The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

Hallucal reduction in sloth lemurs and morphological convergence on orang-utans by Palaeopropithecus


1Cell and Neurobiology, University of Southern California, 2Post-Baccalaureate Premedical Program, Washington University in St. Louis, 3Evolutionary Anthropology, Duke University, 4Anatomical Sciences, Stony Brook University

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It has been claimed that among primates only the orang-utan possesses a “rudimentary hallux”, presumably related to its adaptations for antipronograde positional behaviors. To the contrary, we document here a steep morphocline of hallucal reduction in a clade of strepsirrhines that includes living indriids and subfossil palaeopropithecids (“sloth lemurs”), culminating in a vestigial hallux in Palaeopropithecus. Hallucal metatarsals (Mt1) are described for the first time for Babakotia radofilai (n=4) and Palaeopropithecus ingens (n=1), and these are compared to Indri, Propithecus, Avahi, Pan, Gorilla and Pongo.

The Mt1 of Indri (< 10 kg body mass) is absolutely longer than that of both Babakotia (~20 kg) and Palaeopropithecus (~40 kg). Relative to the length of the fourth metatarsal in strepsirrhines and the third metatarsal in great apes, the Mt1 of Babakotia is much shorter than in all indriids (and the African apes). The Mt1 is also greatly reduced in orang-utans, but this reduction is exceeded by Palaeopropithecus. The prominent peroneal process seen in the Mt1 of living indriids (and other strepsirrhines) has been lost in both sloth lemurs. The diaphysis of the Mt1 is robust in Babakotia but is more gracile in Palaeopropithecus, and the head in Palaeopropithecus is small and flattened as in orang-utans.

Hallucal reduction reinforces other skeletal evidence (e.g., limb proportions and phalangeal curvature) that these two genera of large-bodied sloth lemurs were highly suspensory. Orang-utans are therefore not unique, but still serve as the best primate analogue for Palaeopropithecus, one of the most antipronograde mammals to ever evolve.

Funded in part by The Leakey Foundation (to BAP) and NSF grant BCS 1125507 (to DMB).

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