Evolution of Dermatocranial Shape in Horned Lizards (Phrynosoma)
Abstract
Synapomorphies of the iguanian genus Phrynosoma include morphological, physiological and behavioural characters that are associated with myrmecophagy. One such feature is the dermatocranial horn array which I hypothesized is a key innovation that has promoted the posited adaptive radiation of this genus. To explore this I first described the dermatocranium of Phrynosoma hernandesi and used this as the basis for a review of dermatocranial variation among the species of the genus. Comparative and ontogenetic criteria were used to determine homology among the horns of the dermatocranial horn array. A plesiomorphic horn array, present in the last common ancestor of the genus, was retrieved, and its expression determined in the extant species of the genus. Geometric morphometric analysis of ontogenetic dermatocranial form change in Phrynosoma hernandesi indicated that allometry accounts for ~53% of the total sample form variance, and localized allometric integration of the posterior regions of the dermatocranium is suggested. Sexual shape dimorphism is minimal. Allometry-free shape variance was concentrated in the posterolateral and posterior regions of the dermatocranium. A modularity hypothesis dividing the dermatocranium among six modules is supported.
A phylogenetic geometric morphometric analysis of dermatocranial shape variation of all species of Phrynosoma (excepting P. sherbrookei), and the callisaurine species Holbrookia maculata and Cophosaurus texanus, indicated that the assumption of a Brownian motion evolutionary model does not apply to the evolution of body size and dermatocranial shape in this clade. Evolutionary modes of maximum body length, head length, dermatocranial centroid size at maximum body length, and dermatocranial shape for Phrynosoma alone are non-Brownian motion in nature, suggestive that the genus constitutes an adaptive radiation. Occupation of a phylomorphospace generated for dermatocranial shape is consistent with this conclusion. Phylogenetic shape change early in the evolution of Phrynosoma is, however, inconsistent with the hypothesis that the dermatocranial horns constitute the key innovation for this adaptive radiation, although a well-developed horn array is basal for the genus. I conclude that both the short-horned and long-horned conditions are alternative means of protect the brain by extending the periphery of the dermatocranium.
Description
Keywords
Zoology
Citation
Powell, G. L. (2016). Evolution of Dermatocranial Shape in Horned Lizards (Phrynosoma) (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/26005