University Of Tübingen Study Examines Turtle Body Size Diversity That Evolved Over The Past 200 Million Years

With a shell length of about 100 millimeters, the Areolan Flat-Tortoise (Homopus areolatus), which lives on land, is one of the smallest modern-day tortoise species. The record at the other end of the scale is held by the leatherback sea turtle (Dermochelys coriacea), which is widespread in tropical and subtropical seas and can be up to two meters long. “Among the fossil turtles, the range of body sizes is even more pronounced,” says Dr. Gabriel Ferreira from the Senckenberg Center for Human Evolution and Palaeoenvironment at the University of Tübingen and continues: “We were interested in how this variety of shell sizes developed over the last 200 million years and which factors played the decisive role in this.”

For this purpose, the research team, led by first author Bruna Farina from the Swiss University of Freiburg, created the most comprehensive data collection on turtle body sizes to date: For a total of 795 turtle species, the researchers recorded information on the shell lengths, preferred habitats and the chronological occurrence of the species in the history of the earth . “Previous studies that looked at size development often ignored the fossil species. Our results also include data from 536 extinct species of tortoises, which is essential if you want to understand the evolutionary history and physical adaptations of shell-bearers,” adds Ferreira.

The new study shows that today’s global climate does not appear to have a major impact on the body size of the extant turtles studied. The correlation with paleotemperatures also showed no significant effect on the size of the fossil animals, the study said.

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 ‘Copesh’s law’ that living things tend to increase in size over the course of evolution is not demonstrable for turtles,” explains Ferreira and continues: “The size range of freshwater species has remained correct over the last 200 million years constant. In contrast, tortoises and sea turtles show a much more pronounced variation.” The team explains the different body sizes of tortoises by their ecological diversity and their diverse habitats. The larger terrestrial species would have the advantage of being able to spread more easily. In sea turtles, on the other hand, the upper and lower body size limits seem to be related to physiological constraints, such as thermoregulation or increased lung capacity, and morphological constraints, such as shell size, as well as adaptations to the free water lifestyle. The need to go ashore to lay eggs may also limit the maximum size of sea turtles, the researchers said.

“Our result is very interesting when you consider that the body size of other animal groups – such as dinosaurs or crocodiles – is influenced by climatic factors such as the temperature prevailing in the habitat. It underscores the uniqueness of turtles,” concludes Ferreira.