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Rise of the tetrapods

When the first vertebrates crawled out of the sea and heaved itself onto the land it was one of the most momentous events in the entire history of life on our planet. Many fossils bear witness to the change from aquatic lobe-finned fishes to four-limbed terrestrial creatures, which are described as tetrapods and include amphibians, reptiles, birds and mammals.
Rise of the tetrapods
The TETRAPODS RISING (Tetrapods rising: Linking changes in mandibular form with function across the fish-tetrapod transition) project investigated how changes in the lower jaw during the key evolutionary transition from water to land are linked to mechanical behaviour and ecology.

Computed tomographic scans were conducted on 15 fossil skulls and the heads of 2 extant fish: the European eel (Anguilla anguilla) and pike (Esox Lucius). Visualisation software was used to strip away the matrix from the fossils and muscle and bone tissue in the extant species, and to separate individual bones and teeth. New techniques were developed to create 3D reconstructions of damaged fossil skulls.

High-speed video recordings of eel and pike were used for accurate modelling of feeding behaviour and to validate models. Bite force was recorded using force transducers, while nanoindentation was used to obtain bone material properties from the lower jaw. This information, together with the 3D reconstructions, was used to produce 3D finite element (FE) models of the lower jaws.

The FE models provided deformation, strain and stress under load data, enabling researchers to understand and measure the mechanical behaviour of the lower jaw under feeding loads. Findings from FE models of the lower jaws of early tetrapods, eel, pike and alligator were compared in order to answer questions about the link between changes in the shape of the lower jaw and mechanical behaviour.

Finite element analysis (FEA) was conducted in the lower jaws of early tetrapods, extant fish and alligators. Results support the hypothesis that changes in mandibular morphology during tetrapod evolution resulted in a stronger lower jaw, better adapted to biting. This could be because tetrapods no longer needed to be streamlined once they moved from the water onto land.

In addition, stress was more evenly distributed in the lower jaws of tetrapods than in fish, and total strain energy decreased along the lineage. This indicates that less applied (i.e. muscular) force was expended to deform the model, suggesting a stiffer more efficient structure.

TETRAPOD RISING set the benchmark for performing FEA in a broader evolutionary context than previously attempted. It also established techniques for the 3D reconstruction of fossil skulls. The result was a new understanding of the morphological and functional evolution of the tetrapod lower jaw during the transition from water to land. The large, novel data sets generated by the project will be a major benefit to future researchers.

Related information


Tetrapods, fossils, fish, evolution, lower jaw
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