Xenarthran Pisiform Morphology and its Relation to Hominoid Locomotor Diversity

The extant hominoid wrist joint is defined by ulnar withdrawal and distal relocation of the pisiform. This novel modification eliminates contact between the ulna and the pisiform typically observed in mammals, leaving the sole articulation of the pisiform with the triquetral. Furthermore, the pisiform has undergone a substantial reduction in Homo and Pongo, albeit by distinct developmental means. Humans have lost the pisiform primary center of ossification and growth plate, while these structures remain in the orangutan. The functional significance of these changes is difficult to ascertain. However, slow and cautious climbing lorisine primates have highly derived wrists with reduced pisiforms. The family Xenarthra, consisting of sloths, anteaters, and armadillos, exhibit a diversity of locomotor modes similar to primates. Sloths are obligate suspensors and giant anteaters engage in knuckle- walking. Several studies have noted similarities in xenarthran and hominoid wrist morphology, yet no study has focused on the medial wrist and pisiform. Here we compare these morphologies in hominoids, lorisines, and xenarthrans using uCT on a sample of lorisine and xenarthan adult and juvenile articulated wrists. We conduct a qualitative comparison with hominoids to better understand the adaptive significance of the hominoid medial wrist and diversity of developmental patterns underlying pisiform reduction. We observe numerous similarities with respect to in ulnar withdrawal and distal placement of the pisiform in xenarthrans and hominoids. Interestingly, the sloth pisiform is short similar to humans and orangutans, and the anteater pisiform is projecting and articulates with the triquetral like those of African apes.