Taphonomy, age, and paleoecological implication of a new Pachyrhinosaurus (Dinosauria: Ceratopsidae) bonebed from the Upper Cretaceous (Campanian) Wapiti Formation of Alberta, Canada

Abstract

The Grande Prairie region (Alberta, Canada) includes some of the richest Cretaceous fossil sites in North America, including the recently described bonebed of Pachyrhinosaurus lakustai at the Pipestone Creek locality. Here we describe a new multi-taxa, ceratopsian-dominated bonebed from the region, integrating taphonomic, radioisotopic, and paleoecological data. The bonebed can be traced for 107 m and has been excavated over an area of 40 m2 with an average bone density of 30–50 elements/m2. The new bonebed occurs within Unit 4 of the upper Campanian Wapiti Formation, and 40Ar/39Ar dating provides an age of 71.89 ± 0.14 Ma, thus making the site equivalent in age to the upper Drumheller Member of the lower Horseshoe Canyon Formation of central Alberta. About 88% of vertebrate remains are ceratopsian, and dromaeosaurid, hadrosaurid, troodontid, and tyrannosaurid remains have also been identified. Juvenile material, although scarce, indicates an assemblage of individuals of different ages. Specimens showed no strong preferred two-dimensional orientation but are clearly sorted vertically. Taphonomic and sedimentological interpretation support a complex pre-burial history of preserved elements as well as a depositional setting characterized by persistent waterlogged conditions as those typical of large oxbow lakes or marshy/swampy areas, as well as lacustrine settings within an alluvial plain. Being located more than 450 km inland from the paleo-coastline, the new bonebed represents one of the farthest-known inland occurrence of centrosaurines in North America, further supporting the presence of large aggregations of ceratopsian far from the coastal lowlands of the Western Interior Seaway.