The utility of body size as a functional trait to link the past and present in a diverse reptile clade

Significance: Functional traits are important for forecasting biotic response to climate change. However, the fit of traits to environment is often poorly understood across habits and habitats. We test the functional relationship of body size to climate and the role of metabolism in governing size in turtles. Complex relationships between size and environment in modern turtles suggest that temperature-dependent metabolism is not a primary functional driver of maximum size and that human impacts and other biotic interactions have disrupted modern turtle body size distributions. Using modern trait–environment relationships to estimate paleoenvironments in the fossil record can coarsely predict paleoclimate. Further inclusion of fossil data to examine functional traits may improve the ability to assess and forecast future responses.

Abstract: Understanding the relationships between functional traits and environment is increasingly important for assessing ecosystem health and forecasting biotic responses to future environmental change. Taxon-free analyses of functional traits (ecometrics) allow for testing the performance of such traits through time, utilizing both the fossil record and paleoenvironmental proxies. Here, we test the role of body size as a functional trait with respect to climate, using turtles as a model system. We examine the influence of mass-specific metabolic rate as a functional factor in the sorting of body size with environmental temperature and investigate the utility of community body size composition as an ecometric correlated to climate variables. We then apply our results to the fossil record of the Plio-Pleistocene Shungura Formation in Ethiopia. Results show that turtle body sizes scale with mass-specific metabolic rate for larger taxa, but not for the majority of species, indicating that metabolism is not a primary driver of size. Body size ecometrics have stronger predictive power at continental than at global scales, but without a single, dominant predictive functional relationship. Application of ecometrics to the Shungura fossil record suggests that turtle paleocommunity ecometrics coarsely track independent paleoclimate estimates at local scales. We hypothesize that both human disruption and biotic interactions limit the ecometric fit of size to climate in this clade. Nonetheless, examination of the consistency of trait–environment relationships through deep and shallow time provides a means for testing anthropogenic influences on ecosystems.

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(A) Geographic distributions of species richness (Top) and maximum body size, measured as carapace length (Bottom), for extant turtles. Rectangles denote ranges used for continental-scale ecometric analyses. (B) Turtle fossils from the Shungura Formation of Ethiopia (yellow star on Inset map). (Top) The aquatic trionychid turtle Trionyx cf. triunguis, carapace in dorsal view, rock hammer for scale, Member G. (Bottom) The terrestrial tortoise cf. Centrochelys sp., partial carapace, girdles, and appendicular skeleton in visceral view, with tape measure for scale, Member H.

References

Parker A.K., Müller J., Boisserie J.-R., Head J. 2023 – The utility of body size as a funtional trait to link the past and present in a diverse reptile clade – Proceedings of the National Academy of Sciences, vol. 120, n° 7, e2201948119 – doi.org/10.1073/pnas.2201948119