Osteology and arthrology of the ankle and tarsometatarsus of anoles (Iguania: Anolidae): not convergent with geckos but divergent from the ancestral iguanian condition.

Abstract

The ankle joint of lizards has a complex structure, and its features help to define the Lacertilia. The configuration of this joint in its ancestral state entrains conjoint flexion-extension and long-axis rotation of the pes relative to the long axis of the crus. In Gekko gecko these actions can be decoupled because of derived features of the ankle joint. The increased degrees of freedom of the motions of the pes are associated with the operation of the adhesive toe pads carried on the digits. Among iguanian lizards, the genus Anolis has independently acquired a digital adhesive system that employs toe pads. Geckos and anoles are thus regarded as being convergent in the possession of a digital adhesive apparatus. This raises the question of whether anoles exhibit a similar ankle structure to that of geckos to allow them to deploy their toe pads in a mechanically similar fashion. Comparative analysis reveals that this is not the case, and that Anolis retains an ankle structure very similar to that of its iguanian relatives and non-gekkotan lizards in general. Some differences set its ankle and foot structure apart from those of its closest relatives, but these exaggerate the differences between geckos and anoles rather than lessen them: its ankle joint architecture is more sharply contoured than that of its close iguanian relatives; the ventral peg on the fourth distal tarsal is more extensive; its metatarsals are more gracile in form, relatively longer, and their distal joints are all unicondylar; its fifth metatarsal has a longer shaft and a less prominently sculpted ventral surface; and the meniscus that intervenes between the anterodistal extremity of the astragalocalcaneum and the more medial of the metatarsals is more extensive. These attributes combine to limit degrees of freedom at the ankle joint but provide the digits with greater mobility relative to the metatarsals. Such derived features may prove to be associated with enhanced capabilities for grasping narrow perches, sprinting and jumping, activities common to anoles but much less evident for geckos. The ways in which geckos and anoles negotiate their locomotor environments may be associated with the differences evident in their ankle and tarsometatarsus structure-anoles seemingly using the combination of their toe pads and claws to navigate along and between relatively narrow branches and geckos using broader, more expansive sectors of the substratum. Anoles and geckos have incorporated adhesive toe pads into their locomotor apparatus from structurally different starting points, with the former integrating the adhesive system into a pedal configuration that departs little from the ancestral lacertilian pattern. Beyond the possession of toe pads the pedal structure of anoles exhibits little in the way of convergence with that of geckos.