BIRD AND DINOSAUR TRACKS FROM THE LOWER CEDAR MOUNTAIN FORMATION (BARRIMIAN-APTIAN) OF UTAH, USA

In recent years the Cedar Mountain Formation of Utah has become famous for vertebrate remains. Only a few tracksites have been reported, probably due, in large part, to the fact that the unit is made up largely of mudstones. Most tracks have mainly been the usual iguanodontid-dominated assemblage typical of the early Cretaceous. However, in recent years, a more diverse track assemblage has been reported.

Just outside Arches National Park, bird tracks were discovered in the uppermost Yellow Cat Member (Barrimian) in the summer of 2005. The tracks are preserved on three fallen blocks, although distinctive lithology allowed identification of the track horizon. All the tracks are preserved in concave epirelief on a thin fine-grained sandstone layer at the top of a metre thick erosional based coarser sandstone with a rippled top surface, which may be of crevasse splay origin. The tracks are approximately 30-40mm in length with no traces of interdigital webbing. These are the earliest known bird tracks in North America.

Near the base of the Ruby Ranch Member (Aptian) in Arches National Park itself, a diverse track assemblage is known, including the tracks of sauropods, and possibly didactyl theropods, e.g. dromaeosaurids. These tracks are very deeply impressed in what was evidently a wet muddy cohesive substrate. In addition to these, at a lower stratigraphic level, some enigmatic traces are also preserved, on a ripple-marked sandstone layer tracked by small theropods. These enigmatic traces consist of sets of elongate scratch marks radiating out from a central area and forming ovals or paired crescents 8-10cm in maximum dimension. These have been variously interpreted as tool marks, invertebrate traces or pterosaur feeding traces.

These tracksites provide a more complete view of the early Cretaceous vertebrate assemblages in western North America during the Early Cretaceous. This part of the Cedar Mountain Formation is thought to have been deposited on the margins of a large lake as channels, crevasse splays and lakeshore/floodplain sediments.