Date of this Version
Whiting, E.T. 2016. Constraining Neogene temperature and precipitation histories in the Central Great Plains using the fossil record of Alligator. M.S. Thesis, Department of Earth and Atmospheric Sciences, University of Nebraska-Lincoln, Lincoln, Nebraska, USA.
Most amphibians and reptiles (excluding birds) are poikilothermic; their internal body temperature varies with that of their external environment. This makes them useful as climate proxies, especially when linked to geographic distributions of ambient climate. I evaluate the utility of the extant crocodylian genus Alligator as a paleoclimate proxy for the Central Great Plains (CGP) using species distribution modeling. Alligator is a readily identifiable taxon with a good CGP fossil record during the Neogene (~23–2.6 Ma). Alligator first appeared in the CGP in the late Eocene (~37 Ma), was absent during most of the Oligocene, reappeared in the early Miocene (~19 Ma), and was extirpated in the late Miocene (~9–6 Ma). This history of occurrence and extirpation implies substantial climatic and environmental changes through time.
To quantify these changes, I constructed species distribution models for extant American alligators using 19 climatic variables and the Maximum Entropy algorithm. I found that living Alligator occupies most of its potential geographic range based on modern climatic parameters and is therefore a useful climate proxy. Driest quarter precipitation is the primary variable constraining Alligator distributions, which contrasts with results from other studies suggesting that coldest month mean temperature is the most important factor. Model results and the fossil distribution of Alligator suggest that the CGP witnessed increasing aridity and decreasing temperatures during the late Miocene before the spread of C4 grasslands; this agrees with several other independent proxy reconstructions. The presence or absence of fossil Alligator, used in concert with these other proxies, can therefore help constrain paleoclimatic conditions in the CGP during the Neogene.
Advisor: Sherilyn C. Fritz