Journal of Vertebrate Paleontology 20(4):720–735, December 2000 q 2000 by the Society of Vertebrate Paleontology
ON THE AQUATIC SQUAMATE DOLICHOSAURUS LONGICOLLIS OWEN, 1850 (CENOMANIAN, UPPER CRETACEOUS), AND THE EVOLUTION OF ELONGATE NECKS IN SQUAMATES MICHAEL W. CALDWELL* Paleobiology, Research Division, Canadian Museum of Nature, P.O. Box 3443, Station ‘D’, Ottawa, Ontario, Canada K1P 6P4 ABSTRACT—The marine squamate, Dolichosaurus longicollis, from the Upper Cretaceous (Cenomanian) Chalk deposits of southeast England is redescribed. The elongate neck of D. longicollis is produced by an increased number of cervical vertebrae. Cervical peduncles are elongate, curved and are not fused to the hypapophyses. There is no scapulocoracoid fenestra, the coracoid is not emarginated, and the scapula and coracoid are not fused. The splenial and angular articulate in a ball-and-socket joint similar to that of mosasaurs and Coniasaurus crassidens. The forelimb and pectoral girdle elements show evidence of reduction as compared to the pelvic girdle and rearlimb. Cladistic analysis of six mosasaur taxa, three ‘aigialosaur’ taxa, Coniasaurus crassidens, Coniasaurus gracilodens, and D. longicollis, using 66 characters, found 27 most parsimonious cladograms (MPCs): 122 steps; C.I. 0.648; H.I. 0.352; R.I. 0.669. A Strict Consensus Tree found support for the monophyly of the Mosasauridae and Aigialosauridae; sister-group relationships between coniasaurs, Dolichosaurus, Aigialosauridae and Mosasauridae are an unresolved polytomy. A Majority Rule Consensus Tree finds Dolichosaurus as sistergroup to (C. crassidens, C. gracilodens (Aigialosauridae (Mosasauridae))) in nine (33%) of the MPCs. Lack of support for a more inclusive Dolichosauridae composed of Dolichosaurus 1 (C. crassidens, C. gracilodens) is attributed to the incompleteness of the fossil remains of these three taxa. Presence/absence of a pectoral girdle currently defines the presence/absence of a neck. This definition is insufficient and hypapophyses are found more informative regarding taxic differences and transformational scenarios. The paleobiology of Dolichosaurus is reconstructed as similar to coniasaurs, nothosaurs, and modern sea snakes.
INTRODUCTION Owen (1850) described two monotypic genera of marine lizards, Dolichosaurus longicollis and Coniasaurus crassidens, based on a number of specimens collected from the Lower Chalk (Cenomanian; Upper Cretaceous) of southeast England. Owen used the exceedingly large number of cervical and dorsal vertebrae to diagnose Dolichosaurus, and tooth characteristics to diagnose Coniasaurus. Owen’s diagnoses did not identify any characteristics shared between these two taxa. Nopcsa (1908) restudied Owen’s specimens and added several new specimens to the list of known coniasaurs and dolichosaurs. Unfortunately, for Coniasaurus, Owen (1850) misidentified the tooth-bearing element of the type specimen (a maxilla) as a dentary. Nopcsa’s (1908) re-descriptions built on this error and further confounded the problem by not locating and then incorrectly identifying the type specimen (the type was in Brighton and Nopcsa only worked with specimens at the then British Museum of Natural History). Recently, Caldwell and Cooper (1999) located and redescribed the type maxilla and vertebrae of C. crassidens, and assigned a number of other tooth-bearing elements to Coniasaurus crassidens. Caldwell’s (1999a) description of Coniasaurus gracilodens, sp. nov. highlighted several notable differences with C. crassidens such as tooth shape, maxillary tooth number, and robustness of the maxilla (the maxilla of C. crassidens is much more elongate than that of C. gracilodens). Both species of Coniasaurus are known from disarticulated skulls with isolated vertebral elements, but no more than fragmentary and disarticulated postcranial remains. In contrast, the type and referred specimens of Dolichosaurus longicollis (see below) are articulated postcranial skeletons with only one very fragmentary skull and no preserved teeth. Owen’s (1850) original description included the type (the fragmentary skull and articulated anterior part of the postcranium) * Current address (Effective July 1, 2000): Department of Earth and Atmospheric Sciences, and Department of Biological Science, University of Alberta, Edmonton, Alberta, Canada T6G 2E9.
and a referred specimen (the posterior dorsal, sacral, and caudal vertebral series, and pelvic girdle/hindlimb elements); he considered these two pieces to be the front and back halves of one individual. His justification for this assumption was that both came from the same quarry and were collected at about the same time (Owen, 1850). Phylogenetically, coniasaurs and dolichosaurs have been considered to be nested within basal mosasauroids (mosasaurs and aigialosaurs) (Nopcsa, 1908), or recently, as the sistergroup to mosasauroids (Caldwell, 1999b). The difficulty in resolving these problematic relationships arises from the disparate and non-comparable fossil data forcing the creation and maintenance of two different genera. Dolichosaurus is known from complete postcrania with few skull bones, while both species of Coniasaurus are known from skulls and a small number of disarticulated vertebrae. Recent studies proposing a sistergroup relationship between snakes, coniasaurs, and mosasauroids, a group referred to as the Pythonomorpha (Caldwell and Lee, 1997; Lee, 1997; Lee and Caldwell, 1998; 2000), have highlighted the phylogenetic importance of coniasaurs, dolichosaurs, and other similar Mesozoic marine squamates. The Pythonomorpha excludes the fossorial amphisbaenids and dibamids, which are usually considered to be the closest squamate relatives of snakes (see Bellairs and Underwood, 1951; Wu et al., 1996). This new phylogenetic hypothesis, suggesting that snakes, mosasauroids, dolichosaurs, and coniasaurs may have a common aquatic ancestor, stands in contrast to the previous hypothesis of a fossorial snake origin. And, as a result of this new hypothesis of relationships, features of Dolichosaurus, i.e., the elongate neck, the possession of zygosphenes and zygantra, and the morphology of the intramandibular joint, are synapomorphies, and subsequently, by phylogenetic inference, are homologies. As homologies, these characters are important starting points for reasoned inference and argumentation on the origins of snakes and related forms. Therefore, to address these important issues in squamate phylogeny, this study re-examines the morphology and systematics of Dolichosaurus longicollis Owen, 1850, and its interrelationships with other dolichosaurs, aigialosaurs, and mosasaurs.
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CALDWELL—UPPER CRETACEOUS AQUATIC SQUAMATE METHODS AND MATERIALS Osteological Analysis The holotype specimen required minor preparation to expose the partial skull, braincase, and cervical vertebrae. Drawings and illustrations were made using a dissecting microscope and camera lucida attachment. Photographs were taken by the author and by the photographic department of The Natural History Museum (British Museum), London. Measurements were made using digital calipers. Institutional Abbreviations: BMB, Booth Museum of Natural History, Brighton, Sussex, England; BMNH, The Natural History Museum (British Museum), London, England; BSP, Bayerische Staatssammlung fu¨r Pala¨ontologie und historische Geologie, Mu¨nchen, Germany; FMNH, Field Museum of Natural History, Chicago, Illinois, U.S.A.; GBA, Austrian Geological Survey, Wien, Austria; MCSNT, Museo Civico di Storia Naturale, Trieste, Italy; NMW, Naturhistorisches Museum, Wien, Austria.
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SYSTEMATIC PALAEONTOLOGY SQUAMATA Oppel, 1811 DOLICHOSAURIDAE Gervais, 1852 Genus DOLICHOSAURUS Owen, 1850 Type Species—Dolichosaurus longicollis Owen, 1850 Range—Lower to Upper Cenomanian: Chalk Marl Fm. through Plenus Marls. (Mantelliceras mantelli Zone through the Acanthoceras jukesbrowni Zone [equivalent bio-zones Actinocamax varius Zone through the Holaster subglobosus Zone] (Fig.1). Generic Diagnosis—At least nineteen cervical vertebrae with large posteriorly placed hypapophyses and unfused peduncles; at least thirty-two trunk vertebrae; long, slightly curved ribs, body laterally compressed; scapulocoracoid fenestra absent; forelimb shorter and less robust than hindlimb; humerus subequal in length to two anterior dorsal vertebrae; ectepicondylar foramen absent; at least one pygal vertebra. DOLICHOSAURUS LONGICOLLIS (Figs. 2–12)
Cladistic Analysis The phylogenetic relationships of six genera of mosasaur, three species of aigialosaur, two species of coniasaur, and Dolichosaurus longicollis, were examined by cladistic analysis of a character matrix of 66 osteological characters (Appendices 1 and 2). Justification for limiting the taxonomic scope of this analysis to these twelve terminal taxa is based on more detailed studies of the interrelationships of these taxa with other squamates (Caldwell, 1999b), and with each other (Caldwell et al., 1995; Caldwell, 1996, 1999a; Bell, 1997). The conclusions of previous studies regarding the mosasauroid/pythonomorph affinities of other dolichosaurs (Nopcsa, 1908, 1923; Caldwell, 1999b; Lee and Caldwell, in press) are also accepted as a justification for the taxonomic sample examined in this study. Cladograms were generated by submitting the character matrix derived from study of the above squamates (Appendix 2) to Branch and Bound algorithms used in the computer software application PAUP Version 3.1.1 for the Macintosh (Swofford, 1993). For determining character polarity, an outgroup code was constructed from the character states of Pachyrhachis (Lee and Caldwell, 1998), Varanus salvator (unnumbered specimen, Redpath Museum), and Gerrhonotus leiocephalus (unnumbered specimen, Redpath Museum). Outgroup choice was based on the sistergroup relationships of mosasauroids as postulated by Caldwell (1999b) and Lee (1998). All characters were analyzed unordered and without character weight assignments. Character state transformations were ACCTRAN optimized so that characters appear on more internal nodes, i.e., lower in the tree. This procedure identifies synapomorphies (unequivocal and equivocal character states) for more inclusive clades, rather than being optimized as apomorphies of more exclusive clades or even terminal taxa (DELTRAN). Character states for Aigialosaurus dalmaticus (BSP 1902II501) were based on examination of latex peels and photographs of the type specimen, and from reference to Kramberger (1892), Carroll and DeBraga (1992), DeBraga and Carroll (1993), and Bell (1997). Character states for Aigialosaurus (5Opetiosaurus) buccichi were obtained by examination of the holotype specimen (NMW unnumbered specimen) and from GBA 1901/2, the recently rediscovered counterpart to the holotype specimen, and from Kornhuber (1901). Character states for Carsosaurus marchesetti were obtained by examination of the holotype specimen (MCSNT unnumbered specimen) and from referred specimens MCSNT 11430, 11431, 11432 (in three parts), and from Kornhuber (1893).
Diagnosis—As for genus. Type Locality and Horizon—Collected near Burham, Kent, and is considered to be from the Lower Chalk (Abbotts Cliff Chalk Fm. [North Downs Nomenclature]—Zig Zag Chalk Fm. [South Downs Nomenclature]), Lower Cenomanian (Fig. 1). Holotype—BMNH R 49002 (Figs. 3–7), a small chalk block preserving the articulated cranial and postcranial remains of one individual. The skull is represented by a partial braincase, and fragmentary right and left mandibles. The postcranium is composed of 17 articulated cervical vertebrae and 19 anterior dorsal vertebrae. Referred Material—BMNH R49907 (Figs. 8A, B, 9) Southerham Pit, near Lewes, Sussex; purchased 1879, Capron Collection. BMNH R32268 (Fig. 10A, B) from Burham, Kent, presented by P. de M.G. Egerton, 1856. BMB 0085687 (Fig. 11A, B, C), three prepared blocks, Southerham Pit, near Lewes, Sussex. Remarks—BMNH R49002 was most recently mentioned and figured in part by Milner (1987:pl. 58, fig. 5) in a taxonomic list of reptiles of the English Chalk. In Milner’s description BMNH R49002 and BMNH R49907 were identified as Dolichosaurus longicollis along with BMNH R44141 (the latter has been described as a new species of Coniasaurus by Caldwell, 1999a). The result of the description of BMNH R44141 as a new species of Coniasaurus, is that now only the holotype, BMNH R49002, possesses any cranial material, and that D. longicollis is diagnosed by characters that are not comparable to the known osteology of Coniasaurus. In contrast, both Coniasaurus crassidens (Caldwell and Cooper, 1999), and Coniasaurus gracilodens (Caldwell, 1999a) are both known from diagnostic cranial remains. The associated postcranial elements cannot be differentiated from those of Dolichosaurus, but neither are they diagnostic of Coniasaurus. Dolichosaurus longicollis is diagnosed by an extremely elongate neck composed of at least seventeen cervical vertebrae (the count is likely nineteen: 171atlas-axis), an elongate trunk with at least 32 dorsal vertebrae, and perhaps as high as 38 (Owen 1850, considered BMNH R49002 and R32268 to be one individual), and the relative length of the humerus as compared to two anterior dorsal vertebrae. It is this last character, drawn from the holotype specimen, that allows the referral of BMNH R49907 to Dolichosaurus longicollis. Because the assignment of specimens to D. longicollis is dependent on the possession of specific postcranial characters, and because it is possible that some of these specimens could be reassigned to one or the other
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FIGURE 1. Lower Chalk stratigraphy for the North and South Downs of southeast England (Anglo-Paris Basin), and the localities and approximated stratigraphic positions of known specimens of Dolichosaurus longicollis. 1, BMNH 49002 and BMNH R32268 Lower Chalk, Burham, Kent; 2, BMNH R49907 and BMB 008567. Question mark (?), uncertainty of stratigraphic range of section. Dotted lines, uncertain horizon of specimens. Stratigraphic column compiled from: Owen (1987); Mortimore (1986; 1987; pers. comm.); Robinson (1986, 1987); Hancock et al. (1993); and Obradovich (1993). Abbreviations: CTBE, Cenomanian-Turonian Boundary Event; GM, Glauconitic Marl; MR, Melbourn Rock.
coniasaur species, each postcranial specimen is described independent of the others. The possibility remains that more complete specimens will indicate that either Coniasaurus crassidens or Coniasaurus gracilodens is a senior synonym of Dolichosaurus longicollis. In Owen’s (1850) original description, C. crassidens occurs before D. longicollis in the manuscript and therefore has generic priority. Whatever the nomenclatorial outcome, it is certainly true that there were at least two species (crassidens and gracilodens) of small marine squamates present in the seas that covered the Anglo-Paris Basin during the Cenomanian. DESCRIPTION Skull Parietal (Fig. 6A, B) —The parietal is fragmentary and preserved upside down as a hexagonal, concave plate of dermal bone. The posterior margin is present and preserves the pit for articulation with the supraoccipital. The paired, symmetrical borders on either side of the pit represent the breakage lines for the suspensorial rami of the parietal. It is not possible to deter-
mine the position of the parietal foramen as the anterior portion of the bone is broken away and turned into the matrix. Other margins and suture lines with the frontal are not visible. Braincase (Figs. 6A, B, 7) —The braincase is badly crushed and fragmentary, making exact identification of elements problematic. As a result, useful comparisons with squamates such as Varanus (see Bahl, 1938), mosasaurs (see Russell, 1967), or snakes (Rieppel, 1979) are not possible. However, three interpretations and identifications of the various elements and foramina are given; the presence of an exoccipital-opisthotic is consistent in all three interpretations. Element and foramen identities as given in Figure 7 follow Interpretation/Identification 1, but it is recognized that these identifications are tentative. Interpretation/Identification 1: the braincase is exposed in side view and is broken in saggittal section. If this were accurate, then the bones present would be the left portions of the basioccipital and basisphenoid, a fragment of the left exoccipital-opisthotic and prootic. In most squamate families, including the Mosasauridae, the exoccipital and opisthotic are fused, but the prootic is an independent element (Bahl, 1938; Russell,
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FIGURE 2. Life reconstruction of Dolichosaurus longicollis. Head reconstructed using elements from Coniasaurus crassidens (Caldwell and Cooper, 1999) and Coniasaurus gracilodens (Caldwell, 1999a).
1967); in snakes, the exoccipital is free, but the opisthotic and prootic are fused. The foramina present on the exoccipital-opisthotic might well correspond to the foramina for the eleventh (XI) and twelfth (XII) cranial nerves (Bahl, 1938). Interpretation/Identification 2: the braincase is exposed in dorsal view and is split about mid-height through the prootics and exoccipitals-opisthotics. The right side of the lateral braincase wall is missing as are the processes from the right side of the basioccipital. Fragments of the left supraoccipital, exoccipital and prootic have collapsed onto the floor of the basioccipital-basisphenoid. Interpretation/Identification 3 (Susan Evans, pers. comm.): if this is a medial view of the left half of the braincase, then the element labeled ‘‘ex-op’’ (Fig. 7) could be part of the supraoccipital. The foramen labeled XI might actually be the point where the endolymphatic duct emerges from the supraoccipital (which better fits the angle of emergence of that foramen shown in Fig. 7). In that case, the large foramen labeled XII might be
FIGURE 3. Dorsal view of holotype specimen of Dolichosaurus longicollis (BMNH R49002). Scale bar equals 1 cm.
that for the vagus nerve (CN-X) with the mass to the left of it being the bulk of the exoccipital. Mandible Dentary (Fig. 5A–C)—The right and left dentaries are extremely fragmentary, appear to have been weathered through the middle of the bone on its long axis, and are fixed to the lateral/internal surface of the splenial. There are no teeth remaining on either dentary even though in Owen’s (1850) publication he does figure some small teeth in place on the dentary. Splenial (Fig. 5A–C)—The right splenial is exposed in internal view and is rotated vertically and out of articulation with the postdentary and dentary elements. The posterior end is weathered. The splenial foramen is present just anterior to the articular surface that would have formed the joint with the angular; on the left splenial the splenial foramen is located approximately 4 mm anterior to the angular-splenial joint. The
FIGURE 4. Line drawing of dorsal view of holotype specimen of Dolichosaurus longicollis (BMNH R49002). Abbreviations: br, braincase fragment composed of left prootic, opisthotic, basisphenoid, and basioccipital; H, humerus; 11, 11th preserved cervical vertebra, figured in Figure 12; 17, 17th cervical, likely C19. Scale bar equals 1 cm.
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FIGURE 5. Fragmentary skull elements of holotype specimen of Dolichosaurus longicollis (BMNH R49002): A, lateral view, fragmentary right lower jaw elements; B, lateral view, fragmentary left lower jaw elements; C, detailed lateral view of fragmentary left lower jaw elements as shown in photograph for B. Abbreviations: Ang, angular; Cor, coronoid; Dent, dentary; Splen, splenial; sur, surangular. Scale bar equals 1 cm.
splenial is L-shaped in cross-section. The long side of the bone is exposed medially in a high ascending flange as in Coniasaurus crassidens (see Caldwell and Cooper, 1999). Coronoid (Fig. 5A–C)—The left coronid bears a deep groove for the surangular. It appears to have a fairly long anterior process and would have extended anteriorly along the length of the surangular. The right coronoid is not preserved. Surangular (Fig. 5A)—The right surangular is present but is weathered posteriorly, preserving no details of its contribution to the glenoid facet. As preserved, the element is elongate, with a broad curving crest on the lateral surface that underlies the coronoid process and continues anteriorly to the suture with the dentary. The coronoid process is short but well-defined. The anterior-most portion of the surangular is broken.
Angular (Fig. 5A)—The left angular is a small thin bone that is ventral and medial to the surangular. On the left side the splenial and angular are in articulation. The contact between these two bones forms a ball and socket-like joint; the splenial head, is slightly concave, while the angular head is slightly convex. The joint formed is identical to that of Coniasaurus crassidens (Caldwell and Cooper, 1999), all mosasauroid squamates, and snakes (Lee et al., 1999). Axial Skeleton Cervical Vertebrae (Figs. 3, 4, 12)—There are seventeen cervical vertebrae preserved. However, this does not include the atlas and axis vertebrae as these two elements cannot be iden-
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FIGURE 6. Fragmentary skull and braincase elements of holotype specimen Dolichosaurus longicollis (BMNH R49002): A, dorsal view of head. Note braincase elements; B, detail of braincase elements. Note left prootic, opisthotic, and fragmentary basioccipital-basisphenoid. Abbreviations: Bo, Basioccipital; Cor, coronoid; Ex-Op, fused exocciptal-opisthotic; Par, paritetal; Pr, prootic; soc-p, pit for articulation with the supraoccipital; Splen, splenial; Scale bar equals 1 cm.
tified from among the badly weathered bone fragments located between the mandibular elements. Therefore, the total count is at least nineteen and could be one or two vertebrae higher, particularly if the anterior-most dorsals did not bear ribs that were fixed to the sternum. This however, cannot be ascertained from the specimen as preserved. The individual vertebrae are low and flattened. The neural spines are broken away but do not appear to have been very tall. The articulation of the pre- and postzygapophyses is nearly horizontal. Zygosphenes and zygantra are well developed and the neural arch laminae are notched. In most modern snakes the laminae are straight. The cervicals are shorter and less robust than the dorsal vertebrae; the average length of a cervical vertebra is 9.4 mm as measured along the exposed portion of the neural arch. On average the vertebrae are approximately 6mm wide measured across the top of the neural arch between the pre- and postzygapophyses. Thin, short ribs are present from at least the fourth cervical
vertebrae. Details of the parapophyses of more anterior cervicals have weathered away as have any ribs that may have been present. Intercentra are long, thin and directed posteriorly. The hypapophyses are located on the posterior portion of the centrum. Dorsal Vertebrae (Figs. 3, 4)—There are nineteen dorsal vertebrae preserved. It is not possible to determine the number of ribs that articulated with the sternum, nor to establish which ribs and vertebra is actually the first dorsal as no articulations and chondral cartilages are preserved. The dorsal vertebrae are larger than the cervicals in both length and width. The average length of a dorsal vertebra is 9.6 mm, measured along the neural lamina, and 10.4 mm, measured along the length of the centrum from the ventral lip of the cotyle to the end of the condyle. On average, the vertebrae are approximately 6mm wide measured across the top of the neural arch between the pre- and postzygapophyses. Vertebrae narrow immediately behind the parapophyses and do not become wider or narrower at the condyle. The condyle is circular and the
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FIGURE 7. Fragmentary left wall of braincase, holotype specimen of Dolichosaurus longicollis (BMNH R49002). The element not stippled is unknown, but may be a fragment of the decensus parietalis. Abbreviations: Bo, Basioccipital; Ex-Op, fused exocciptal-opisthotic; fBa, foramen/canal for Basilar artery; Pr, prootic; XI, foramen for 11th cranial nerve; XII, foramen for 12th cranial nerve. Scale bar equals 5 mm.
cotyle is slightly elliptical in the horizontal plane. The articulation of the pre- and postzygapophyses are in the horizontal plane and the accessory articulations of the zygosphene-zygantrum are well developed. The ribs of the dorsal vertebrae increase in size from anterior to posterior. The largest rib is approximately 41 mm long and about 2 mm wide at the proximal end. Appendicular Skeleton (Figs. 3, 4)—Fragments of the sternal cartilages, coracoid, and scapula are present. However, details of these elements are not visible. The only appendicular element that is well preserved is the right humerus, a robust bone with well developed muscle attachments. The humerus is 15 mm long, 6.1 mm wide across the proximal head, and 5.3 mm wide at the distal end. The ratio of humerus length to the length of an average dorsal vertebra is 1.44:1. The distal epiphyses are absent and likely did not fuse to the bone. The proximal epiphysis appears fused to the bone as this area is rounded and presents a similar morphology to that of other squamates. There is no evidence of either an ectepicondylar foramen or groove. BMNH R49907 This specimen was collected at the Southerham Pit, near Lewes, Sussex. It was purchased by the Natural History Museum, London, in 1879, as part of the Capron Collection. Forelimb and Pectoral Girdle (Figs. 8, 9)—The right scapula and right coracoid are well preserved and in articulation. The elements are not co-ossified, but rather are sutured to each other as in Coniasaurus gracilodens (Caldwell, 1999a) and the aigialosaur, Carsosaurus marchesetti (see Caldwell et al., 1995). The scapula is small and the scapular blade descends into the matrix. Nopcsa (1908) interprets this small scapula to
be a procoracoid element; however, there is no justification for this identification. The principal difference between the scapulocoracoid of Dolichosaurus longicollis and that of coniasaurs or aigialosaurs, is the apparent lack of a scapulocoracoid fenestra; such a feature is strongly pronounced in aigialosaurs, more weakly manifest in Coniasaurus gracilodens. The right coracoid is still in articulation with the supracoracoid cartilage although the exact margins of this calcified tissue are not obvious. The coracoid margin is not emarginate, and the bone is perforated anteromedial to the glenoid fossa by a single coracoid foramen. There are three identifiable rib attachment points on the sternal cartilage in addition to the paired xiphisternal cartilages present at its extreme posterior tip. As compared to the aigialosaur Carsosaurus marchesetti with five rib articulations and a pair of xiphisternal processes (Caldwell et al., 1995), it is fair to conclude that Dolichosaurs longicollis shows reduction of the pectoral girdle elements. The shafts of the right and left humeri are present, but the proximal and distal ends have been broken away and details of their articular surfaces are unknown. The right radius is narrow proximally, and widens distally to form a preaxial to postaxially elongate boot. The right and left ulnae appear to be slightly wider proximally (preaxial to postaxial) than distally. The radius and ulna appear to be approximately equal in length. Carpal elements are present on the left limb, though poorly preserved. An elliptical ulnare and possible fragments of a pisiform are present. Metacarpals two and three are approximately half the length of the ulna, and several phalangeal elements are also preserved. Dorsal Vertebrae (Figs. 8, 9) —There are six dorsals and the fragments or impressions of three other poorly preserved more posterior dorsals. There are no unique features of these vertebrae that are not better preserved on other specimens. Length and width measurements were not taken. Sacral Vertebrae—Two poorly preserved sacral vertebrae and ribs are present (Fig. 8A). The anterior rib is slender and angled posteriorly, while the second sacral rib is more robust and is angled anteriorly. What may be a fragment of the ilium lies on top of the vertebral centra. Caudal Vertebrae—There are ten caudals with transverse processes and partial neural spines (Fig. 8A). Unfortunately, it is not possible to determine the presence of pygal vertebrae. BMNH R 32268 (Fig. 10) This specimen was found near Burham, Kent, in the same quarry as the holotype. Rear Limb and Girdle Elements—Portions of the girdle and limb elements are present for the both left and right sides of the body. The ilium is posteriorly elongate as is common in most squamates with the exception of mosasaurs and snakes. In mosasaurs the posterior iliac crest and process are lost, and the anterior, superior iliac process is greatly enlarged (see Russell, 1967); in snakes that still possess pelvic girdle rudiments, the ilium is often anteriorly elongate (pers. obs.). The sacral ribs are stout, elongate elements with broadly rounded articular heads. The foot of the second rib is much more expanded than the first. The second is angled slightly anterior, while the first is angled slightly posterior. The articular heads appear to touch each other at the contact with the ilium. The left femur is badly crushed and the proximal portion is missing; the element is approximately 1.5 cm long as preserved. The distal end of the right femur is well preserved and very broad distally with well-developed articular surfaces. The shaft of the right femur is missing. Dorsal Vertebrae—Nineteen posterior dorsal vertebrae are preserved. They differ in no visible way from the more anterior
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FIGURE 8. Dolichosaurus longicollis (BMNH R49907), Lower Chalk, Southerham Pit, Lewes, Sussex: A, ventral view of complete specimen; B, detail view of pectoral girdle and anterior dorsal vertebrae. Abbreviations: s1, 1st sacral rib and vertebra; s2, 2nd sacral rib and vertebra. Scale bar equals 1 cm.
dorsals preserved on the holotype. The average centrum length is 9.5 mm with the vertebrae decreasing in length approaching the sacrum. The average width of the neural arch is 5.4 mm and the average width across the centrum, posterior to the parapophysis, is 4.5 mm. Sacral Vertebrae—There are two sacral vertebrae each bearing very well developed sacral ribs. The more anterior rib is slightly more slender than the posterior one. The posteriormost rib is expanded distally into a wide and slightly flattened end. The sacral vertebrae are much shorter than the dorsals (approximate 6.5 mm in length). Caudal Vertebrae—Four caudal vertebrae are preserved. Transverse processes are present on the most anterior vertebra, but are broken on the other three. Facets for the haemal ribs/
arches are found on the second caudal but not on the first. This would indicate the presence of at least one pygal vertebra in Dolichosaurus. Haemal facets are present on the most posterior portions of long, raised crests that run the length of the ventral surface of the centrum. Between these crests is a flat shallow triangle-shaped region. The presence of facets cannot be evaluated on the two more distal caudals as the ventral surfaces are not preserved. The two caudals for which lengths are obtained are longer than the sacrals, but shorter than the dorsals (7.2 mm and 7.8 mm). BMB 008567 (Fig. 11) This specimen was collected at the Southerham Pit, near Lewes, Sussex, and was prepared in three blocks, the anterior
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FIGURE 9. Dolichosaurus longicollis (BMNH R49907), Lower Chalk, Southerham Pit, Lewes, Sussex: detailed line drawing of pectoral girdle, left forelimb, and anterior dorsal vertebrae. Small arrows indicate the large and well preserved sternal cartilages. Abbreviations: cl, clavicle; Co, coracoid; S, scapula; cCc, calcified supracoracoid cartilage; stc, calcified sternal cartilages; ul, ulnare? Scale bar equals 1 cm.
block in dorsal view, the middle block in ventral view, and the posterior block in dorsal view. Rear Limb and Girdle Elements—Portions of both the right and left girdle and limb elements are present (Fig. 11B, C). There are remnants of seven phalangeal elements, the distal and proximal portions of the femur, tibia, fibula and left ilium are present on the block 2. Block 3 has a very complete femur (approximately, 1.5 cm in length) that has a definite bend in the mid-shaft of the bone. This is not an artifact of either preservation or preparation, but is instead a real feature of the bone. It may represent a paleopathology. There is also a fragment of the distal epiphysis, and a number of unidentifiable fragments of the phalanges and metatarsals. Dorsal Vertebrae—There are thirty-two dorsal vertebrae preserved (Fig. 11A, B). These vertebrae differ in no visible way from the more anterior dorsals preserved on either the holotype or the other referred specimens. The individual vertebrae narrow immediately behind the parapophyses and do not become wider or narrower at the condyle. The condyle is circular and the cotyle slightly elliptical. Measured along the midline of the centrum, the length appears to average around 8.1 mm. The vertebrae become shorter towards the sacrum. The average width of the neural arch is 4.9 mm and the average width across the centrum, posterior to the parapophysis is 3.6 mm. The width across the parapophyses is 8.7 mm. Sacral Vertebrae—There are two sacral vertebrae each bearing very well developed sacral ribs (Fig. 11B). The more anterior rib is slightly more slender than the posterior one. The posterior rib is expanded distally into a wide and slightly flattened end. Near the base of the sacral rib and along the posterior border there is a small recess and groove for the lymphatic system of the pelvic region. The sacral vertebrae are much shorter than the dorsals (approximately 6.5 mm in length).
FIGURE 10. Dolichosaurus longicollis (BMNH R32268), Lower Chalk, Burham, Kent: A, ventral view of specimen; B, detail view of pelvic girdle and posterior dorsal vertebrae. Abbreviations: lF, left femur; rF, right femur; ril, right ilium; P, pygal vertebra; s1, 1st sacral rib and vertebrae; s2, 2nd sacral rib and vertebra. Scale bars equal 1 cm.
Caudal Vertebrae (Fig. 11B, C)—There are at least thirtytwo articulated caudal vertebrae preserved on the second and third block; more may be present but are badly weathered (Fig. 11B, C). The first caudal vertebra also bears a stout pair of transverse processes and there are no caudal peduncles visible; this vertebra is considered to be a pygal. It is possible that several vertebrae were lost between the second and third block when the specimen was excavated. Broad, flat, transverse processes, preserved on all caudals, become gradually shorter and slimmer posteriorly. Facets for the haemal ribs/arches have been destroyed by weathering. Small facets may be present on the first but this is difficult to ascertain as the posterior region is broken. The caudals are longest just behind the sacrum (6.8 mm) and shortest at the most posterior extent of the tail (4.6 mm). PHYLOGENETIC ANALYSIS Results Cladistic analysis of the data matrix (Appendix 2) resulted in 27 most parsimonious cladograms (122 steps) with a Consistency Index (C.I.) of 0. 648, a Homoplasy Index (H.I.) of 0.352, and a Retention Index (R.I.) of 0.669.
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FIGURE 11. Dolichosaurus longicollis (BMB 008567), Lower Chalk, Burham, Kent. Most complete postcranial skeleton currently known, in 3 blocks. A, dorsal view of anterior dorsal vertebrae and ribs; B, ventral view of mid to posterior dorsal vertebrae and pelvic girdle; C, dorsal view of caudal vertebrae and elements of right hindlimb. Abbreviations: rF, right femur; lil, left ilium; P, pygal vertebra; s1, 1st sacral rib and vertebrae; s2, 2nd sacral rib and vertebra. Scale bar equals 1 cm.
The topology of the Strict Consensus Tree (Fig. 13A) supports the monophyly of the Mosasauridae (Russell, 1967; Bell, 1997; Caldwell, 1996): (Halisaurus (Ectenosaurus (Clidastes, Mosasaurus)), Tylosaurus, Platecarpus)). The Aigialosauridae is also reconstructed as a distinct clade, though their ingroup relationships are unresolved. No support is found for suggestions of aigialosaur paraphyly (Bell, 1997) as the clade is well supported and differentiated in all cladograms relative to the Mosasauridae, Dolichosaurus, and coniasaurs. Dolichosaurus and both coniasaurs form an unresolved five-branch polytomy with the Mosasauridae and Aigialosauridae. A Majority Rule Consensus Tree (Fig. 13B) shows that in fifty-six percent of the trees, aigialosaurs and mosasaurs are found to form a clade distinct from Dolichosaurus and coniasaurs (Fig. 13B); this clade is conventionally referred to as the Mosasauroidea. In thirty-three percent of the trees Aigialosau-
rus dalmaticus and Aigialosaurus [5Opetiosaurus] buccichi form a monophyletic genus; as was noted by Caldwell et al. (1995), generic characters separating A. dalmaticus from A. [5O.] buccichi cannot be identified although the species are identifiable. Dolichosaurus is reconstructed as the most basal member of the clade, with coniasaurs ([Coniasaurus crassidens, Coniasaurus gracilodens] in forty-four percent of the trees) in the sistergroup position to the Mosasauroidea (thirty-three percent of the trees). DISCUSSION Phylogeny and Origins Caldwell (1999b), in an analysis of higher-level squamate phylogeny, found the clade coniasaurs1Mosasauroidea (Aigialosauridae and Mosasauridae) in all eighteen shortest clado-
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FIGURE 12. Left lateral view of 11th cervical vertebrae Dolichosaurus longicollis (BMNH R49002): Abbreviations: hy, hypapophysis; p, peduncle; poz, postzygapophysis; prz, prezygapophysis; zg, zygantrum.
grams, and snakes to be the sistergroup of that clade (Serpentes (Coniasaurus, Mosasauroidea)) in twelve of those eighteen shortest cladograms. A similar treatment of terminal taxa and clades was also presented by Caldwell and Lee (1997) and Lee and Caldwell (1998) in their examination of the relationships of Pachyrhachis problematicus Haas, 1979, where they referred to the snake-mosasauroid clade as the Pythonomorpha (see also Lee, 1997, 1998). Despite the recent efforts devoted to including fossils in analyses of squamate phylogeny, many theoretical problems remain
concerning the interrelationships of mosasauroids, coniasaurs, and snakes. For example, Lee (1998), in contrast to Caldwell (1999b) and Caldwell et al. (1995), finds mosasauroids, and by extension coniasaurs and dolichosaurs (see Caldwell, 1999a; Caldwell and Cooper, 1999, and this study), to be nested within Varanoidea. Such a phylogenetic position for dolichosaurs and other mosasauroids indicates a very different evolutionary history for basal varanoids and their sistertaxa than is currently recognized from the fossil record (see Norell and Gao, 1997). The degree of difference between Caldwell and Lee’s hypotheses requires further examination of other dolichosaurs (see Lee and Caldwell, 2000), other fossil varanoids, basal anguids, and the varanoid or non-varanoid characteristics of snakes. A far more contentious issue is the snakes-as-pythonomorphs hypothesis supported by Caldwell (1999b), Lee (1997, 1998), and Lee and Caldwell (1998, 2000). Recently, Zaher (1998), and Zaher and Rieppel (1999) have argued that Pachyrhachis is a derived macrostomatan snake rather than a primitive snake (contra Caldwell and Lee, 1997). Zaher (1998) and Zaher and Rieppel (1999) base these assertions on their reinterpretation of the skeletal anatomy of Pachyrhachis. Zaher and Rieppel (1999) support the idea that snakes are the sistergroup to amphisbaenids and dibamids, not mosasauroids and coniasaurs. Zaher (1998) has been addressed in detail by Caldwell (2000) and will not be examined further here. However, it is important to examine the data and ideas put forward by Zaher and Rieppel (1999) that are relevant to the phylogenetic relationships and anatomy of Dolichosaurus. As was shown by Caldwell (2000) regarding Zaher (1998), the salient problem with Zaher and Rieppel’s (1999) study is that they provide no evidence (data matrices, characters, etc.) in support of their phylogenetic hypothesis. Zaher and Rieppel (1999:834; fig. 2) list synapomorphies for their snake clades, but oddly enough the synapomorphy list contains none of the ‘undoubted’ macrostomatan characters of Pachyrhachis that they stated were misinterpretations by Caldwell and Lee (1997),
FIGURE 13. Concensus Trees of 27 cladograms (122 steps) showing ingroup relationships of 12 fossil squamate taxa using 66 osteological characters. A, Strict Consensus Tree. B, Majority Rule Consensus Tree. Numbers on branches indicate the percentage of most parsimonius cladograms showing that particular branching pattern.
CALDWELL—UPPER CRETACEOUS AQUATIC SQUAMATE Lee and Caldwell (1998), and Lee (1998). Does this mean these characters were not found as true synapomorphies in a cladistic analysis (for which there is no evidence of having been undertaken)? Or perhaps the undoubted macrostomatan features of Pachyrhachis, which were described by Lee and Caldwell (1998:1550) were found homoplastic, were not included in the unpublished data matrices? If not, and why not? In either case it is impossible to recover this information as the data is absent. As with Zaher (1998), until character lists, state assignments, and data matrices are available such inductively derived phylogenetic conclusions must be deemed as primary hypothetical statements, and not deductive statements worthy of tests of falsification (see Kluge, 1997). In relation to testable statements of phylogeny, Dolichosaurus longicollis presents a suite of morphological characteristics that should prove informative when analyzed for more global character congruence with snakes, mosasaurs, and other squamates (see Lee and Caldwell, 2000). However, the current paucity of good Dolichosaurus fossils that possess both skulls and jaws, is frustrating when compared to the skulls and jaws known for two species of Coniasaurus. It is equally problematic that the reverse condition exists for the postcranium: absence of postcrania for Coniasaurus versus complete postcrania for Dolichosaurus. The possibility remains that these taxa are congeneric. Synonomizing the two genera would reduce the three species to one genus and two species. Dolichosaurus/Coniasaurus would then be a small, very elongate, limb reduced, marine squamate, of Cenomanian age, with a head very similar to an aigialosaur, an intramandibular joint synapomorphic with mosasauroids and snakes, and an elongate neck following a similar pattern to that found in snakes (neck elongation is discussed below). The polytomy (Fig. 13) currently obscuring reconstructions of dolichosaur-coniasaur relationships would disappear. The remaining question is whether or not such a taxon would be reconstructed as a basal mosasauroid, or in a sistergroup position to a clade containing crown-group snakes (see Lee and Caldwell, 2000). Neck Elongation and Elongate Squamates The long neck of Dolichosaurus is unique among non-snake squamates. This aquatic lizard has doubled the number of cervical vertebrae from the primitive squamate count of eight, and increased the number of dorsal vertebrae as well (32 to 38 if the association of BMNH R49002 and R32268 is correct as Owen [1850] supposed). The question is whether or not an increased number of cervicals and dorsals are informative on broader problems of squamate phylogeny and evolution. Elongation of the squamate body is accomplished by changing the length of any or all of the three body regions: neck, trunk, and tail. Each region appears able to independently change the number, and to a lesser degree, the size of its respective vertebrae in order to effect elongation. The terminology for axial variation uses a number of landmarks or reference points to define the beginning and end of a particular region. Cervicals are vertebrae anterior to the first vertebra with a rib contacting a sternal cartilage (see below for more detail). Dorsals are vertebrae immediately posterior to the last cervical and anterior to the first sacral. Presacrals are all vertebrae (cervicals and dorsals) anterior to the first sacral vertebrae; this term is applied to most squamates, but is most accurate when applied to those with no sternal attachments but a definite pelvic girdle (e.g., pygopodid geckos). The term precloacals is applied to amphisbaenids and snakes as there are little or no remnants of the pectoral girdle, and the sacral vertebrae have been lost; precloacals are all vertebrae anterior to the cloaca. Sacrals are vertebrae with transverse processes contacting the ilium. Caudals are all vertebrae posterior to the last sacral. Mosasaurs have no
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sacral vertebrae, but possess a unique pygal vertebra series located between the last dorsal, and the first caudal. Pygals are differentiated from dorsals and caudals by the absence of free ribs and haemal arches. For typical limbed squamates (typical teiids, varanids, lacertids, etc.), the cervical count is 8 to 9, while the dorsal count is from 20 to 36. Aigialosaurids have 7 to 8 cervicals and approximately 20 dorsals; mosasaurs average 7 to 8 cervicals and 22 to 35 dorsals (Carroll and deBraga, 1992; Caldwell et al., 1995; Holmes, 1996). The only non-snake squamates with comparable numbers of presacrals are the limbless dibamids, amphisbaenids, anguids, pygopodids, and scincids (including feylinids and acontines). However, none of these groups show an increased number of cervicals. Instead, these taxa reduce the number of cervicals, ranging from four in Dibamus and four to five in most amphisbaenids, to seven in feylinids (Hoffstetter and Gasc, 1969). For limbless or limb-reduced non-snake squamates, the dorsal count increases while the cervical count decreases; in limb-reduced anguids the dorsal count is 68, while in amphisbaenids it reaches 160. For snakes, the precloacal and postaxial counts begins at 120 and reaches 320 (some fossil forms are over 400; for more detail see Hoffstetter and Gasc, 1969). The details of some specific examples are informative on modes of elongation. Neck elongation in extant Varanus results from an increase in the relative size of the cervical vertebrae (nearly twice as long as the anterior dorsals), not an increased number. In comparison, neck elongation in Dolichosaurus results from the addition of at least 11 vertebrae to the primitive count of 8, for a total of at least 19. Dibamus has only four cervicals, produced either by loss in absolute number, or the anterior translocation of the pectoral girdle. It is clear that Dolichosaurus is unique among limbed squamates as none of the latter have a presacral/precloacal count including 19 or more cervicals. The current definition of cervical vertebrae restricts its usage and identification to all vertebrae anterior to the first ‘dorsal’ vertebra, with the first dorsal being identified as the first vertebra that articulates with a sternal rib (Hoffstetter and Gasc, 1969). Therefore, the presence/ absence and position of the sternal rib, and the pectoral girdle of which it is a part, dictates the presence/absence of a neck. For Varanus, dolichosaurs, and other elongate non-snake squamates such as Dibamus, it is comparatively easy to recognize the mode of neck elongation (increased number and/or size of cervical vertebrae) because they have either a complete or partial pectoral girdle. Squamates, in particular snakes, that no longer possess any remnant of the pectoral girdle, are therefore without a neck and cervical vertebrae. In short, the neck ceases to exist because a language is created that abstracts it to the point of erasing it. Such definitions are undesirable and problematic as they do not define cervical vertebrae based on their characteristics, but rather in terms of their association, or lack of association, with a suite of appendicular elements. For snakes, quite apart from comparisons with Dolichosaurus, this semantic obfuscation makes it impossible to discuss neck evolution, or to discover the cladogenic distribution of characters of the neck and cervical vertebrae with the matching region in putative sistergroups, i.e., with dolichosaurs (this study), or amphisbaenids (Zaher and Rieppel, 1999). In light of the sistergroup relationships of snakes and mosasauroids as hypothesized by Caldwell (1999b) and Lee (1998), it is important that some standard of comparison between the ‘necks’ of snakes and Dolichosaurus be established. A neck is a body region that contains a variety of organ systems such as the hyoid apparatus, the esophagus, the trachea, a discrete series of arteries, veins, motor nerves, and so on. As snakes still possess all of these structures, it can only be concluded that snakes have not lost their necks, only their pectoral
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girdles and forelimbs. Redefining the neck in terms of features possessed by the constituent vertebra resolves this problem, and allows snake evolution to be better understood in the context of neck elongation and forelimb loss. Squamate cervical vertebrae have hypapophyses and intercentra. I recommend here, that their presence, position, morphology, and number be reconsidered as informative on the beginning and end of the cervical series in limbed and limbless squamates. This position is contrary to Hoffstetter and Gasc (1969) who argue that hypapophyses and their characteristics are so variable that they are of no utility in characterizing the presence of a neck and/or cervical region. A survey of hypapophyseal variation is therefore necessary to support my position. Not counting the atlas-axis vertebrae, hypapophyses are present on one to three vertebrae in Heloderma, and on seven or eight vertebrae in Varanus (sometimes extending onto the first dorsal [pers. obs.]). Typical limbed squamates show a count of six to seven vertebrae with hypapophyses. According to Hofstetter and Gasc (1969) Feylinia has nine while Anniella has ten to eleven; Acontias and Dibamus have seven while the limbless anguids have six. It is interesting to note that counting the number of hypapophysis-bearing vertebra shows a much closer correspondence to the primitive squamate cervical count of eight, than the corresponding cervical number derived by reference to the pectoral girdle (see above). The question then is which is more reliable. The features of cervical vertebrae, or the position of the pectoral girdle. Let us weigh the evidence by examination of Dibamus. Dibamus has four cervicals based on the position of the pectoral girdle, it seven cervicals derived from counting the hypapophyses. If the primitive squamate count is approximately eight cervicals with hypapophyses, and the first sternal attachment is on the ninth vertebra, then it can be hypothesized that primitive dibamids possessed eight cervicals, and that along the way lost the expression of the hypapophysis on the eighth; the pectoral girdle has been anteriorly translocated providing mechanical stability for the neck during burrowing. Dibamus retains a nearly primitive cervical count, while showing a derived state for pectoral girdle positioning. For an animal that burrows with its head a short neck is a functional advantage. The same reasoning suggests that the low hypapophyseal count of scolecophidians and other burrowing snakes is also derived. For squamates with long necks, indexed by the presence of a large number of hypapophyses, the opposite conclusion is reached, and the opposite scenario applies. The number of hypapophysis-bearing vertebrae increase, but not as a result of burrowing adaptations. An elongate neck functions differently than a short neck. Modern snakes present the ‘elongate neck condition’, not the shortened, primitive neck of Dibamus. The number of vertebrae with hypapophyses ranges from all precloacal vertebrae to only the first 4 or 5 immediately posterior to the occipital condyle (Hoffstetter and Gasc, 1969); the primitive snake Pachyrhachis has at least 18 (see Lee and Caldwell, 1998) vertebrae with hypapophyses, similar to the number in Dolichosaurus (contra Zaher and Rieppel’s (1999) recent dismissal of any notable cervical zonation in Pachyrhachis). Comparing the number of hypapophysis-bearing vertebrae of Dolichosaurus with Pachyrhachis and other snakes highlights the limitations and deficiencies of the current definition of cervical vertebrae. If such a definition is used, Pachyrhachis and all other snakes must be coded ‘not applicable’ for Character 65 (Appendix 1). As I have argued, it is the definition that creates the ‘not applicable’ state assignment, and not the morphology. In short, if cervical synapomorphies are present between Dolichosaurus and snakes they are impossible to detect not because they do not exist, but because of the language applied to the character state distinction.
Under the freedom of a more accurate definition, snakes can be seen as having dramatically elongated the neck, rather than having lost it. For most tetrapods the neck is relatively mobile and usually capable of a wider range of rotational movement than other axial regions, particularly if the pre- and postzygapophyseal surfaces are horizontal. Horizontal zygapophyses are present throughout the column in most modern snakes. It is also important to note that snakes possess zygosphene and zygantral articulations in association with horizontal zygapophyseal facets. The only other squamates sharing this character complex are dolichosaurs. The increased number of cervicals and attendant hypapophyses pass the test of similarity for snakes and Dolichosaurus. For snakes, the unique evolutionary feature is the cervicalization of a significant proportion of the presacral column thus creating a very long neck. The next step is the test of congruence as outlined by Patterson (1982) (see Lee and Caldwell, 2000). Dolichosaur Paleobiology Dolichosaurus was a small marine squamate, likely no more than 0.5–1.0 meters in total length. As is likely the case for Dolichosaurus and other putative dolichosaurs (see Nopcsa, 1908, 1923; Caldwell, 1999a; Caldwell and Cooper, 1999), these lizards had long narrow heads, and elongate necks, bodies, and tails (Dal Sasso and Pinna, 1997; Dal Sasso and Renesto, 1999). As such, Cenomanian dolichosaurs were elongate, limb-reduced marine lizards with small, thin heads, that may have served them very well as predators feeding in crevices and narrow spaces such as might exist in coral reefs and on rocky shores. In this role, dolichosaurs would have occupied a niche similar to that of the earlier nothosaurs (see discussion in Caldwell and Cooper, 1999), to that occupied by extant sea snakes (Greene, 1995), and to the one proposed for the Cenomanian marine snakes Pachyrhachis (Caldwell and Lee, 1997; Lee and Caldwell, 1998; Scanlon et al., 1999), and Pachyophis (Lee et al., 1999). The degree of limb reduction (pectoral versus pelvic, and overall reduction in limb size), suggests that dolichosaurs spent much of their time in water, and may have relied very heavily on lateral undulations of the body to generate force for anguilliform locomotion. Their limbs appear to have been too small to have generated any significant motive force either on land, or in the water. While of no obvious adaptive advantage, the reduction of the forelimb and girdle as compared to the rearlimb, may be a common feature shared with early snakes (Caldwell and Lee, 1997; Lee and Caldwell, 1998). ACKNOWLEDGMENTS For assistance while gathering data, I wish to thank S. Chapman, J. Cooper, A. Currant, J. Evans, and C. Price. For assistance in editing the manuscript I wish to thank A. Nicholson and R. Holmes. I also thank S. Evans and R. Carroll for extremely helpful suggestions and for declining anonymity in the review process. In particular I wish to acknowledge the photographs taken, and prints provided to me, by the Photographics Department at the Natural History Museum (London). This research was supported by a Natural Sciences and Engineering Research Council of Canada (NSERC) Postdoctoral Fellowship and by a Canandian Museum of Nature, Research Advisory Council Grant (No. RS34) to the author. LITERATURE CITED Bahl, K. N. 1938. Skull of Varanus monitor (Linn.). Records of the Indian Museum, Journal of Indian Zoology 39:133–174. Bell, G. L. 1997. A phylogenetic revision of North American and Adriatic Mosasauroidea; pp. 293–332 in J. M. Callaway and E. L. Nicholls (eds.), Ancient Marine Reptiles, Academic Press, San Diego.
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saurs. Peabody Museum of Natural History, Yale University Bulletin 23:1–241. Scanlon, J. D., M. S. Y. Lee, M. W. Caldwell, R. Shine. 1999. The palaeoecology of the primitive snake Pachyrhachis. Historical Biology 13:127–152. Swofford, D. L. 1993. PAUP: Phylogenetic Analysis Using Parsimony, Version 3.1.1. Laboratory of Moelcular Systematics, Smithsonian Institution, Washington D.C. Wu, X. -C., D. B. Brinkman, and A. P. Russell. 1996. Sineoamphisbaena hexatabularis, an amphisbaenian (Diapsida: Squamata) from the Upper Cretaceous redbeds at Bayan Mandahu (Inner Mongolia, People’s Republic of China), and comments on the phylogenetic relationships of the Amphisbaenia. Canadian Journal of Earth Sciences 33:541–577. Zaher, H. 1998. The phylogenetic position of Pachyrhachis within snakes (Squamata, Serpentes). Journal of Vertebrate Paleontology 18:1–3. , and O. Rieppel. 1999. The phylogenetic relationships of Pachyrhachis problematicus, and the evolution of limblessness in snakes (Lepidosauria, Squamata). Comptes rendus de l’academie des sciences, Sciences de la terre et des plane`tes 329:831–837. Received 27 August 1999; accepted 7 June 2000. APPENDIX 1 Character and character state descriptions. 1. Bony predental rostrum on premaxilla: short and obtuse (0); distinctly protruding (1). 2. Size of premaxillary foramen on rostrum: small (0); large (1). 3. Dorsal keel on internarial bar of premaxilla: absent (0); present (1). 4. Nasal bones: present (0); absent (1). 5. Internarial process of frontal: not constricted (0); very constricted (1). 6. Frontal: broad and short (0); long and narrow (1). 7. Frontal forms part of margin of nares: not on margin (0); forms part of margin (1). 8. Frontal with sagittal crest: absent (0); low (1); high, and well developed (2). 9. Shape of frontal ala: narrow and pointed (0); broad, rounded (1). 10. Fronto-parietal suture: interlocking sutures (0); overlapping flanges, no sutures (1). 11. Parietal, dorsal surface: short (0); elongate (1). 12. Parietal table: rectangular to trapezoidal, sides convergent (0); triangular, straight sides contact anterior to suspensorial rami (1). 13. Parietal foramen size: small (0); large (1). 14. Parietal foramen position: near to center of parietal table (0); close to suture (1); touching suture (2); crosses suture including frontal (3). 15. Posterior shelf of parietal: distinct shelf, projects between suspensorial rami (0); shelf absent (1). 16. Parietal suspensorial ramus, greatest width: vertical or oblique (0); horizontal (1). 17. Prefrontal, suborbital process: absent or very small (0); large overhanging wing (1). 18. Prefrontal contact with postorbital frontal: no contact (0); contact (1). 19. Postorbitofrontal, transverse dorsal ridge: absent (0); present (1). 20. Maxillary tooth count: 20 to 24 (0); 17–19 (1); 15–16 (2); 12–14 (3). 21. Posterior terminus of maxillo-premax suture: with first to fourth maxillary tooth (0); between fourth and ninth tooth (1); even with or posterior to ninth tooth (2). 22. Ascending process of maxilla: recurved wing of maxilla dorsolaterally overlaps anterior end of prefrontal (0); process absent (1). 23. Posteroventral process of jugal: absent (0); present (1).
24. Pterygoid tooth row: teeth arise from main shaft of pterygoid (0); teeth arise from thin pronounced ridge (1). 25. Length of quadrate stapedial process: short (0); moderate length (1); long (2). 26. Constriction of quadrate stapedial process: distinct (0); none (1). 27. Fusion of quadrate stapedial process to ventral process: absent (0); present (1). 28. Quadrate tympanic rim size: large, almost as high as quadrate (0); smaller, 50–65% of quadrate height (1). 29. Quadrate anteroventral condyle modification: no upward deflection of anterior edge of condyle (0); distinct deflection (1). 30. Pterygoid process of basisphenoid: process relatively narrow with articular surface facing mostly anterolaterally (0); process thinner, more fan-shaped with posterior extension of articular surface (1). 31. Length of basioccipital tuber: short (0); long (1). 32. Dentary tooth number: .19 (0); 18–15 (1); 14–13 (2); ,12 (3). 33. Sub-dental shelf of dentary: shelf low (0); shelf raised to median height (1); equal in height to lateral wall (2). 34. Coronoid shape: slight dorsal curvature, posterior wing not wide fan-shape (0); very concave above, posterior wing expanded (1). 35. Coronoid medial wing: does not reach angular (0); contacts angular (1). 36. Surangular-coronoid buttress: low and thick (0); high and thin (1). 37. Surangular-articular suture position: behind condyle (0); middle of glenoid (1). 38. Foramina on lateral aspect of retroarticular process: none (0); one to three (1). 39. Tooth surface, medial aspect: finely striate (0); not striated (1). 40. Tooth facets: absent (0); present (1). 41. Tooth carinae: weak (0); strong and elevated (1). 42. Atlas neural arch: notch in anterior order (0); no notch (1). 43. Atlas synapophysis: extremely reduced (0); large and elongate (1). 44. Condyle inclination, dorsal vertebrae: inclined (0); vertical (1). 45. Condyle shape: depressed (0); slightly depressed (1); rounded (2). 46. Number of sacral vertebrae: two (0); one (1). 47. Caudal neural spines: uniformly short (0); several dorsally elongate mid-tail region (1). 48. Haemal arch articulations: unfused (0); fused (1). 49. Scapulocoracoid size: bone approximately equal in proximo-distal length (0); scapula about half length of coracoid (1). 50. Scapula width: no anteroposterior widening (0); distinct fan-shaped expansion (1); extreme widening (2). 51. Scapula posterior emargination: gently concave (0); deeply concave (1). 52. Scapulocoracoid fusion: bones fused (0); not fused (1). 53. Humerus length relative to distal width: elongate, 3 to 4 (0); shortened, 1.5 to 2 (1); length and width equal (2); distal width greater (3). 54. Humerus postglenoid process: absent or small (0); distinctly enlarged (1). 55. Humerus deltopectoral crest: single ridge (0); two separate insertion areas (1). 56. Humerus pectoral crest: located anteriorly (0); located medially (1). 57. Humerus entepicondyle: absent (0); present as a prominence (1). 58. Radius shape: radius not expanded (0); slightly expanded (1); broadly expanded (2). 59. Ulna contact with centrale: excluded by broad ulnare (0); contacts centrale (1). 60. Radiale size: large and broad (0); reduced or absent (1). 61. Carpal reduction: six or more (0); five or less (1). 62. Pisiform: present (0); absent (1). 63. Metacarpal I expansion: spindle shaped, elongate (0); broadly expanded (1). 64. Tooth crown inflation: not inflated (0); crown moderate to thick inflation [bulbous] (1). 65. Number of cervical vertebrae: 7–9 (0); 10–20 (1). 66. Zygosphenes and zygantra: where present, consistently expressed throughout length of axial skeleton (0); present, but show reduction from caudal region towards head (1).
CALDWELL—UPPER CRETACEOUS AQUATIC SQUAMATE
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APPENDIX 2 Character matrix with character state distributions. Character Taxon
5
10
15
20
25
30
35
outgroup A. dalmaticus A. buccichi Carsosaurus Halisaurus Clidastes Tylosaurus Mosasaurus Ectenosaurus Platecarpus Coniasaurus crassidens Coniasaurus gracilodens Dolichosaurus longicollis
00000 ??010 0???0 ????? 01001 10011 21111 11111 10011 00011 ????? ????1 ?????
11000 11100 11100 ????? 11100 10111 01211 00111 10111 01011 ????? 11100 ?????
00000 1?101 10?0? ????? 00100 10011 10011 10011 11100 ??101 ????? 0??0? ?????
00000 0000? 0?0?? ????? 000?0 11002 10113 11103 10011 10013 ????1 ??0?2 ?????
00000 ?00?2 0?0?2 ????2 20?12 10011 11101 10?10 11101 00102 ?0??? 00?0? ?????
00100 ?0000 ?00?? 000?? 0110? 0011? 10111 0010? 011?1 1001? ????? ????? ?????
00000 0?00? ?0000 ????? ??110 11211 03200 12211 02210 03200 ?00?0 ????? ???00
40
45
50
55
60
65
outgroup A. dalmaticus A. buccichi Carsosaurus Halisaurus Clidastes Tylosaurus Mosasaurus Ectenosaurus Platecarpus Coniasaurus crassidens Coniasaurus gracilodens Dolichosaurus longicollis
00010 01??0 0??00 00??? 01010 10011 00001 10011 00101 00101 0??00 ???00 0?0??
00000 1?0?? 1??0? ?1?01 0?100 10112 100?1 10112 1??1? 11011 0??01 0??01 ???01
00000 0??00 00000 0?000 11111 ?1101 ???11 1?101 ???01 ???01 ????? ???00 00000
00000 0?00? 0100? 01000 12101 02211 12101 ?2??? 1221? 12201 ????? 01??? 01000
00000 ?0000 00000 00000 00101 01210 101?? ??210 ?1201 1?201 ????? ????? 00000
00001 00000 00000 00000 11000 00100 ???00 0?100 01000 11000 ???1? ???1? ??0?1
0 1 1 1 1 1 1 1 1 1 0 ? 0