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Metapodial or Phalanx? An Evolutionary and Developmental Perspective on the Homology of the First Ray's Proximal Segment

Authors

  • PHILIP L. RENO,

    Corresponding author
    1. Department of Anthropology, The Pennsylvania State University, University Park, Pennsylvania
    • Correspondence to: Philip L. Reno, Department of Anthropology, 409 Carpenter Building, University Park, PA 16802.

      E-mail: philreno@psu.edu

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  • WALTER E. HORTON Jr.,

    1. Department of Anatomy and Neurobiology, Northeast Ohio Medical University, Rootstown, Ohio
    2. School of Biomedical Sciences, Kent State University, Kent, Ohio
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  • C. Owen LOVEJOY

    1. School of Biomedical Sciences, Kent State University, Kent, Ohio
    2. Department of Anthropology, Kent State University, Kent, Ohio
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  • Conflicts of interest: None.

Abstract

The first mammalian metapodial (MP1) has periodically been argued to actually be a phalanx, because the first ray has one less element than the four posterior rays, and because the MP1 growth plate is proximal like those of all phalanges, rather than distal as in metapodials 2–5. However, growth plates are formed at both ends in non-therian tetrapod metapodials, and phylogenetic analysis demonstrates that growth plate loss is a therian synapomorphy that postdates the establishment of the mammalian phalangeal formula. These data, along with results of developmental and morphological studies, suggest that the MP1 is not a phalanx. The singular, proximal growth plates in MPs 2–5 are likely to be an adaptation to dynamic erect quadrupedal gait which was characterized by conversion of the posterior metapodials into rigid struts with the carpus/tarsus. While the adaptive significance of the reversed ossification of MP1 is less clear, we present three functional/developmental hypotheses. J. Exp. Zool. (Mol. Dev. Evol.) 320B:276–285, 2013. © 2013 Wiley Periodicals, Inc.

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