TAPHONOMY OF A UNIQUE MULTIGENERATIONAL TRICERATOPS BONEBED FROM EASTERN WYOMING (USA): NEW INSIGHTS FROM A MULTI-PROXY PERSPECTIVE

Bonebeds of chasmosaurine ceratopsian dinosaurs are surprisingly rare compared to the high relative abundance of isolated material of this group in the fossil record. Here, we describe a large bonebed of Triceratops horridus with at least five individuals, abundant postcranial material preserved (>30%) and skeletons displaying features of subadult and adult ontogenetic stages. Hence, this fossil locality can shed new light on the osteology, ontogeny and possible gregarious behaviour of this iconic horned dinosaur. Since 2013, Naturalis Biodiversity Center has been excavating this bonebed, which is situated in eastern Wyoming (USA) and comprises fluvial sediments from the lower part of the Upper Maastrichtian Lance Formation. Five field campaigns and a 3D survey including drone photogrammetry resulted in a high-resolution 3D model of the excavation site, incorporating the spatial position of >1100 bone elements and >500 sediment samples. Integrating this 3D framework with a multi-proxy study of osteohistology and sedimentological and geochemical analysis allows for a detailed taphonomic reconstruction. The bone layer is c. 0,5 m thick and stretches over an area of c. 200 m2, displaying disarticulated but associated skeletal remains with little evidence of scavenging. Grain-size, loss-on-ignition, XRF and palaeobotanical data suggests a paludal environment that experienced two separate flooding events. We propose that the Triceratops individuals in this bonebed were mired, possibly as a group, and post-mortem rapidly buried by two crevasse splays of coarse silt to very fine sand. This explains both the lateral bone distribution and the rare preservation of small postcranial elements such as unguals.