Unique pathological phalangeal fusion in the chalicothere subfamily Chalicotheriinae and the interphalangeal immobilization in chalicotheres

Chalicotheres are bizarre extinct herbivore mammals closely related to today's rhinoceroses, tapirs and horses. The family Chalicotheriidae includes two subfamilies, the Chalicotheriinae and the Schizotheriinae. Some members of the schizotheriines form a duplex bone by fusing the proximal and the medial phalanges of the second digit of the hand. Here, we report the only known fused set of proximal and medial phalanges in a specimen of the subfamily Chalicotheriinae from the Late Miocene of Höwenegg in Germany. This specimen has been the center of some confusion regarding its identity and the nature of the fusion. In our study, we conduct a detailed comparison to chalicotheriines and schizotheriines, identifying the specimen as a chalicotheriine and attributing its fusion to a pathology. We additionally acquired CT scans of this specimen and two schizotheriines to compare the internal structure of the fused phalanges, which revealed great differences between the fusion in schizotheriines and the pathologically fused Höwenegg specimen. Furthermore, we found that both subfamilies show a trend towards immobilizing their digits, expressed in different ways, with chalicotheriines forming a notched-joint between the phalanges in some digits that hinders their movement and schizotheriines regularly fusing the phalanges.