The hooked element in the pes of turtles (Testudines): a global approach to exploring primary and secondary homology

Authors

  • Walter G. Joyce,

    Corresponding author
    1. Department of Geosciences, University of Fribourg, Fribourg, Switzerland
    • Department of Geosciences, University of Tübingen, Tübingen, Germany
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  • Ingmar Werneburg,

    1. Department of Geosciences, University of Tübingen, Tübingen, Germany
    2. Paläontologisches Institut und Museum, Universität Zürich, Zürich, Switzerland
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  • Tyler R. Lyson

    1. Department of Geology and Geophysics, Yale University, New Haven, CT, USA
    2. Department of Vertebrate Zoology, National Museum of Natural History, Smithsonian Institution, Washington, DC, USA
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Correspondence

Walter G. Joyce, Department of Geosciences, University of Fribourg, 1700 Fribourg, Switzerland. E: walter.joyce@unifr.ch

Abstract

The hooked element in the pes of turtles was historically identified by most palaeontologists and embryologists as a modified fifth metatarsal, and often used as evidence to unite turtles with other reptiles with a hooked element. Some recent embryological studies, however, revealed that this element might represent an enlarged fifth distal tarsal. We herein provide extensive new myological and developmental observations on the hooked element of turtles, and re-evaluate its primary and secondary homology using all available lines of evidence. Digital count and timing of development are uninformative. However, extensive myological, embryological and topological data are consistent with the hypothesis that the hooked element of turtles represents a fusion of the fifth distal tarsal with the fifth metatarsal, but that the fifth distal tarsal dominates the hooked element in pleurodiran turtles, whereas the fifth metatarsal dominates the hooked element of cryptodiran turtles. The term ‘ansulate bone’ is proposed to refer to hooked elements that result from the fusion of these two bones. The available phylogenetic and fossil data are currently insufficient to clarify the secondary homology of hooked elements within Reptilia.

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