Project EARTH-17-IR1: Deciphering the Evolution of Symmetry in Early Echinoderms

Supervisors: Dr Erin Saupe, Department of Earth Sciences; Dr Imran Rahman, Museum of Natural History; Professor Paul Smith, Museum of Natural History

Symmetry is an integral aspect of organismal biology that is reflected in the construction of the basic body plans of all animals, which originated over half a billion years ago. The vast majority of living species are bilaterally symmetrical, but a few groups display other types of symmetry. Echinoderms (starfish, sea urchins and the like) are particularly noteworthy; they are unique among bilaterians in exhibiting pentaradial symmetry as adults, having departed radically from the bilateral ancestral body plan. Moreover, the phylum possesses a rich fossil history that includes bilateral, asymmetrical, triradial and pentaradial forms. As a result, echinoderms are an excellent model group in which to study the evolution of symmetry, with implications for uncovering the evolutionary significance of symmetry in animals more broadly.

figure-1-evolutionary-relationships-of-early-fossil-echinoderms

Figure 1. Evolutionary relationships of early fossil echinoderms.

Based on their extensive fossil record, it has been proposed that echinoderms evolved through successive bilateral, asymmetrical, triradial and then finally pentaradial stages (Figure 1). However, the evolutionary significance of pentaradial symmetry in echinoderms remains unclear. One early suggestion was that pentaradial symmetry evolved as an adaptation to a sessile, suspension-feeding mode of life, but this idea has never been rigorously evaluated. The student will thoroughly test this hypothesis using a diverse toolkit that includes computed tomography, computational fluid dynamics, and environmental modelling. More specifically, select fossil echinoderms will be imaged using high-resolution X-ray tomography, revealing their morphology in unprecedented detail. Computer modelling will be used to analyse feeding efficiency and stability in these same fossil taxa, informing on the functional performance of forms with different types of symmetry. Moreover, an extensive dataset characterising the ecological and environmental parameters of early echinoderms will be assembled and interrogated. The student will determine whether different ecological categories (i.e. tiering, motility, feeding) and environmental parameters (e.g. water depth, oxygen, nutrient flux) are associated with different types of symmetry. The results obtained from these diverse but complementary analyses will transform our understanding of the evolution of symmetry in echinoderms in particular, with implications for animal body plan assembly in general.

The student will be provided with state-of-the-art training in 3-D imaging and analysis of specimens, including computed tomography, digital visualization and computational fluid dynamics. In addition, the student will receive training in quantitative methods for analysing ecological and environmental datasets. More general guidance on technical writing, public speaking and teaching will be offered. This combination of specialist research competencies and generic transferable skills will make the student well suited for a range of high-profile positions within the scientific community.

Further Reading:

Rahman, I.A., Zamora, S., Falkingham, P.L. & Phillips, J.C. 2015. Cambrian cinctan echinoderms shed light on feeding in the ancestral deuterostome. Proceedings of the Royal Society B 282: 20151964.

Smith, A.B. & Zamora, S. 2013. Cambrian spiral-plated echinoderms from Gondwana reveal the earliest pentaradial body plan. Proceedings of the Royal Society B 280: 20131197.

Zamora, S. & Rahman, I.A. 2014. Deciphering the early evolution of echinoderms with Cambrian fossils. Palaeontology 57: 1105–1119.