Evolutionary Osteology and Relationships of the Horned Lizard Genus Phrynosoma (Family Iguanidae)

Abstract

pores. The sceloporines are divided into four subgroups. The Phrynosoma subgroup appears closest to the Uma-Callisaurus-Holbrookia subgroup. Both subgroups lack lacrimal and postfrontal bones and clavicular hooks. The Uma subgroup lacks the first pair of cervical ribs, which are retained in Phrynosoma. In Phrynosoma the skeleton is greatly specialized, particularly in respect to the palatal and temporal regions of the skull, bony cephalic spines, and the pectoral skeleton where the sternal apparatus is flattened and expanded. Detailed osteological comparison of the 14 species of Phrynosoma indicates significant intrageneric differences in many cranial elements. There is also considerable variability in the number of postcranial serial elements. On the basis of skeletal morphology, Phrynosoma orbiculare is the most primitive living form and may be considered close to the ancestral stock from which the other 13 species arose. One major radiation took place in Mexico, and a second to the north occurred after a coronatumorbiculare stock invaded the southwestern United States and northern Mexico. The latter line gave rise to coronatum, solare, modestum, cornutum, m'callii, and platyrhinos, with coronatum becoming restricted to the Pacific Coast and Baja California. The present distribution of Phrynosoma is correlated with the development of the Madro-Tertiary geofloral associations which dominate most arid and semi-arid regions of the southwestern United States and northern Mexico.