New theropod display arena sites in the Cretaceous of North America: Clues to distributions in space and time

Previously-unknown large scale scrapes attributed to Cretaceous theropod dinosaurs from the Naturita Formation (formerly the Dakota Sandstone) of western Colorado were recently named as Ostendichnus bilobatus and interpreted as evidence of “nest scrape display,” a type of courtship behavior previously known only in extant avians. However, comparatively little is known of the morphology, distribution and preservation potential of either modern or ancient nest scrapes. Further study of the initially described samples combined with new discoveries brings the total number of known in Colorado sites to five, one with two scrape-bearing levels. Combined, these sites preserve a total of more than 100 recognizable scrapes from all these sites. We also identify the first O. bilobatus-like scrape from the Cretaceous of Canada. Although variable, a majority of the large sample of Colorado scrapes have the diagnostic characteristics of O. bilobatus, with two lateral troughs separated by a median ridge, and are sufficiently distinct to allow measurement of salient features such as scrape size, depth, and median ridge and average trough width. These provide data which indicate that theropod nest scrapes range from ∼50 to ∼200 cm in length and up to ∼25 cm in depth, presumably indicate dinosaurs of different sizes, and variable time and energy spent in creation of individual scrapes. Scrape orientations are highly variable. Three of the sites occur at about the same stratigraphic level, although they are ∼3.0-∼6.0 km apart, suggesting that display arena sites may have been large, involving many dinosaurs and repeat activity in sequential breeding seasons. High-precision U-Pb zircon analyses by the CA-ID-TIMS method from a volcanic ash bed above the scrape bearing levels at Roubideau Creek (Colorado) yielded a weighted mean 206Pb/238U date of 97.689 ± 0.037 Ma (2σ internal error) and indicate a Cenomanian age for O. bilobatus scrapes in western Colorado.