Thursday, November 4, 2010

Cordylid Lizard Fossils, Phylogeny, and Sensitivity to Climate Change

The saurian family known as the Girdled Lizards (Cordylidae) has about 80 species with common names like, Armadillo Lizards, Sungazers, and Flat Lizards. They are mostly rock-dwelling scinciform lizards that have at least some dermal armor. Living members are endemic to Sub-Saharan Africa and have been traditionally placed in four  genera: the Flat Lizards, Platysaurus with exceptionally depressed bodies; the African Grass Lizards, Chamaesaura, reduced limb species with long tails; the Crag Lizards, Pseudocordylus, crevice-dwelling lizards; and the Armadillio Lizards of the genus Cordylus, heavily armored species. But, there are fossil species known from the early to middle Miocene of Europe: Palaeocordylus bohemicus and Bavaricordylus ornatus.  Böhme (2010) has reported on the fossil species at Sandelzhausen Germany, a site that produced remains of several cordylids. The Miocene of Sandelzhausen had a quite different climate than it does today. It was semi-arid to sub-humid, with 571-847 mm of mean annual precipitation making it subtropical with mean annual temperatures ranging from 18°C to 20.8°C, mean cold month temperatures as high as 13.3°C and mean warm month temperatures as high as 28.1°C.

A recent molecular study has produced a new view of the cordylids. Stanley et al. (2010) used three nuclear and three mitochondrial genes from 111 specimens, representing 51 of the 80 known taxa and recovered a comb-like tree with 10, well-supported, monophyletic lineages.Their taxonomic reassessment divides the family into 10 genera that correspond to the well-supported lineages. They recovered evidence that supports two subfamilies within the Cordylidae, the egg-laying Platysaurinae and the live-bearing Cordylinae. A preliminary dating analysis place the diversification of the Cordylinae between 35 and 25 million years ago, during the Oligocene a period when southern Africa would have been cool and dry. This may explain why these lizards evolved viviparity, one hypothesis suggests the ability to bear live young is an adaptation to cold environments, allowing the live-bearing female to carefully regulate the temperature of the developing embryos by shuttling between warm and cool microhabitats. The authors found that the reproductive strategies of the two cordylid subfamilies are consistent with this pattern, the viviparous cordylines inhabit temperate areas, at higher altitudes and higher latitudes, while the oviparous platysaurines are restricted to the tropics and subtropics. The Oligocene was cool and dry and may have been an ideal environment for the cold-adapted, rock-dwelling, stem cordylines.

The challenges lizards face from a warming planet was recently examined by Tolley (2010), she reviewed recent research and specifically addressed the cordylids because they are rock dwelling species, and rocks heat rapidly and cool slowly. Thus, the Cordylidae could be more susceptible to temperature increases than ground-dwelling or arboreal lizards because their microhabitats would be unusable for longer periods of time. The situation for cordylines may be worse than other lizards because viviparous lizards are thought to be more sensitive to temperature changes and believed twice as likely to go extinct as egg-laying species. Extinctions are not predicted to be high for members of the Cordylidae at present, nor over the next 40 years, but their risk of extinction is expected to increase by the year 2080.

Literature
Böhme, M. 2010. Ectothermic vertebrates (Actinopterygii, Allocaudata, Urodela, Anura, Crocodylia, Squamata) from the Miocene of Sandelzhausen (Germany, Bavaria) and their implications for environment reconstruction and palaeoclimate. Paläontologische Zeitschrift, 84:3-41, DOI: 10.1007/s12542-010-0050-4.

Stanley, E. L., A. M. Bauer, T. R. Jackman, W. R. Branch and P. Le Fras N. Mouton. 2010. Between a rock and a hard polytomy: Rapid radiation in the rupicolous girdled lizards (Squamata: Cordylidae)
Molecular Phylogenetics and Evolution, doi:10.1016/j.ympev.2010.08.024.

Tolley, K. A. 2010. Can lizards beat the heat, or will they go extinct? South African Journal of Science 2010;106(5/6), 2 pages. DOI: 10.4102/sajs. v106i5/6.278. This article is available at: http://www.sajs.co.za



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