SPHARM, a New Computational Approach for Locomotor Signal Identification in 15 MYA fossil primates from Maboko Island, Kenya

Purpose: Maboko Island, western Kenya preserves a diverse collection of seven catarrhine proximal humeri from the middle Miocene (~15 Ma). Proximal humeri are extremely rare in the Miocene (25-5 mya), limited to only four specimens outside of Maboko. Identified taxa include cercopithecoids (Victoriapithecus), nyanzapithecines (Mabokopithecus), small-bodied "apes” ("Micropithecus”), and large-bodied hominoids (Kenyapithecus), all of which provide important insight into diversity of locomotor patterns among middle Miocene catarrhines.

Methods: This project uses weighted spherical harmonics analysis (SPHARM), a landmark-free automated method to explore locomotor signals in the Maboko sample. Meshes of five intact humeri were compared to a sample of 94 extant catarrhines and platyrrhines, spanning a range of locomotor modes (suspensory arborealism, terrestrial quadrupedalism, arboreal quadrupedalism, and knuckle walking). Principal components analyses (PCA) were run on the associated SPHARM coefficients to explore differences among these locomotor groups.

Results: Preliminary assessment shows extant suspensory arboreal primates clustering away from both knuckle-walking and quadrupedal group along PC1 and PC2. Along PC2, further separation was observed between arboreal and terrestrial quadrupeds. The Maboko specimens show distinctions between all five of the fossil primates, particularly between two taxa previously suggested to be arboreal (Mabokopithecus and "Micropithecus”) and two taxa documented as terrestrial quadrupeds (Victoriapithecus and Kenyapithecus).

Conclusion: Though preliminary, this analysis provides insight into the diversity of catarrhine locomotor behavior during the middle Miocene, reinforcing previous descriptions of locomotor partitioning among the Maboko specimens. Future research that includes early and late Miocene taxa may shed light into diversity of catarrhine locomotor behaviors that spanned the Miocene epoch.