Exostinus, Cope, 1873

Sahni, Ashok, 1972, The vertebrate Fauna of the Judith River formation, Montana, Bulletin of the American Museum of Natural History 147 (6), pp. 319-416 : 355

cf. Exostinus sp.

Figure 8E, F

Specimen AMNH 8498 has been referred tentatively to Exostinus (fig. 8E, F). It resembles the iguanid Paraderma in some features, particularly in the relatively fewer number of teeth, in the general shape of the maxilla, and in the sculpturing of the osteodermal scutes. Assignment to Exostinus is based primarily on the morphology of the tooth bases which are smooth and lack the prominent infolding present in parasaniwids.

Most of the maxilla, except for the dorsal edge and the posterior elongation, is present. Externally, the nasal process of the maxilla rises steeply and then gently slopes to the posterior. Ventral to the five superior alveolar foramina present, the external surface of the maxilla is smooth. Dorsal to the foramina, a few large osteodermal scutes are separated by unsculptured sinuous depressions. The osteoderms are polygonal in shape and extend anteriorly to the nasal process where they accentuate the steep rise of the process. The ornamentation of the scutes is fairly regular and different from that found in E. Iancensis and E. serratus . It consists of anteroposteriorly elongate pits separated by irregular and "hummocky" ridges.

Internally, only the tooth bases of five teeth are present. The tooth bases are very unlike those of the Cretaceous iguanids Parasaniwa and Paraderma in that they are neither expanded nor externally folded into a number of ridges. Eight or nine teeth are estimated to have been originally present in the maxilla; this is far fewer than the number found in species of Exostinus , but is comparable with the number found in the maxilla of Paraderma .

DIscUSSION: The Judith River Formation material of Exostinus falls within the intraspecific range of variation of E. lancensis from the Maestrichtian. The species was originally described by Gilmore (1928, p. 23) and tentatively placed in the genus Exostinus . The genus was proposed by Cope (1873b) on the basis of a specimen consisting of a frontal, jugal, and dentary with teeth. The type was obtained from the Oligocene "Oreodon beds" of the White River Formation, Colorado. At that time Cope made no comments on the familial position of the genus. Later Cope (1900) compared it with Xenosaurus grandis and suggested that Exostinus should be placed in the Xenosauridae . Gilmore (1928), although realizing the evidence was inconclusive, placed the genus in the Iguanidae . McDowell and Bogert (1954, p. 32) clearly demonstrated the xenosaurid affinities of Exostinus , although they were skeptical as to the relationship of the Cretaceous species. Estes (1964) accepted their view regarding the placement of Exostinus in the Xenosauridae and established E. lancensis as a valid species. He also synonymized Harpagosaurus parvus ( Gilmore, 1928, p. 156) and Prionosaurus regularis ( Gilmore, 1928, p. 159) with E. lancensis .

The genus ranges in age from the Campanian through the Oligocene. E. rugosus was described from the Paleocene Polecat Bench Formation of Wyoming by Gilmore (1942), who was uncertain as to the generic identification. Estes (1965, p. 105) restudied the Princeton material and confirmed Gilmore's tentative assignment to the genus. The genotypic species from the Oligocene of Colorado, E. serratus , is found also in Wyoming and adjoining states and is similar in many respects to Recent Xenosaurus . The ancestry of Xenosaurus can be traced to the Cretaceous E. lancensis . The Paleocene E. rugosus is probably on an evolutionary side line as E. serratus appears to be closer to E. lancensis than to the Paleocene species.

Figures

FIG. 8. A. Chamops segnis. AMNH 8486, labial view of left dentary. B. Lingual view. Both x 5. C. AMNH 8488, lingual view of left maxilla. D. Labial view. Both x 5. E. cf. Xenosauridae, AMNH 8498, labial view of left maxilla. F. Lingual view. Both x 5. G. Peltosaurus piger, AMNH 8495, lingual view of left maxilla, x 5. H. AMNH 8494, lingual view of right dentary, x 5. I. Paraderma bogerti, AMNH 8504, lingual view of right maxilla. x 5. J. Parasaniwa wyomingensis, AMNH 8499, lingual view of left dentary, x 5. K. Leptochamops denticulatus, AMNH 8491, lingual view of right maxilla, x 5. L. Exostinus lancensis, AMNH 8497, lingual view of right maxilla, x 5. M. Deinodon horridus, AMNH 8513, posterior view of isolated tooth. N. Lateral view. Both x 13. 0. AMNH 8515, lateral view of anterior dentary tooth. P. Posterior view. Both x 13. Q. AMNH 8514, posterior view of?premaxillary tooth. R. Lateral view. Both x 12. S. Paronychodon lacustris, AMNH 8522, lingual view of isolated tooth. T. Labial view. Both x 12. U. Kritosaurus cf. K. breviceps, AMNH 8525, lateral view of isolated tooth. V. Posterior view. Both x 1 2. W. Procheneosaurus altidens, AMNH 8527, labial view of isolated tooth. X. Anterior or posterior view. Both x 12. Y, Z.?Sebecosuchia, AMNH 8549, views of an solated tooth. Both x 10.