69 (2): 113 –143 © Senckenberg Gesellschaft für Naturforschung, 2019. 2019 VIRTUAL ISSUE on Recent Advances in Chondrocranium Research | Guest Editor: Ingmar Werneburg Diverging development of akinetic skulls in cryptodire and pleurodire turtles: an ontogenetic and phylogenetic study Ingmar Werneburg 1, 2, *, Wolfgang Maier 3 1 Senckenberg Center for Human Evolution and Palaeoenvironment (HEP) an der Eberhard Karls Universität Tübingen, Sigwartstr. 10, 72076 Tü- bingen, Germany — 2 Universität Tübingen, Fachbereich Geowissenschaften, Hölderlinstraße 12, 72074 Tübingen, Germany — 3 Universität Tübingen, Fachbereich Biologie, Auf der Morgenstelle 28, 72076 Tübingen, Germany — *
[email protected] Submitted September 5, 2018. Accepted January 15, 2019. Published online at www.senckenberg.de/vertebrate-zoology on February 19, 2019. Published in print on Q2/2019. Editor in charge: Uwe Fritz Abstract 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 ossifed 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.