Caldwell, 1999b

Zoological Journal of the Linnean Society (1999), 127: 423–452. With 13 figures Article ID: zjls.1998.0161, available online at http://www.idealibrary.com.on

Redescription, palaeobiogeography and palaeoecology of Coniasaurus crassidens Owen, 1850 (Squamata) from the Lower Chalk (Cretaceous; Cenomanian) of SE England MICHAEL W. CALDWELL∗ Department of Geology, The Field Museum, Roosevelt Road at Lakeshore Drive, Chicago, Illinois 60605–2496, U.S.A. and Department of Biological Sciences, Biological Sciences Center, University of Alberta, Edmonton, Alberta, Canada, T6G 2E9 JOHN A. COOPER Booth Museum of Natural History, Brighton BN1 5AA Received July 1996; accepted for publication July 1998

Type and referred specimens of Coniasaurus crassidens from the Lower Chalk (Upper Cretaceous; Cenomanian) of southeast England, are re-described. The type is a left maxilla associated with 14 dorsal vertebrae. The maxilla is elongate, bears a low ascending process, and has a long and posteriorly positioned external narial margin. The first maxillary tooth is pointed and bears a groove on the labial face; more posterior maxillary teeth are increasingly rounded and bulbous, and have a single groove on the labial face. Mandibles assigned to Coniasaurus cf. C. crassidens possess teeth of similar form; mandibular bones include the dentary, splenial, angular, coronoid, prearticular, and surangular. A number of features show important similarities to later mosasaurs and contemporaneous groups such as dolichosaurs. These new data provide a very different picture of coniasaurs and their mode of life in the early Upper Cretaceous. The functional morphology of coniasaur teeth is unique and shows occlusion between the lingual platforms of the upper teeth with the crowns of the lower teeth.

Coniasaurs can be considered as analgous to small sauropterygians in terms of general morphology, habitats, and trophic structure. Coniasaur distributions in the Cenomanian and Turonian of Europe and North America are similar to the palaeobiogeographic patterns of other organisms living in the Tethys and SuperTethys Seaway.  1999 The Linnean Society of London

ADDITIONAL KEY WORDS:—squamates – dolichosaurs – mosasaurs – palaeoecology – palaeobiogeography.

∗ Corresponding author. Present address: Paleobiology, Research Division, Canadian Museum of Nature, P.O. Box 3443, Stn.D, Ottawa, Ontario, Canada, K1P 6P4. Email: mcaldwell@mus-nature.ca 0024–4082/99/120423+30 $30.00/0

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M. W. CALDWELL AND J. A. COOPER CONTENTS

Introduction . . . . . . . . . . . . .

Lower Chalk stratigraphy and palaeoceanography Lithostratigraphy . . . . . . . . . .

Palaeoceanography . . . . . . . . .

Coniasaurus localities: North and South Downs . .

Biostratigraphy . . . . . . . . . .

Cenomanian-Turonian Boundary Event . .

Material and methods . . . . . . . . .

Systematic palaeontology . . . . . . . .

Skull . . . . . . . . . . . . . .

Postcranial skeleton . . . . . . . . .

Conclusions . . . . . . . . . . . . .

Palaeobiogeography . . . . . . . . .

Palaeobiology: locomotion and dental function Phylogenetic relationships . . . . . . .

Acknowledgements . . . . . . . . . .

References . . . . . . . . . . . . .

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INTRODUCTION

Coniasaurus crassidens was described by Sir Richard Owen (1850) from two specimens collected from marine deposits of the Lower Chalk Group (Upper Cretaceous; Cenomanian) exposed in the Clayton Chalk Pit near Brighton, East Sussex, England (Fig. 1). The type of C. crassidens, as identified and described by Owen (1850), consisted of a right dentary with teeth, and 13 associated but disarticulated dorsal vertebrae. Owen’s (1850) brief description diagnosed C. crassidens on the basis of its bulbous tooth morphology. Several other specimens, assigned to the species but not described, included associated and articulated vertebrae on two blocks of chalk (Owen, 1850: pl. 37, fig. 18), a small and poorly preserved dentary element, and a small block of chalk with four very large, associated vertebrae; neither of the latter specimens were figured. The assignment of the figured vertebral series (Owen, 1850: pl. 37, fig. 18) to C. crassidens was based on the description, “. . . of apparently the same species . . .” (Owen, 1850:387). All of these specimens, with the exception of the fragmentary dentary described from the collection of Frederick Dixon (1799– 1849), were part of the collection of Henry Catt (1823–1905), who in 1863 changed his name to Henry Willett to comply with the terms of a will of which he was a beneficiary. Although the earlier work of Egerton, Dixon, and Owen all refer to Willett under his original name of Catt, all references subsequent to 1863 use the name Willett.

Henry Willett deposited his collection of Chalk fossils in the Brighton Museum in April 1860, prior to the museum’s inauguration in November of that year by Richard Owen. Willett subsequently published his catalogue (Willett, 1871) in which three Coniasaurus specimens, including the types, appear. The only subsequent literature reference to the location of the holotypes was given by Woodward and Sherborn (1890), who note that the holotype, “. . . associated jaws, teeth and vertebrae . . .”, are in the Brighton Museum. There is no indication that any other authors studying coniasaurs or dolichosaurs ever located and re-described the type.

Examination of the holotype of Coniasaurus crassidens (BMB 007155), and the referred vertebral series (BMB 007157 and 012485), immediately showed that Owen

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Figure 1. Lower Chalk stratigraphy for the North and South Downs of southeast England (AngloParis Basin), and the localities and approximated stratigraphic position of Coniasaurus specimens. 1.

NHM R3421, Plenus Marls, H. subglobosus Zone, Peter’s Pit, Wouldham, Kent; 2. NHM R1937, Lower Chalk (Lower Greys), Blue Bell Hill, Burham, Kent; 3. NHM R62, Chalk, Hart Hill, Charing, Kent. 4. BMB 007155 (the holotype) and BMB 012485, Chalk Marl, Clayton Chalk Pit, near Brighton, East Sussex. 5. NHM R25790, Chalk, Washington, near Worthing, West Sussex. Question marks (?) indicate uncertainty in stratigraphic range of the outcrop from which coniasaurs are known; Dotted lines between question markes indicate the range of uncertainy regarding the provenance of the specimen in that outcrop. Stratigraphic column compiled from: Owen (1987), Mortimore (1986, 1987; pers. comm.), Robinson (1986, 1987), Hancock, Kennedy, & Cobban (1993), and Obradovich (1993).

Abbreviations: CTBE, Cenomanian-Turonian Boundary Event; GM, Glauconitic Marl; MR, Melbourn Rock.

(1850) had misidentified the tooth-bearing element of the holotype. It was not a right dentary but rather a left maxilla. Unfortunately, because of Owen’s (1850) mis-identification and the difficulty of locating the type, the assumption has been that an excellent mandible (actually a pair but the right is fragmentary and has been ignored in the literature) in the collections of the Natural History Museum, London, collected in 1906 by G.E. Dibley (see Dibley, 1918) and described by Nopcsa (1908), was Owen’s holotype. Inaccurate and incomplete characterization

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T 1. Literature survey making formal or informal reference to the classification of Coniasaurus crassidens Owen, 1850. The category and taxon are in the left column and the author(s) are in the right. With the exception of Bell et al. (1982) these classifications have apparently been based solely on observations of English Chalk specimens of C. crassidens Category and Taxon

Authority

‘iguanian’ Suborder Lacertilia, Family non. det. ‘varanoid affinities’, provisionally Dolichosauridae Suborder Sauria, incertae sedis. ‘dolichosaurs’ Mosasauridae Iguanidae ?Aigialosauridae dolichosaurs sensu Nopcsa (1908) Superfamily Varanoidea, Family Dolichosauridae ‘dolichosaurs’ Superfamily Varanoidea, Family Dolichosauridae ?Aigialosauridae

Owen, 1850 Lydekker, 1888 Nopcsa, 1908 Camp, 1923 McDowell & Bogert, 1954 Romer, 1956 Romer, 1966 Rieppel, 1980 Bell et al., 1982 Estes, 1983 Milner, 1987 Carroll, 1988

of coniasaur morphology has subsequently produced great confusion regarding their squamate affinities (Table 1).

In his original description Owen (1850) made no definite statement regarding the relationships of Coniasaurus crassidens, but rather made a tentative reference to ‘iquanian’ affinity based on tooth morphology. He made no attempt to derive characters from any other osteological elements of the holotype or referred specimens.

Authors subsequent to Owen, working without the holotype and with an incomplete and incorrect data set, have been equally as uncertain though a consensus places Coniasaurus within the equally as poorly understood Dolichosauridae (Table 1). As a dolichosaur, Coniasaurus is pigeon-holed as a derived aquatic varanoid with no particular affinity to mosasaurs and uncertain affinities to any one varanoid taxon (Nopcsa, 1908, 1923; Fejevary, 1918; Bell, Murry & Osten, 1982; Milner, 1987; Evans, 1994). However, coniasaurs have also been classified as mosasaurs by McDowell & Bogert (1954) and as the sister-taxon to mosasauroids by Polcyn & Bell (1994).

As shown by Caldwell et al. (1995) the commonly held view that mosasauroids are derived varanoid lizards is not conclusively supported by the available data. A test of the most recent hypothesis of coniasaur affinities to mosasaurs by Polcyn and Bell (1994), in the context of mosasaur and other squamate characters has led to the re-examination of coniasaurs and dolichosaurs.

The search to discover the monophyletic group that includes these taxa, along with other putative aquatic fossil varanoids, has required this re-description and characterization of Coniasaurus crassidens Owen, 1850. The stratigraphy and palaeoceanography of the English Lower Chalk of southeast England is reviewed and discussed in the context of palaeoceanographic events occurring during the Cenomanian in the Anglo-Paris Basin (Robinson, 1987). The biostratigraphy and morphology of known specimens of C. crassidens are described. A discussion of the functional morphology of coniasaur teeth and an examination of coniasaur palaeobiology is also given.

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LOWER CHALK STRATIGRAPHY AND PALAEOCEANOGRAPHY

Lithostratigraphy Chalk is a whitish carbonate rock formed from the fossil skeletons of planktonic algae known as coccoliths. In southeast England, Upper Cretaceous deposits of chalk carbonates are divided into the formations of the North and South Downs (Fig. 1; [Mortimore, 1983, 1986, 1987; Robinson, 1986, 1987]); both of these exposures of chalk are part of the larger Anglo-Paris Basin. The English chalk ranges in age from the base of the Cenomanian through the Lower Campanian.

The Cenomanian Chalks of the South Downs vary considerably in thickness (Mortimore, 1983). They can be up to 80–100 meters thick at Lewes, East Sussex, and at Amberley, West Sussex. In coastal sections near Eastbourne, East Sussex, these beds are reduced to approximately 50 meters. Near Dover, Kent, the Cenomanian chalks of the North Downs are approximately 82 m thick (Robinson, 1987), and thin inland to approximately 48 m at Hart Hill, Charing, Kent (Kennedy, 1969).

The basal contact of the Lower Chalk is with the Upper Albian Gault Clay Formation. In the North (Robinson, 1986) and South Downs (Mortimore, 1986) the contact of Lower and Middle Chalk is at the upper contact of the Plenus Marls with the Shakespeare Cliff Member and Melbourne Rock respectively (Fig. 1). The Cenomanian-Turonian Boundary is marked by the entry of Mytiloides replacing Inoceramus pictus (Fig. 1).

Nomenclature of formations and members in the North and South Downs chalks is currently undergoing revision and investigation. Mortimore (1986) retained the traditional nomenclature for the Lower Chalk of the South Downs, effectively raised the Lower Chalk to group status, and divided it into the basal Glauconitic Marl Formation, Chalk Marl Formation, Grey Chalk Formation, and the Plenus Marls (Fig. 1).

Robinson (1986, 1987), in a series of exchanges with Mortimore (1986, 1987), argued that the Lower Chalk of the North Downs can be correlated with that of the South Downs but that the chalk beds differ significantly and therefore that the nomenclature should vary as well. Following Robinson (1987) for the North Downs chalk, the Glauconitic Marl and Chalk Marl Formations of the South Downs are subsumed within the East Wear Bay Chalk Formation, the Grey Chalk correlates with the Abbott’s Cliff Chalk Formation, and the Plenus Marls are equivalent in both the North and South Downs (Fig. 1). Current British Geological Survey nomenclature for the southern chalks applies the names West Melbury Marly Chalk to the Chalk Marl, and Zig Zag Chalk to the Grey Chalk (pers. comm. R.N. Mortimore); this nomenclature is also given in Figure 1.

Palaeoceanography The chalks of the Anglo-Paris Basin preserve information on palaeoceanographic events with significant biotic repercussions. These events are marked by carbonisotope excursions in the Middle Cenomanian (Paul et al., 1994), and major carbon and oxygen isotope excursions in the Late Cenomanian at the top of the Plenus Marls (Lamolda, Gorostidi & Paul, 1994). The latter excursions, in association with

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other markers (sharp and continued declines in nanoplankton abundance and diversity), are interpreted as the product of a major regression and drop in sea-level, of declining sea water temperatures, and of a decrease in productivity in the AngloParis Basin (Lamolda et al., 1994). Contact of the Plenus Marls with the overlying Melbourn Rock is recognized as a time-diachronous erosional contact. Through these two units, significant biotic and geologic changes are recognized and interpreted to represent a major extinction horizon known to as the Cenomanian-Turonian Boundary Event (CTBE).

CONIASAURUS LOCALITIES: NORTH AND SOUTH DOWNS

Biostratigraphy For several specimens of Coniasaurus, collection data are insufficient to allow placement in a specific member, horizon, or ammonite zone (Fig. 1). Data usually indicate a pit or outcrop, closest town or village, and the county of provenance (Fig. 1). All of the specimens were collected in the 19th century from inland localities such as pits or roadcuttings, many of which are now eroded, overgrown, or filled (Kennedy, 1969). However, four of the specimens can be placed with confidence in specific members of parts of a formation (Fig. 1) based on the following original accession data: NHM R3421, Lower Chalk, Peter’s Pit, near Wouldham, Kent, H. subglobosus Zone, (see also Dibley, 1918); NHM R1937, Lower Chalk (Lower Greys) Blue Bell Hill, Burham, Kent; BMB 007155 and 012485, Chalk Marl, Clayton Chalk Pit, Clayton near Brighton, East Sussex.

Dibley (1918) describes the horizon, and his collection, of NHM R3421 from the Holaster subglobosus zone of Peter’s Quarries, Wouldham, Kent (alias Peter’s Pit; see Kennedy [1969]), within the ‘A. plenus Marls’, now known as the Plenus Marls (Robinson, 1986). NHM R1937, found in association with chelonian limb elements, fragments of Ornithocheirus (a pterodactyl), and sharks teeth, is from the lower part of the Grey Chalk Fm., just below the Plenus Marls as exposed at Blue Bell Hill, Burham, Kent. The associated materials Dibley (1918) describes have not been located, but it is possible that more complete data regarding the horizon of collection was included with them.

Three specimens bear very little collection data: NHM R62, Chalk, Hart Hill, Charing, Kent; NHM R25790, Chalk, at Washington, Worthing, West Sussex, Dixon Collection; BMB 007156, Middle Chalk, Falmer near Brighton, collection Henry Willet.

Published descriptions of the Washington, West Sussex, chalk pits (Gaster, 1937; Kennedy, 1969) highlight the Lower Chalk contact with the underlying Gault.

Section descriptions do not indicate the presence of chalk of an age later than Cenomanian. Therefore, for NHM R25790, the locality at Washington is likely to be Lower Chalk, Chalk Marl Fm. Kennedy (1969) indicates that the contact of the Gault Clay-Lower Chalk (East Wear Bay Chalk) passes through Hart Hill, at Charing, Kent; Lower Chalk exposures are significant in pits and roadcuts in that area. It is probable that NHM R62 was collected from either the East Wear Bay Chalk or Abbott’s Cliff Chalk (using the nomenclature of Robinson [1986, 1987]).

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Based on the examination of geological maps and stratigraphic descriptions of the chalk deposits of the South Downs, it is reasonable to conclude that for BMB 007156, the assignment of the locality to the Middle Chalk is probably accurate though far from specific (Gaster, 1937; Kennedy, 1969). Based on the features of this specimen (see below), we also do not assign it to Coniasaurus contra Owen (1850).

Cenomanian-Turonian Boundary Event (CTBE) At this time, we have not identified any specimens of Coniasaurus from chalk horizons above the Plenus Marls (Fig. 1; see above). Even though the number of known specimens is limited, it is apparent that there is a change in the distribution of Coniasaurus at the CTBE in the Anglo-Paris Basin. The same restriction to the Cenomanian is found for all specimens of Dolichosaurus longicollis so far examined (Caldwell; per. observ.); all are from horizons within the Lower Chalk.

Several questions are raised. Is the Cenomanian distribution of Coniasaurus causally linked to the end-Cenomanian extinction? Is the absence of Coniasaurus in the English Turonian the result of regression and erosion of deposits containing Coniasaurus fossils? Or, finally, is the absence of Coniasaurus due to unknown migrations and colonization in different parts of the globe. These questions remain very difficult to determine. However, when addressing them, it should be remembered that current estimates of the duration of the CTBE are between 250–300 000 years. All of the above factors may well be implicated.

In contrast to the fossil record of Coniasaurus in the English Cenomanian, Coniasaurus sp. is known from Cenomanian through Middle Turonian rocks in North America (Bell, Murry & Osten, 1982). The global distribution of Coniasaurus is discussed in the concluding remarks of this paper.

MATERIAL AND METHODS

The holotype and referred materials of Coniasaurus crassidens required minor repreparation. All work was done by the second author in the Booth Museum of Natural History in Brighton. Some matrix was removed and a form of pyrite decay, leaving a brownish, iron-oxide residue common on vertebrate fossils from the chalk, had attacked the specimen and surrounding matrix. Specimens at the Natural History Museum, London, required no extra preparation. Drawings and illustrations were made by the first author using a microscope and camera lucida. Photographs were taken by the first author and by photographic technicians employed by The Natural History Museum (British Museum), London. Measurements were made using digital calipers. Museum abbreviations: (AMNH) American Museum of Natural History, New York, New York; (BMB) Booth Museum of Natural History, Brighton, East Sussex, England; (NHM R), Natural History Museum, London, England; (FMNH) The Field Museum, Chicago, Illinois.

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Squamata Oppel, 1811 Mosasauroidea Genus Coniasaurus Owen, 1850 Type Species. Coniasaurus crassidens Owen, 1850 Range. Lower to Upper Cenomanian: Chalk Marl Fm. through Plenus Marls.

(Mantelliceras mantelli Zone through the Acanthoceras jukesbrowni Zone [equivalent biozones Actinocamax varius Zone through the Holaster subglobosus Zone) [Fig.1].

Emended diagnosis. Long-snouted squamate; maxilla long and low, with approximately eighteen dental alveoli; long premaxillary contact with maxilla; external narial opening long and posteriorly positioned on maxilla; ascending process of maxilla low and posteriorly positioned; anterior maxillary teeth elongate, pointed and recurved; mandible very elongate with no curvature of ventral margin; sub-dental shelf thin and reduced; coronoid anteroposteriorly elongate; splenial-angular articulation with vertical margins; splenial elongate, narrow anteriorly, and broadly exposed in medial view; inferior alveolar foramen enclosed by dorsal ossifications of splenial; angular broadly exposed in medial view; Meckel’s groove closed along length of dentary; anterior dentary teeth not as bulbous as posterior teeth; more posterior teeth swollen, bulbous, with labial sulcus, posteriorly directed apex, narrow lingual shelf; well-developed zygosphenes and zygantra; centrum not constricted anterior to condyle; neural arch of dorsal vertebrae notched in lateral profile.

Coniasaurus crassidens Owen, 1850 Figures 2, 3, 5, 6, 7B & C, 11 Coniasaurus crassidens Owen, 1850; p1. 37; figs. 19, 19a, 19a′, and 20.

Diagnosis. Characterized by heterodont maxillary tooth characters; anterior-most maxillary tooth gracile, recurved, and bears long lateral groove beginning at or near gum-line and extending to tip; posterior maxillary and mandibular teeth bulbous and lingually expanded; fine crenulations on anterodorsal carina of tooth crown; crown with deep sulcus on anterolateral (labial) face, posteriorly directed apex, and broad lingual shelf.

Type locality and horizon. Collected by H. Catt, (H. Willett Collection), from the Clayton Chalk Pit, near Brighton, East Sussex, Chalk Marl Fm. (Lower Chalk), Lower to Middle Cenomanian.

Holotype. BMB 007155 (Figs. 2; 5; 6; 11A–C), a small chalk block bearing a left maxilla, preserved in left lateral view, and 14 disassociated dorsal vertebrae preserved in various views (Willett, 1871; No. 9, p. 10).

Re-description of the holotype maxilla. The left maxilla, originally identified as a right dentary (Owen, 1850), is long and low, rising slowly to produce a comparatively low ascending process for the maxillary contact with the prefrontal. Prefrontalmaxillary contact was long and descended fairly low across the cheek. Small facets,

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4 1

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8 mx

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Figure 2. Holotype of Coniasaurus crassidens BMB 007155. A, photograph of left maxilla and fourteen associated vertebrae; B, line drawing of left maxilla and 14 associated vertebrae; numbers 1–14 highlight the vertebrae as counted from left to right. Since Owen’s (1850) description, six teeth have been lost from the holotype maxilla. Dotted lines on photograph (A), indicate region of ascending process broken off during recent preparation. Scale bars=1 cm.

likely for the lacrimal, jugal, palatine, and ectopterygoid, are preserved. The maxilla forms a thin, shallow wall, lateral to the narial cavity; a thin horizontal shelf is present slightly above the tooth row and projects into the narial/buccal cavity. At the anterior tip of the maxilla is the weathered remnant of a small process produced by the rostral extension of the mesial maxillary shelf: the premaxillary process. Eight teeth, some complete, and the alveoli for ten more teeth, indicate a total of 18 maxillary teeth. The maxilla is fractured between the third and sixth maxillary teeth resulting in the loss of some bone and tooth material.

Referred material. NHM R 1937 (Fig. 7B, C): left maxilla with six complete teeth, root of a seventh, and anterior alveoli for two more teeth, from the Lower Chalk, Abbott’s Cliff Fm., Hay Cliff Member (Lower Greys), Blue Bell Hill, Burham, Kent; presented

by S.J. Hawkins, July, 1891. BMB 012485 (Fig. 3A), four associated vertebrae, and BMB 007157 (Figs 3B, 11D), 14 dorsal vertebrae (ten articulated, four associated), from the Clayton Chalk Pit, near Brighton, East Sussex, Chalk Marl Fm., Lower Chalk (Willett, 1871; No. 11, p. 10).

Remarks. We prefer a rigid diagnosis for the formal referral of specimens to a genus and species. As such, the holotype provides details only on characters of the maxilla and fourteen dorsal vertebrae. Specimens referred by us to Coniasaurus crassidens possess characters that can unequivocally be compared with the holotype. The teeth of NHM R1937 show morphologies comparable to the teeth of the type. The dorsal

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C

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Figure 3. Dorsal vertebrae referred to Coniasaurus crassidens by Owen (1850). These vertebrae were not designated as paratypes. A, BMB 012485, four associated dorsal vertebrae prepared onto a separate block. BC, BMB 007157. B, articulated dorsal series, 4 in the left series, 10 in the right (anterior is to the left); C, line drawing of the series of 10 dorsals. Numbers 1–10 indicate the vertebrae of the largest articulated series as counted from left to right (anterior to posterior). Scale bars=1 cm.

vertebrae of BMB 007157 cannot be distinguished from those of the holotype: both are similar in size, the shape of the condyle and centrum, and the notching of the neural spines. Though vertebral characters are usually not diagnostic of a specific squamate taxon, the notching of the neural spine is considered here to support Owen’s (1850) original referral of BMB 007157 to C. crassidens.

Coniasaurus cf. C. crassidens Owen, 1850.

Figures 7–9 Material. NHM R3421 (Figs 8, 9), associated right and left mandibles; fragmentary right mandible (dentary and splenial) with nine teeth, and relatively complete left mandible (dentary, splenial, coronoid, angular, surangular, prearticular) with 13 teeth; collection data reads Lower Chalk, Peters Pit, Wouldham (Burham), G.E. Dibley Collection, collected October of 1906 (H. subglobosus Zone). NHM R62 (Fig. 7A), a fragment of left dentary with three complete teeth, fragments of a fourth and a total of eight alveoli; collection data reads Hart Hill, Charing, Kent. NHM R25790, fragmentary left dentary and portions of left splenial; collection data reads, Chalk, Washington, Worthing, West Sussex (not figured).

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Figure 4. Composite reconstruction, in left lateral view, of the non-neurocranial portions of the skull of Coniasaurus. Elements with dotted borders are reconstructed from undescribed skull bones (frontal, prefrontal, parietal, maxilla, squamosal) of a new species of Coniasaurus (in prep.), and by reference to Dolichosaurus longicollis (coronoids, surangular, articular) and mosasauroids. The maxilla is reconstructed from BMB 007155 and NHM R1937; the mandible is based on NHM R3421. Abbreviations: E na, external nares; St f, supratemporal fenestra.

Remarks. As discussed above, we are conservative in our assignment of specimens to crassidens. We do not accept that tooth characters from a left maxilla can serve to diagnose species-level assignment for other isolated tooth-bearing elements unless those elements are a maxilla of similar morphology. An articulated or reasonably associated mandible and maxilla is required to positively assign the known dentigerous specimens to a species. Characteristic tooth morphologies are found among many extant squamates (i.e. teiids, scincids, iguanians) but none of these can be used to assign an individual maxillary or mandibular dentition to a specific species in exclusion of a priori knowledge of both upper and lower dentitions. Associated and/ or articulated mandibles and maxillae have not been found for Coniasaurus. Therefore, the assignment of NHM R3421 to C. crassidens by Nopcsa (1908), and of NHM R62

by Lydekker (1888), is rejected. Characteristics of these mandibles cannot be compared to the holotype maxilla, nor to the referred specimens, and are provisionally referred to the genus for comparison to the species. Both NHM R3421 and NHM R62 are from the Middle to Upper Cenomanian (Fig. 1) and are significantly larger (approximately 30% larger) then the holotype maxilla.

Against the above criteria, NHM R25790 (Owen, 1850) is only comparable to the species and cannot be assigned to it. The specimen is a fragmentary left dentary, contains no teeth but has alveoli for eight teeth, and cannot be diagnosed compared to the type or referred specimens.

Description. Description of Coniasaurus cf. C. crassidens is based on the type and referred specimens from the Chalk Marl deposits near Brighton, and from the referred materials from the Plenus Marl Fm. (H. subglobosus Zone) near Charing and Wouldham, Kent (Fig. 1).

Skull The skull of Coniasaurus crassidens is represented by two left maxillae. Using all referable and comparable specimens, in association with skull elements from a new species of Coniasaurus (in preparation), a reconstruction is made of the skull (Fig. 4).

This reconstruction utilizes tooth characters of C. crassidens but not those of the new, undescribed species of Coniasaurus.

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Maxilla. The holotype maxilla is complete anteriorly though a small non-tooth bearing portion may be missing from the premaxillary process (Fig. 6A). The maxilla is very long and low in lateral view. The premaxillary process is short and recessed from the lateral wall of the maxilla (Fig. 6A, B). The maxilla has a very elongate contact with the premaxilla but is not fused (Fig. 4). This margin is smooth, shows no sign of sutures or fusion to the premaxilla, and curves dorsally to form the margin of the external bony nares. The lateral wall of the maxilla, ventral to the narial emargination, is relatively straight and low; dorsally it overhangs the internal narial space (Fig. 6A). The narial emargination begins at about the sixth maxillary tooth and terminates above the 13th maxillary tooth. The ascending process is relatively low (Fig. 5A, B), arising from the posterior quarter of the maxilla above the 14th maxillary tooth and terminating above the 16th; at this point the maxilla descends at about a 45 degree angle to a presumed position below the orbit.

Facets for the premaxilla (Fig. 5B), lacrimal, jugal, palatine, and ectopterygoid (Figs 6C; 7B, C) are recognized. However, facets for the prefrontal are not readily observed. Further preparation, required to observe the prefrontal articulation of the holotype maxilla, was deemed too difficult.

There are seven to eight maxillary foramina for maxillary rami of the fifth nerve.

In both the holotype, BMB 007155, and the referred specimen, NHM R1937, the preserved and comparable maxillary foramina are located above the 12th, 15th, 17th and 18th maxillary teeth. The maxillary foramina lying above the 17th and 18th maxillary teeth are anterior to the presumed facet for the lacrimal bone, and ventral to the facet for the jugal (Fig. 7C). The anterior portion of R1937 is broken (Fig. 7B, C) revealing a pair of canals separated by a thin septum. The lateral canal is compressed and ovate while the medial canal is circular. The lateral canal is likely the canal for the maxillary ramus of the Vth nerve and the more medial canal that of the lacrimal duct. In Varanus and Lanthantotus the lacrimal duct is paired (Rieppel, 1980, 1983) while in all other squamates the lacrimal duct is a single canal. However, once the duct enters the maxilla, it does so via a single canal in both varanoids and other squamates (pers. observ.).

In Varanus, the maxillary branch of the Vth Cranial Nerve, the inferior orbital artery, and the maxillary vein pass through the palatine foramen. Upon exiting the palatine anteriorly, the arteries and the nerve cross the dorsal surface of the orbitalpalatine process of the maxilla, along the ventro-medial surface of the ascending process of the maxilla (ventral to the lacrimal) and into the alveolar canal of the maxilla (Bahl, 1938). The macroteiids Dracaena (FMNH 207657), Tupinambis (FMNH 140193), and Callopistes (FMNH 9922), do not possess a palatine foramen that leads to the alveolar canal of the maxilla; the alveolar canal communicates directly with the orbital space; a foramen through the palatine is therefore unnecessary for the passage of nerves and arteries. In Iguana and the scinicid Egernia (FMNH 35146), the palatine foramen is present and formed medially by the palatine and laterally by the maxilla. In mosasaurs the palatine foramen has never been observed (Russell, 1967), but there is a groove across the dorsal surface of the orbital-palatine process of the maxilla that leads into the alveolar canal. Viewed posteriorly, the fractured posterior surface of R1937 shows a groove in the correct position to enter the alveolar canal of the maxilla. However, this portion of the specimen is not prepared anteriorly to demonstrate the presence of the foramen, though the groove begins at the presumed position of articulation with the palatine. A similar groove is found on the maxilla of the holotype of Coniasaurus (Fig. 5C).

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A

mx

2 3

1

4

5

6 7

8

12 11 13 14

9 10

16 17 18

15

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C fL

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fJ

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Figure 5. Detail of holotype left maxilla (BMB 007155) of Coniasaurus crassidens. A, line drawing; B, photograph showing general details; C, superior view of posterior extent of maxilla; arrows indicate facets for the articulations with the prefrontal, lacrimal, jugal, and ectopterygoid; dotted line indicates portion of ascending process broken during preparation. Scale bars=1 cm.

The dental shelf of the maxilla is narrow and in occlusal view appears to be straight. There is no emargination of the dental shelf to accommodate contact of the maxilla with the vomer at the posterior margin of the opening leading into the cavity for the Jacobsen’s Organ, the palaeochoanate condition. In Varanus, the maxilla is emarginated and forms a descending semi-circular crest at the contact with the vomer (Bahl, 1938). In the mosasaur Platecarpus, the maxilla is straight as in Coniasaurus.

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B

Figure 6. Holotype maxilla of Coniasaurus crassidens BMB 007155. A, photo-detail of anterior portion of maxilla showing anteriormost maxillary tooth with posterolateral groove and premaxillary-vomerine process of maxilla; B, detailed line drawing of posterolateral groove. Scale bars: A=1 cm; B=1 mm.

Maxillary dentition. There are alveoli for 18 maxillary teeth. In the holotype, most of these alveoli are filled by complete or broken teeth; in NHM R1937, only six complete teeth, and the root of a seventh, are preserved (Fig. 7B, C). Patterns of tooth replacement in the maxilla cannot be determined.

Tooth implantation is of the pleurodont-type but there is a build-up of bone around the base of the tooth that extends out from the lingual wall of the maxilladental shelf, to form a partial socket for the tooth. This socket brackets about onethird to one-half of the diameter of the tooth (this condition was difficult to illustrate due to the preservation of the holotype, however, a comparable condition is observed in the mandible [Figs 8, 9]).

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C fL mf m 1 2

Figure 7. Tooth-bearing elements of Coniasaurus. A, NHM R62, Coniasaurus cf. C. crassidens, fragment of a left dentary bearing four teeth and portions of two others (Plenus Marls, Lower Chalk, Hart Hill, Charing, Kent); B & C, NHM R1937, C. crassidens. fragment of a left maxilla with six complete teeth (Lower Chalk [Lower Greys] Blue Bell Hill, Wouldham, Burham, Kent). Abbreviations: fL, lacrimal facet; mf, maxillary foramina; 1, upper maxillary alveolus; 2, lower maxillary alveolus. Scale bars= 1 cm.

The maxillary dentition is also heterodont. The anterior most maxillary tooth is elongate, laterally compressed, and terminates in a point. There is a relatively deep sulcus on the labial surface. It cannot be determined if there is a corresponding sulcus on the lingual surface (Fig. 6A, B).

Teeth in the middle of the maxilla are more bulbous and labio-lingually expanded.

The tooth tip is rounded and there is a long crest/carina starting anteriorly and running posteriorly to the tooth tip; a similar though shorter carina descends along

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Figure 8. Left and right mandibles of Coniasaurus cf. C. crassidens NHM R 3421. A, lateral view of left mandible; B, medial view of left mandible; C, occlusal/dorsal view of left mandible (anterior is to the left); D, lateral view of fragmentary right mandible. Scale bar in white (=1 cm) refers to A–C; black scale bar (=1 cm) refers to D.

the posterior margin of the tooth (Figs 5A, B; 6A). The anterior carina has a number of small crenulations or wrinkles in the enamel. Midway down the length of the carina a small labial sulcus, parallel to the long axis of the tooth, is present on the tenth maxillary tooth; the sulcus also characterizes more posterior maxillary teeth (Fig. 6A).

Only a single posterior maxillary tooth remains in BMB 007155 (six remain in

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Figure 9. Line drawings of left and right mandible of Coniasaurus cf. C. crassidens NHM R3421. A–C, Left Mandible: A, ventral view (anterior is to the right); B, lateral view; C, medial view. D, Right Mandible, lateral view. Abbreviations: an, angular; c, coronoid; d, dentary; me gr, Meckel’s groove; pa, prearticular; sa, surangular; sp, splenial. Scale bar=1 cm.

NHM R1937). Since the original description six teeth have been lost from the type maxilla (cf. Owen, 1850; pl. XXXVII, fig. 19). The remaining posterior-most tooth, tooth 15 (Figs 2A, B, 5A, B), is large and bulbous and bears a deep labial sulcus that separates the anterior one-fifth of the tooth from the posterior four-fifths (cf. the dentary teeth illustrated in Figs 8–10). In labial view, if the anterior inflation is ignored, the tooth is roughly conical and nearly symmetrical (Fig. 5A, B). In occlusal view, the apex of the tooth is offset labially and posteriorly; lingually, the tooth expands into a broad, bulbous, platform (cf. the dentary teeth illustrated in Fig. 10).

Maxillary teeth of NHM R1937 (Fig. 7B, C) are slightly less bulbous anterior to posterior than teeth in the same position in the holotype (Fig. 5A,B) but still show the lingual expansion.

Mandible. Morphology of the mandible of Coniasaurus cf. C. crassidens (Fig. 4) is based on the following descriptions of NHM R3421, associated right and left mandibles (Figs 8, 9).

The right mandible is still imbedded in the matrix and is represented by a partial dentary and anterior portion of the splenial (Figs 8D, 9D). The dentary preserved two mandibular foramina. The broken anterior margin reveals the alveolar canal for the mandibular ramus in a position superior to Meckel’s Groove. Meckel’s

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Figure 10. Mandibular dentition from the left mandible of NHM R3421. A, lateral view. B, occlusal view. Arrows and ‘f’ indicate position of wear facets on the tip and lateral tooth faces. Abbreviations: a, alveolous; f, facets; sh, lingual shelf.

Groove is enclosed by the dentary anteriorly, while ventrally it is enclosed by the tightly fitting splenial. Nine teeth are preserved along with the alveoli for three more teeth.

The left mandible includes a mostly complete dentary, partial coronoid, surangular, prearticular, angular, and complete splenial.

The dentary is long and shallow in lateral view, with no appreciable curvature along the ventral margin, and can be described as a long, thin wedge. Dorsally, at the position of the third-last tooth, the dentary deepens as it rises towards the coronoid. The posterior margin of the dentary makes a small, dorso-lateral and dorso-medial contact with the coronoid, a broad, overlapping contact with the surangular, a broad contact with the splenial, and likely an internal contact with the prearticular (cf. mosasaurs [Russell, 1967]). On the lateral surface there are four mandibular foramina. The dental shelf is dorso-ventrally deep and labio-lingually shallow. The sub-dental shelf is very small.

The splenial is almost covered by the dentary in lateral view; only the articular head of the splenial is exposed where it contacts the angular. In medial view, and beginning posteriorly, the dentary and splenial form the ventral margin of the mandible. At about the mid-point of its length, the dentary forms the medial mandibular margin. Along this length, the splenial is ventral and slightly recessed into the dentary. Meckel’s Groove is completely covered by the splenial to a point below the fourth to third most-anterior dentary tooth (Fig 9A, C). The splenial twists slightly at this point so that in medial view its most anterior tip is visible.

Meckel’s Groove is not exposed as the dentary pinches together around the groove closing it over. This pinching together of the dorsal and ventral lips of Meckel’s Groove produces a canal; this canal is visible (in broken section) at the anterior end of both the left and right dentaries (Fig. 9D). However, it should be noted that there is some slight flaring of Meckel’s Groove towards the anteriormost end of the dentary (Fig. 9A). This flaring may represent ventral exposure of Meckel’s Groove if it is not an artifact of preservation.

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Posteroventrally the splenial meets the angular. The contact between them is vertical medially and laterally; ventrally the contact is also flat. Contact between the angular and splenial may have formed a true transverse intramandibular joint though the arthrodial nature of this joint is unknown. The external morphology of these elements, and their shared contact, is indistinguishable from that of mosasaurs, aigialosaurs, and snakes (McDowell & Bogert, 1954; Russell, 1967; deBraga & Carroll, 1993). Morphology of the disarticulated elements is unknown.

Superior and anterior to the intramandibular joint the splenial produces a large curving flange that contacts the prearticular and coronoid posteriorly, and anteriorly makes a long curving contact with the dentary. A small anteriorly emarginated recess is located dorsal to the articular head and ventral to the dorsal process/ flange. The splenial bears two foramina; the anterior-most is the alveolar recess. What is unique is that the splenial has overgrown the recess postero- and anterodorsally to restrict the size of the opening. Both foramina of the splenial are level with the bottom of the dentary tooth row.

The coronoid is robust and appears to be equal in size laterally and medially.

The ascending process is broken (Figs 8A–C, 9B, C) but when originally described

by Nopcsa (1908) was present (see reconstruction Fig. 4); there is no evidence of the posteromedial process. Two small anterior processes are present on the anteromedial and antero-lateral faces. The medial process onlaps the dentary while the lateral process contacts but does not overlap the dentary; both of these processes are very small compared to the antero-medial and antero-lateral processes of the coronoid of teiids, varanids, or other scleroglossans (pers. observ.). The major articulations of the coronoid are laterally and medially with the surangular and prearticular-articular.

More of the angular is exposed medially than laterally. An articular head, complementing the morphology of the articular head of the splenial, is present on the antero-ventral extreme of the angular and is visible in lateral, medial, and ventral view (Figs 8A, B, 9A–C). In medial view the angular has a tall thin flange that overlaps the prearticular; it is impossible to determine the height of this flange or the possibility of contact with the coronoid. A small foramen, slightly below the level of the tooth row, is located posterior to the articular head. Anteriorly, the angular expands dorsally to form a small but pronounced process articulating with a small facet on the surangular. Postero-laterally, along the narrowly exposed dorsolateral margin, the angular bears an elongate facet for the surangular. The posterior extent of the angular is unknown.

The surangular is exposed laterally and is overlapped by the coronoid, angular, and dentary. The element is slightly displaced relative to the angular thereby exposing the articular facet for the angular.

The articular-prearticular is known only from the medially exposed fragment of the prearticular. As in mosasauroids (see above) the prearticular likely extends forward across the intramandibular hinge to contact the dentary.

Mandibular dentition. Along the alveolar groove of the left dentary of NHM R3421 there are 13 teeth and five empty alveoli, for a total of 18 teeth (Figs 8A–C, 9B, C). The broken anterior end likely was long enough to accommodate three to four more teeth (see the reconstruction in Fig. 4) giving a reconstructed total of 21–22 teeth. This number corresponds with the variation between the maxillary and dentary tooth count observed in mosasaurs (Russell, 1967) and with the number of

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dentary teeth considered present in aigialosaurs, i.e. 22 by Carroll & deBraga (1992) and deBraga & Carroll (1993). Tooth implantation is pleurodont, though the teeth are set into deep sockets in the lateral wall of the dentary. These sockets bracket one-third to one-half of the diameter of the tooth (Figs 8B, C, 9C, 10B). The partial sockets are deepest in the middle of the dentary but become shallower anteriorly and posteriorly. Large amounts of cement-like bone serve to hold the teeth in place along the lingual margin. There are no pits present at the base of the teeth, no obvious replacement teeth, nor any indication of the pattern of tooth replacement.

Wear facets are present on both the labial surface and on the posteriorly inclined tip of the tooth crowns (Figs 8D, 10A).

A significant degree of heterodonty is observed for the dentary teeth as was observed for maxillary teeth. Anterior dentary teeth are more gracile in all dimensions then are the more posterior, very bulbous, and very asymmetrical posterior teeth.

All teeth are best described as bipartite: the anterior portion comprises a quarter to a third of the tooth, is thinner than the posterior portion, and in labial aspect is separated from the posterior part by a sulcus or groove that descends from the crown to near the gum-line in larger teeth (these sulci are present on eight of the mid-dentary teeth [Figs 8A, 9, 10A]). The posterior portion of the tooth makes up the bulk of the swollen tooth body and forms a rounded, posteriorly inclined tip, that overhangs the anterior portion of the next-most posterior tooth. A crest or carina is formed between the anterior part of the tooth and the tooth tip. This carina may bear small wrinkles or crenulations in the enamel. These crenulations descend down the carina until they contact the sulcus; they should not be confused with serrations such as those seen in Varanus (Bahl, 1938). In contrast to extant varanoids (Heloderma, Varanus, and Lanthanotus), but similar to other squamates, plicidentine is not apparent on the teeth of Coniasaurus. Because the tip of the tooth overhangs the next-most posterior tooth, and because the anterior swollen portion is quite high relative to the height of the tooth, the crenulated carina is raised at a very shallow angle relative to the alveolar margin of the dentary. The effect is that the crowns of the teeth form a long, continuous, occlusal surface (Figs 8C, 10B). In occlusal view the more posterior teeth of the dentary dentition show a unique morphological feature of the lingual face of the tooth. Ventral to the swollen crown the shaft of the tooth is enlarged lingually to form a shallow, inclined platform; the crown is offset labially.

Postcranial skeleton Vertebrae. Fourteen dorsal vertebrae are preserved in both the holotype (BMB 007155) and referred (BMB 007157 and 012485) specimens. The longest series of articulated dorsals is counted at ten in BMB 007157. All vertebrae are uniformly procoelous.

The cotyle is a deep, broad, oval surface. The dorsal lip of the cotyle overhangs the ventral cotyle margin (Fig. 11A, B) giving a slightly angled appearance to the joint between articulated vertebrae (Fig. 3B, C). The observed obliquity does not approach the condition observed in extant varanoids. The condyle is rounded and only slightly compressed laterally (Figs 2B, 3C, 11B, D). The condyle shows articular surfaces on both the ventral and dorsal margins. The centrum is differentiated from the condyle by a slight emargination and constriction, and the condyle does not expand

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Figure 11. Dorsal vertebrae of Coniasaurus crassidens. BMB 007155 A–C. A, anterior view of dorsal vertebra #1 from the type series (see Fig. 2); B, lateral view of dorsal vertebra #10 from the type series (see Fig. 2), anterior is to the left; C, dorsal view of dorsal vertebra #5 from the type series (see Fig. 2), anterior is to the left. BMB 007157. D, lateral view of dorsal vertebra #10 from the referred series (see Fig. 3), anterior is to the left. Abbreviations: pa, parapophyses; Poz, postzygapophyses; Prz, prezygapophyses; z, zygosphenes. Scale bar=1 cm.

past the width of the centrum. The emargination indicates a difference in the nature of the finished bone on the centrum as compared to the endochondral or articular bone on the condyle.

The centrum is elongate and cylindrical. Ventral to the parapophyses the centrum expands to form the margins of the cotyle. On the ventral surface of the centrum two small foramina penetrate the centrum at a mid-point along its length.

The neural arch is deep and expands dorsally into a broad platform to support the pre- and postzygapophyses and parapophyses (Fig. 11C). The zygapophyses are inclined at approximately 30–35 degrees to the horizontal. The parapophyses are dorso-ventrally elongate and laterally directed.

The neural spine is short and in several vertebrae appears to be bear a v-shaped notch in the middle of the spine (Fig. 3B, C). Well developed zygosphenes and zygantra are present anteriorly and posteriorly, respectively, on the neural pedicle (Fig. 11C, D). The articular faces of the zygosphene-zygantrum are inclined at approximately 70 degrees to the horizontal.

The neural arch lamina separating the zygosphenes is relatively wide and in dorsal view forms a very shallow v-shape along the anterior margin directly above the neural canal (Fig. 11C). Non-serpent squamates uniformly show notching or embayment of the neural arch lamina between the zygosphenes in those taxa that possess these accessory articulations, i.e. lacertids, teiids, gymnophthalmids, some

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large iguanids, some cordyline cordylids (Estes et al., 1988), mosasauroids (Carroll & deBraga, 1992; Bell, 1993), Dolichosaurus (pers. observ.), and Coniasaurus.

In contrast, the intra-zygosphene neural arch lamina of snakes is not notched. There is great variation on this character in extant snakes, but Cretaceous snakes such as Dinilysia patagonica (Rage & Albino, 1989) and Pouitella pervetus (Rage, 1988) show no notching of this margin. Neither of the above snakes possess accessory processes or epizygapophyseal spines. However, a snake vertebra from the Barremian (Lower Cretaceous) of Spain (Rage & Richter, 1994), reputedly the oldest known snake, shows a definite v-shaped notching of the intra-zygosphene, neural arch lamina. The pre- and postzygapophyseal facets are not horizontal, nor is there any evidence of accessory processes or epizygapophyseal spines. Rage & Richter (1994) also note that the exact angle of the zygapophyseal facets is unknown due to crushing.

An isolated vertebra from the Cenomanian of France was referred to the Dolichosauridae by Rage (1989, fig. 1A–E). This vertebra is similar in all respects to the vertebrae described above for Coniasaurus crassidens with the exception of the width of the intra-zygosphene neural arch lamina; the margin between the zygosphenes is notched but it is comparatively much narrower. The distance between the zygosphenes of Dolichosaurus longicollis (pers. observ.) and a new species of Coniasaurus (in prep.) is less than that observed for the known vertebrae of C. crassidens.

However, so few vertebrae are known for Coniasaurus that it is difficult to estimate the degree of vertebral variation of this feature. Adding to this ambiguity, the described and referred material of Dolichosaurus longicollis (pers. observ.) is articulated, thereby making it exceptionally difficult to observe variations in the width of the zygosphene platform. cf. Dolichosauridae Gervais, 1852 Material. BMB 007156: Four very large, associated, dorsal vertebrae with very long neural spines (Willett, 1871: No. 10, p. 10).

Locality and horizon. Southeast England, from Falmer, near Brighton, East Sussex, collected by H. Catt (H. Willett Collection) from horizons described as Middle Chalk.

Remarks. Owen (1850) made reference to four associated vertebrae from the Middle Chalk (Lower Turonian), at Falmer near Brighton, and assigned them to Coniasaurus crassidens. These vertebrae are three times larger than the holotype vertebrae, are significantly younger, assuming the lithostratigraphic horizon data to be modestly accurate, are badly weathered, and are comparable to the vertebrae of the types and referable materials of Coniasaurus and Dolichosaurus. We have compared them to the Dolichosauridae but these vertebrae are also equal in size to the dorsal vertebrae of aigialosaurs (Carroll & deBraga, 1992; Caldwell et al., 1995).

CONCLUSIONS

Palaeobiogeography Coniasaurus crassidens is recognized from fossil remains found throughout the Cenomanian-aged Chalk of England. Fossil squamates with similar swollen teeth

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Figure 12. Cenomanian Palaeocoastline Map (c. 98–93 mya), distributions of known coniasaur specimens, and the current direction and position of the Tethys and SuperTethys Seaway. Solid lines indicate the coastlines of Cenomanian continents and land masses. Dotted lines outline the margins of modern coastlines by reconstructing modern continental bodies in the positions they were in during the Cenomanian. Open Stars indicate localities from modern continents where coniasaur fossils have been found. Arrows indicate the presumed current direction of the Tethys and SuperTethys Seaway (Map redrawn from Smith et al. [1994]).

are found in Cenomanian to Middle Turonian rocks in Texas (Bell, Murry & Osten, 1982; Polcyn & Bell, 1994), and Cenomanian to Turonian rocks of the mid-western United States (Bell, pers. comm.). Globally, the earliest record of Coniasaurus is from the Lower Cenomanian chalks of Southern England while the latest occurrence is from the Turonian of South Dakota from rocks deposited in the North American Interior Seaway (Fig. 12).

This distribution of fossils of Coniasaurus is spatially and temporally similar to that observed for invertebrates such as inoceramids (Voigt, 1995) and rudistid bivalves ( Johnson & Kauffman, 1990; Kauffman & Fagerstrom, 1993). The palaeobiogeography of marine organisms in the Cenomanian can be correlated to reconstructed patterns of circum-global currents through the Tethys Seaway and across the widening Atlantic Ocean in what has become known as the SuperTethys (Kauffman & Fagerstrom, 1993; Smith & Funnel, 1994). Palaeo-currents have been hypothesized as moving from east to west through the Tethys and SuperTethys based on sedimentological studies conducted between the Carpathian Mountains and Spain (Fo¨llmi & Delamette, 1991).

Invertebrates such as bivalves are very efficient colonizers of new habitats. Larvae spend a good part of their development as zooplankton and can travel great distances depending on the strength and direction of the prevailing water currents. For example, rudist bivalves are thought to have originated in the European Tethys region sometime in the Berriasian to Valanginian of the Lower Cretaceous. The first record of this group in the Caribbean SuperTethys Seaway is in the Barremian, approximately 10 to 12 Mya after their European origin ( Johnson & Kauffman, 1990; Kauffman & Fagerstrom, 1993). It is concluded that rudist larvae dispersed

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Figure 13. Reconstruction of a coniasaur-dolichosaur based on skull materials of Coniasaurus crassidens and an undescribed species of Coniasaurus, and from postcranial remains of Dolichosaurus longicollis (Owen, 1850). Scale bar=5 cm.

across the widening Atlantic Ocean on currents of the westward flowing TethysSuperTethys Seaway and radiated into a number of reef building niches left open by extinctions among hermatypic corals.

The Cenomanian through Turonian distribution of Coniasaurus shows a similar pattern of migration and dispersion. The earliest fossils are Lower Cenomanian and within the European Tethys are found throughout the Cenomanian to at least the Cenomanian-Turonian boundary. If the palaeobiogeography of other dolichosaurs is considered as well, the Cenomanian can be seen as a time of diversification and radiation of these small marine squamates within the Tethys Seaway habitats of Southern Europe.

Stratigraphic and palaeobiogeographic data indicate that from this centre of origin these small aquatic squamates migrated west to the inland seaway of North America. During the process of this migration it seems unlikely that they would have successfully traversed the Atlantic despite the predominant direction of the SuperTethys flow. A more likely scenario is that these migrations were accomplished by following the palaeocoastlines of western Africa to South America and then north, and the coastline of numerous small Baltic islands and continents to the coast of Eastern North America (Fig. 12). This hypothesis is consistent with one proposed for the global distribution of later Cretaceous mosasaurs and for the Turonian distribution of for russellosaurine and mosasaurine-like aigialosaurs (Caldwell & Bell, 1994) The absence of coniasaurs and dolichosaurs from Turonian and younger rocks in Europe is likely an artifact of the focus on fossil collecting in association with the rarity of fossils of these animals. However, it is also possible that these small squamates suffered extinction or near-extinction in the European Tethys during the large-scale extinction event recognized at the Cenomanian-Turonian Boundary (Lamolda et al., 1994). The Turonian record of Coniasaurus in North America may be representative of those taxa that survived extinction by virtue of earlier migrations.

Palaeobiology: locomotion and dental function Coniasaurus was a small animal with a relatively elongate head and was probably no more than 0.5 meters in length (Fig. 13). It may well have had an elongate neck and body similar to that reported for Dolichosaurus (Owen, 1850) and other putative dolichosaurs such as Pontosaurus and Acteosaurus (Nopcsa, 1908, 1923). Reconstructed with a long head, neck, body, and tail, Coniasaurus and other dolichosaurs (Fig. 13)

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look very much like small nothosaurs or pachypleurosaurs (see Carroll, 1985). The limbs of nothosaurs and pachypleurosaurs are reduced in size relative their bodies and they had very long necks and relatively long tails. Nothosaurs are considered to have been angulliform swimmers that kept their limbs tightly pressed to the sides of their bodies and used powerful lateral undulations of the tail and body to move through the water. It is not assumed for nothosaurs that they were obligatorily aquatic as they still possessed well developed limbs. Therefore, these small sauropterygians are thought to have been able to move about on land quite easily, perhaps for reproduction, basking, or some food gathering. Nothosaurs and pachypleurosaurs are found in Middle to Upper Triassic rocks around the globe, but in particular are well known from the marine rocks deposited in the European Triassic Tethys Seaway (Rieppel, 1993). Many of these deposits are considered to have been formed in lagoons or offshore environments in close proximity to the Triassic palaeoshorelines.

In comparison, coniasaur limbs remain unknown, but data on dolichosaurs indicate that the limbs were relatively short compared to other squamates, and that the neck, body, and tail were quite long. Coniasaurus had a long head (Fig. 4) that if reconstructed with a dolichosaur-like body describes an animal that is physically similar to a nothosaur or pachypleurosaur. Coniasaurs and dolichosaurs are both known from rocks that were deposited in relatively shallow marine environments along the coastlines of land masses bordering the Cretaceous European Tethys and the Cretaceous North American Inland Seaway and part of the SuperTethys. From these similarities it is concluded that coniasaurs and dolichosaurs locomoted in a similar manner to small sauropterygians, and likely filled niches in marine habitats similar to those occupied by the now extinct small sauropterygian forms: small marine carnivores preying on small vertebrates and invertebrates and living in near shore environments.

The most notable morphological difference between coniasaurs and small sauropterygians, and between coniasaurs and other squamates, is the morphology of their teeth. It is likely that this difference would have led to differences in the specific trophic strategy adopted by coniasaurs as compared to sauropterygians or other squamates.

At least six unique tooth characters are present in Coniasaurus: (1) large, swollen, bulbous crowns; (2) large labial sulcus; (3) wear facets on the lateral faces of the mandibular teeth; (4) wear facets on the tips of the mandibular teeth; (5) posterior overgrowth of teeth and tooth-tips, in both the maxillary and mandibular dentition; (6) development of a carinate occlusal margin. A fifth factor, not easily seen on the maxillary dentition of the holotype (see Owen [1850] pl. 37, fig. 20), can be observed on the maxillary dentition of a new species of Coniasaurus: the formation of lingual shelves or platforms below the crown of mid to posterior maxillary teeth. The morphology can be understood by examining the last two to three teeth of the mandibular dentition of NHM R3421 (Fig. 10B).

Reconstructing this dental battery provides an interesting picture of dental function in Coniasaurus. In most extant lizards, when the mouth is closed, the mandibular dentition does occlude with the maxillary dentition, but rather lies lingual to it when the jaws are passively closed (an exception is the macroteiid Dracaena guianensis. The wear facets on the lateral surface of the mandibular teeth indicate a similar passive position for Coniasaurus (Figs 8–10), and further that this wear was produced actively, that is, during biting. The wear facets on the tips of the mandibular dentition

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indicate differing dental function compared to maxillary teeth because wear facets are not present on the tips of maxillary teeth.

In Coniasaurus, the source of the apical wear facets may well be from occlusion of the mandibular dentition against the lingual platforms of the maxillary teeth (Fig. 10A,B) Occlusion as displayed by these fossils appears to have been relatively imprecise and to have been accomplished by only a small number of posteriorly located maxillary and mandibular teeth. In association with some form of limited occlusion, another modification to dental morphology would also have affected dental function in Coniasaurus. A long, ragged, shearing or crushing surface is created by the way in which the tooth tips overhang the next-most posterior tooth. Each tooth also presents an elongate carina leading to that tip. Functionally, the carinate surfaces of both the upper and lower dentition would have formed effective shearing margins, while occlusion of the lingual shelf of the upper tooth with the swollen tip of lower tooth provided Coniasaurus a crushing function. Shearing and crushing would have been accomplished in a single bite.

Though a functional analogue is not obvious among other extant or fossil squamates, the macroteiid Dracaena guianensis (FMNH 207657) shows a similar, pronounced form of heterodonty. The anterior maxillary and mandibular teeth are gracile, recurved and pointed, while the posterior teeth are very large, inflated in all dimensions, and have flattened crowns. The large flattened teeth, in association with a short, robust mandible, deep maxilla, and a large m. adductor mandibulae complex, allow Dracaena to feed successfully on hard-shelled invertebrates. This interaction of dental, osteological, and mycological adaptations are apparent in other mollusc-feeding squamates such as extant and extinct species of Varanus (Auffenberg, 1988; Clos, 1995), and the mosasaur Globidens (Russell, 1967, 1975).

In contrast, the maxilla and mandible of Coniasaurus are gracile, elongate bones, with shallow lateral profiles (Fig. 4). Unfortunately, the mandibular fossa/adductor fossa is not preserved and muscle attachments to other postdentary bones are not clear. However, based on the available data, it is reasonable to conclude that Coniasaurus could not have fed upon hard-shelled invertebrates with an efficiency similar to Globidens or Dracaena. The length of the jaw is much more similar to modern predators who snap quickly at passing prey items, depending on the speed of the jaw-closing muscles and the length of the gape in order to secure prey. As such, Coniasaurus could clearly have fed upon numerous varieties of small crustaceans and fishes; fossils for all these groups are known from the chalk (Milner, 1987; Morris, 1987). Just exactly what trophic strategies were employed by Coniasaurus, and how disparate niches were partitioned between similar and dissimilar species, cannot be known.

Massare (1987) examined tooth morphologies in large Mesozoic marine reptiles: mosasaurs, ichthyosaurs, plesiosaurs, pliosaurs, and marine crocodiles. She concluded that tooth crown morphology showed high diversity between and within these groups of reptiles, that convergence was common, and further, that crown morphologies reflected a great diversity of predator types. Four criteria: (1) kind of reptile with tooth type, (2) stomach contents, (3) type of wear, and (4) inferred function, contributed to the assessment of tooth types. Five general functions were identified: crush, crunch, smash, pierce, and cut. From the types of teeth, and their inferred function, seven predator guilds were hypothesized: Crush, Crunch, Smash, Pierce I, Pierce II, Cut, and General.

Coniasaurus is certainly not a large marine reptile and so perhaps is not best

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assessed against Massare’s guild-membership criteria, but it is nevertheless an interesting exercise. The dentition of Coniasaurus must be assessed as a whole due to the high degree of heterodonty. All teeth bear carinae except for the most anterior, thin, recurved teeth. All teeth do possess a lateral sulcus to varying degrees. the posterior maxillary and mandibular teeth are swollen labially and lingually, and there is a definite labial shelf. Wear facets are present on the crowns of both the upper and lower dentitions, lingually on the upper, and labially on the lower. Against Massare’s (1987) criteria, if Coniasaurus were large, its dentition would place it in two guilds, if not three, i.e. Crush-Cut, or Crush-Cut-Crunch.

An important comparison is with small sauropterygians. Most of these nothosaurs and pachypleurosaurs have teeth similar to those of Massare’s Pierce I and II guilds, but several pachypleurosaurs (i.e. Anarosaurus heterodontus) have teeth that are closely spaced and have inflated crowns with crenulations and multiple carinae (Rieppel & Kebang, 1995). The jaw is also elongate and relatively thin and straight. None of these teeth are inflated or bulbous to the degree seen in Coniasaurus, and the presence or absence of a lingual shelf cannot be confirmed. Still, this Triassic marine reptile is a reasonable analog to Coniasaurus. Among later Mesozoic marine reptiles, small and large, the dentition of Coniasaurus is clearly very unique and very specialized.

The close temporal and spatial relationship of Coniasaurus to other dolichosaurs, and to early mosasauroids such as ‘aigialosaurs’ (Caldwell & Bell, 1994) suggests that species from these closely related taxa were able to partition resources in shared habitats, and to continue to radiate and evolve under those constraints. Niche partitioning may well have been due to differences in feeding habits and prey selection; this conclusion is based on the morphology and function of coniasaur teeth as described above versus the tooth morphology observed in ‘aigialosaur’ taxa (see Carroll & deBraga, 1992; deBraga & Carroll, 1993). What is clearly evidenced by this discussion is the need to examine tooth morphology in small marine reptiles for insights into the palaeoecology of these animals.

Phylogenetic relationships The phylogenetic relationships of Coniasaurus have remained unresolved because of poor characterization in the literature and an absence of studies with the goal of presenting empirical studies of new and described specimens. This study has attempted to rectify the problems for phylogenetic analysis created by improper identifications, mis-characterization of parts, and the presence of new but undescribed specimens. We consider this study to be a step in the protocol of examining and describing known materials so that a correct characterization of the genus and included species is available for phylogenetic analysis. The results of a phylogenetic analysis are included in a second study detailing a new species of Coniasaurus (Caldwell, 1999).

ACKNOWLEDGEMENTS

For assistance while gathering data, we thank S. Chapman, A. Currant, J. Evans, C. Price, and V. Sowiak. We thank R.N. Mortimore for critical reviews of our

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discussion and description of Chalk litho- and biostratigraphy. MWC also thanks M. Wilson and J. Clark for use of their lab space and equipment. This research was supported by a Natural Sciences and Engineering Research Council of Canada (NSERC) Post Graduate Scholarship and by an NSERC Postdoctoral Fellowship to MWC.

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