desert drepanosaurs – svp abstract 2013

From the abstract
Chure et al. wrote: “Drepanosaurids are enigmatic diapsids from the Late Triassic of Asia, Europe, and North America. Here we report on a new form from the Nugget Sandstone based on multiple three-dimensional, articulated skeletons with disarticulated skulls, found lying side-by-side.  The new taxon combines diagnostic features of other drepanosaurids (Dolabrosaurus, Megalancosaurus, and Drepanosaurus.) The Nugget form is most closely related to Drepanosaurus. Synapomorphies with that genus include 1) short, plate-like ulna, 2) greatly elongated radiale and ulnare replacing the ulna and forming part of the forearm, 3) hypertrophied manual ungual 2. Striking autapomorphies of the new taxon are 1) large maxillary and dentary teeth reminiscent of Trilophosaurus in being tall, much wider than long, and bearing small labiolingually arranged apical cusps, 2) manual digit 1 opposable and bearing hypertrophied ungual at least as large as that on 2, 3) large, well defined pleurocoels on posterior dorsal centra, 4) dorsal vertebrae prezygapophyses fused into a narrow, midline process at the neural spine base. Striking dimorphism is seen in the pes, with an abbreviated opposable digit I in some specimens but not others. This dimorphism is not ontogenetic but may be sexual. Drepanosaurids are generally viewed as arboreal and chameleon-like, but many specializations in the Nugget taxon (and Drepanosaurus) are similar to adaptations for burrowing and digging in extant small mammals and suggest a similar habit. In addition, these are the first drepanosaurids from an erg environment, indicating drepanosaur ecology was more diverse than previously envisioned. The age of the Early Jurassic erg deposits in western North America is poorly constrained due to lack of age diagnostic fossils or datable crystals. The Saints and Sinners Quarry is approximately 55m above the base of the eolian part of the Nugget, within interdunal sediments situated between large cross-bedded eolian packages. Elsewhere, drepanosaurids are restricted to the Late Carnian through Late Norian. These Nugget drepanosaurids suggest that either a significant part of the formation is Triassic or that the group extended into the Jurassic. In either case, the Nugget material is likely the geologically youngest record of the group.”

Notes:
Earlier we established that the elbow bone was a previously overlooked sesamoid common to many drepanosaurids, not the displaced ulna. Lots of outgroups have the sesamoid and it gets larger closer to Drepanosaurus (Fig. 1). Otherwise it’s exciting to think about seeing these new desert drepanosaurs.

References
Chure D, Britt B, Engelmann G, Andrus A and Scheetz R 2013. Drepanosaurs in the desert: multipled skeletons of a new dreapanosauris from the eolian nugget sandstone (?Late Triassic – Early Jurassic), Saints and Sinners Quarry, Utah: morphology, relationships, and biostrategraphic implications.