A study by the University of Fribourg suggests that the shell of turtles significantly restricts their evolutionary development, explaining their low species diversity compared to other animal groups.

Although turtles originated over 230 million years ago in the Triassic period, there are only around 350 species today. In contrast, there are more than 10,000 bird species. In their new study, published in the journal Ecology & Evolution, Guilherme Hermanson and Dr. Serjoscha Evers from the University of Fribourg examine the relationship between body size and limb length in turtles, highlighting how these proportions limit the animals’ evolutionary potential.

The shell as an evolutionary constraint
While other animals’ limbs grow and adapt flexibly with body size (allometry), turtles exhibit a fixed body-limb proportion (isometry). «Our measurements of over 200 turtle species, both living and extinct, show that the ratio of shell length to limb length has barely changed over millions of years», explains Hermanson. The shell acts as an evolutionary ‘dead end’: it prevents turtles from developing new body forms, such as winged or limbless varieties.

Exceptions among sea turtles
Sea turtles show slight variations, as they use their front flippers for swimming. Using statistical scaling models, the researchers were also able to estimate the size of extinct species, suggesting that the maximum shell length of sea turtles is around 2.2 meters —similar to that of the modern leatherback turtle. Sea turtles may not be able to grow larger because they need to return to land to lay their eggs, where larger animals overheat more quickly. Marine animals like whales and dolphins or extinct reptiles like plesiosaurs and ichthyosaurs didn’t have this limitation: they gave birth to their young in the water and could therefore grow much larger.

Study:
Hermanson G, Evers SW. 2024. Shell constraints on evolutionary body size-limb size allometry can explain morphological conservatism in the turtle body plan. Ecology & Evolution, 14. https://doi.org/10.1002/ece3.70504