NERC GW4+ DTP PhD studentship: Evolution’s First Rule: Testing for a Driven Trend in Complexity among Tetrapods

This project is one of a number that are in competition for funding from the NERC Great Western Four+ Doctoral Training Partnership (GW4+ DTP). The GW4+ DTP consists of the Great Western Four alliance of the University of Bath, University of Bristol, Cardiff University and the University of Exeter plus six Research Organisation partners: British Antarctic Survey, British Geological Survey, Centre for Ecology and Hydrology, the Met Office, the Natural History Museum and Plymouth Marine Laboratory. The partnership aims to provide a broad training in earth and environmental sciences, designed to train tomorrow’s leaders in earth and environmental science. For further details about the programme, please see http://nercgw4plus.ac.uk/.

At least 37 fully-funded studentships that encompass the breadth of earth and environmental sciences are being offered to start in September 2017 across the GW4+ DTP.

Supervisors:
Main supervisor - Prof Matthew Wills (University of Bath)
Co-supervisor(s): Prof Paul Barrett (The Natural History Museum), Dr Marcello Ruta (University of Lincoln), Dr Nicholas Longrich (University of Bath)

Project description:

As popularly understood, evolution is largely synonymous with increasing complexity. But is this really the case? The maximum complexity of animals is undeniably greater today than when they first evolved (1). However, the most marked increase in complexity occurred during the Cambrian ‘Explosion’ (with a step-like rapidity that vexed Darwin). Two things in particular remain unclear, however. The first is whether the Cambrian marked a unique gear-change in the evolution of animal body-plans (e.g. sponges are less complex than jellyfishes, which are less complex than molluscs, vertebrates and so on), or whether an overarching trend for increasing complexity persisted from the Cambrian to the present day. The second is whether this process was purely the result of passive diffusion from some lower bound, or whether there was a widespread and driven trend for parallel change in multiple, independent lineages. If the latter, then increasing complexity may be viewed as an evolutionary ‘rule’ of the widest generality, similar to those already documented for increasing maximum size (Cope’s rule) and early high disparity [2]. Initial investigations [3] reveal a driven pattern of increasing complexity in crustaceans on this timeframe (now a text-book example [4]), but the generality and implications of this finding are highly controversial.

This project will focus on tetrapods (limbed vertebrates) because of their excellent fossil record and because their bodies comprise differentiated but serially homonomous units (vertebrae; ribs) and similarly patterned anterior and posterior appendicular skeletons (limbs; girdles). The student will use a variety of approaches to describe the distribution and specialisation of these elements across all groups of living and fossil tetrapods. S/he will initially focus upon discrete character codings and indices of the differentiation of vertebrae and ribs (Fig. 1A). These data will allow us to express complexity both in terms of the serial specialisation of somites, and by plotting the morphological diversity of vertebrae within a single individual relative to empirical morphospaces (Fig. 1B) encompassing all realised forms. The excellent record of tetrapods means that we will be able to calibrate a supertree of major groups against the appearance of lineages in deep time, and thereby track the parallel and convergent evolution of similar morphologies and similar levels of complexity more broadly. This approach will also highlight constraints upon the evolution of bodyplans, and the manner in which different clades have circumvented these. For example, all mammals (except sloths) have just seven neck vertebrae. Elongation of the neck has been achieved by radically different mechanisms in mammals (e.g., giraffes and indricotheres) compared with birds (e.g. ostriches) and many reptiles (which have much greater developmental flexibility).