Thursday, March 9, 2017

[Paleontology • 2017] A Jurassic Stem Pleurodire, Platychelys oberndorferi, Sheds Light on the Functional Origin of Neck Retraction in Turtles


Platychelys oberndorferi Wagner, 1853

Life reconstruction represents Platychelys oberndorferi as a fast ram feeder and emphasizes the resemblance of this taxon with the extant matamata (Chelus fimbriatus) and snapping turtles (Chelydra serpentina and Macrochelys temminckii). 

Artwork by P. Röschli. DOI:  10.1038/srep42376 

Abstract
Modern turtles are composed of two monophyletic groups, notably diagnosed by divergent neck retraction mechanisms. Pleurodires (side-necked turtles) bend their neck sideways and protect their head under the anterior margin of the carapace. Cryptodires (hidden-necked turtles) withdraw their neck and head in the vertical plane between the shoulder girdles. These two mechanisms of neck retraction appeared independently in the two lineages and are usually assumed to have evolved for protective reasons. Here we describe the neck of Platychelys oberndorferi, a Late Jurassic early stem pleurodire, and find remarkable convergent morphological and functional similarities with modern cryptodires. Partial vertical neck retraction in this taxon is interpreted to have enabled fast forward projection of the head during underwater prey capture and offers a likely explanation to the functional origin of neck retraction in modern cryptodires. Complete head withdrawal for protection may therefore have resulted from an exaptation in that group.

Figure 3: Life reconstruction of Platychelys oberndorferi in its palaeoenvironment.
 This reconstruction represents Platychelys oberndorferi as a fast ram feeder and emphasizes the resemblance of this taxon with the extant matamata (Chelus fimbriatus) and snapping turtles (Chelydra serpentina and Macrochelys temminckii).  
Artwork by P. Röschli. DOI:  10.1038/srep42376 

Figure 2: Neck mobility in Platychelys oberndorferi.
(a) Only-known complete individual of Platychelys oberndorferi from the Late Jurassic of Eichstätt, Germany (undescribed; coll. Stefan Schäfer, Puchheim, Germany), illustrating the peculiar morphology of this species [photo by H. Tischlinger]. The biomechanichal reconstructions
(be) represent specimen NMB So.596, for which the shell, two cervical vertebrae, and one caudal vertebra are known (elements in white). Other parts of the skeleton (elements in grey) where either derived from other specimens or reconstructed based on adjoining elements (see Methods). (b) Maximal protraction of the neck. (c) Neutral position of the neck (maximum overlap of zygapophyses). (d) Probable maximal retraction of the neck. (e) Extreme hypothesis of maximal retraction of the neck (see Biomechanical analysis). Reconstructions by P. Röschli. 
  

Jérémy Anquetin, Haiyan Tong and Julien Claude. 2017. A Jurassic Stem Pleurodire Sheds Light on the Functional Origin of Neck Retraction in Turtles.
Scientific Reports. 7, 42376. DOI:  10.1038/srep42376

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