Diverging development of akinetic skulls in cryptodire and pleurodire turtles: an ontogenetic and phylogenetic study

Title: Diverging development of akinetic skulls in cryptodire and pleurodire turtles: an ontogenetic and phylogenetic study

Creators: Ingmar Werneburg & Wolfgang Maier 

Submitted September 5, 2018.
Accepted January 15, 2019.
Published online at www.senckenberg.de/vertebrate-zoology on February 19, 2019.
Published in print Q2/2019.

DOI: 10.26049/VZ69-2-2019-01 

Published by Senckenberg Gesellschaft für Naturforschung

Date (Publication Year): 2019

Resource Type (General): TEXT

Resource Type (optional): Vertebrate Zoology, Scientific Article

Description: Extant turtles (Testudines) are characterized among others by an akinetic skull, whereas early turtles (Testudinata) still had kinetic skulls. By considering both ontogenetic and evolutionary adaptations, we analyze four character complexes related to the akinetic skull of turtles: (1) snout stiffening, (2) reduction of the basipterygoid process, (3) formation of a secondary lateral braincase wall, and (4) the fusion of the palatoquadrate cartilage to the braincase. Through ontogeny, both major clades of modern turtles, Pleurodira and Cryptodira, show strikingly different modes how the akinetic constructions in the orbitotemporal and quadrate regions are developed. Whereas mainly the ascending process of the palatoquadrate (later ossified as epipterygoid) contributes to the formation of the secondary braincase wall in Cryptodira, only the descending process of the parietal forms that wall in Pleurodira. This is related to the fact that the latter taxon does not develop an extended ascending process that could ossify as the epipterygoid. Whereas the palatoquadrate directly fuses to the braincase in pleurodires by forming appositional bone of quadrate (called Eßwein-fixation herein), both structures are ventrally bridged by the pterygoid in cryptodires. Phylogenetic evidence, including fossil data, suggests that the cryptodiran type of skull fixation is plesiomorphic for crown turtles and that the pleurodire condition evolved secondarily within the crown. Embryonic neck muscle activity may be the cause of this shift of the palatoquadrate. Hidden-necked retraction in cryptodires pulls the palatoquadrate in a posterodorsal and -medial direction during ontogeny, whereas side-necked retraction in pleurodires redirects the palatoquadrate posterolaterally and -medially. These different muscle forces may result in differing positions of the palatoquadrate in relation to the braincase and eventually result in the two different attachment types. Moreover, the general construction of the jaw adductor chamber is affected by alternative fusion modes, which secondarily result in two different types of the trochlear system of the external jaw musculature. Related to that, changes in feeding habit through turtle evolution may have also triggered increasing stabilizattion of the skull. Palatoquadrate fixation, finally, was an important prerequisite for the reduction of the exocranial bones of the temporal skull region.

Keywords: Appositional bone; basipterygoid articulation; Cryptodira; Eßwein-fixation; heterochrony; ontogeny; ossification; palatoquadrate; paleontology; Pleurodira; Proganochelys; secondary lateral braincase wall; secondary palate; stem-Testudines; Testudinata; Zuwachsknochen

Citation: Werneburg, I., Maier, W. (2019). Diverging development of akinetic skulls in cryptodire and pleurodire turtles: an ontogenetic and phylogenetic study. Vertebrate Zoology, 69(2): 113-143. https://doi.org/10.26049/VZ69-2-2019-01