Morphology and articular configuration of the ceratopsid (Dinosauria: Ornithischia) lower hindlimb as revealed by a specimen from the Upper Cretaceous Dinosaur Park Formation of southern Alberta, Canada.

Ceratopsid dinosaurs, such as Triceratops, are well-known for the striking horns and frills that are ubiquitous among members of the clade. But their postcranium has received little attention by comparison. Many questions persist regarding surprisingly fundamental aspects of the anatomy of the distal part of the hindlimb, such as the proper configuration of the metatarsals and the number and position of the distal tarsals. Here we use a taxonomically indeterminate but nearly complete lower hindlimb of a ceratopsid from the Upper Cretaceous Dinosaur Park Formation of southern Alberta, Canada, originally collected more than a century ago but never previously described, to address some of these uncertainties. The bones were CT scanned and segmented to create digital models that were then positioned based on morphological fit and comparisons with articulated specimens to establish the most plausible arrangement of the lower hindlimb elements. Proximally, the fibula articulated with the posterior surface of the laterally directed cnemial crest of the tibia and the lateral surface of the lateral condyle. The only distal tarsals present, distal tarsals III and IV, articulated with the proximal ends of the corresponding metatarsals, and their proximal surfaces were likely convex and concave, respectively. The metatarsals were closely appressed to one another throughout their lengths. The phalanges are asymmetric in varying ways and to varying degrees. Unguals of some ceratopsids have a visible shelf between the shaft and blade on the posterior side, while others, including the specimen described here, do not. The anatomical and arthrological insights obtained in this investigation of the distal part of the ceratopsid hindlimb will inform future biomechanical studies and contribute to comparisons of crural and pedal osteology among various ceratopsid taxa.