PhD: The Deep Evolution of Cambrian Ecdysozoans

The ‘Cambrian explosion’ some 542 Mya witnessed the apparently simultaneous appearance of most major animal phyla in the fossil record, with a rapidity that vexed Darwin. Many of the most intriguing Cambrian fossils discovered in the last thirty years come from exceptionally preserved fossil assemblages in Southwest China, which are increasingly shedding new light on our understanding of this pivotal period in Life’s history. Controversy has centred on the phylogenetic relationships and evolutionary implications of these fossils (Ma et al., 2015), and what they imply about the bodyplan diversity or disparity of these first flourishings of animal life (Wills et al., 2013). By far the most diverse and abundant of all animal clades (both in the Cambrian and today) are the Ecdysozoa (moulting animals) containing the arthropods, priapulids, lobopodians and nematodes. Unfortunately, we know little about the manner in which these groups first evolved. This project will systematically explore the record of these early ecdysozoans to seek fossils that may exemplify intermediate or transitional forms. Two groups will be of particular interest; the lobopodians (a stem group of the euarthropods) and the cycloneuralians (variously armoured worms).

Material held at the NHM, China and elsewhere will be described or re-examined, and used to evaluate, update and expand discrete character matrices developed by all supervisors and others. These will be used to produce new phylogenies and visualisations of Cambrian ecdysozoan morphospace. The student will also apply classical morphometric tools (e.g., eigenshape analysis) to the mineralised elements (scalids, spines, sclerites and other hard parts). We will use these data to quantify the anatomical complexity and regionalisation of the bodies of individual species (testing for putative driven trends through time that may mirror those hypothesised within arthropods) and to derive independent estimates of morphological disparity within groups (complementing those obtained from discrete characters).

The project will also intergrade morphological (from living and fossil species) and genomic data to reconstruct an accurate timescale for the major splitting events within Ecdysozoa and to investigate the evolutionary links of the innovations of genotypes and phenotypes. The student will also carry out some decay experiments to test how the decay process affect the description of the fossil anatomy and the final phylogenetic results.

^1. X. Ma, Hallucigenia's head. Nature 523, 38-39 (2015).
^2. M. A. Wills, S. Gerber, M. Ruta, M. Hughes, The disparity of priapulid, archaeopriapulid and palaeoscolecid worms in the light of new data. Journal of evolutionary biology 25, 2056-2076 (2012).
^3. T. H. P. Harvey, X. Dong, P. C. J. Donoghue, Are palaeoscolecids ancestral ecdysozoans? Evolution & Development 12, 177-200 (2010).
^4. O. Rota-Stabelli et al., Ecdysozoan Mitogenomics: Evidence for a Common Origin of the Legged Invertebrates, the Panarthropoda. Genome Biol Evol 2, 425-440 (2010).