Ankylosuchus chinlegroupensis: a new Badly Named Doswelliid Archosauromorph From Upper Triassic Texas

A new doswelliid archosauromorph from the Upper Triassic of West Texas.

Authors:

Lucas et al

Abstract:

Ankylosuchus chinlegroupensis is a new genus and species of heavily-armored archosauromorph from the Otischalkian Colorado City Formation of the Chinle Group in Howard County, West Texas. The incomplete holotype skeleton consists of cranial and pelvic (?) elements, vertebral centra, a partial limb shaft and numerous osteoderms. The skull elements reveal a thick, heavily armored braincase and skull roof with parasagittal crests. The relatively short sacral vertebrae suggest an animal less than 1 meter in total body length, not including the tail. The morphology of the osteoderms does not match any currently known armored archosauromorph, but it is most similar to doswelliids. Most of the osteoderms possess large, closely packed pits that form no obvious pattern. Some osteoderms have raised, linear ridges running across them and others have anterior laminae with faint patterning on the articular surface. Some of the osteoderms are tightly sutured to each other via digitate sutures; all are relatively thick. The patterning of the ostoderms matches well with that of doswellids in being coarse, deeply incised and mostly composed of equal-sized pits and in the possession of anterior laminae. Even so, these osteoderms are readily distinguished from those of Doswellia, the only doswelliid previously reported from the Chinle Group, by their coarser pitting, greater thickness and (at least in some osteoderms) fusion with laterally adjacent osteoderms along their mutual sutural boundaries. A. chinlegroupensis is derived from the oldest strata of the Texas Chinle Group, the Otischalkian, whereas the genus Doswellia is known from the Chinle Group in Texas, New Mexico and Utah, in strata of Otischalkian-Adamanian (late
Carnian) age. Doswelliids are very rare, but visible components of global Triassic faunas. They include Tarjadia (= Archeopelta) from the Berdyankian (Ladinian) of Argentina and Brazil, Doswellia from the Otischalkian-Adamanian of the American Southwest and the Otischalkian of the Newark Supergroup in the eastern USA, and now Ankylosuchus from the Otischalkian of West Texas.