Cranial kinesis in the miniaturised lizard Ablepharus kitaibelii (Squamata: Scincidae)

Cranial kinesis refers to intracranial movements in the vertebrate skull that do not concern the jaw joint, the middle ear, or the hypobranchial skeleton. Different kinds of cranial kinesis have been reported for lizards, including mesokinesis, metakinesis, amphikinesis (simultaneous meso- and metakinesis), and streptostyly. Streptostyly is considered relatively widespread within lizards, while mesokinesis has been documented only for geckos, varanids, and anguids. The present study investigates cranial kinesis in the miniaturised scincid Ablepharus kitaibelii by integrating morphological and experimental data. Based on microCT, we provide a description of skull osteology. Cranial joints were studied with histology, which results in the first detailed description of cranial joint histology for a member of the Scincidae. Taken together, morphological data indicates a high potential for amphikinesis and streptostyly, which was also corroborated by skull manipulations. High-speed cinematography demonstrated that mesokinesis occurs during food uptake, processing, and intraoral transport cycles. Bite force measurements showed prolonged and reasonably hard biting even at large gape. Based on this data we formulate a model of the amphikinetic Ablepharus skull mechanism, which provides an extension of Frazzetta's quadric-crank model by placing a special emphasis on metakinesis. According to this model, we hypothesize that metakinetic intracranial movements may provide a means for reducing strain in jaw adductor muscles. Presented hypotheses can be addressed and tested in future studies.