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Reconstructing the mass and thermal ecology of North American Pleistocene tortoises

Published online by Cambridge University Press:  01 May 2019

Donald A. Esker ,
Steve L. Forman  and
Dava K. Butler
Donald A. Esker
Affiliation:
Department of Geology, Baylor University, Waco, Texas 76798, U.S.A. E-mail: Don_Esker@Baylor.edu, Steven_Forman@Baylor.edu
Steve L. Forman
Affiliation:
Department of Geology, Baylor University, Waco, Texas 76798, U.S.A. E-mail: Don_Esker@Baylor.edu, Steven_Forman@Baylor.edu
Dava K. Butler
Affiliation:
College of Letters and Science, Montana State University, Post Office Box 172880 Bozeman, Montana 59717, U.S.A. E-mail: DavaKB@gmail.com
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Abstract

Researchers often interpret the presence of tortoises in Pleistocene assemblages as evidence of an interglacial age, based on an assumption that these fossils indicate thermic climates, as modern giant tortoises require. Since the Paleocene, tortoises have been common components of terrestrial fossil assemblages and have repeatedly evolved species of giant size. Whereas extant giant tortoises are found only on islands off the coasts of South America and Africa, at least two species persisted in North America until the terminal Pleistocene. These tortoises, Hesperotestudo crassiscutata and Gopherus “hexagonatus,” both of which reached carapace lengths of >1 m, were distributed across the southern United States. This study provides new metrics to derive quantitative weight estimates from measurements of the tortoise shell. The linear measurements of 69 anatomical features of the shells of 108 live tortoises indicate that the regression between straight carapace length and weight is most significant, with a maximum r2 > 0.99. This regression is useful for tortoises that weigh between 1.8 and 339 kg. This mass estimate, coupled with a heat dissipation rate derived from thermoregulation modeling, provides estimates of how long tortoises can maintain a viable body temperature at low ambient temperatures. Depending on size, a tortoise can survive a maximum of 2.3 to 33 hours of freezing temperatures, which corresponds to a mean annual temperature ≥22°C and a mean winter low temperature ≥7.5°C. This analysis infers warmer temperatures at Pleistocene sites with fossil tortoise occurrences than previous qualitative estimates.

Type
Articles
Information
Paleobiology , Volume 45 , Issue 2 , May 2019 , pp. 363 - 377
Copyright
Copyright © The Paleontological Society. All rights reserved 2019 

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Footnotes

Data available from the Dryad Digital Repository: https://doi.org/10.5061/dryad.2d58rr0

References

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