The neck of the Late Jurassic Platychelys oberndorferi and the origin of neck retraction mechanisms in turtles
Creators: name: Anquetin, Jérémy
name: Haiyan Tong
name: Claude, Julien
Publisher: name: figshare
Year: 2016
Subjects: Paleontology

description: Modern turtles are divided into two main clades, which find their origin as early as the Middle Jurassic. These two groups have independently developed complex double-bend neck retraction mechanisms. Pleurodires (side-necked turtles) fold the neck sideways and tuck the head under the anterior margin of the carapace. Cryptodires (hidden-necked turtles) withdraw the neck and head in the vertical plane between the shoulder girdles. Since each neck retraction mechanism offers some measure of protection for the neck and head in extant turtles, it is usually assumed that these mechanisms evolved mainly for protective reasons. Here, we describe the neck of Platychelys oberndorferi, a Late Jurassic stem pleurodire from Germany and Switzerland, and find major morphological similarities with modern cryptodires in the posterior part of the neck. A biomechanical analysis reveals that this taxon was able to withdraw the neck in the vertical plane, but that this retraction was incomplete. The head and anterior part of the neck cannot be withdrawn within the shell, which undermines the potential protective value of neck retraction in this taxon. Platychelys oberndorferi is a peculiar turtle that shows remarkable morphological convergences with the modern matamata (Chelus fimbriata) and the snapping turtles (Chelydra serpentina and Macrochelys temminckii). This strongly suggest that Platychelys was an ambush predator practising ram and/or suction feeding. We therefore suggest that vertical neck retraction primarily evolved in this taxon to enable fast forward projection of the head and improve capture of darting prey. The functional origin of neck retraction mechanisms in turtles has never been fully explored. The apparition of the complex double-bend mechanisms of pleurodires and cryptodires for purely protective reasons is relatively unlikely. The horizontal folding of pleurodires actually offers little more protection that the lateral head tuck of stem turtles, whereas in cryptodires the protection is only effective when the retraction is complete, which necessitates substantial morphological changes from the ancestral cervical morphotype. The case of Platychelys offers a likely explanation to the origin of complex neck retraction mechanism in turtles, especially for cryptodires. Neck retraction could 14th Swiss Geoscience Meeting, Geneva 2016 have evolved gradually as a way to enable fast forward thrust of the head during prey capture in the water. Partial neck retraction would already provide an advantage, as demonstrated by the very specialized ecological niche occupied by Platychelys. Once a primary mechanism of neck retraction was in place, it would be easier to obtain complete head withdrawal for protection via natural selection. We therefore suggest that vertical neck retraction in cryptodires may have evolved primarily in order to enable fast forward projection of the head during underwater feeding, and that protection of the head by complete withdrawal within the shell is actually an exaptation.