A NEW ICHNOSPECIES OF AQUATILAVIPES FROM THE ALBIAN-CENOMANIAN DAKOTA FORMATION OF NORTHEASTERN UTAH

Mid-Cretaceous tracks of a new ichnospecies of Aquatilavipes have been recovered from the uppermost Dakota Formation near Dinosaur National Monument, Utah.  The footprints are preserved in current-ripple laminae of fine-grained quartz sandstone capped by a very-fine silt layer. The laminae were formed in a marginal-marine environment.

The horizon contains eight tracks that range in size from 22.5 mm to 31.3 mm in length and from 28.1 mm to 38.7 mm in width. The footprints have an average divarication of 118 degrees and a width 32 percent greater than length. These two measurements distinguish the tracks from that of a small dinosaur, as dinosaurs typically have a divarication of less than 100 degrees and a length greater than width. All eight tracks appear similar in morphology but a step pattern of four differs slightly in size, suggesting the presence of more than one bird within the horizon.  Additional partial bird footprints and three parallel slashes made by the claws of what appears to be an animal similar to Aquatilavipes are found approximately 60 cm below the trackway horizon.  The lower horizon has yet to be described or recovered from the field.

The newly described bird tracks are only the fifth locality identified as the ichnogenus Aquatilavipes and the first in the United States. The Utah tracks increase the range for the paleobiogeography of Aquatilavipes and create a greater understanding of the environments in which the ichnogenus occurs. The trackway is the first evidence of vertebrates from the Dakota Formation of Utah.  Aquatilavipes is among the earliest shorebird traces known.