THEROPOD COURTSHIP BEHAVIOR PRESERVED BY MICROBIAL MATS, CRETACEOUS DAKOTA SANDSTONE, COLORADO, USA

The Cretaceous (Cenomanian) Muddy Formation exposures at Dinosaur Ridge, Morrison, Colorado preserve a diverse suite of well-preserved vertebrate, invertebrate, and microbial traces which occur alongside one another in coastal plain and tidally influenced deposits. Recent studies at Dinosaur Ridge describe two Theropod mating display traces (Ostendichnus bilobatus), which are characterized by large bilobate grooves up to 2 meters in length. The trace-making behavior is analogous to seasonal mating display behaviors in extant ground-nesting birds. We report the occurrence of an additional Ostendichnus in the type section at Dinosaur Ridge within a very fine-grained sandstone, 4.5 m below the Muddy Formation-Mowry Shale contact. The mating display trace— studied using direct observations, high-resolution digital photographs, and 3D photogrammetric models— is composed of two bilobate depressions measuring 116 cm long, 76 cm wide, 15.42 cm deep, and contain 17 individual scratch marks. Various scratch directions suggest the organism rotated or pirouetted around the display trace. The sedimentary surface also includes two generations of fossil epibenthic microbial mats which likely aided preservation of Ostendichnus via microbe-mediated calcite and hematite precipitation and record the disturbance and mechanical redistribution of the original mat by dinoturbation, which was followed by overgrowth and stabilization. Microbial mat fragments and chips were concentrated in debris heaps along the posterior of Ostendichnus and individual scratches. Erosional pockets bordering the trace likely propagated after the trace making behavior before being overgrown by a second epibenthic mat generation. Overgrowth of Ostendichnus is characterized by a second, thin (0.5-1.5 cm) epibenthic mat, subsequently overprinted by Magnoavipes, Rhizoliths, and a diminutive Rosselia-Archaeonassa ichnocoenosis. Our results indicate the trace was produced in a moist to water-undersaturated substrate within a sandy supratidal flat. Lateral heterogeneities in the bedding plane bioturbation index and size-diversity index derived from invertebrate traces in both epibenthic mat generations suggest a depositional environment subjected to seasonal variation in sediment reworking and deposition.