but beautifully preserved premaxilla (PIMUZ T2806), and a fragmentary palate (PIMUZ T4776) showing the palatal dentition, and a fragmentary dentary (PIMUZ T4777; Peyer, 1931b). A second specimen, an incomplete and disarticulated skeleton, was the object of a preliminary note (Peyer, 1931a) in which the name Paraplacodus broilii was first published, designating the specimen as holotype for its species (PIMUZ T4773). Later, the holotype was described in greater detail (Peyer, 1935), together with a third incomplete and disarticulated skeleton (PIMUZ T4774), the ‘specimen B’ of Peyer (1935). The complete and articulated skeleton (PIMUZ T4775), described

by Kuhn-Schnyder (1942), has a strongly compressed and poorly preserved skull (Fig. 2). The description of the cranial anatomy of Paraplacodus given below is primarily based on the skull from Munich (Fig. 1; BSP 1953 XV 5; a cast is kept at the Field Museum, FMNH PR 2102), preserved in left lateral view, in combination with information obtained from the specimens PIMUZ T4773 and T4774.

Most information on the postcranial skeleton is obtained from the specimen PIMUZ T4775, supplemented by an incomplete yet articulated postcranial skeleton (PIMUZ T4827). Additional information comes from the specimens described by Peyer (1935), including the holotype.

Institutional acronyms are: BMNH, British Museum (Natural History); BSP, Bayerische Staatssammlung fu¨r Pala¨ontologie und historische Geologie, Munich; FMNH, Field Museum of Natural History, Chicago; PIMUZ, Pala¨ontologisches Institut und Museum der Universita¨t, Zu¨rich.

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Sauropterygia Owen, 1860 Placodontia Cope, 1871 Paraplacodontidae Peyer & Kuhn-Schnyder, 1955 Paraplacodus Peyer, 1931a Diagnosis. Three long, pointed and strongly procumbent teeth on each premaxilla; seven hemispheric maxillary teeth; dermatocranial cheek region strongly excavated, jugal L-shaped; quadratojugal absent; postfrontal extending far anteriorly along dorsal margin of orbit; two elongated, pointed and strongly procumbent anterior dentary teeth followed by seven hemispheric teeth; low coronoid process with the coronoid exposed in lateral view; dorsal ribs with distinct, fan-shaped and overlapping uncinate processes; ilium with narrow and tall dorsal process; osteoderms absent.

Paraplacodus broilii Peyer, 1931a Holotype. PIMUZ T4773; incomplete skeleton.

Stratum typicum and locus typicus. Grenzbitumen-horizon, Anisian–Ladinian boundary, Middle Triassic; Valporina, Monte San Giorgio, Kanton Tessin, Switzerland.

Diagnosis. Same as for genus, of which this is the only known species.

Morphological description.

The skull of Paraplacodus (Fig. 3) is relatively high and narrow, as is that of Placodus, unlike the dorsoventrally depressed and broadened skull of cyamodontoids.

Paraplacodus therefore groups with Placodus in Meyer’s (1863) ‘Macrocephali.’ As noted by Zanon (1989), and Pinna (1989), Paraplacodus shows a more pronounced ventral emargination of the cheek region than any other placodont, which together with the narrow, L-shaped jugal, corroborates diapsid affinities of the Placodontia.

The premaxillaries of Paraplacodus form an anteriorly projecting rostrum, distinctly set off from the remainder of the preorbital skull by a step visible in lateral view. A similar step, at the height of the external naris, is also observed in Placodus.

Each premaxilla of Paraplacodus carries three elongated, slightly curved, strongly procumbent, and pointed teeth, of which the anteriormost one is the longest. The presence of a rostral constriction, a synapomorphy of sauropterygians in general, remains equivocal for Paraplacodus for reasons of incomplete and/or distorted preservation. However, the palate of specimen T4773, preserved in ventral view, shows a slight constriction of the lateral margin of the maxilla at the level of the internal naris, which may indicate the presence of a rostral constriction.

The left external naris is distinct in the specimen BSP 1953 XV 5. It is an almost circular opening, as high (7.5 mm) as it is wide (7.6 mm), which differs from Placodus, where the external naris is much higher than it is wide (Rieppel, 1995). The premaxilla forms the anterior margin of the external naris. The premaxillarymaxillary suture is distinct, as it trends from the anteroventral margin of the external naris in an anterolateral direction towards the margin of the upper jaw. The area above and behind the external naris has been subject to severe crushing which

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Figure 3. The skull of Paraplacodus broilii Peyer (BSP 1953 XV 5). Scale bar=20 mm. Abbreviations: an, angular, c, coronoid; d, dentary; f, frontal; j, jugal; m, maxilla; n, nasal; p, parietal; pm, premaxilla; po, postorbital; pof, postfrontal; prf, prefrontal; q, quadrate; sa, surangular, sq, squamosal.

renders the identification of separate cranial elements diďŹƒcult. The maxilla forms a relatively high ascending process between the external naris and the orbit (also seen in the disarticulated maxilla of specimen PIMUZ T4774). It defines the posteroventral and posterior margin of the external naris, and broadly enters the anteroventral margin of the orbit. Dorsally, it narrows to a point as it enters between the nasal and the prefrontal.

Anterodorsal to the external naris, an anteroventral projection of the nasal can be identified, overlapping the premaxilla. Forming the dorsal margin of the external naris, the nasal extends posterodorsally on the prefacial skull, but extensive damage obscures details of its relation with the frontal and prefrontal respectively. It remains

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unknown whether the nasals were fused in Paraplacodus, as they are in Placodus, but from the general proportions of the skull it can be inferred that the nasals are relatively large elements in Paraplacodus, as they also are in Placodus.

At about mid-height of the anterior margin of the orbit, the ventral tip of the prefrontal is distinct. The posterior end of the prefrontal is well exposed at the anterodorsal corner of the orbit. It appears bifurcated, and must have embraced the anterior tip of the postfrontal in the undistorted skull. The postfrontal is readily identified as an element located dorsal and posterodorsal to the orbit. With a posteroventrally directed process, it defines the posterodorsal margin of the orbit.

An anterior process extends far anteriorly along the dorsal margin of the orbit, defining most of its dorsal margin, and most probably contacting the prefrontal at the anterodorsal corner of the orbit. In Placodus, prefrontal and postfrontal also contact each other dorsal to the orbit, but more posteriorly, at about the midpoint of the longitudinal diameter of the orbit. The anterior extent of the postfrontal along the dorsal margin of the orbit is an autapomorphy of Paraplacodus.

A deep concavity in the posterior margin of the postfrontal, slightly displaced in the specimen BSP 1953 XV 5, must have defined the anterior margin of the upper temporal fossa. As in Placodus, but unlike in cyamodontoids (except for Henodus, which obliterated the upper temporal fenestra), the upper temporal fossa of Paraplacodus is not very much larger than the orbit. In the specimen BSP 1953 XV 5, the longitudinal diameter of the (left) upper temporal fossa is 40.5 mm, that of the (left) orbit is 31.9 mm. The postorbital forms most of the posterior margin of the orbit, while the jugal is located at the posteroventral corner of the orbit. In Paraplacodus, the jugal forms an L-shaped element, with an anterior process lining the posteroventral margin of the orbit, and a dorsal process which enters into the postorbital arch.

Unlike in Placodus, the anterior process of the jugal does not extend to a level in front of the anterior margin of the orbit. According to Zanon (1989) the jugal of Paraplacodus enters only minimally into the temporal arch, whereas Pinna (1989) shows a contact of the jugal and quadratojugal in the ventral margin of the temporal arch. My own observations (Rieppel, 1995, fig. 46) concur with Zanon’s (1989) in that the dorsal tip of the jugal can be identified in the anterodorsal corner of the cheek emargination (Fig. 3). Other sutures are more difficult to identify in the temporal arch, especially in the heavily varnished original of specimen BSP 1953 XV 5, but a cast of that specimen (FMNH PR 2102) reveals more structural detail.

A posteroventral process of the squamosal is readily identified as it caps the cephalic condyle of the quadrate. The participation of the squamosal in the formation of the posterior and posterolateral margin of the upper temporal fossa again is distinct, and so is the squamosal–postorbital contact somewhat behind the midpoint of the dorsal margin of the temporal arch. From this point, the irregular postorbital– squamosal suture, although partially obscured by breakage, can be followed across the temporal arch trending anteroventrally before curving backwards in its lower most part. The squamosal thus extends across the entire height of the temporal arch, and a quadratojugal is absent (as is also indicated in a drawing of the temporal region of Paraplacodus by the late Robert Zanon). Paraplacodus shares the absence of a quadratojugal with Placodus (Rieppel, 1995), but a quadratojugal is retained in cyamodontoids (Rieppel, 1995, fig. 22, and work in progress). The anteroventral tip of the squamosal remains separated from the posterodorsal tip of the jugal in Paraplacodus (Fig. 3), which corroborates Zanon’s (1989) conclusion that the contact of the jugal with the squamosal is secondary where it occurs, as for example in

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Figure 4. The palatines of Paraplacodus broilii Peyer (holotype, PIMUZ T4773) in ventral view. The anterior ends of the palatines (to the left) define the posterior margin of the confluent internal nares.

Scale bar=10 mm.

Placodus. The deep emargination of the temporal region, the L-shaped jugal which does not contact the squamosal, as well as the absence of a quadratojugal in contact with the jugal, postorbital, and squamosal (as postulated by Pinna, 1989), confirm the diapsid status of Paraplacodus.

The diapsid status of Paraplacodus is further corroborated by the lateral exposure of the quadrate below and in front of the squamosal, revealing its weakly concave posterior margin. In front of and deep to the shaft of the quadrate, an expanded anteromedial flange of the quadrate is exposed, which meets the quadrate ramus of the pterygoid in an overlapping contact, as is also the case in Placodus. Severe compression of the skull does not allow to identify any more details of this overlap, nor is it possible to ascertain whether palatoquadrate cartilage persisted in the adult Paraplacodus as it does in other placodonts (Rieppel, in press).

The anterior part of the palate of Paraplacodus is known from the holotype (PIMUZ T4773) and from the second specimen (PIMUZ T4774) described and illustrated

by Peyer (1935). Each maxilla carries seven subspherical teeth, separated from the premaxillary teeth by a distinct diastema. The labial side of the tooth crown may be drawn out into a weakly expressed pointed tip, particularly in the anterior maxillary teeth. The medial surface of the tooth crown forms a lingually sloping shoulder. Each palatine carries a minimum of four subspherical teeth with a blunt tooth crown. The most salient character of the anterior palate of Paraplacodus are the confluent internal nares, a character shared with Placodus.

Inspection of the holotype (PIMUZ T4773) revealed well preserved and articulated anterior parts of both palatines (Fig. 4). These elements become rather narrow anteriorly, and the anterior margin of both palatines combine to form the gently curved posterior margin of the confluent internal nares. There is no indication of any

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breakage of possible anteromedial processes of the palatines, or of a palatine–vomer contact, which would have indicated an original separation of the internal nares.

Nor are there any indications of facets on the palatines for the reception of the posterior tips of the vomers. The presence of confluent internal nares can thus be safely assumed for Paraplacodus (Peyer, 1935).

The mandible is well exposed in lateral view in the specimens PIMUZ T4774 and BSP 1953 XV 5, whereas that of the holotype is broken. Sutures are very difficult, in most cases even impossible, to identify. In general, the mandible of Paraplacodus is relatively deep and massive (Fig. 3). The mandibular symphysis carries four elongated, sigmoidally curved, strongly procumbent and pointed teeth, two on each dentary, which match the premaxillary teeth in shape and arrangement. Behind a distinct diastema, each dentary carries a total of seven subspherical crushing teeth which match the maxillary teeth in their morphology. Peyer’s (1935, fig. 1) reconstruction of the lower jaw, molded by the preparator Chr. Strunz, shows an elongated mandibular symphysis which remains narrower than the premaxillary rostrum and which shows no indication of a rostral constriction. It also shows the posterior dentary teeth to bite against the groove defined by the maxillary teeth laterally, and the palatine teeth medially. Although this seems to represent a functional arrangement, it should be borne in mind than no articulated lower jaw is known for Paraplacodus.

Paraplacodus differs from other placodonts, and in particular from Placodus, by a much less developed coronoid process, which also reflects a lesser degree of durophagy. And whereas in Placodus the coronoid process of the dentary obscures the coronoid in lateral view, the coronoid of Paraplacodus is exposed laterally behind the coronoid process of the dentary. A series of mental foramina opens below the dentary teeth on the lateral surface of the dentary. The delineation of the surangular, angular, prearticular and splenial remains obscure for Paraplacodus, but the retroarticular process can be seen to form a much more massive structure (Fig. 3) than the slender retroarticular process characteristic of Placodus.

Vertebral counts were established by Kuhn-Schnyder (1942) for the only articulated specimen available (PIMUZ T4775). The number of cervical vertebrae cannot be precisely established. Six cervicals can be identified, an incomplete number. These are followed by 21 dorsal vertebrae, three sacrals, and 54 caudals. The vertebral centra are slightly constricted ventrally and laterally, and their terminal articular surfaces are distinctly amphicoelous, but as far as can be determined, not notochordal.

Short and ‘knobby’ dia- and parapophyses are distinct on an anterior cervical centrum of specimen BSP 1953 XV 5, exposed in lateral view. Other cervical vertebrae are less well exposed, and do not allow the identification of relevant morphological detail. In several specimens, the vertebral centra have separated from the neural arch, and expose the sutural facets for the pedicels of the neural arch.

With respect to this character, Paraplacodus resembles the eosauropterygians more closely than Placodus. In the latter genus, the centrum forms low ridges which define the lateral margins of the neurocentral canal, and which receive the neural arch pedicels. In Paraplacodus, both cervical and dorsal centra carry flat facets for the neural arch pedicels which also show some very weak broadening reminiscent of the cruciform or ‘butterfly shaped’ structure characteristic of eosauropterygians. The neural canal is slightly constricted at the middle of the centrum.

Within the dorsal region, the transverse processes of the neural arches are

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Figure 5. Postcranial elements of Paraplacodus broilii Peyer (holotype, PIMUZ T4773). A, isolated dorsal neural arch; B, isolated dorsal rib. Scale bars=20 mm.

elongated and distally expanded (Fig. 5A), more distinctly so than in Placodus. In Paraplacodus, the length of the transverse processes increases from the anterior dorsal region towards its middle section, behind which it decreases again. In the anterior dorsal region of specimen T4775, the width across both transverse processes is 44.5 mm, in the middle section the width is 60.1 mm, in the posterior dorsal region it is 57.2 mm. The same trend is observed in specimen PIMUZ T4827. Compression of the material does not allow the identification of details of the orientation of the pre- and postzygapophyses (Peyer, 1935; Kuhn-Schnyder, 1942), which in Placodus show a changing inclination in an antero-posterior trend. However, Peyer (1935) established the presence of accessory, hyposphene–hypantrum articulations between the dorsal vertebrae on the holotype, which was confirmed by personal inspection.

An isolated dorsal neural arch with a total height of 32.4 mm, and a total width of 64 mm, shows the high (9.8 mm) and narrow (3.1 mm) neural canal characteristic for placodonts. A deep and wide trough, undivided by a vertical internal septum, lies at the base of the neural spine, above the neural canal and below the prezygapophyses, forming the hypantrum. Another isolated dorsal neural arch, exposed in posterior view, shows a crushed hyposphere projecting from the base of the neural spine, below the postzygapophyses. Specimen PIMUZ T4774 shows again an isolated dorsal neural arch in anterior view, which is somewhat better prepared and preserved the distinct ridges which run from below the prezygapophysis to the neural arch pedicels, defining the lateral margins of the hypantrum. These ridges lack the anteriorly projecting lappets that are present on an enigmatic vertebra from the Muschelkalk of Makhtesh Ramon (Rieppel, Mazin & Tchernov, 1999, fig.

54C), which was tentatively referred to Paraplacodus by Haas (1975).

As shown by specimen PIMUZ T4775, the neural spines are relatively low yet elongated and fairly thick sagittal blades which are of trapezoidal shape in lateral view throughout the cervical and dorsal region. Their height increases somewhat towards the middle section of the dorsal region, and slightly decreases again towards the sacrum. The uniform appearance of the neural spines changes in the caudal region (Fig. 6). Their height increases abruptly in the third and succeeding caudal vertebrae, which caused the tail to rotate relative to the sacrum as the carcass became embedded in sediment. As a consequence of that rotation, the tail is exposed in lateral view, whereas the dorsal and sacral region are exposed in dorsal view. In the anterior caudal region (Fig. 6A), the neural spines are higher than they are long, and they slant slightly posteriorly. In the middle section of the tail (Fig. 6B), the

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Figure 6. Caudal vertebrae of Paraplacodus broilii Peyer (PIMUZ T4775). A, anterior tail section; B, middle tail section; C, distal tail section. Scale bars=20 mm.

neural spines have an elongate and low base which quickly narrows to a slender dorsal process which is slightly curved in an anterior direction. Further back, the slender dorsal process which forms the neural spine becomes progressively bent backwards, running parallel to the vertebral centrum at the tip of the tail (Fig. 6C).

The first chevron is carried by the fourth caudal vertebra. As described by KuhnSchnyder (1942), the anterior haemapophyses articulate with the posteroventral aspect of their respective centra. In the middle section of the tail, the chevron articulation bridges the contact between two successive vertebral centra, while they articulate with the anteroventral aspect of the centra in the posterior part of the tail. The distal end of the haemapophyses is distinctly broadened in Paraplacodus. In the anterior part of the tail, each chevron expands into a semicircular distal plate with a straight ventral margin (Fig. 6A). In the middle section of the tail, the ventral margin of the semicircular plate becomes distinctly concave, such that the chevron appears to expand into short and curved anteroventral and posteroventral processes (Fig. 6B). Further back still, the chevrons expand distally into short and slender anteroventral and posteroventral processes which are aligned along the longitudinal axis of the centrum (Fig. 6C). In general, the neural and haemal arches contribute to a lateral flattening of the tail, especially in its anterior and middle portion, which

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Figure 7. The pectoral girdle of Paraplacodus broilii Peyer. A, left clavicle (PIMUZ T4775); B, right coracoid (PIMUZ T4827). Scale bars=20 mm.

indicates the support of subaqueous locomotion through lateral undulation of the tail.

In Placodus, the first cervical rib is associated with the axis; no specimen of Paraplacodus is well enough preserved to allow the unequivocal identification of the first cervical rib and its associated vertebra. In specimen BSP 1953 XV 5, the anteriormost cervical ribs are located immediately behind the quadrate. As in all sauropterygians, the cervical ribs are double headed, and they carry a free ending anterior process.

The dorsal ribs of Paraplacodus are highly characteristic (Fig. 5B). They expand into a distinct uncinate process along their posterior margin. This expansion starts close to the proximal articular head of the rib, and becomes broader distally. It terminates with an irregularly notched and ridged ventral margin at about two thirds of the length of the rib. If articulated, the uncinate processes of succeeding ribs tightly overlap. As described by Kuhn-Schnyder (1942), the 14th dorsal vertebra still carries a well developed uncinate process, which is strongly reduced on the 15th dorsal rib, and absent on the 16th rib. The three sacral ribs dier from the caudal ribs by their distinct distal expansion.

Several specimens of Paraplacodus show disarticulated clavicles. This is a generally slender, strongly curved or boomerang-shaped element (Fig. 7A). Its shanks, which enclose an angle of approximately 75°, taper at both ends. Specimen PIMUZ T4774 in particular shows a close similarity of the clavicle to the same element of Placodus, with a posterior lamellar expansion of the anteromedial process partially filling the space between the two shanks (Peyer, 1935). This broadening of the anteromedial process of the clavicle cannot be identified on specimen PIMUZ T4775 (Fig. 7A).

Specimen BSP 1953 XV 5 preserves a curved, elongated element located between the skull and the right scapula, which may represent the right clavicle. If correctly identified, this clavicle appears much less angulated than that observed in other specimens but rather evenly curved, although this may be an artifact of preservation as the element is broken across cervical vertebrae. It also shows a relatively broad termination of the anteromedial process, whereas the posterolateral process,

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Figure 8. The appendicular skeleton of Paraplacodus broilii Peyer. A, right humerus (PIMUZ T4775); B, left zeugopodium and carpus (PIMUZ T4775); C, right humerus (holotype, PIMUZ T4773); D, right femur (holotype, PIMUZ T4773). Scale bars=20 mm.

contacting the scapula, appears more slender and tapering. There seems to be some potential for variation of the shape of the clavicle in Paraplacodus. The interclavicle of Paraplacodus remains unknown.

Specimen BSP 1953 XV 5 shows the beautifully preserved left scapula in lateral view. Its ventral margin overlaps the distal end of the posterodorsal process of the clavicle, which indicates that the clavicle was applied against the medial surface of the scapula, as was described and illustrated by Peyer (1935), and as is also the case in Placodus and other sauropterygians. However, the shape of the scapula differs from that of Placodus, as was also noted by Peyer (1935) and Kuhn-Schnyder (1942), in that the ventrally expanding glenoidal portion is distinctly set off from the posterodorsally extending dorsal blade. The two components of the scapula are separated by a constricted neck region, which together with the posterodorsal orientation of the dorsal blade results in a deeply concave posterior margin of the scapula. The coracoid of Paraplacodus again differs from that of Placodus in being more elongate (Fig. 7B). The best preserved coracoid is shown by the specimen PIMUZ T4827 (Fig. 7B). It is 1.5 times as broad as it is long. Its anterior margin is sigmoidally curved, its posterior margin slightly concave in the proximal part.

The lateral (glenoidal) margin is weakly concave, the medial (symphyseal) margin is evenly curved. A weak indentation on its lateral (glenoidal) margin indicates the passage of the supracoracoid nerve between the scapula and the coronoid.

The humerus is well exposed in several specimens (Fig. 8A, C) and most distinctly differentiated in the specimen PIMUZ T4775 (Fig. 8A). The humerus is strongly

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curved, more strongly so than in Placodus, but its distal expansion is equally prominent.

The preaxial margin of the humerus of Placodus is rather straight, while its deeply concave postaxial margin still results in the appearance of a generally curved humerus (Rieppel, 1995, fig. 42). In Paraplacodus, the anterior margin of the humerus is convex, its posterior margin concave, more like in other sauropterygians. The deltopectoral crest is well developed. Dorsoventral compression of the bone may have obscured the ectepicondylar groove, or else it was weakly developed. The entepicondylar foramen is absent. The left radius and ulna are again well exposed in the same specimen (Fig. 8B). The radius has a curved appearance due to a distinct angulation of the proximal part of its preaxial margin. Its postaxial margin is evenly concave. The ulna is a rather straight element, somewhat more lightly built than the radius, with a biconcave diaphysis. Together, the radius and ulna enclose a distinct spatium interosseum. Between the distal ends of the radius and ulna, and the proximal ends of the metacarpals, a total of four ossifications are preserved. Three of these are closely associated, and exposed in dorsal view (Fig. 8D). The largest element must represent the intermedium. It carries a distinct concavity on its proximal margin, indicating the passage of the perforating artery proximal to the intermedium, between the distal ends of the radius and ulna. Next to the intermedium lies the ulnare of intermediate size, which in turn articulates with the smallest of the three ossifications, the fourth distal carpal. The fourth ossification preserved in the general area of the carpus is not associated with the other three elements, and lies perpendicular to the bedding plane, exposing its articular margin. It could represent a fourth carpal ossification (third distal carpal) or, less likely, a much displaced phalanx. With three, perhaps even four carpal ossifications, Paraplacodus differs from Placodus for which a single carpal ossification has been described by Drevermann (1933), perhaps an incomplete number. The metacarpals and phalanges are incompletely preserved, but still allow the minimal reconstruction of the plesiomorphic phalangeal formula 2–3–4–5–3 for the manus (Kuhn-Schnyder, 1942).

The ilium of Paraplacodus is an autapomorphic structure quite different from that of other sauropterygians (Fig. 9B). It forms an essentially vertical strut with concave lateral margins. The ventral portion is expanded to from the shallow dorsal half of the acetabulum. This acetabular portion is separated from a dorsal process by a distinctly constricted neck. The dorsal process expands dorsally, but not as much as to reach the width of the ventral acetabular portion, and it ends in an obliquely sloping dorsal margin. The pubis is well preserved in the specimen PIMUZ T4827 (Fig. 8C). It is an elongated element with weakly concave anterior and posterior margins. Its ‘waisted’ appearance resembles the pubis of other sauropterygians more closely than the shorter, more rounded element characteristic of Placodus. But as in Placodus, the slit-like obturator foramen remains open in the adult. The ischium, is again of typical sauropterygian structure with a narrow lateral (acetabular) head, and a wide medial (symphyseal) expansion which results in distinctly concave anterior and posterior margins (Fig. 9A). By comparison to Placodus, both the pubis and ischium are less expanded to form plate-like elements, which results in a much more distinct thyroid fenestra in Paraplacodus, the plesiomorphic condition. A comparison of Paraplacodus with Placodus and other sauropterygians shows that the thyroid fenestra is secondarily reduced in Placodus (Sues, 1987).

Both femora are well preserved in specimen PIMUZ T4775. The element is more lightly built and somewhat shorter than the humerus (Fig. 8D). It is weakly curved,

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O. RIEPPEL

Figure 9. The pelvic girdle of Paraplacodus broilii Peyer. A, left ischium and femur (PIMUZ T4775); B, ilium (holotype, PIMUZ T4773); C, left pubis (PIMUZ T4827). Scale bars: A=20 mm, B=10 mm, C=20 mm.

and shows a well developed internal trochanter distinctly set o from the proximal articular surface by an intertrochanteric fossa (Fig. 9A), a morphology which is closely comparable to the femur of Placodus (Rieppel, 1995, fig. 42B). Tibia and fibula are relatively long and slender elements. The tibia is somewhat longer relative to the femur than is the radius relative to the humerus. The tibia has a relatively straight preaxial margin and a concave postaxial margin. The diaphysis of the fibula is biconcave. Together, the tibia and fibula enclose a distinct spatium interosseum.

The right and the left hind-limbs lie parallel to each other to the left side of the axial skeleton. The left and the right tarsus thus come to lie in juxtaposition to one another. Combined, the two tarsi preserve three tarsal ossifications. The left calcaneum is in articulation with the left fibula, leaving the right tarsus with an astragalus and a calcaneum in a slightly shifted position relative to the right tibia and fibula. A fourth distal tarsal cannot be identified, and appears to have been

PARAPLACODUS AND THE PHYLOGENY OF THE PLACODONTIA

649

absent (not ossified). If this interpretation is correct, Paraplacodus would share with Placodus the presence of only two tarsal ossifications. The metacarpals of the left foot are incompletely preserved, along with a single phalanx. The phalangeal formula for the pes of Paraplacodus therefore remains unknown.

Both Placodus and Paraplacodus show gastral ribs, composed of five elements each, and in both taxa, the two most lateral elements on either side of the gastral rib are strongly angulated, enclosing an angle of almost 90° (Rieppel & Zanon, 1997). In Placodus, a row of osteoderms is aligned along the dorsal midline of the body, capping the tips of the neural arches. Dorsal osteoderms are absent in Paraplacodus (specimen PIMUZ T4827).

Comparisons Paraplacodus shares with Placodus some important characters of the cranial and postcranial skeleton, such as the high and narrow skull, the large nasals, the relative size of the upper temporal fossa the confluent internal nares, the presence of accessory intervertebral (hyposphene–hypantrum) articulations, the broad distal expansion of the humerus, the distinct internal trochanter on the femur, and the strongly angulated lateral gastral rib elements. However, it also shows important dierences from Placodus, such as the low coronoid process, exposing the coronoid in lateral view, the low neural spines, the nature of the neurocentral suture, the shape of the chondral elements in the pectoral and pelvic girdles, and the absence of osteoderms (other than the gastralia). It would therefore seem to be important to re-assess the monophyly of the Placodontoidea, which are to include Paraplacodus and Placodus, as opposed to a hypothesis which would place Placodus closer to cyamodontoids than Paraplacodus, and thus place Paraplacodus as the sister-taxon of all other known placodonts.

Unfortunately, the postcranial skeleton of cyamodontoids remains very poorly known, mostly for preservational reasons. Articulated material of cyamodontoid placodonts (Cyamodus: Pinna, 1980, 1992; Psephoderma: Pinna & Nosotti, 1989) comes from deposits which yielded severely dorsoventrally compressed specimens, and many details of postcranial structures are obscured by the extensive dermal armour.

However, some characters can be identified. Given their extensive armour, cyamodontoid dorsal vertebrae tend to carry low neural spines, but their transverse processes are much enlarged and curved, sometimes even fused with the dorsal ribs (Psephoderma: Pinna & Nosotti, 1989), an autapomorphy of the group. Details of the intervertebral articulations remain unknown. According to Pinna & Nosotti (1989), the chevrons articulate with the anteroventral aspect of their respective centrum in Psephoderma, as is also the case in the posterior part of the tail in Paraplacodus. Distal caudal vertebrae of Placodus did not carry any chevrons; more anterior caudal vertebrae show articular facets for the chevrons on their posteroventral aspect (Drevermann, 1933), as is also the case in Paraplacodus.

In limb girdle morphology, cyamodontoids more closely resemble Placodus than Paraplacodus in some characters. Probably again correlated with the development of a carapace, the scapula is reduced by comparison to placodontoids. In Cyamodus, the scapula is still rather large, and its contours, although ill preserved, resemble those of the scapula of Paraplacodus more closely than that of Placodus, in that it is a little more slender and its posterior margin more deeply concave (Pinna, 1980). For Psephoderma, it is described as a simple vertical strut with a limited ventral glenoid

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expansion (Pinna & Nosotti, 1989). In both genera, however, the coracoid is a rounded plate of bone with an open notch for the coracoid foramen, very much like the coracoid in Placodus, and unlike the more elongate and slender coracoid of Paraplacodus. The ilium is very incompletely known for Cyamodus (Pinna, 1980), but in Psephoderma, the ilium expands dorsal to its acetabular portion into a short and stubby, dorsally somewhat expanding blade (Pinna & Nosotti, 1989). A similar morphology is indicated for the ilium of Placochelys ( Jaekel, 1907). In cyamodontoids, however, the ventral pelvic elements, pubis and ischium, are expanded rounded plates, the pubis with an open obturator foramen, closely resembling the pubis and ischium of Placodus. Given the morphology of the ventral pelvic element, the thyroid fenestra is much reduced in cyamodontoids, as it also is in Placodus.

The stylopodial and zeugopodial elements of the front- and hind-limbs are relatively somewhat shorter in cyamodontoids as compared to placodontoids. The humerus is well known in three-dimensional preservation for Placochelys ( Jaekel, 1907), and reasonably well known for other genera (Pinna, 1980; Pinna & Nosotti, 1989), and it resembles the placodontoid humerus in its pronounced distal expansion, the presence of an ectepicondylar groove, and the absence of an entepicondylar foramen. However, as in Placodus, the humerus of cyamodontoids is much less distinctly curved than it is in Paraplacodus. In cyamodontoids and in Placodus, the preaxial margin of the humerus is rather straight, whereas it is distinctly convex in Paraplacodus. As far as they are known, radius, ulna, and the manus do not offer distinctive similarities or differences between placodontoids and cyamodontoids.

The femur of Placochelys is again well known in three-dimensional preservation, and although the original material described by Jaekel (1907) is lost, casts are kept at the Natural History Museum London (BMNH R 4070, 4074). The femur shows a reduction of the internal trochanter as compared to Placodus and Paraplacodus, which results in a femur morphology more closely comparable to eosauropterygians (e.g. Simosaurus: Rieppel, 1994, fig. 63A). The badly crushed material of Psephoderma (Pinna & Nosotti, 1989) conveys the impression of a sturdier femur, but this may be an artifact of preservation. There is no indication that the internal trochanter was as well developed as it is in placodontoids. As far as they are known, tibia, fibula, and the pes do not offer distinctive similarities or differences between placodontoids and cyamodontoids.

Gastral ribs are present in cyamodontoids, but there is no indication of a strong angulation of the lateral elements. Cyamodontoids share with Placodus the presence of osteoderms, but these are developed into a carapace and, in some taxa, into a separate tail shield and plastron.

CLADISTIC ANALYSIS

A recent review of the cranial anatomy, phylogeny, and historical palaeobiogeography of the Cyamodontoidea resulted in a data matrix for 67 characters for the analysis of placodont interrelationships (Rieppel, 2000). In order to test the monophyly of the Placodontoidea, this data matrix was amended with 11 additional characters which result from the description and discussion above (characters 52 through 62). Since this data matrix was designed to test primarily cyamodontoid interrelationships, but is now used to test placodont interrelationships in general,

PARAPLACODUS AND THE PHYLOGENY OF THE PLACODONTIA

651

T 1. Data matrix for the analysis of the phylogenetic interrelationships of placodonts. See text for character definitions 12345

67891 0

11111 12345

11112 67890

22222 12345

22223 67890

33333 12345

Pachypleurosaurs

00001

00000

00000

000??

01001 01101

1???0 1?100

1??00 10000

00000 00000

000??

000??

Placodus

10001

00100 1 01010 00000 1 11 00000

1?000

Simosaurus Nothosaurus

11000

00000

00000

11111

Paraplacodus Cyamodus

00001 20000

Henodus Macroplacus

202?0 200?1

00000 1 1 10000 001?0 1 00000 1 0?0??

01110 1 10110 10110

1???0 ?0110 1 ??110 12111

0??0?

11011 1 0?0?1 0?011

0??00 00100 1 1?1??

0010?

01001 21210 2 ??43? ?1220

Placochelys

21110

11111

01021

01111

??120

Protenodontosaurus

20011

01010 1 00010

?0??0 11011 2 10?1?

000??

1 00111 00102

0211?

01110

11320

Psephoderma

2110?

1??20

00002

1111?

0?021 1 0?0?1

011?1

??220

33334 67890

44444 12345

44445 67890

55555 12345

55556 67890

66666 12345

66 67

Pachypleurosaurs Simosaurus Nothosaurus

?0000 ?0000 ?0000

11100 01100 01100

0?000 0?000 0?000

00000 00000 00000

10000 10000 10000

00 00 00

Placodus Paraplacodus

00001 0??01

00000 ?????

00000 ??001

00000 00000 00000 1 00120 00010

01111 0111?

11 ?1

Cyamodus Henodus Macroplacus Placochelys Protenodontosaurus

00000 1?000 11100 01110 01100

10012 10102 ?????

01112 110??

21121 21111 211??

2112? ?11??

10111 101?0 ??1?1 101?1 ??1?1

11 11 11 11 11

Psephoderma

11110

20101 ?1001 101??

11010 10000 2 11010 2

10111 10000 1 ?1011 ????1 ????? ????1 ?????

00112

11121

??111

101?1

11

and is being rooted on sister-taxa of placodonts (i.e. eosauropterygians, including pachypleurosaurs and the genera Simosaurus and Nothosaurus), placodont synapomorphies were also added to the data matrix (characters 63 through 67, from Rieppel & Zanon, 1997). Furthermore, the characters of the species of Cyamodus are combined and coded at the generic level, which renders some characters with relevance to the reconstruction of cyamodontoid interrelationships uninformative.

These characters are deleted from the list of character definitions given below. For postcranial characters, Cyamodus was coded using C. hildegardis. The character definitions are the following (the data matrix is given in Table 1): (1) Osteoderms absent (0), osteoderms present (1), carapace present (2).

(2) Dividing the total length of the skull by the total height of the skull yields a ratio smaller (0), or larger (1) than 3.

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(3) Rostrum relatively short and broad (0), or narrow and distinctly elongated (1), or spatulate (2).

(4) The ventral surface of the premaxilla is level with the ventral surface of the maxilla (0), or the rostrum is distinctly downturned (1) (Merck, 1997).

(5) The premaxilla extends backwards for more (0), or less (1) than half of the length of the ventral margin of the external naris (Merck, 1997).

(6) Nasals in contact along midline of skull (0), or separated from one another by large posterior (nasal) processes of the premaxilla.

(7) Anterior end of maxilla does not (0), or does (1) expand medially to form most of the dermal floor of the external naris.

(8) The anterior tip of jugal does (0), or does not (1) extend anteriorly along the ventral margin of the orbit beyond the midpoint of the longitudinal diameter of the orbit.

(9) Pineal foramen placed in centre of skull table (0), displaced anteriorly on skull table (1), or is displaced anteriorly with frontal entering its anterior margin (2).

(10) Anterolateral processes of frontals well developed (0), or reduced (1).

(11) Parietal without (0), or with (1) distinct anterolateral processes embraced by postfrontal and frontal.

(12) Frontals do not (0), or do (1) reach posteriorly beyond the level of the anterior margin of the upper temporal fossa.

(13) Partietal skull table constricted in its posterior part, i.e. with concave lateral margins (0), or square, i.e. with straight lateral margins in its posterior part (1).

(14) Posterolateral margin of postfrontal weakly concave and evenly curved (0), or deeply concave and angulated (1).

(15) Postfrontal enters upper temporal fossa (0), or is excluded from upper temporal fossa by a narrow (1), or broad (2) contact of the postorbital with the parietal.

(16) Postorbital extends along lateral margin of temporal fossa to a level in front of or at the midpoint of the longitudinal diameter of the upper temporal fossa (0), or further back (1).

(17) The vertical part of the suture separating the maxilla from the jugal is located behind the level of the posterior margin of the orbit (0), behind the level of the midpoint of the longitudinal diameter of the orbit but in front of the posterior margin of the latter (1), or at the level of the midpoint of the longitudinal diameter of the orbit (2). The jugal in placodonts may form a slender, tapering anterior process which lines the ventral margin of the orbit (character 8 above). This process is distinctly set o from the vertical suture which separates the jugal from the maxilla over the greater part of its height, which is the character addressed here.

(18) The dorsal process of epipterygoid is narrow (0), or broad (1).

(19) The base of epipterygoid is sutured predominantly to the pterygoid (0), or to the palatine (1).

(20) The postorbital does not (0) or does (1) form a medioventral process which abuts against the lateral surface of the epipterygoid at the posterodorsal margin of the foramen interorbitale.

(21) The longitudinal diameter of the upper temporal fossa is less than twice (0), or at least twice (1) the longitudinal diameter of the orbit (in the adult).

PARAPLACODUS AND THE PHYLOGENY OF THE PLACODONTIA

653

(22) The epipterygoid does not (0), or does (1) form a posterior dorsal process which contacts the squamosal at anterodorsal corner of posttemporal fossa.

(23) The epipterygoid is always fully ossified in the adult (0), or may be incompletely ossified in the adult (1). The incompletely ossified epipterygoid occurs in some cyamodontoids. If present, the epipterygoid appears as a bipartite element, the two parts separated by a cleft that must have been filled with cartilage in the adult (Nosotti & Pinna, 1998) (24) The (neomorph) otic process of the squamosal is absent (0), extends to the midpoint of the ventral margin of the posttemporal fossa (1), or extends beyond the level of the medial margin of the posttemporal fossa (2) (in lateral view of the skull).

(25) A palatoquadrate cartilage recess is absent (0), or present (1).

(26) A basiorbital furrow is absent (0), or present (1).

(27) The palatine does not (0), or does (1) contact the quadrate along the lateral margin of the palatoquadrate cartilage recess.

(28) The pteroccipital foramen is absent (0), or present (1).

(29) The prootic is not (0), or is (1) exposed in occipital view of the skull.

(30) Premaxillary teeth are present (0), or absent (1).

(31) Anterior premaxillary and dentary teeth pointed (0), chisel-shaped (1), or bulbous with anterior transverse ridge (2).

(32) A diastema separating premaxillary and maxillary teeth is absent (0), or present (1).

(33) Seven or more (0), five to three (1), two (2), one (3), or no (4) maxillary teeth (tooth).

(34) More than three (0), three (1), two (2) or one (3) pair(s) of palatine teeth.

(35) Anterior palatal tooth plate(s) small and rounded (0), or transversely enlarged (1). The anteriormost palatine tooth plates are small and have a crown with a circular circumference (rounded crown) in cyamodontoids. By contrast, the anterior palatine tooth plates in Paraplacodus and Placodus are larger relative to the posterior palatine tooth plates, and have a irregularly rhomboidal shape, i.e. a crown with a transverse diameter exceeding the longitudinal diameter.

(36) The ratio of the longitudinal to the transverse diameter of the posterior palatine tooth plate less (0), or equal/more (1) than 1.4 (in the adult).

(37) Maxilla without (0), or with (1) anterior process extending into rostrum in ventral view.

(38) Ventral surface of rostrum flat (0), or concave (1).

(39) Ventral surface of rostrum without (0), or with distinct grooves leading up to internal nares (1).

(40) Internal nares separated (0) or confluent (1).

(41) Ectopterygoid present (0) or absent; if absent, palatine extends laterally at the anterior margin of the subtemporal fossa to meet the jugal (1), or jugal extends medially to meet the palatine (2).

(42) The ratio of the length of palatal exposure of pterygoid relative to length of palatine is less (0), or more (1) than 0.3.

(43) The posttemporal fossae are relatively large (0), or reduced (1) due to expansion of occipital exposure of parietal, squamosal, and opisthotic.

(44) The squamosal buttress against which abuts the distal tip of the paroccipital process is absent (0), or present (1).

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(45) The posteroventral tubercle is absent (0), or present (1) at the distal tip of the paroccipital process.

(46) The exoccipitals do not (0), or do (1) meet above occipital condyle (above the basioccipital).

(47) The basioccipital tuber and the ventral opisthotic flange remain separate (0), or meet each other (1) ventral to passage of internal carotid.

(48) Anterior tip of dentary dentigerous (0), or edentulous (1) (Merck, 1997).

(49) The coronoid remains well separated from lower margin of the mandible (0), or closely approaches the lower margin of mandible (1).

(50) The retroarticular process is long and slender (0), or robust (1), or short and sloping (2). The retroarticular process of Placodus is elongate and slender, i.e. distinctly set off from the remaining part of the lower jaw, which results in a deep concavity in the posteroventral margin of the mandible (0). In Paraplacodus, the retroarticular process is equally elongate with a horizontal dorsal margin, but it is less distinctly set off from the remaining lower jaw, and hence deeper and more robust (1). The retroarticular process of cyamodontoids is relatively shorter, stubby, and has a dorsal surface which slopes in a posteroventral direction (2).

(51) Tubercular osteoderms, secondarily fused to the underlying bone, are absent (0), or present along the posterior margin of the upper temporal fossa only (1), or present on lateral surface of posterior part of temporal arch also (2).

(52) Quadratojugal present (0), or absent (1).

(53) Jugal–squamosal contact absent (0), or present (1).

(54) Coronoid process absent (0), distinct but low (1), or very high (2).

(55) Number of dorsal vertebrae: 19 or more (0), 15 or less (1).

(56) Hyposphene–hypantrum articulation absent (0), or present (1).

(57) Chevrons articulating with posteroventral (0), or anteroventral (1) aspect of centrum.

(58) Coracoid elongated, with more or less concave anterior and posterior margins (0), or rounded plate of bone (1) (59) Thyroid fenestra large (0), or reduced by expansion of pubis and ischium to form rounded plates of bone (1).

(60) Preaxial margin of humerus curved (0), or rather straight (1).

(61) Internal trochanter distinctly set off from proximal end of femur by intertrochanteric fossa (0), or intertrochanteric fossa much reduced, absent (1).

(62) Lateral gastral ribs without (0), or with (1) distinct angulation.

(63) Crushing tooth plates absent (0), or present (1).

(64) Diastema between symphyseal and posterior dentary teeth absent (0), or present (1).

(65) Palatines separated by pterygoids (0), or meeting in medial suture (1).

(66) Pterygoids longer (0), or shorter (1), than palatines.

(67) Median gastral rib element angulated (0), or straight (1).

A total of four analyses were performed using the software package PAUP version 3.1.1. developed by David L. Swofford (Swofford, 1990; Swofford & Begle, 1993).

All searches were done implementing the branch-and-bound search option, and uninformative characters were always ignored. For two analyses, all multistate characters were unordered, and these were rooted alternatively on an all-0-ancestor, and on successive sister-groups of the Placodontoidea, i.e. the eosauropterygian taxa

ep

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655

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us H

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od ac Pl

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us

co

ur sa Pa r

ho ot N

M ac

du s

us

s ru au os m

Si

pa ch y

pl

eu ro

sa ur s

PARAPLACODUS AND THE PHYLOGENY OF THE PLACODONTIA

Figure 10. The phylogenetic interrelationships of placodonts, showing the paraphyly of Placodontoidea (Paraplacodus plus Placodus). For further discussion, see text.

Pachypleurosauria, Simosaurus, and Nothosaurus. In two further analyses, the multistate characters 1, 35, and 36 were ordered. This procedure implies the following a priori hypotheses of character evolution. Ordering character 1 implies the absence of osteoderms to be plesiomorphic for placodonts, and the appearance of a single row of osteoderms in Placodus to precede the evolution of a carapace in cyamodontoids.

Ordering the characters 33 and 34 assumes a regular trend in the reduction of the maxillary and palatine dentition.

Deleting the eosauropterygian taxa from the analysis, and rooting the search on an all-0-ancestor (characters 12, 21, 32, 56, 63, 64, 66, and 67 uninformative and hence ignored) yielded a single most parsimonious tree with a Tree-Length (TL) of 111 steps, a Consistency Index (CI) of 0.775, and a Retention Index of 0.667. The Placodontoidea was found to be paraphyletic, with Placodus more closely related to the Cyamodontoidea than Paraplacodus (Fig. 10). Deleting the all-0-ancestor, and rooting the analysis on the three eosauropterygian taxa (character 16 uninformative and hence ignored), yielded a single most parsimonious tree with a TL of 135 steps, a CI of 0.733, and a RI of 0.7. The hierarchy of the tree was the same: (Paraplacodus (Placodus, Cyamodontoidea). With all multistate characters unordered, Paraplacodus, Placodus fall and the monophyletic Cyamodontoidea form a basal trichotomy in a tree only one step longer than the most parsimonious solution. The clade including Placodus and the Cyamodontoidea therefore has a decay index of 1.

The second set of analyses paralleled the first two searches, but with the characters 1, 33 and 34 ordered. Rooting the analysis on the all-0-ancestor (characters 12, 21, 32, 56, 63, 64, 66, and 67 uninformative and hence ignored), yielded a single most parsimonious tree again, with a TL of 112, a CI of 0.768, and a RI of 0.683. The tree topology was the same as in the previous two searches (Fig. 10). Rooting the tree of the selected eosauropterygian taxa (character 16 uninformative and hence

656

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ignored) yielded again one single most parsimonious tree, with a TL of 136 steps, a CI of 0.728, and a RI of 0.715. The tree topology remained the same again: (Paraplacodus (Placodus, Cyamodontoidea). With three multistate characters ordered, Paraplacodus, Placodus and the monophyletic Cyamodontoidea form a basal trichotomy in a tree three steps longer. Ordering multistate characters 1, 33, and 34 therefore increased to decay index for the clade including Placodus and the Cyamodontoidea to 3.

As is clear from the description given above, Paraplacodus shares with Placodus a number of potential synapomorphies which are absent in cyamodontoids, such as the high skull, large nasals, the loss of a quadratojugal, the confluent internal nares, the hyposphene–hypantrum articulation (coded unknown for cyamodontoids), and the strongly angulated lateral gastral rib elements (Rieppel & Zanon, 1997). Placodus on the other hand shares with cyamodontoids a number of potential synapomorphies which are absent in Paraplacodus, such as the secondary squamosal-jugal contact, the reduction of the maxillary teeth to five or fewer, the tall cornoid process, the rounded coracoid, the rather straight preaxial margin of the humerus, the reduction of the thyroid fenestra, and osteoderms. These character conflicts may explain the low decay indices, in spite of the fact that the monophyly of the clade including Placodus and the cyamodontoids is supported by several unambiguous synapomorphies (with a consistency index of 1).

With the tree rooted on the three eosauropterygian taxa, all multistate characters unordered, and DELTRAN character optimization implemented, Placodus was found to share with the Cyamodontoidea the following synapomorphies (unambiguous synapomorphies, with a ci=1, are denoted with an asterisk): ∗31(1), premaxillary teeth chisel-shaped; 42(0), short palatal exposure of the pterygoid (coded unknown for Paraplacodus); ∗53(1), secondary jugal–squamosal contact present; 54(2), tall coronoid process (this would be an unambiguous synapomorphy were it not reversed in the highly derived genus Henodus); ∗59(1), reduction of the thyroid fenestra by expansion of pubis and ischium; ∗60(1), preaxial margin of humerus rather straight; 65(1), palatines meet in ventromedial suture (coded unknown for Paraplacodus, reversed in Henodus), ∗66(1), pterygoids shorter than palatines (coded unknown for Paraplacodus).

Ordering the multistate characters 1, 33, and 34 adds these to the list of synapomorphies shared by Placodus and cyamodontoids: ∗1(1) osteoderms present (this character is questionably optimized, as the potential to have dermal ossifications may well be plesiomorphic in placodonts with respect to sister-taxa of the Sauropterygia, and lost in Paraplacodus); 33(1), five or less maxillary teeth; ∗34(1), three or less palatine teeth.

DISCUSSION

The configuration of the temporal region in placodonts has to the present day remained a matter of contention (summarized in Pinna, 1989, and Rieppel, 1995).

The significance of the configuration of the temporal region has also played an important role in the discussion of placodont relationships among reptiles in general (Kuhn-Schnyder, 1980). The presence of a contact between jugal and quadratojugal, and their juxtaposition to the postorbital and squamosal respectively, has been cited

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as evidence that placodonts never had a lower temporal fossa, and hence could not be nested within diapsids (Kuhn-Schnyder, 1980; Pinna, 1989). Alternatively, the presence of an L-shaped jugal, which remains separate from the squamosal, and the deep embayment of the cheek region in Paraplacodus has been thought to indicate a diapsid status of placodonts (Zanon, 1989). Zanon (1989) also pointed out that the limited dorsal and posterior extent of the jugal in Paraplacodus indicates that the contact of the jugal with the squamosal is secondary in those placodonts where it occurs (also supported by Mazin, 1982). Following this line of reasoning, placodonts such as Placodus would have secondarily roofed over the cheek region, which seems also to be reflected in the broad and fan-shaped contours of the jugal, and in the covering of the quadrate in lateral view by dermal bone (Rieppel, 1995). The secondary expansion of dermal elements in the temporal region of the skull is carried to its extreme in Henodus among all placodonts, a taxon which obliterates the upper temporal fossa (Rieppel, in press).

The temporal region of Placodus has been subject of various interpretations. Broili (1912) figured a postorbital and a much expanded jugal which meet an even more expanded squamosal in the formation of the broad temporal arch. Sues (1987) concurred with Broili’s (1912) identification of postorbital and jugal, but separated a narrow dorsal squamosal, defining the posterolateral margin of the upper temporal fossa, from a broad quadratojugal, which would cover the quadrate in lateral view.

Pinna (1989) shows a small squamosal which is restricted to the posterior margin of the upper temporal fossa, while the quadratojugal would have much expanded, forming the entire posterior part of the temporal arch, and entering the posterior lateral margin of the upper temporal fossa. Rieppel (1995) noted that there is not one skull of Placodus kept in public repositories which allows the unequivocal delineation of a quadratojugal from a squamosal, and accepted Broili’s (1912) conclusion that a quadratojugal is absent (or fused with the squamosal?) in Placodus.

The interpretation of the temporal region of the skull of Paraplacodus, with an Lshaped jugal, and a temporal bar formed by the postorbital and squamosal only, indicates that it shares with Placodus the lack of a quadratojugal. Furthermore, the configuration of the temporal region of Paraplacodus suggests the loss of a lower temporal arch, and a secondary closure of the cheek region in other placodonts, including Placodus, which results in a secondary jugal–squamosal contact.

This hypothesis conflicts with an alternative view, which would interpret the reduction of the dermal covering of the cheek region in Paraplacodus as the result of ventral emargination. In order to test these two hypotheses, the coding for Placodus (used as paradigm for placodonts in general) in a more inclusive data matrix (Rieppel, 1998) was alternated between the presence of a lower temporal fossa with reduced lower temporal arch versus the absence of the lower temporal fossa. The data matrix of Rieppel (1998, character 27) codes the lower temporal fenestra as absent (0), present and closed ventrally (1), present and open ventrally (2). This multistate character was ordered, because the loss of a lower temporal arch logically requires the prior presence of a lower temporal fenestra. Using Paraplacodus as the relatively most plesiomorphic placodont, characterized by the loss of the lower temporal arch (code 2 for Placodus, paradigmatic taxon for placodonts) results in a tree which is one step shorter than the assumption that Paraplacodus and with it placodonts in general would be characterized by the ventral emargination of the cheek region (code 0 for Placodus, i.e. absence of the lower temporal fenestra). The assumption of a loss of the lower temporal arch in Paraplacodus, and hence in placodonts in general,

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is therefore somewhat more parsimonious, and is in accordance not only with the configuration of the temporal region in Paraplacodus, but also with its position as the sister-taxon of all other known placodonts (Peyer & Kuhn-Schnyder, 1955).

As both, eosauropterygians ( Jaekel, 1910; Kuhn-Schnyder, 1967) as well as placodonts are characterized by the loss of the lower temporal arch, the latter in fact becomes a synapomorphy of Sauropterygia. However, since the Younginiformes are removed from the Sauria (Laurin, 1991; Laurin & Reisz, 1995), and the presence of a complete lower temporal arch is secondary in those rhynchocephalians where it occurs (Whiteside, 1986), it seems possible that the loss of the lower temporal arch is a synapomorphy of the Lepidosauromorpha with the Sauropterygia nested at the base of this clade (Rieppel, 1994,1998; but see Merck, 1997, for an alternative view).

ACKNOWLEDGEMENTS

I thank H. Rieber and H. Furrer from the Palaeontological Institute and Museum of the University of Zu¨rich for granted me access to the material of Paraplacodus in their care. The skull of the specimen BSP 1953 XV 5 was studied when it was on loan to G. Pinna, Museo Civico di Storia Naturale, Milano. A cast of the skull was made by the late R. Zanon, and deposited in the Field Museum by J. Hopson. John Merck allowed me to cite his unpublished thesis. N.C. Fraser read an earlier draft of the manuscript, offering much helpful advice and criticism. To all of these colleagues go my sincere thanks. This study was supported by NSF grants DEB9419675 and DEB-9815235.

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