THE BIOSTRATIGRAPHIC AND PALEOECOLOGICAL SIGNIFICANCE OF A NEW DREPANOSAUR FROM THE TRIASSIC-?JURASSIC NUGGET SANDSTONE OF NORTHEASTERN UTAH

The Saints and Sinners Quarry, in the Nugget Sandstone near Dinosaur National Monument in northeastern Utah, has yielded thousands of bones of a new theropod dinosaur. This locality occurs in an interdunal interval well within the dominantly eolian upper part of the Nugget Ss. Recently, we have also recovered remains of small, non-dinosaurian vertebrates, including a remarkable, articulated, partial skeleton that is unmistakably identifiable as a drepanosaur. Drepanosaurs are a small, enigmatic group of reptiles characterized by a number of bizarre and highly distinctive features throughout the skeleton.

Drepanosaurs are geographically widespread across Europe, North America, and Asia, but are temporally restricted to the Triassic (Late Carnian through Late Norian), with the highest stratigraphic occurrence in the Rock Point Member of the Chinle Fm. The new taxon is stratigraphically higher than this, occurring about 55 meters above the base of the eolian part of the Nugget Ss. This represents a range extension for the Drepanosauromorpha, but whether that extension is still higher in the uppermost Triassic or into the Lower Jurassic cannot be determined without other evidence. The presence of the ichnotaxon Brachychirotherium in the basal beds of the Nugget in the study area and elsewhere has been interpreted as indicating a Triassic age for at least the lower part of the Nugget. The upper, eolian beds of the Nugget Sandstone have been considered to be part of the Lower Jurassic, although there is no strong evidence for this, but the presence of drepanosaurs in the Saints and Sinners Quarry suggests that at least the lower part of the eolian beds could be Triassic.

The presence of drepanosaurs in an interdunal facies within the eolian depositional environment of the Nugget also presents a paleoecological problem. Previously described drepanosaur taxa do not occur within eolian depositional environments, and morphologic characteristics of the animals have been interpreted as indicative of arboreal habitat. Possible explanations for these apparent inconsistencies may be that drepanosaurs were more diverse taxonomically and ecologically than the record indicates, or that the principal adaptations of the group were correlated with common aspects of environments that appear strikingly dissimilar to us.