# The unique ability of modern turtles to retract their head



## Cowboy_Ken (Nov 26, 2014)

Systematic Biology, 11/2014

Ingmar Werneburg 1,2 *
Laura A.B. Wilson3,*,
William C. H. Parr4 
Walter G. Joyce5

+ Author Affiliations

1Paläntologisches Institut und Museum der Universität Zürich, Karl-Schmid-Strasse 4, 8006 Zürich, Switzerland; 2Museum für Naturkunde, Leibniz-Institut für Evolutions- & Biodiversitätsforschung an der Humboldt-Universität zu Berlin, Invalidenstraße 43, 10115 Berlin, Germany; 3School of Biological, Earth & Environmental Sciences, University of New South Wales, Kensington NSW 2052, Australia; 4Surgical and Orthopaedic Research Laboratories, Prince of Wales Clinical School, University of New South Wales, Randwick , NSW 2031, Australia; and 5Department of Geosciences, University of Fribourg, 1700 Fribourg, Switzerland 
↵*Correspondence to be sent to: Paläntologisches Institut und Museum der Universität Zürich, Karl-Schmid-Strasse 4, 8006 Zürich, Switzerland; E-mail: [email protected] or School of Biological, Earth and Environmental Sciences, University of New South Wales, Kensington NSW 2052, Australia; E-mail: [email protected]

Abstract

The unique ability of modern turtles to retract their head and neck into the shell through a side-necked (pleurodiran) or hidden-necked (cryptodiran) motion is thought to have evolved independently in crown turtles. The anatomical changes that led to the vertebral shapes of modern turtles, however, are still poorly understood. Here we present comprehensive geometric morphometric analyses that trace turtle vertebral evolution and reconstruct disparity across phylogeny. Disparity of vertebral shape was high at the dawn of turtle evolution and decreased after the modern groups evolved, reflecting a stabilization of morphotypes that correspond to the two retraction modes. Stem turtles, which had a very simple mode of retraction, the lateral head tuck, show increasing flexibility of the neck through evolution towards a pleurodiran-like morphotype. The latter was the precondition for evolving pleurodiran and cryptodiran vertebrae. There is no correlation between the construction of formed articulations in the cervical centra and neck mobility. An increasing mobility between vertebrae, associated with changes in vertebral shape, resulted in a more advanced ability to retract the neck. In this regard, we hypothesize that the lateral tucking retraction of stem turtles was not only the precondition for pleurodiran but also of cryptodiran retraction. For the former, a kink in the middle third of the neck needed to be acquired, whereas for the latter modification was necessary between the eighth cervical vertebra and first thoracic vertebra. Our paper highlights the utility of 3D shape data, analyzed in a phylogenetic framework, to examine the magnitude and mode of evolutionary modifications to vertebral morphology. By reconstructing and visualizing ancestral anatomical shapes, we provide insight into the anatomical features underlying neck retraction mode, which is a salient component of extant turtle classification.


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