The areole flat-bottomed turtle (Homopus areolatus), one of the smallest modern turtle species, is only 100 millimeters long. In contrast, the largest turtle in the world measures a proud two meters, it is the leatherback turtle, which is widespread in tropical and subtropical seas. How come these size differences? Gabriel Ferreira from the Senckenberg Center for Human Evolution and Palaeoenvironment at the University of Tübingen investigated this together with an international team of researchers: “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 795 turtle species that are still alive or have already died out, the researchers therefore recorded information on the shell lengths, preferred habitats and the time when the species appeared in the history of the earth.
Their study shows that the global climate appears to have little effect on the body size of the extant turtles studied. This applies to the present time, but also to the paleoclimate. The correlation with temperatures prevailing in prehistory also showed no significant effect on the size of the fossil animals. 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 cannot be demonstrated for turtles,” explains Ferreira. “The size range of freshwater species has remained fairly constant over the past 200 million years. In contrast, tortoises and sea turtles show a much more pronounced variation.” Ferreira: “Our result is very interesting when you consider that the body size of other animal groups – such as dinosaurs or crocodiles – depends on climatic factors such as those prevailing in the habitat temperature. It underlines the uniqueness of turtles.”