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

Lead Research Organisation: University of Bristol
Department Name: Earth Sciences


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.
Description The significance of this study is that FE-models of avian jaws, even with simple material properties, can reproduce reliable patterns of bone strain. This has implications for the growing numbers of biologists, anthropologists and palaeontologists using finite element methods to understand skeletal evolution and function.

Major milestones achieved:

1. First recording of strain in the skull of birds using a subject-specific testing rig

2. First determination of pecking forces in a large bird (so far data only exists for pigeons)

3. CT data and data from experimental strain recording used to create the first validation of an avian finite element model.

4. Validation study shows that strain patterns, orientation, and, if material properties are correct, magnitude, can be achieved in an 'in-silico' virtual loading system. This has important implications for the growing number of people using FEA to explore the biomechanics of living and fossil organisms. The paper describing these results is in review at the Journal of Anatomy and formed the basis of an invited talk at a symposium on Craniofacial Biomechanics organized by the York-Hull Medical School in July 2009.

5. I organised two conference symposia on topics related to this research: (1) 'Functional morphology at the intersection between biology and engineering' at the 3rd International Palaeontological Congress, London, June 2010; (2) 'Limitations of computational reconstructions in musculoskeletal evolution' at the 9th International Congress of Vertebrate Morphology, Punta del Este, Uruguay, July 2010.
Exploitation Route The finite element method (FEM) offers biologists and palaeontologists a new technique to explore the function and morphological evolution of the animal skeleton but the main drawback is that for most taxa we have no idea how well our results reflect reality. What this research has provided is a detailed understanding of how accurately FEA can replicate strain in the jaws of birds subject to quasi-functional, controlled loading conditions. This research is of major interest to palaeontologists, zoologists, anthropologists and even orthopaedic scientists that are interested in the function of the vertebrate skeleton.
Sectors Other

Description The key paper resulting from this work has been cited 17 times and is still the only FE validation of a bird skull or postcrania.
First Year Of Impact 2011
Sector Other
Impact Types Cultural

Description Australian Science TV show: Catalyst 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact I featured in the popular Australian science show, Catalyst, discussing my work on the ostrich jaws:

I was filmed in the lab running strain gauge experiments on the ostrich jaw, and then explaining the need for validation of finite element and other biomechanical approaches on fossils. Filming took at day of my time, which was offered free of charge.

not aware of any direct impact
Year(s) Of Engagement Activity 2008