Reconstructing the mass and thermal ecology of North American Pleistocene tortoises

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.

Zooarchaeological investigation of the Hoabinhian exploitation of reptiles and amphibians in Thailand and Cambodia with a focus on the Yellow-headed Tortoise (Indotestudo elongata(Blyth, 1854))

“…In order to reliably estimate the body size of the archaeological individuals of Indotestudo sp., we built size estimation equations based on the models of what was previously done for Southeast Asian monitor lizards (Bochaton et al, 2019a), and recently on the size and weight of species of tortoises (Codron et al, 2022; Esker et al, 2019). These approaches are more powerful than considering isolated measurements (e.g., Klein and Cruz-Uribe 1983) to describe the size of subfossil animal populations because: 1) they enable us to take into account several measurements from different anatomical parts to reconstruct the body size structure of a past population, and 2) they convert measurements taken on the skeleton into a variable used to describe the size of modern individuals, which makes it easier to make comparisons needed to address biological questions.…”

Zooarchaeological investigation of the Hoabinhian exploitation of reptiles and amphibians in Thailand and Cambodia with a focus on the Yellow-headed Tortoise (Indotestudo elongata (Blyth, 1854))

“…In order to reliably estimate the body size of the archaeological individuals of Indotestudo sp., we built size estimation equations based on the models of what was previously done for Southeast Asian monitor lizards (Bochaton, Hanot, et al, 2019), and recently on the size and weight of species of tortoises (Esker et al, 2019;Codron et al, 2022). These approaches are more powerful than considering isolated measurements (e.g., Klein and Cruz-Uribe 1983) to describe the size of subfossil animal populations because: 1) they enable us to take into account several measurements from different anatomical parts to reconstruct the body size structure of a past population, and 2) they convert measurements taken on the skeleton into a variable used to describe the size of modern individuals, which makes it easier to make comparisons needed to address biological questions.…”

Quaternary megafauna extinctions altered body size distribution in tortoises

“…Giant tortoises are common in the fossil record since the late Palaeogene [20][21][22] and causal explanations for their extinction during the Quaternary have been investigated [14]. The potential causes of extinction of large tortoises in the late Quaternary include hominin and human exploitation concomitant with global and local changes in climate and habitat [23,24].…”