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An Enigmatic (Synapsid?) Tooth from the Early Cretaceous of New South Wales, Australia

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An Enigmatic (Synapsid?) Tooth from the Early Cretaceous of New South Wales, Australia Journal of Vertebrate Paleontology 23(1):232±237, March 2003 q 2003 by the Society of Vertebrate Paleontology AN ENIGMATIC (SYNAPSID?) TOOTH FROM THE EARLY CRETACEOUS OF NEW SOUTH WALES, AUSTRALIA WILLIAM A. CLEMENS1, GREGORY P. WILSON1, and RALPH E. MOLNAR2 1Museum of Paleontology and Department of Integrative Biology, University of California Berkeley, California 94720-4785, [email protected]; 2Queensland Museum, P.O. Box 3300, South Brisbane QLD 4101, Australia, and Museum of Northern Arizona, Flagstaff, Arizona 86011 ABSTRACTÐLargely fragmentary fossils from sites in New South Wales, Victoria, and Queensland, Australia doc- ument terrestrial and marine vertebrate faunas of Aptian±Albian age. The natural cast of a large tooth from the Griman Creek Formation, Lightning Ridge, New South Wales, records the presence of a hitherto unknown member of the fauna. Although reference to one of the groups of crocodyliforms that evolved complex, mammal-like postcanine teeth cannot be excluded, the fossil more likely represents a species of synapsid. In some respects it is similar to lower postcanines of traversodontids. Greater morphological similarities to upper molars of dryolestids make reference of this tooth to this group more likely. Current Mesozoic Laurasian and Gondwanan fossil records include mammals with cheek teeth of similar large size. INTRODUCTION ited knowledge of the Australian Early Cretaceous fauna. Mea- surements made to the nearest 0.1 millimeter (mm) were ob- Early Cretaceous terrestrial faunas from Australia are known tained with dial calipers. primarily from fragmentary specimens of Albian age. Discov- eries at Lightning Ridge and several other localities in New South Wales, Australia, document the presence of dipnoans, SYSTEMATIC PALEONTOLOGY teleosts, chelonians, crocodyliforms, ornithopods, saurischians, Preservation birds, and monotremes (Dettmann et al., 1992; Willis, 1993; Flannery et al., 1995). Approximately contemporaneous (Ap- AMF 118621 (Fig. 1) is a fully opalized cast of a tooth of a tian±Albian) sites along the southern coast of Victoria have pro- vertebrate. The surface texture of its crown varies from smooth duced remains of actinopterygians, lung®sh, temnospondyls, to a lightly textured or pitted pattern resembling rugose areas turtles, plesiosaurs, crocodyliforms, pterosaurs, theropods, or- of some kinds of teeth (Fig. 2). Small areas of its surface are nithopods, birds, and mammals (Rich et al., 1999; Rich and abraded. Few of the opalized bones from the Griman Creek Vickers-Rich, 2000). Marine Cretaceous rocks in central Formation show evidence of transport prior to ®nal burial, how- Queensland have yielded pterosaur, ornithopod, ankylosaur, ever, they exhibit a clear bias towards small, equi-dimensional sauropod, bird, and mammal material (Molnar, 1991). elements (such as centra and phalanges) that could be trans- In 1991, Mrs. Elizabeth Smith showed one of the authors ported easily (unpublished analysis by REM). Also, the fossil (REM) an opalized cast of a tooth. The specimen had been might have suffered slight abrasion during collection for it was found that year by Mr. Edward Long in the Albian opal-bearing recovered after being separated from the clay matrix in a tum- rocks of the Griman Creek Formation exposed in a mine at bling tank. Smith (1999:48) noted that surface detail of many ``Vertical Bill's'' claim in the Three-Mile Field, Lightning of the larger bone specimens from Lightning Ridge is Ridge, New South Wales (Smith, 1999). Analysis of its char- ``. poorly preserved, pitted and corroded.'' Some of the pit- acters indicates that it is the replica (natural cast) of a tooth of ting in the surface of the tooth might be the result of transport a vertebrate, hitherto unrecognized in Australian, Aptian±Al- prior to burial or reworking. Acid attack from ground water or bian terrestrial faunas. Nor does it appear to belong to any passage through a carnivore's gut also might have damaged aquatic vertebrate, either ®sh (elasmobranchs, actinopterygians, some areas of the tooth. We conclude that the pitted areas on or sarcopterygians) or marine reptile (ichthyosaurs or saurop- the crown probably re¯ect the general morphology but not the terygians), known from Australia at this time. Comparisons de- original surface pattern, and that only a few areas appear to scribed below suggest it is most likely the tooth of a synapsid. have been modi®ed by abrasion. Its greatest similarities in derived characters are to upper mo- lariform teeth of dryolestid mammals. Description The cast of the occlusal surface of the crown has a triangular MATERIALS AND METHODS outline (Fig. 1). On the basis of comparisons with dentitions of Initially, its discoverer, Edward Long, kindly permitted casts a variety of vertebrates, detailed below, we ®nd the closest re- to be made of the fossil and distributed for study (e.g., Queens- semblance of this tooth are to upper molars of dryolestid mam- land Museum, Brisbane, fossil collection, QM F33564). Eliza- mals and use them to provide a basis for orientation of AMF beth Smith (1999:23) published a photograph of the specimen 118621. Following this working hypothesis, the two large cusps (``Vertical Bill's Beastie''). The original specimen was curated de®ne the transverse axis of the crown with the largest cusp in the collections of the Australian Museum, Sydney (AMF buccal. From the lingual side of the large buccal cusp a low 118621). Although the specimen apparently represents a new ridge extends to the base of the smaller lingual cusp. If refer- taxon, we refrain from assigning it a formal binomial because ence to the dryolestids proves correct, the buccal cusp would of the uncertainties surrounding its identi®cation and our lim- be the stylocone and the lingual cusp the paracone. A weak 232 CLEMENS ET AL.ÐAN ENIGMATIC SYNAPSID? TOOTH 233 FIGURE 1. Natural cast of AMF 118621 from Lightning Ridge, New South Wales, Australia. A, anterior view; B, occlusal view. Illustrations based on a cast of the specimen. crest extending from the paracone toward the presumed metas- groups that have been documented in the Mesozoic faunas of tylar region bounds a posterior basin. A deeper basin is present Australia and other southern continents (by REM and GPW). anterior to the main transverse ridge and is bounded anteriorly Similarities were found only with the dentitions of ``notosuchi- by a low crest. Clear evidence of a parastylar cusp is lacking, an'' crocodyliforms. These taxa either possess simple, canini- but the surface of the specimen in this area appears to have form teeth or multicusped teeth that can be mistaken for those been slightly abraded. of synapsids (Wu and Sues, 1996; Gomani, 1997; Buckley and When viewed laterally, the walls of the base of the crown Brochu, 1999). In fact, one taxon based on teeth originally iden- are convex with, apparently, a distinct boundary between the ti®ed as mammaliaform was later shown to be a crocodyliform crown and root(s). Below the crown there is a small, irregular (Carvalho, 1994). In view of these morphological similarities, pyramidal projection, which is interpreted as a postmortem ar- the possibility that AMF 118621 derives from a crocodyliform tifact and not a trace of the root(s). The morphology of the rim must be explored. of the base of the crown suggests that at the time of death either If the ridge connecting the main cusps of AMF 118621 was the root(s) had not been formed or had been resorbed. not oriented transversely but oriented anteroposteriorly, the In the orientation assumed here, AMF 118621 is 12.2 mm in specimen shows some similarities to molariform teeth of Ma- maximum anteroposterior length, and 14.0 mm in maximum lawisuchus mwakasyungutiensis from Malawi (Gomani, 1997) width. and Candidodon itapecuruense from Brazil (Carvalho, 1994), both of Early Cretaceous age. Speci®cally, the molariform teeth Comparisons of these ``notosuchians'' have a single major central cusp, a Vertebrates Other than Synapsids The specimen was very small posterior cusp, and a minor anterior cusp, aligned compared with teeth of elasmobranchs, actinopterygians, sar- approximately anteroposteriorly. Although AMF 118621 has copterygians, temnospondyls, and reptiles, particularly those two obvious cusps, each situated at an end of the crown, a third FIGURE 2. Stereophotograph of AMF 118621 in occlusal view from Lightning Ridge, Australia. 234 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 23, NO. 1, 2003 minor anterior cusp is not evident. Likewise, the low crests bial crest and the principal lingual cusp, posterior to which lies described above could be interpreted as marginal cingula as a concavity. In contrast, the bulbous in¯ated cusps of AMF described for ``notosuchian'' teeth, but the low crests in AMF 118621 show no evidence of anteroposterior crests or a large 118621 are not multicusped and do not extend anterior to the central concavity. principal cusp, as they do in ``notosuchian'' teeth. Furthermore, A recently described traversodontid from the Late Triassic of its crown is far more in¯ated than the laterally compressed mo- Belgium, Habayia halbardieri, resembles AMF 118621 in gen- lariform teeth of these ``notosuchians.'' Finally, these crocod- eral morphology and the bulbous morphology of its cusps (God- yliform teeth are substantially smallerÐabout half the sizeÐ efroit, 1999). A striking dissimilarity of this and some other than AMF 118621. Other molariform ``notosuchian'' teeth of European, Late Triassic traversodonts is their minute size; the more comparable size, such as those of Chimaerasuchus par- transverse widths of the postcanines of Habayia are less than adoxus from the Cretaceous of China (Wu and Sues, 1996), are one millimeter. ovoid rather than triangular in occlusal view and contain three, Of the gomphodonts, the tritylodonts were the only clade anteroposterior rows of cusps, unlike AMF 118621. known to have diversi®ed during the Jurassic, particularly in As far as we can determine, none of the known ``notosuchi- eastern Asia and North America. Recently, Early Cretaceous ans'' has complex cheek teeth closely resembling AMF 118621.
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