(Image: Pixabay CC0)A study by the University of Fribourg suggests that turtles’ shells are a major hindrance to their evolutionary development, which would explain the low number of species compared with other animal groups.
Although turtles first appeared over 230 million years ago, in the Triassic period, only around 350 species are known today. By comparison, there are over 10,000 species of birds. In their new study, published in the journal Ecology & Evolution, Guilherme Hermanson and Serjoscha Evers from the University of Fribourg examine the relationship between body size and limb length in turtles, highlighting how these proportions restrict the evolutionary potential of these animals.
The shell as an evolutionary constraint
Whereas the limbs of other animals grow and adapt according to body size (allometry), turtles have a fixed proportion between body and limbs (isometry). Our measurements of over 200 species of turtle, both living and extinct, show that the ratio of carapace to limb length has remained virtually unchanged 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.
Sea turtle exceptions
Sea turtles show slight variations, as they use their front flippers to swim. Using statistical scaling models, researchers were also able to estimate the size of extinct species, suggesting that the maximum shell length of sea turtles could reach around 2.2 meters - comparable to that of the modern leatherback. Sea turtles may not be able to reach a larger size, as they have to return to land to lay their eggs, where larger animals overheat more quickly. Marine animals such as whales and dolphins, or extinct reptiles like plesiosaurs and ichthyosaurs, had no such constraints: they gave birth to their young in the water, and were thus able to reach much larger sizes.
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
