Caitilin McCracken’s Master's Thesis

Sauromalus spp. [Reptilia, Squamata, Iguanidae, Sauromalus: Chuckwalla]. Photos courtesy of http://helios.bto.ed.ac.uk/bto/ desertecology/lizards.htm

My Master’s thesis concerns the chuckwalla, genus Sauromalus. Chuckwallas are large, strict herbivore, iguanid lizards. According to the most current systematic study of chuckwallas (Hollingsworth, 1998), the genus Sauromalus consists of five species: S. ater, S. hispidus, S. klauberi, S. slevini, and S. varius. The geographic distribution of chuckwallas closely follows the limits of the Mojave and Sonoran deserts combined, in addition to 31 islands in the Gulf of California, Mexico (Sea of Cortés). One species, Sauromalus ater, is primarily continental, and the remaining four species are insular endemics.

Chuckwallas are a morphologically varied group of lizards, some species being considered gigantics (S. hispidus and S. varius), attaining an adult body size analogous to their large iguanid relatives. Other species of Sauromalus are smaller, growing to be only half this size as adults (S. ater, S. klauberi, and S. slevini). Chuckwallas are among the most conspicuous of squamates in the southwestern United States and northwestern Mexico. Fortunately, they employ a unique defensive strategy of seeking shelter in rock crevices and inflating the lungs. This results in the chuckwalla becoming tightly wedged between rocks, thereby making removal by a predator nearly impossible.

The chuckwallas’ distinctive lifestyle, varied geographic distribution, assorted morphology and number of species, makes this an appealing group for study. Some past analyses of chuckwallas have focused upon aspects of distribution, taxonomy, systematics and morphology (Avery and Tanner, 1964; Bostic, 1971; Cliff, 1958; Etheridge, 1982; Gates, 1968; Grismer et al., 1994, 1995; Hollingsworth et al., 1997; Hollingsworth, 1998; Lawler et al., 1995; Petren and Case, 1997; Robinson, 1974; Shaw, 1945; Soulé and Sloan, 1966; Sylber, 1985; Tanner and Avery, 1964).

My Master’s thesis will consist of a comparative osteological description of isolated cranial elements, noting the range of observed variation in the expression of morphological features of Sauromalus spp.. As S. klauberi and S. slevini are not available to me in skeletonized and disarticulated form, my analysis is focused solely upon S. ater, S. hispidus and S. varius. An analysis of isolated elements allows for a more detailed examination of the surface structure of the bones, as well as allowing for an assessment of internal structures and sutures. Morphological differences may relate to the firmness of articulation between bones, greater thickness of individual bones and/or variation in the nature of processes, facets or shelves. Moreover, intraspecific variation may include ontogenetic, individual, and sexual differences. If the analysis reveals discrete morphological characters, this may permit specific-level identification of Sauromalus.

In addition to a comparative osteological description of isolated cranial elements of Sauromalus, an osteological comparison of Sauromalus to Ctenosaura will also be made. Furthermore, I will present an evaluation of Sauromalus fossils, recovered from Rampart Cave located in the Grand Canyon of Arizona. A CT scan of Sauromalus, performed at the High-Resolution X-Ray Computed Tomography Facility at the University of Texas at Austin (UTCT) will also be included.

Avery, D. F. and W. W. Tanner. 1964. The osteology and mycology of the head and thorax regions of the obesus group of the genus Sauromalus Dumeril (Iguanidae). Brigham Young University Science Bulletin, Biology Series 5:1-30.

Bostic, D.L. 1971. Herpetofauna of the Pacific coast of north central Baja California, Mexico, with a description of a new subspecies of Phyllodactylus xanti. Transactions of the San Diego Society of Natural History 16 (10): 237-264.

Cliff, F. S. 1954. Variation and Evolution of the Reptiles Inhabiting the Islands in the Gulf of California, Mexico. Ph. D. Dissertation, Stanford University, Stanford, California.

Etheridge, R. E. 1982. Checklist of the iguanine and Malagasy Iguanid Lizards. Pp. 7-37. In G. M. Burghardt and A. S. Rand (Eds.), Iguanas of the World: Their Behavior, Ecology, and Conservation. Noyes Publication, Park Ridge, New Jersey, U.S.A.

Gates, G. O. 1968. Geographical distribution and character analysis of the Iguanid lizard Sauromalus obesus in Baja California, Mexico. Herpetologica 24:285-288.

Grismer, L. L., J. A. McGuire, and B. D. Hollingsworth. 1994. A report on the herpetofauna of the Vizcaíno peninsula, Baja California, México, with a discussion of its biogeographic and taxonomic implications. Bulletin of Southern California Academy Sciences 92:45-80.

Grismer, L. L., K. R. Beaman, and H. E. Lawler. 1995. Sauromalus hispidus. Catalogue of American Amphibians and Reptiles 615.

Hollingsworth, B. D., C. R. Mahrdt, L. L. Grismer, B. H. Banta, and C. K. Sylber. 1997. The occurrence of Sauromalus varius on a satellite islet of Isla Salsipuedes, Gulf of California, México. Herpetological Review 28:26-28.

Hollingsworth, B. D. 1998. The Systematics of Chuckwallas (Sauromalus) with a Phylogenetic Analysis of Other Iguanid Lizards. Herpetological Monographs 12, 38-191.

Lawler, H .E., K. R. Beaman, and L. L. Grismer. 1995. Sauromalus varius Dickerson. Catalogue of American Amphibians and Reptiles 616.

Petren, K. and T. J. Case. 1997. A phylogenetic analysis of body size evolution and biogeography in chuckwallas (Sauromalus) and other iguanines. Evolution 51:206-219.

Robinson, M. D. 1972. Chromosomes, Protein Polymorphism, and Systematics of Insular Chuckwalla Lizards (genus Sauromalus) in the Gulf of California, Mexico. Unpublished Ph. D. Dissertation, University of Arizona.

Shaw, C. E. 1945. The chuckwallas, genus Sauromalus. Transactions of the San Diego Society of Natural History 9(28):285-288.

Smits, A. W. 1985. Behavioral and dietary responses to aridity in the chuckwalla, Sauromalus hispidus. Journal of Herpetology

Soulé, M. E. and A. J. Sloan. 1966. Biogeography and distribution of the reptiles and amphibians on islands in the Gulf of California, Mexico. Transactions of the San Diego Society of Natural History 14(11):137-156.

Sylber, C. K. 1985. Feeding Habits, Reproduction, and Relocation of Insular Chuckwalla Lizards. Unpublished Ph. D. Dissertation, Colorado State University.

Tanner, W. W. and D. F. Avery. 1964. A new Sauromalus obesus from the upper Colorado Basin of Utah. Herpetologica 20:38-42.

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