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Ossification patterns in the tetrapod limb--conservation and divergence from morphogenetic events.

Nadia B Fröbisch1

  • 1Redpath Museum, McGill University, 859 Sherbrooke Street West, Montreal H3A2K6, Canada. nadia.stoecker@mcgill.ca

Biological Reviews of the Cambridge Philosophical Society
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PubMed
Summary

Tetrapod limb ossification patterns differ between amniotes and urodeles. While early skeletal development isn't fully recapitulated in ossification, conserved patterns in anamniotes suggest ancestral traits and phylogenetic signals.

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Area of Science:

  • Developmental Biology
  • Paleontology
  • Comparative Anatomy
  • Evolutionary Biology

Background:

  • Tetrapod limb development exhibits two main patterns: postaxial dominance (amniotes, anurans) and preaxial dominance (urodeles) in skeletal element formation.
  • Mesenchymal condensation and chondrification are critical early stages in tetrapod limb skeletal development.
  • The relationship between these early developmental events and the subsequent ossification sequence is not fully understood across tetrapod phylogeny.

Purpose of the Study:

  • To investigate the sequence of limb ossification in various tetrapod taxa.
  • To compare ossification sequences with patterns of mesenchymal condensation and chondrification.
  • To assess the conservation and divergence of ossification patterns from early morphogenetic events and identify potential phylogenetic signals.

Main Methods:

  • Literature survey of ossification sequences in the fore- and hindlimbs of selected tetrapod species.
  • Comparative analysis of ossification patterns against known data on mesenchymal condensation and chondrification.
  • Detailed description and comparison of limb ossification in the fossil amphibian Apateon sp. with extant taxa.

Main Results:

  • A direct step-by-step recapitulation of condensation and chondrification during limb ossification is refuted.
  • Conserved aspects of early skeletal patterning, such as digit formation sequence, are evident, particularly in anamniotes.
  • Amniotes display weaker coupling between ossification sequence and earlier condensation/chondrification events compared to anamniotes.
  • The stronger correlation in anamniotes may represent a plesiomorphic trait for tetrapods.
  • Limb ossification patterns reveal shared trends among major tetrapod clades, suggesting phylogenetic signals.
  • Apateon sp. exhibits preaxial dominance, similar to extant salamanders, with an ossification sequence closely matching modern urodeles.

Conclusions:

  • Limb ossification sequences are not simple recapitulations of earlier developmental stages but show conserved and divergent patterns.
  • The strong coupling of ossification with condensation/chondrification in anamniotes might be an ancestral tetrapod trait.
  • The ossification sequence of Apateon sp. supports preaxial dominance in early tetrapods and highlights evolutionary links with modern urodeles.