VERTEBRATE FAUNA OF THE TRIASSIC-JURASSIC ERG: INSIGHTS FROM AN EOLIAN LAGERSTATTEN (Invited Presentation)

Deposits of the great erg that covered much of what is now southwestern North America and span the Triassic-Jurassic boundary include the Glen Canyon Group and Aztec Sandstone to the south and the Nugget Sandstone to the north. Included within the Glen Canyon Group, amid dominantly eolian units, the fluvial Moenave and Kayenta formations have provided the most abundant and diverse vertebrate faunas from this interval. They include several kinds of dinosaurs and other reptiles; advanced therapsids; amphibians; bony fish and sharks. In contrast, the eolianites, especially of the Navajo and Nugget formations, have a sparse record of vertebrate fossils.

The most widespread and abundant occurrences of vertebrates in the dune sands are traces (tracks of dinosaurs, pterosaurs, other reptiles, advanced synapsids, and possible vertebrate burrows). Vertebrate body fossils are very rare and, with one exception, amount to a handful of isolated specimens of theropod and prosauropod dinosaurs, crocodylomorphs, and tritylodonts. The one exception is the Saints and Sinners (S&S) lagerstatten that has produced abundant bones representing several reptilian taxa, some of which are spectacularly well-preserved.

The S&S quarry in the Nugget Sandstone in northeastern Utah provides the best record of the erg’s vertebrate fauna. Microvertebrates, primarily a sphenosuchian and a drepanosaur (both represented by many articulated skeletons) along with two sphenodontian taxa comprise most of the diversity of the fauna. Abundant bones of a coelophysoid theropod, a few teeth indicative of a larger theropod and a partial skull of a dimorphodontid pterosaur are also present. While some of these animals could have roamed widely, the abundant small vertebrates suggest a resident population in a persistent lake/oasis environment.

The overall vertebrate fossil record of the erg shows ecological segregation within the ecosystem, as the synapsids that left tracks on the dune slip face surfaces are not present in the lagerstatten and the tracks of the taxa found in the lagerstatten are not present on the dune slip faces. The interdunal facies represent a favorable environment within the erg, and the lake a particularly favorable climatic episode. As in today’s ergs, vertebrate life was concentrated in the interdunes.