WHY BONE BEDS ARE BETTER INDICATORS OF HOW CERATOPSIDS LIVED THAN HOW THEY DIED

Gregarious behavior in Ceratopsia, as inferred from bone beds (BBs), or the close association of multiple skeletal remains, was a shared trait throughout their history (Yinlong, early Late Jurassic to Triceratops, latest Cretaceous). Although many ceratopsids are known from typically monodominant BBs, centrosaurine BBs are generally larger (>MNI), more numerous, and found over larger geographical ranges than chasmosaurine BBs. Almost every derived centrosaurine taxa is known from multiple BBs.

Numerous detailed sedimentological and taphonomic analyses have been conducted on BBs of the late Campanian Albertan centrosaurs Centrosaurus (20+), Coronosaurus (2), Pachyrhinosaurus (2), and Styracosaurus (1); however, not all of the data has been published. Although the BBs are found in different formations, their entombing lithosomes are invariably channel sandstones or overbank mudstones reflecting the alluvial to coastal plain environments in which they were living during the Campanian transgressive phase of the Western Interior Seaway (WIS). Material in the BBs is always disarticulated with the exceptions of the highly fused skulls of Pachyrhinosaurus, or the fused nasals and supraorbitals of the other taxa. Abrasion is typically moderate although the ends of limb elements are broken, and the smaller, lighter skeletal elements are usually recovered in relatively reduced numbers suggesting winnowing. Shed large theropod teeth are common (up to ~10%) in the BBs, but tooth-marked bones are rare (~<5%) suggesting a postmortem scavenging event with more carcasses than the theropods could process. The gestalt of the taphonomic indicators suggests that these dinosaurs lived in gregarious herds composed of very young to mature individuals that died together. Their corpses were then subjected to scavenging, disarticulated by decay, and then redeposited relatively close to the point of death by a subsequent high water event. Episodic, large scale flooding of the low-lying flood plain adjacent to the WIS is a plausible mechanism for drowning large numbers of herding animals, although a definitive sedimentological signature has yet to be determined. Other factors, including illness or drought, cannot be discounted, but definitive evidence for these mechanisms for ceratopsid mass death is rarely recovered.