Cranial osteology of Exostinus serratus (Squamata: Anguimorpha), fossil sister taxon to the enigmatic clade Xenosaurus

Abstract

Within the squamate clade Anguimorpha, Xenosaurus is an enigmatic taxon combining several apparently primitive features with a highly specialized set of autapomorphies. This combination makes the fossil record along the Xenosaurus stem particularly important for resolving the relationships of Xenosaurus with other anguimorphs, and between species of Xenosaurus. Exostinus serratus Cope, 1873 from the Oligocene of the western United States is currently hypothesized to be the immediate sister taxon to Xenosaurus. The cranial osteology of this pivotal taxon is described here for the first time from all known material, using high-resolution X-ray computerized tomography (CT) scanning to visualize individual elements. Exostinus serratus displays a mosaic of ancestral anguimorph features that are transformed in Xenosaurus, as well as unique synapomorphies shared with Xenosaurus. The region of the external nares is less transformed than in Xenosaurus, as are the general proportions of the bones surrounding the cartilaginous nasal capsule. However, the forms of the teeth and of the osteodermal sculpture, as well as several details of maxillary and mandibular morphology, are distinctly Xenosaurus-like. Several autapomorphies are also present, including an abbreviated tooth row and an unusually wide palatal shelf of the maxilla. An understanding of the ways in which the non-ossified tissues of the head influenced the development of the bones is crucial to interpreting their morphology. Furthermore, subtle anatomical features often provide important comparative information, and are emphasized herein.

A Phylogenetic Approach to Ontogeny and Heterochrony in the Fossil Record: Cranial Evolution and Development in Anguimorphan Lizards (Reptilia: Squamata)

“…For the current dataset, although a number of very young and large adult X. platyceps were available, no subadult skeletons could be located. Xenosaurus skeletons in general are rare, although considerably less so than skeletons of Shinisaurus , which had to be excluded from this study (Bhullar, ). Two fossils were selected for evaluation based on their uncertain ontogenetic stages.…”

Variation in the Position of the Jugal Medial Ridge Among Lizards (Reptilia: Squamata): Its Functional and Taxonomic Significance

“…This condition is found in Restes rugosus (Fig. A), Exostinus serratus (Bhullar, ), and Xenosaurus newmanorum (Bhullar, ; Fig. B).…”

An assemblage of lizards from the Early Cretaceous of Japan

“…These dental features distinguish Asagaolacerta from other Kuwajima Formation lizards and from described Late Jurassic and Early Cretaceous taxa from Europe (e.g., Hoffstetter, 1967;Evans and Searle, 2002), North America (Nydam, 2002;Nydam andCifelli, 2002a, 2002b), and China , 2010. Asagaolacerta tricuspidens resembles North American Late Cretaceous borioteiioids like Obamadon gracilis, Tripennaculus sp., Socognathus brachyodon and Chamops segnis (Longrich et al, 2012) in having tricuspid teeth, but differs in lacking any swelling of the tooth bases, and in having tooth crowns that are proportionally smaller in relation to the tooth bases.…”

“…For abbreviations, see Material and Methods. and Norell, 2000) in having proportionally more gracile jaw, proportionally longer and narrower frontals and parietals, and absence of parietal foramen; resembles the Late Cretaceous Carusia intermedia (Borsuk-Białynicka, 1985) and Exostinus serratus (Bhullar, 2010) in having frontals that are anteriorly narrow and posteriorly broad, with strong sub-olfactory processes, in the strong concavity of the maxillary narial margin, and, for Carusia but not Exostinus, in the shape of the dentary coronoid process, but Kuroyuriella differs from both in that the frontals are paired rather than fused and lack coarse tubercular sculpture, there is no sculpture on the maxilla, the parietal is of a different morphology (square and largely excluded from the upper temporal fenestra by postorbitofrontal facets, no parietal foramen) and sculpture pattern (low relief rather than coarse and tubercular); resembles Late Cretaceous genera Parmeosaurus scutatus and Hymenosaurus clarki (Gao and Norell, 2000) in gracile dentary with straight long axis, but differs from former in having uni-rather than tricuspid teeth, free posterior edge on dentary coronoid process, greater orbital emargination of frontals, FIGURE 4. Kuroyuriella mikikoi gen. et sp.…”