Body size histories of Shungura Formation reptiles in biotic and abiotic environmental context
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Abstract
To investigate reptile body size as an ecological indicator and the relationships between size and environmental variables through time, we compared patterns of maximum size from the Plio-Pleistocene Shungura Formation of Ethiopia. For this previously-undescribed reptile fossil record, we provided estimates of body mass for pythonid snakes, aquatic pelomedusid and trionychid turtles, terrestrial testudinid turtles, whose carapace lengths reach over 1m, and crocodylians including Crocodylus, cf. Mecistops, and the tubulirostrine Euthecodon, which is the largest known crocodylian from the Early Pleistocene. Body size maxima in aquatic taxa correspond to lake level, with large body size observed in aquatic turtles, crocodylians, and hippopotamids during lake high stands on the north side of the Turkana Depression. However, these semi-aquatic groups display heterogenous trends over time and relationships to hydrologic proxies, indicating that their differential niches in these aquatic habitats were linked to different conditions and food resources. Terrestrial tortoises (Testudinidae) exceed 100 kg in mass in three members of the Shungura Formation, but are absent at large sizes between 2.3 Ma and 2.1 Ma, during the main period of hominid stone tool production. We tested for correlation between reptile maximum sizes, mammal maximum sizes and faunal metrics, δ18O and δ13C records from paleosols and mammal tooth enamel, and paleotemperature estimates across members of the Shungura Formation. After correction for multiple comparisons, no correlation tests between reptile size and paleoenvironment or mammal metrics are significant. However, high correlation coefficients between size maxima and paleosol δ18O values suggest temporal coincidence between size change and shifts in hydrological regimes and evaporation levels. These results identify links between maximum size in reptiles and local environments that, if confirmed by trends in other settings, could be utilized in the future as paleoenvironmental proxies for terrestrial vegetation and aquatic habitats.
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Relationships between environment and morphology—particularly body size—are a common topic of study in vertebrate paleoecology. In the face of contemporary anthropogenic climate change, the fossil record is a key to understanding what the future might hold for today’s ecosystems. Similarly, questions around the origin of humans are often framed against the backdrop of local, regional, and global environmental change. This contribution by Parker et al. (2025) explores relationships between body size and local environmental proxies for reptiles in the Shungura Formation of Ethiopia during the Plio-Pleistocene (~3.5 to 1.2 Ma). This formation has particular significance for understanding human evolution, alongside an extensive non-human vertebrate fossil record and rigorously documented geological context.
In this study, the research team …
Relationships between environment and morphology—particularly body size—are a common topic of study in vertebrate paleoecology. In the face of contemporary anthropogenic climate change, the fossil record is a key to understanding what the future might hold for today’s ecosystems. Similarly, questions around the origin of humans are often framed against the backdrop of local, regional, and global environmental change. This contribution by Parker et al. (2025) explores relationships between body size and local environmental proxies for reptiles in the Shungura Formation of Ethiopia during the Plio-Pleistocene (~3.5 to 1.2 Ma). This formation has particular significance for understanding human evolution, alongside an extensive non-human vertebrate fossil record and rigorously documented geological context.
In this study, the research team tested for correlations between the maximum size of a variety of reptiles, including snakes, turtles, and crocodylians, with faunal variables and environmental proxies across multiple levels of the Shungura Formation. Due to the necessity of correcting for multiple comparisons as well as relatively small sample sizes, no tested relationship between body length and other variables was statistically significant. Nonetheless, some correlation values are quite strong, suggestive still of a tight relationship in some cases, such as a negative correlation between pelomedusid turtle size and inferred lake level. Unfortunately, but not unexpectedly, the incomplete nature of individual fossils complicated size estimation. As one example, crocodylian body length was estimated from a regression incorporating skull length. However, skull length often had to be estimated, such as from alveolar spacing in snout fragments. Although this undoubtedly introduces error, it is a practical necessity for maximizing sample size in this case.
The current stage of work does not have much for statistically definitive answers, but it points the way to great potential for future studies of reptile size evolution in the Shungura Formation. Certainly, additional collecting will be a critical part of this. Although ungulates and hominids often get much of the glory in studies of environmental change in eastern Africa, this new contribution hints at how crocs, turtles, and snakes are another important piece of the puzzle.
References
Parker, A., Boisserie, J.-R., Müller, J., Brochu, C. A., and Head, J. J. (2025). Body size histories of Shungura Formation reptiles in biotic and abiotic environmental context. PaleorXiv, 2p6e3, ver. 3 peer-reviewed by PCI Paleo. https://doi.org/10.31233/osf.io/2p6e3_v3
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