Variation in Pisiform Morphology

Author:

Gavazzi, Lia Michelle

Area of Honors:

Biological Anthropolgy

Degree:

Bachelor of Science

Document Type:

Thesis

Thesis Supervisors:

Timothy Michael Ryan, Thesis Supervisor
Timothy Michael Ryan, Thesis Honors Advisor
Kenneth Gale Hirth, Faculty Reader

Keywords:

pisiform
growth plate
morphology
xenarthra
lorisine
hominoid
evolution and development
histology
tetrapod

Abstract:

The pisiform is unique among carpal bones because it is the only one to form a secondary center of ossification and growth plate, a configuration that is present across nearly all mammalian lineages. The human pisiform has undergone major morphological changes including the lost of its growth plate and an ossification center. What is typically a rod-shaped bone in many mammalian species is more akin to a pea shape in our species. This drastic change in development and morphology has a number of implications for humans, however the functional consequences of pisiform growth plate loss are still not understood. The pisiform is severely underrepresented in most skeletal literature, despite its relative importance. Unique human pisiform morphology is often correlated to locomotor behaviors or tool use, although direct associations between carpal morphology and specific behaviors have yet to be established. To understand the developmental history of the pisiform, it is imperative to look beyond the scope of mammals. Investigation of reptilian and amphibious species reveal a complicated and non-linear developmental history of the pisiform. Evolutionary research frequently relies on a diverse number of species to act as proxy for the subject of interest. In the context of development, it is beneficial to compare species across multiple taxa with similar skeletal morphology to identify similarities or differences in development trajectories. In a remarkable case of parallel evolution, the family Xenarthra, comprised of sloths, armadillos, and anteaters contains several species who share morphological similarities with extant hominoids. This includes a reduced pisiform in each species of extant sloths. Our research indicates that the giant anteater pisiform develops along a typical mammal trajectory with the secondary center of ossification and a growth plate. The two-toed and three-toed sloth pisiform does not indicate any secondary center in ontogeny, leading us to conclude that the growth plate has additionally been lost in these species.