Many-to-one mapping of locomotor mode: a case study in the pelvic morphology of two basal hyaenodonts

Abstract Extant carnivorans are often used as ecological analogs for hyaenodonts because of their similar size and diet. However, it is unclear whether this comparison extends to locomotor mode. Unlike carnivorans, hyaenodonts have relatively short limbs and revolute lumbar articulations. These systematic anatomical differences across clades can skew the results of ecomorphological analyses if only carnivorans are used. We tested similarity in the shape of pelvis, the anchor for most major hindlimb muscles, between two hyaenodonts, Sinopa sp. indet. and Thinocyon velox Marsh, 1872, and a range of extant mammals including carnivorans. We used linear morphometrics to compare pelvis shape among 20 small to medium-sized mammals across 10 orders and the two hyaenodonts. We then tested the correlation between pelvis shape and multiple categorization schemes for locomotion. Both hyaenodonts had elongate obturator foramina and shorter ilia without the large gluteal fossa seen in carnivorans. Based on a cluster analysis, Sinopa sp. indet. was most similar in shape to the bobcat, Lynx rufus (Schreber, 1777), but Thinocyon velox did not cluster closely with any one extant mammal or Sinopa . Of the locomotion categories, only digitigrady had a relationship with pelvis shape. Despite showing some similarity in overall shape with carnivorans, both hyaenodonts had distinct features in the pelvis that could suggest differences in musculature between the two clades. These results suggest that carnivorans are imperfect ecomorphological analogs to hyaenodonts for analyses of locomotion due to differences in each clade’s ancestral pelvic morphology. Thus, any assessment of hyaenodont locomotion and postcrania would benefit from incorporating a larger taxonomic breadth of small to medium-sized mammals.