Study undermines evolutionary rule

 (Image: Pixabay CC0)
(Image: Pixabay CC0)

According to Cope’s rule, today’s animal species are on average larger than older species of the same genus. A large-scale study led by a researcher at the University of Fribourg has just demonstrated that this is not the case in turtles.

Paleontologists have noticed that, in the course of their evolution, certain species tend to get bigger and bigger. For example, Hyracotherium, an ancestor of the horse that lived some 50 million years ago, was no more than 20 cm at the withers - much smaller than today’s horses. This tendency of lineages to evolve towards a larger body size is known as Cope’s rule. However, this rule does not apply to turtles, as demonstrated by a major study published in the scientific journal Ecology and Evolution, led by Bruna M. Farina of the Biology Department at the University of Fribourg.

A more exhaustive study than ever

Turtles are an ideal field in which to study the evolution of body size. With over 357 known living species, there are many lineages to study in order to trace their evolutionary processes. What’s more, turtles are characterized by a remarkable disparity in size: the smallest living species(Chersobius signatus) doesn’t exceed 10 centimetres, while the largest (Dermochelys coriacea) can exceed 2m20. Turtle fossils, meanwhile, show an even wider range: the shell of Stupendemys geographicus can exceed 2m80! Bruna M. Farina explains: "This is why we felt it was essential to take into account the diversity of fossils when studying turtles, something that previous studies have rarely done.In the end, with the support of four colleagues from Brazilian, American and German institutions, the researcher studied 795 species, including 536 extinct ones, a corpus more than twice the size of previous studies. It’s even the largest database of its kind to date", adds the University of Fribourg paleontologist.

Neither smaller nor larger, quite the contrary

This extensive study shows that:

  • There is no evidence of directional changes in turtle body size in either direction.
  • Changes in temperature do not appear to affect turtle size.
  • Habitat, on the other hand, plays a significant role: freshwater turtle body sizes remain homogeneous over time, while those of marine and terrestrial turtles show marked variations.

A rule but not a universal law

Bruna M. Farina has come to a conclusion that nuances an old concept in paleontology. These results show that Cope’s rule rarely applies to vertebrates, with the exception of certain mammalian lineages and pterosaurs, an extinct order of flying reptiles", concludes Bruna M. Farina. The study also provides a better understanding of the particular evolution of turtles. For the researcher from the University of Fribourg, the data gathered will serve as a basis for further analyses that will further clarify the evolutionary history of turtles.