A new theropod dinosaur lek in the Cretaceous Dakota Sandstone (Dinosaur Ridge, Colorado, USA)

Abstract

High-resolution aerial imagery captured by drones document new non-avian theropod mating display scrapes (Ostendichnus) along two stratigraphic levels of the Upper Cretaceous (Cenomanian) Dakota Sandstone at Dinosaur Ridge, Colorado, USA. We describe and identify their morphologies, orientations, and patterns of clustering that include overprinting. Twenty-five additional scrapes are documented on 2 stratigraphic surfaces (24 on surface 2b, colloquially the “Dinosaur Courtship” surface; and 1 on surface 2h). The additional scrapes on surface 2b elevates its status to a fossilized nest display arena or lek. Most of the scrapes have the diagnostic “bilobate” shape, but some are bowl-shaped consistent with the description of Ostendichnus-like traces and contain well-detectable toe claw marks in all but one of the structures. A new means to determine the direction the trace maker faced while creating the scrapes (azimuth) is proposed for Ostendichnus lacking footprints, anterior divergence of claw marks, posterior tapering of troughs, and a sand crescent pile of excavated sediment at the posterior of the trace. Analysis of the Dinosaur Ridge sites and of 3 previously known lek sites in western Colorado—Club Gulch (n = 2) and Roubideau Creek— permit insight into the social behavior of the trace making theropods by comparisons to modern avian analogs, particularly those of lekking, nest scrape display, a combined nest building and display activity, and nest building that involves digging, and― in some cases― reconfiguration using available materials.