Shell size: how turtles evolved over the last 200 million years Diversity of turtle body size studied

Homopus areolatus - one of the smallest recent turtle species.
Homopus areolatus - one of the smallest recent turtle species.

With a shell length of about 100 millimeters, the land-dwelling areolate flat-shelled turtle (Homopus areolatus) is one of today’s smallest turtle species. The record at the other end of the scale is held by the leatherback turtle (Dermochelys coriacea), which is common in tropical and subtropical seas and can reach up to two meters in length. Among fossil turtles, the range of body sizes is even more pronounced," says Dr. Gabriel Ferreira of the Senckenberg Centre for Human Evolution and Palaeoenvironment at the University of Tübingen, and continues: "We were interested in how this diversity of carapace sizes evolved over the last 200 million years and which factors played the decisive role in this."

To this end, the research team, led by first author Bruna Farina of the Swiss University of Fribourg, compiled the most comprehensive collection of data on turtle body sizes to date: For a total of 795 turtle species, the researchers recorded information on carapace lengths, preferred habitats and the temporal occurrence of the species in the history of the earth. Previous studies that focused on size evolution often ignored fossil species. Our results also incorporate data from 536 extinct turtle species - which is essential if you want to understand the evolutionary history and physical adaptations of the shell bearers," Ferreira adds.

The new study shows that today’s global climate does not appear to have a major impact on the body size of the recent turtles studied. Correlation with paleo temperatures also did not indicate a significant effect on the size of the fossil animals, the study says.

Instead of a climatic influence, the ecology and habitat preferences of the turtles are decisive for their body size, according to the study. "The assumption known as ’Cope’s law’ that living things tend to increase in body size over the course of evolution is not demonstrable for turtles," Ferreira explains and continues: "The size spectrum of freshwater species has remained fairly constant over the last 200 million years. In contrast, land and sea turtles show much more pronounced variation." The team explains the different body sizes of land turtles by their ecological diversity and diverse habitats. The larger land-dwelling species would have the advantage of being able to disperse more easily. In sea turtles, on the other hand, the upper and lower limits of body size appear to be related to physiological constraints, such as thermoregulation or increased lung capacity, and morphological constraints, such as carapace size, as well as adaptations to the open-water lifestyle. It is also possible that the need to go ashore to lay eggs limits the maximum size of sea turtles, the researchers said.

"Our result is very interesting, considering that body size in other animal groups - such as dinosaurs or crocodiles - is influenced by climatic factors, such as the temperature prevailing in the habitat. It highlights the uniqueness of turtles," Ferreira concludes.


Farina, B. M., Godoy, P. L., Benson, R. B. J., Langer, M. C., & Ferreira, G. S. (2023). Turtle body size evolution is determined by lineage-specific specializations rather than global trends. Ecology and Evolution , 13, e10201.