Since Darwin, we have known that evolutionary adaptation is reflected in the appearance and function of species’ bodies under environmental changes. One of the questions commonly asked by evolutionary biologists is how body shape relates to a specific ecological feature, such as diet. In a recent study published in the journal Evolution, Guilherme Hermanson and his team at the University of Freiburg looked at the environmental factors that affect the shape of turtle skulls.
The skull is an important part of the animal body, as it interacts very directly with the environment, especially during food acquisition and processing. In turtles, this relationship is likely accentuated by the fact that they do not use legs to collect and manipulate their food. Turtles are sometimes overlooked in evolutionary studies. This is partly due to the fact that they have a strange body, largely enclosed in a shell. But it is precisely this feature that makes them interesting from an evolutionary point of view,’ says senior researcher Serjoscha Evers. Turtles have a lot to reveal about the link between evolution and body shape. They live in many environments, and their fossils show that they have repeatedly conquered a variety of terrestrial and marine habitats over the nearly 230 million years of evolutionary history. These recurrent environmental changes provide an excellent subject for testing the influence of adaptation on skulls.
Modern 3D analysis
The team used a state-of-the-art approach that combines 3D skull models of living and extinct turtle species with statistical models linking skull shape to ecology. oeThis is the first study to combine 3D skull models of turtles with sophisticated statistical tools. We were thus able to show that even groups made up of a small number of species, such as turtles, can be used as models to understand the major concepts of evolutionary theory, such as adaptation’, says Guilherme Hermanson about the approach taken. The results indicate that the shape of the skull is determined by multiple influencing factors. In particular, the researchers observed that the skull shape of related turtles often has more similarities than that of more distant species - one of the main postulates of evolutionary theory.
Strong impact of diet and feeding patterns
However, ecological adaptations can intervene and lead to additional changes in the shape of the skull. The study concludes that diet and feeding methods have a strong influence. For example, aquatic turtles have long skulls and forward-facing eyes, useful for hunting prey underwater. In addition, turtles that eat hard-shelled animals, such as snails, have jaws with large crushing surfaces, used instead of teeth to break down the shells. In addition, scientists have found that various influencing factors act on different parts of the skull. While there are many dietary adaptations in the mouth, eyes and overall head size, differences in neck retraction affect the back of the skull to which the relevant muscles are attached.
Ecological factors not decisive
If all these influences have measurable effects on the shape of the turtle’s skull, Guilherme Hermanson’s team found that the ecological factors considered explained only a quarter of the observed differences: a large part of the visible differences in shape must be attributed to non-adaptive factors such as kinship links between turtle species.
Scientists used their findings to postulate the ecology of extinct turtles, using well-preserved fossil skulls. The skulls studied indicate that neck retraction (the ability of turtles to fully retract their heads under their protective shells) probably emerged during the Middle to Upper Jurassic geological era, 150 million years ago. During this period, turtles moved into aquatic habitats for the first time. The team therefore suggests that the diversification of turtles in these habitats is linked to the acquisition of neck retraction, which allowed species to exploit new ecological niches, in which individuals evolved to the diversity of forms observed today.
Thanks to the combination of different 3D models and statistical analyses, it is now possible to explore questions that could not be answered ten years ago. This original method contributes to the development of new scientific approaches to the theory of evolution.
Hermanson G, Benson RBJ, Farina BM, Ferreira GS, Langer MC, Evers SW (2022): Cranial ecomorphology of turtles and neck retraction as a possible trigger of ecological diversification. Evolution, https://doi.org/10.1111/evo.14629
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