FIRST REPORT OF PTEROSAUR TRACKS FROM THE DAKOTA GROUP (CRETACEOUS) OF COLORADO

Although more than 60 fossil footprint localities are known from the ‘middle' Cretaceous (Late Albian-Early Cenomanian) Dakota Group, ‘Dinosaur Freeway' of Colorado and northeastern New Mexico, the vast majority yield only ornithopod (Caririchnium), theropod (Magnoavipes) and crocodilian footprints. Bird tracks (Ignotornis) are known from only one site in this region, and from another site in northeastern Utah.

We herein report the first ichnological evidence of pterosaurs, from the Dakota Group. Four specimens were recovered from two adjacent sites at John Martin Reservoir in southeastern Colorado. The traces include two relatively complete pes tracks associated with swim traces and two incomplete pes swim traces. All tracks are associated with layers heavily bioturbated by invertebrates.

Several of the ten known John Martin track sites also yield crocodile swim traces, consisting of relatively large, predominantly three-toed toe impressions up to 24 cm long and 17 cm wide. By contrast the pterosaur tracks are small (7 cm long and 3.5 cm wide) with four distinct toe impressions. Two of the footprints run continuously into four elongate parallel scrape marks (swim traces).

It is rare to find such elongate four-toed swim tracks with identifiable complete footprints, since it requires the trackmaker to make two types of traces (swimming and ‘walking') in a small area. However, this small Dakota assemblage is very similar to a well-described pterosaur swim track assemblages from the Upper Jurassic of Utah, also associated with invertebrate traces. Thus, we infer that the pterosaurian trackmakers were likely feeding while floating in shallow water and using their feet for propulsion.