Chamops segnis Marsh, 1892a

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 : 352

Chamops segnis Marsh, 1892a

Figure 8 A-D

A dentary, AMNH 8486 from Clayball Hill, is referred to Chamops segnis on the basis of its resemblance to the type dentary. The jaw is broken posteriorly and contains 13 worn and broken teeth (fig. 8A, B). The Meckelian fossa is open and extends anteriorly to the second most anterior tooth. The roof of the Meckelian fossa consists of a thick border which considerably widens anteriorly in the form of a lingual expansion when seen in dorsal view. The lower border of the dentary is only slightly curved. The teeth are subpleurodont and tricuspid. The tricuspid nature of the teeth cannot be made out in AMNH 8486, because of excessive wear. A number of other dentary fragments, however, indicate that the teeth are similar to those referred to the genus by Estes (1964). The tooth row is heterodont; the anterior five or six teeth are anteroposteriorly compressed, whereas the posterior ones are less so and possess rounded tooth bases. The linguolabially compressed crowns have a prominent central cusp with smaller anterior and posterior accessory cusps delineated by short but distinct ridges. The anterior cusps are larger than the posterior cusps at the anterior end of the dentary; the anterior and posterior cusps are the same size toward the posterior end. Faint vertical striations are more prominent on the lingual than the labial side of the crowns. Labially the dentary is smooth except for the presence of prominent mental foramina. The foramina are situated closer to the dorsal edge of the dentary. T-he six foramina extend posteriorly to the ninth or tenth tooth from the front.

Chamops maxillae, which vary considerably in size, are present in the collection. AMNH 8487 from Clambank Hollow, is a partial tooth-bearing border of a maxilla. It corresponds closely to UCMP 46033, described in detail by Estes (1964, p. 106). AMNH 8488, also from Clambank Hollow, is considerably more robust than AMNH 8487 and probably corresponds to the more massive dentaryjust described (fig. 8C, D). The anterior part of the maxilla, bearing four teeth, is preserved. The teeth are quite large and show signs of wear. The anterior maxillary teeth are directed backward and are slightly recurved but are otherwise morphologically similar to corresponding teeth on the dentary.

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.