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Does reptile body size track climate change over millions of years?


Climate change particularly affects ectothermic reptiles that cannot regulate their internal temperature metabolically. But it is difficult to anticipate how the unprecedented pace of current climate change will affect these animals based only on recent data. We need a larger temporal context based on historical data spanning deeper time scales and past episodes of rapid climate change. Body size is a suitable response variable to test in such investigations because data from extant taxa can predict body size for fossil specimens from the same lineages. We can also reconstruct past climate parameters from terrestrial and marine proxies. Metabolic theory for ectothermic vertebrates predicts that maximum body size should correlate with environmental temperature over ecological time scales. Here, I test this theory on an evolutionary time scale, using both paleotemperature and paleoprecipitation, for two different higher order reptile groups occupying different habitats. I hypothesize that maximum snout-vent length (SVL) in terrestrial lizards and semiaquatic crocodyliforms tracks temperature and precipitation over geologic time intervals, and that these patterns emerge across both regional and local geographic scales.

I measured 283 lizard and 280 crocodyliform fossil specimens from intermontane basins across the Western Interior of North America through the Paleogene (66-23 million years ago), which spans several warming and cooling events. Most of the fossil record of these animals consists of individual cranial or limb bones. I therefore collected an additional extensive dataset of measurements from extant specimens to develop regression equations for estimating SVL from isolated anatomical elements. I applied these methods to reconstruct lizard body sizes through the Paleogene using the available fossil record (Chapter 1). I then used similar methods to investigate whether deep time body size evolution patterns in terrestrial lizards compared to those of contemporaneous amphibious crocodyliforms, and to see how those patterns compared at regional vs. local geographic scales (Chapter 2). Finally, I collected over 100 estimates each for mean annual paleotemperature (MAPT) and paleoprecipitation (MAPP) and tested for correlation between these variables and maximum body size in lizards or crocodyliforms (Chapter 3).

My results indicated that during the warmest interval in the early Eocene, maximum lizard body size increased to almost one meter, even rivaling some co-occurring crocodyliforms in body size. Maximum lizard SVL demonstrated a positive linear relationship with local terrestrial temperature within basin assemblages over geologic time scales but did not correlate as strongly with temperatures averaged across the Western Interior. In contrast to the lizards, maximum crocodyliform SVL (about 2 meters) was consistently high across the intermontane basins through the Paleogene and indicated a strong relationship to paleoprecipitation rather than paleotemperature. Large-bodied crocodyliforms were most abundant in localities that hosted large bodies of water at the time of deposition. Maximum body size and diversity decreased for both lizards and crocodyliforms in the early Oligocene, when the Western Interior experienced cooling and aridification. Neontological studies of lizard and crocodylian ecology and physiology corroborate these paleontological observations. These results offer new evidence that climate variables affect body size in ectothermic reptiles on evolutionary time scales, which deepens our understanding of these dynamics on ecological time scales. Studies that integrate data across time scales and biological hierarchies and can inform conservation efforts under current rapid climate change.

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