Surface structure of fossil and Recent epidermal scales from North American lizards of the genus Sceloporus (Reptilia, Iguanidae). Bulletin of the AMNH ; v. 156, article 4
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Date
1976
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New York : American Museum of Natural History
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Abstract
"More than 2000 fossil epidermal scales from lizards of the genus Sceloporus have been found in the American Southwest (Arizona, California, Nevada) in 35 fossil packrat (Neotoma) middens, ranging in age from 21,700 [plus or minus] 500 to 2720 [plus or minus] 100 radiocarbon years. We compared the surface structure (using a scanning electron microscope), dimensions, and shapes of the fossil scales with scales from seven Recent species of Sceloporus (S. occidentalis, S. undulatus, S. graciosus, S. jarrovii, S. orcutti, S. clarkii, and S. magister). Our objective was to identify the species of Sceloporus represented by the fossil scales, with a view to improving our understanding of the communities that formerly inhabited the fossil localities. In addition, we discovered several fossil scales that are referable to Sauromalus obesus, Crotaphytus, and Cnemidophorus. Controlled observations demonstrate that the scales of the seven Recent species we examined have rather similar surface micro-ornamentation, as do the fossils. Such specific differences as do exist are subtle and difficult to resolve on most of the fossils. Work with living and preserved S. occidentalis and S. undulatus revealed extreme intraspecific variation in scale surface ornamentation that is correlated with the shedding cycle. Scale surfaces that are newly exposed upon molting are highly ornamented with spinules, pits, and partitions that represent superficial cell boundaries. This ornamentation disappears or deteriorates drastically in time, but reappears after the animal molts. This extreme variation must be accounted for in making interspecific comparisons. The most reliable method for studying surface micro-ornamentation on Sceloporus scales is to work with living captives and examine their newly exposed scale surfaces immediately following molting. Burstein, Larsen, and Smith (1974) proposed that certain characteristics of surface micro-ornamentation of Sceloporus scales may be very useful in systematics at lower levels of the taxonomic hierarchy and are unidirectional in evolution; they also concluded which traits represent primitive character-states. These authors, however, worked strictly with scales from preserved specimens, and they did not properly account for variation. Our direct comparisons of shed scales with newly exposed scale surfaces obtained concurrently from the same living individuals demonstrate that many of the character-states utilized by these authors are nothing more than differences that normally appear on a single scale at different periods in the shedding cycle. Examination of gross features revealed that, when isolated, certain scales with distinctive morphology (dimensions, shapes) are useful for distinguishing among the seven Recent species, but even at best their utility is limited. Small scales cannot be identified with certainty because most of the small scales on large lizards cannot be distinguished readily from the larger scales on small lizards. Certain large scales can be determined as not having been deposited by the smaller species if the sizes and shapes of the scales clearly exceed the maximum limits observed on the smaller species. Thus, gross morphology can be used to limit the number of candidate species represented by certain large scales in a given sample of fossils, and in some instances geographic distribution is useful to limit the possibilities further. The scales in 11 samples of fossils were such that we could not confidently identify the species of Sceloporus they represented. However, 24 samples ranging in age from 9770 [plus or minus] 160 to 17,610 [plus or minus] 290 radiocarbon years, from Coconino, Mohave, and Yuma counties, Arizona, and from San Bernardino County, California, contained large scales that probably were deposited by Sceloporus magister, which is found today at or very near each of the fossil localities. Sceloporus magister, and some other species of reptiles (Sauromalus obesus, Gopherus agassizi) whose remains have been found in some of the same fossil pack-rat middens, occur primarily in desert and chaparral communities today. However, plant fossils from the same deposits suggest that in the Pleistocene these reptiles occurred often in relatively mesic woodland in which the lowest winter temperatures were not very different from those at the same sites today, but in which summer temperatures were cooler than those prevailing today. Scale functions are discussed briefly. We suspect that one of the important functions of the large, keeled, spinose and imbricating scales of Sceloporus, and possibly of their micro-ornamentation, is to provide the proper surface contact with the environment, particularly in regard to friction. This scutellation may be particularly advantageous to lizards that seek shelter from predators in deep crevices among rocks and vegetation, where efficient entry into cover and the ability to cling there may be enhanced by the structure of the scales"--P. 455-456.
Description
p. 453-513 : ill., map ; 26 cm.
Includes bibliographical references (p. 512-513).
Includes bibliographical references (p. 512-513).
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