Animals Tortoise shell severely restricts the evolution of tortoises

The shell of turtles has severely restricted their evolutionary development. This explains their low species diversity compared to other animal groups, as two researchers from the University of Freiburg shed light on in their study in the journal "Ecology & Evolution".

Although turtles evolved 230 million years ago in the Triassic period, there are only around 350 species today. In comparison, there are 10,000 bird species. The two researchers investigated the relationship between body size and limb length in turtles and how these proportions limit the evolutionary possibilities of the animals, as the University of Fribourg (Unifr) announced on Monday.

Tortoises show a fixed body-limb proportion, while in other animals the limbs grow and adapt flexibly with body size. "Our measurements on over 200 turtle species, both living and extinct, show that the ratio of shell to limb length has hardly changed for millions of years," scientist Guilherme Hermanson was quoted as saying in the press release. The shell acts as an evolutionary dead end. It prevents turtles from developing new body forms such as flightless or limbless variants.

Sea turtles show slight deviations as they use their front flippers for locomotion in the water. Using statistical scaling models, the researchers were also able to estimate the size of extinct species and assume that the maximum carapace length of sea turtles is around 2.2 meters, wrote Unifr. Similar to today's leatherback turtles.

Sea turtles may not be able to grow any larger as they have to return to land to lay their eggs, where larger animals would overheat more quickly, it added. Marine animals such as whales and dolphins do not have this limitation. They give birth to their young in the water and have been able to grow much larger as a result.

SDA