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Metapodial or Phalanx? An Evolutionary and Developmental Perspective on the Homology of the First Ray's Proximal Segment
Author(s) -
RENO PHILIP L.,
HORTON WALTER E.,
LOVEJOY C. Owen
Publication year - 2013
Publication title -
journal of experimental zoology part b: molecular and developmental evolution
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.823
H-Index - 63
eISSN - 1552-5015
pISSN - 1552-5007
DOI - 10.1002/jez.b.22506
Subject(s) - phalanx , anatomy , synapomorphy , biology , quadrupedalism , ossicle , phylogenetic tree , genetics , clade , middle ear , gene
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.