Postcranial ontogenetic data and the origin of primates

Stafford and Thorington (1998) and Hamrick (2001) presented important data on the hand proportions in developing and adult mammals. Using a ternary plot (Hamrick 2001), relative metacarpal, proximal, and intermediate phalanx lengths among fossil and extant taxa deliver an elegant possibility of distinguishing primates from other "archontan" mammals. Primates differ from flying and gliding mammals in having much longer proximal phalanges relative to their metacarpals and are unique among the sampled mammals in having elongated proximal phalanges relative to their metacarpals. A comparative analysis of hand development in the mouse lemur Microcebus murinus and other meta- and eutherian mammals reveals that "interspecific variation in relative digit and metapodial proportions has high-developmental penetrance; that is, adult differences are observed at early ontogenetic stages'' (Richardson 1999 p 348). Hamrick's (2001) results suggest an evolutionary scenario that describes an "invasion'' of the fine branch niche based on a hand with a short palm and long fingers yielded by a change in digital ray patterning and segmentation.

Recent advances in developmental genetics elucidate patterns of postcranial growth in primates. Another study is based on a comparison of developmental data of CD1 mice fetuses and Rhesus macaques: Hallgrimsson et al. (2002) demonstrated the evidence for morphological integration of the limbs as serially homologous structures by reporting the covariation structure of forelimb and hindlimb skeletal elements. This proves that link structures between the limbs are caused by developmental modules, producing the covariation that would be needed to be overcome by selection for divergence in fore- and hindlimb morphology.

Since primates have more distally concentrated limb mass than most other mammals, Raichlen (2005) studied the unique kinematics of primates by testing a longitudinal ontogenetic sample of baboons (Papio cynocephalus). He concluded that the evolution of primate quadrupedal kinematics was tied to the evolution of grasping hands and feet.

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