Mechanisms of Pisiform Reduction in Mammals

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Most mammalian pisiforms are large, elongated carpal bones with two ossification centers and an associated growth plate, similar to the calcaneus in the ankle. Short pisiforms are rare among mammals, but can be found in humans, orangutans (Primates: Pongo sp.), two‐toed sloths (Pilosa: Choloepus sp.), and three‐toed sloths (Pilosa: Bradypus sp.). These groups utilize diverse forms of locomotion; humans are bipedal, orangutans are quadrumanous climbers, and sloths are suspensory quadrupeds. Human pisiforms form from a single ossification center and have likely lost a primary ossification center and the associated growth plate. Given the rarity of short pisiforms among mammals, little is known about the functional implications or developmental mechanisms underlying this convergent morphology in species with such diverse locomotor modes and evolutionary histories. The purpose of this study is to compare ossification patterns in orangutans and sloths with the mechanism underlying human pisiform truncation. We compare pisiform ossification data from human, orangutan, and sloth museum specimens. Unlike humans, orangutan pisiforms retain two centers of ossification but appear to form the secondary center of ossification later than other apes, possibly shortening the growth period for the pisiform. Sloths form a primary center of ossification with an unusual, pitted surface, which may represent an abnormal subchondral surface. In non‐human primates (including orangutans) and giant anteaters (Pilosa: Myrmecophaga tridactyla), which represent the closest living taxonomic group to sloths, pisiform and calcaneus ossification is generally correlated with epiphyses forming at similar times within individuals. In contrast, we found only a single ossification center corresponding to sloth pisiforms at all stages of calcaneal epiphyseal formation. Thus, the single pisiform ossification center appears to precede calcaneal epiphysis formation. This indicates that the sloth pisiform is homologous to the primary center of ossification of the calcaneus and other mammalian pisiforms, and that it fails to form a secondary center. This is in contrast to human pisiforms, which fail to form the primary ossification center. Humans, orangutans, and sloths are all unusual among mammals for having short pisiforms and each group achieves pisiform reduction through unique mechanisms.Support or Funding InformationThis research was funded by the National Science Foundation (BCS‐1540418 and BCS‐1638812).