To Move or not to move: cranial joints in european gekkotans and lacertids, an osteological and histological perspective

Lepidosaurs are frequently described as having highly kinetic skulls,and different forms of cranial kinesis have been described as being characteristic of their radiation. The model of amphikinesis proposed by Frazzetta (J Morphol 1962; 111:287???319, which was long considered a synapomorphy of the large suborder Sauria, is now much debated given its uncertain distribution among the various lizard taxa and the lack of data about its morphological correlates. In this article, we analyze the anatomical correlates of different forms of cranial kinesis, with particular regard to the putative saurian amphikinesis, describing the possible diverse skull movements of several species of European gekkotans (Hemidactylus turcicus, Mediodactylus kotschyi, and Tarentola mauritanica) and lacertids (Lacerta agilis, L. bilineata, Podarcis muralis, P. siculus, and Teira dugesii). Using serial and whole-mount histology, we found clear differences between gekkotans and lacertids in the structure of several cranial joints underlining the existence of two degrees of intracranial mobility. The lacertid species possess the anatomical features for streptostyly (quadrate joints) and metakinesis (parietal-supraoccipital and parabasisphenoid-pterygoid joints) and lack the anatomical correlates for mesokinesis (mobility of frontal-parietal and palatine-pterygoid joints) and amphikinesis (coupled mesokinesis, metakinesis, and streptostyly). In contrast, geckos present all the anatomical correlates for amphikinesis as described by the traditional quadratic crank model. Finally, we present a comprehensive summary of the different forms of squamate cranial kinesis, advancing two alternative hypotheses about the evolutionary origin of amphikinesis.