Cranial functional morphology of Archaeopteryx and the biomechanical triggers of avian evolution

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Archaeopteryx lithographica is the famous 'missing-link'. Brought to the attention of the world's scientific community only two years after publication of Darwin's On the Origin of the Species, the famous half-bird, half-reptile fossil was hailed as the ultimate evidence for evolution. The study proposed here will reveal new information about the ecology and evolution of this enigmatic fossil, using a rigorously quantified modelling approach. In some respects we know a good deal about Archaeopteryx. It is the most basal bird, and descended from within the theropod dinosaurs. It has a mosaic of bird and theropod characters: a bird-like brain capable of complex motor control, wings with asymmetrical, aerodynamic flight feathers but clawed hands, an elongated tail and jaws lined with teeth rather than a beak. A century and a half later there are still many things that we don't know about Archaeopteryx. How well could it fly? Was it terrestrial and took off from the ground up, or did it live in the trees? Could it run up tree trunks flapping its wings like partridges do? Are changes in function of the skeleton mirrored by changes in the skull? Did cranial evolution keep pace with the rest of the skeleton? Was the Archaeopteryx skull functioning as a bird or a dinosaur? Bones are generally adapted to the loads they experience, and the resulting stress and strain can dictate the shape of the skeleton. Despite lying at the base of a major radiation, we know nothing concrete about how the skull morphology of Archaeopteryx relates to its function. Deducing this would inform on its feeding behaviour, ecology and the selective pressures driving cranial evolution. The aim of this study is to deliver this knowledge. One method to decipher functional clues in the skeleton is the engineering technique Finite Element Analysis (FEA). Users build a digital model of the structure they wish to test (i.e. bridge, Archaeopteryx skull), apply elastic properties and loading forces that mimic the elasticity and behaviour (i.e. biting) to be tested. The analysis calculates strain and stress within the structure in response to these loading forces. This study will use FEA to test whether the skull of Archaeopteryx is adapted for pecking or biting and tearing, and estimate force of its bite, could it penetrate insect cuticle or hard seeds for example? This is important, as it will inform on ecological niches of the earliest flier and if these were predominately terrestrial or arboreal. The study will compare whether Archaeopteryx was functionally similar to a bird or a non-avian theropod, and test assertions that avian skull flexibility (kinesis) was present in Archaeopteryx. This is crucial to understanding the sequence and timing of the acquisition of avian characters, whether the skull evolves apace with the remaining skeleton and whether the skulls of early birds were more morphologically diverse than their non-avian theropod ancestors. The problem with FEA is that we know little of how well model results reflect reality, and which input parameters matter the most. Because of this, I will verify the accuracy of FEA first. I will load a dead, defleshed ostrich skull with carefully quantified force and experimentally measure bone strain. I will measure the elasticity of cranial bone, force exerted by the adductor muscles and use CT scans to accurately reconstruct the ostrich skull in FE-software. I will then apply the same loads to the FE-model and quantify the difference in orientation and magnitude of the experimental versus FE-model derived strain. By altering elastic properties, loads and muscle force in a sensitivity analysis, I will see which parameters influence model results the most. The resulting information can be used to make extinct animal FE-models as rigorous as possible based on what we know of the input parameters, the results being of importance to all those interested in FEA in biology.