Cranial kinesis in the miniaturised lizardAblepharus kitaibelii(Squamata: Scincidae)

Abstract

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 mesokinesis and metakinesis) and streptostyly. Streptostyly is considered relatively widespread within lizards, whereas mesokinesis has been documented only for geckos, varanids and anguids. The present study investigated cranial kinesis in the miniaturised scincid Ablepharus kitaibelii by integrating morphological and experimental data. Based on micro computed tomography, 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, the morphological data indicate 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 angles. Based on these data, we formulate a model of the amphikinetic A. kitaibelii skull mechanism, which provides an extension of Frazzetta's quadric-crank model by placing a special emphasis on metakinesis. According to this model, we hypothesise 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.

Cranial anatomy of the Galápagos marine iguana Amblyrhynchus cristatus (Squamata: Iguanidae)

“…In other miniaturized lizards, such as Anolis (iguanian), Ablepharus (skink), and Sphaerodactylus (gecko), the crista is weakly developed (cf. Daza et al, 2008; Handschuh et al, 2019; IP pers. obs.…”

Phylogenetic history influences convergence for a specialized ecology: comparative skull morphology of African burrowing skinks (Squamata; Scincidae)

“…It narrows anteriorly until it reaches a small foramen on the dorsal surface. The cartilaginous processus ascendens likely lies inside this tube as documented in Ablepharus [ 52 ]. Typhlacontias gracilis has a small parietal fossa visible posteriorly (Fig.…”

“…Although skinks have generally been thought to lack cranial kinesis [ 73 ], anatomical studies have pointed to the potential for it in some species, including several acontines [ 10 , 44 ], and experimental work has demonstrated it in a small species [ 52 ]. Our work provides further evidence for the potential of mesokinesis in some skinks (closure of upper temporal fenestrae, reduction of jugal and postorbital, gap between cristae cranii and parietal downgrowths permitting movement).…”

Lizards as models to explore the ecological and neuroanatomical correlates of miniaturization

“…Reduction of skeletal elements throughout the body, such as the carpals and phalanges (Hanken, 1985;Yeh, 2002), have given credence to the hypothesis that deossification is a consequence of miniaturization, arising from truncated development or stunted growth rate (Hanken, 1993;Hanken & Wake, 1993). Alternatively, skull deossification could provide greater cranial kinesis and space for enlarged jaw adductor musculature to achieve adequate gape and bite force in feeding (Rieppel, 1984;Bhullar & Bell, 2008;Handschuh et al, 2019). Hyperossification, on the other hand, might be a compensation for structural weakening of the skeleton due to reduced ossification (Hanken, 1993;Maglia et al, 2007;Fröbisch & Schoch, 2009).…”