According to Cope’s rule, today’s species are on average larger than their distant ancestors. A broad study by a researcher at the University of Fribourg has just shown that that is not the case among turtles.

Paleontologists have noticed that certain species tend to grow in size as they evolve over time. Hyracotherium, for instance, an ancestor of the horse which lived some fifty million years ago, measured no more than 20 cm at its withers, significantly smaller than present-day horses. This tendency of population lineages to increase in body size over evolutionary time is known as Cope’s rule (after the paleontologist Edward Drinker Cope). Cope’s rule, however, does not apply to turtles, as a large-scale study published in the scientific journal Ecology and Evolution has recently demonstrated. The study was led by Bruna M. Farina of the University of Fribourg’s Department of Biology.

The most comprehensive study to date
Turtles (Testudines) represent a field that is especially fertile for studying the evolution of body size. There are over 357 known living species and therefore numerous lineages that can be studied to trace evolutionary processes at work. In addition, turtles vary remarkably in size. The smallest living species, the speckled Cape tortoise (Homopus signatus, more commonly called in the scientific literature Chersobius signatus) usually measures no more than 10 cm in length, while the largest, the leatherback sea turtle (Ermochelys coriacea), can exceed 2.2 m. Turtle fossils offer even greater variance. The shell of the extinct Stupendemys geographicus can measure over 2.8 m long! “That’s why it seemed essential to us to take into consideration the range of fossils when studying turtles, which earlier studies rarely did,” Ms. Farina explained. In the end, Ms. Farina, with the support of four colleagues from Brazilian, American and German institutions, studied 795 species, including 536 extinct ones, a corpus that is over twice as large as those involved in earlier studies. “It’s even the largest database of its kind assembled until now,” the University of Fribourg paleontologist cheerfully added.

Neither smaller, nor larger, quite the contrary

From this extensive study, it appears that:

• There is no proof of directional evolution in the body size of turtles, neither from small to larger, as Cope’s rule posits, nor the reverse.
• Variance in environmental temperature seems to have no effect on the size of turtles.
• Habitat, on the other hand, plays a significant role: body size of freshwater turtles remains uniform over time, while sea turtles and tortoises (land turtles) show noticeable evolutionary variations.

A rule but not a universal law
Ms. Farina has reached a conclusion that qualifies a long-standing concept in paleontology. “These results demonstrate that Cope’s rule applies only rarely to vertebrates, except for certain lineages of mammals and pterosaurs, an extinct order of flying reptiles”, Ms. Farina noted. The study also allows us to better understand the specific evolution of turtles. For the University of Fribourg researcher, the data collected will serve as the basis of further analyses and thus throw new light on the evolutionary history of turtles.