Geometric Morphometry of the Breast-Shoulder Apparatus of Greater Antillean Anolis Lizards: In Situ Investigation of a Composite Skeletal Assemblage and Its Relationship to Ecomorphological Categorization

The breast-shoulder apparatus (BSA) poses a particular challenge for morphological comparisons, since its skeletal components display a great degree of mobility with respect to one another. We employed computed tomography to visualise the skeletal elements of the BSA of Anolis lizards from the Greater Antillean islands, and use four different methods to compare its structural morphology: geometric morphometric analysis of (a) isolated elements of the BSA, (b) a common shape space projection, (c) the BSA in situ, adherent to a standardised configuration, and (d) qualitative-descriptive comparison. Our findings of morphological variability of the BSA relate directly to functional-morphological demands imposed by the microhabitat specialisation of these lizards. Surprisingly, the combination of the mobile elements of the BSA into one in situ structure did not reveal a greater disparity between groups than the analysis of the moieties in isolation. However, examining this structure in situ added spatial information that could not be examined with previous approaches. Trunk-ground anoles are characterised by the relative anteroposterior extent of the presternum, interclavicle, and coracoid, likely impacting the muscle vectors and power output of humeral pro- and retractors, and emphasising humeral adduction in trunk-ground anoles. Shape differences that indicate a trajectory from trunk-crown, to crown-giant, to twig anoles are continuous. These consist of complementary shape variation in all skeletal elements of the BSA and describe a relatively lateromedially narrower BSA and an anteroposteriorly shorter epicoracoid towards twig forms, without altering the direction of the major muscle vectors in this region.