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. 1997 Oct 28;94(22):11998-2001.
doi: 10.1073/pnas.94.22.11998.

Phalangeal Curvature and Positional Behavior in Extinct Sloth Lemurs (Primates, Palaeopropithecidae)

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Free PMC article

Phalangeal Curvature and Positional Behavior in Extinct Sloth Lemurs (Primates, Palaeopropithecidae)

W L Jungers et al. Proc Natl Acad Sci U S A. .
Free PMC article

Abstract

Recent paleontological discoveries in Madagascar document the existence of a diverse clade of palaeopropithecids or "sloth lemurs": Mesopropithecus (three species), Babakotia (one species), Palaeopropithecus (three species), and Archaeoindris (one species). This mini-radiation of now extinct ("subfossil") lemurs is most closely related to the living indrids (Indri, Propithecus, and Avahi). Whereas the extant indrids are known for their leaping acrobatics, the palaeopropithecids (except perhaps for the poorly known giant Archaeoindris) exhibit numerous skeletal design features for antipronograde or suspensory positional behaviors (e.g., high intermembral indices and mobile joints). Here we analyze the curvature of the proximal phalanges of the hands and feet. Computed as the included angle (theta), phalangeal curvature develops in response to mechanical use and is known to be correlated in primates with hand and foot function in different habitats; terrestrial species have straighter phalanges than their arboreal counterparts, and highly suspensory forms such as the orangutan possess the most curved phalanges. Sloth lemurs as a group are characterized by very curved proximal phalanges, exceeding those seen in spider monkeys and siamangs, and approaching that of orangutans. Indrids have curvatures roughly half that of sloth lemurs, and the more terrestrial, subfossil Archaeolemur possesses the least curved phalanges of all the indroids. Taken together with many other derived aspects of their postcranial anatomy, phalangeal curvature indicates that the sloth lemurs are one of the most suspensory clades of mammals ever to evolve.

Figures

Figure 1
Figure 1
(a) Measurements needed to calculate (b) radius of curvature R and included angle of curvature θ; (c) calculations of R and θ.
Figure 2
Figure 2
Representative examples of proximal phalanges of indroid primates (top to bottom: Archaeolemur, Indri, Babakotia, and Palaeopropithecus).
Figure 3
Figure 3
Cladogram of indroid primates with average degree of phalangeal curvature indicated. A high degree of phalangeal curvature is a synapomorphy of the sloth lemurs (palaeopropithecids).
Figure 4
Figure 4
Curvature of the proximal phalanges in living primates and subfossil indroids. The average for θ is indicated (★ for sloth lemurs, • for all other species) along with ±1 SD. Note the high degree of curvature in all species of extinct sloth lemurs.

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