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Skull form and function in therizinosaur dinosaurs, and the convergent evolution of herbivory in theropods

Supervisors: Dr E. J. Rayfield and Professor L. M Witmer (Ohio, USA)

Computed tomography image of Erlikosaurus andrewsi.
Do not use or distribute this image without permission of EJ Rayfield

Therizinosaurs are an enigmatic clade of theropod dinosaurs, found in Cretaceous deposits in Asia and North America (1). They have previously been allied to prosauropod and ornithischian dinosaurs, however subsequent discoveries have firmly placed the group within the Maniraptoria, as a derived theropod clade. The reason for such taxonomic uncertaintly is that therizinosaurs possess an unusual suite of characters including an elongated neck, wide pelvis and long arms tipped with sickle-shaped claws that were lined with primitive feathers. These features coupled with the presence of blunt leaf-shaped teeth suggest a diet of plants, rather than meat. Furthermore, their wide pelvic regions may have supported a large, fermenting gut, and the sickle claws may have been utilized for cropping vegetation, rather than predation.

In many clades (e.g. basal amniotes, ornithischian dinosaurs) the evolution of herbivory is frequently considered to be a ‘key innovation’, releasing organisms from the constraints of a carnivorous or insectivorous diet, creating access to new ecological niches, promoting diversification (species richness) and morphological variation (disparity). Herbivory is thought to have existed in at least three derived clades of theropod dinosaurs, yet the role of herbivory in theropod dinosaur diversification (including the lineages closely related to the origin of birds) has not been explored. Recently a number of morphological features associated with becoming a herbivorous theropod were proposed (2). These include tooth loss in the upper and lower jaw, the presence of a rhamphotheca beak-like structure, a convex dentary, conical rostral teeth, and a U-shaped dentary symphysis. All features are thought to have been acquired convergently in at least three theropod lineages and their correlated appearance strongly suggests a role in the functional acquisition of herbivory. However, it is not known if and indeed how such features convey a functional advantage, and at what stage of their development or acquisition does this advantage appear. To deduce whether this is the case is the purpose of this project.

This will be achieved by firstly conducting an analysis of the biomechanics and function of the therizinosaur crainum, focusing primarily on Erlicosaurus, but also utilizing anatomical and CT scan data from cranial material of the basal therizinosauroid Falcarius (3) and the therizinosaurid Nothronychus (4). This will be achieved through a finite element analysis (FEA) of the skull and jaws, in conjunction with detailed reconstruction of soft tissues such as adductor (jaw closing) musculature. FEA is an engineering technique, also frequently used in orthopaedic medicine, that reconstructs stress and strain within a digital model after the application of functional loads. It is becoming more frequently used to examine the biomechanics of fossil organisms, including dinosaurs (see 5,6). By using digital FEA models, there exists the ability to digitally manipulate model morphology and run successive analyses to test what is the effect of adding, removing or modifying cranial features to stress and strain in the skull and jaw. If modifying a feature makes the skull more stressed, or weaker, then it is possible that this modification is not advantageous. The FE-models will be manipulated to add or remove features associated with herbivory such as the U-shaped, downturned and convex dentary, loss of teeth, development of a beak and conical teeth. These analyses can be run using a ‘hypothesis testing’ approach, creating a testing specific hypotheses that certain features of anatomy highlighted as convergent characters will indeed confer a functional advantage during FE-simulated feeding.

The advent of dietary plasticity at this stage in the theropod clade now proposed to be linked to cladogenesis and morphological diversification in the group. So far research has mainly focused on descriptive and taxonomic work; this project offers a means to fully qualify such assertions with the application of functional morphology and biomechanics.

Training

Candidates with a strong background in palaeontology, biology or geology will be well-suited to this project. Training will be provided in how to execute a successful research programme, in a ddition to specific training in tomographic (CT) reconstruction techniques (both in house and in the lab of Prof. Witmer), and finite element analysis.

Key references

Last updated: 9/11/09