PhD: Nature, Green in Tooth and Claw: Reconstructing the Origin of Moulting Animals with New Phylogenetic Approaches

In the opening chapters of the Phanerozoic æon, the principal members of the cast belong to Superphylum Ecdysozoa. The ecdysozoan fossil record comprises complex burrows, mineralized and carbonaceous cuticular elements, exceptionally preserved soft-bodied compression fossils, and phosphatized microfossils of complete, often juvenile, individuals. The meaning of this fossil record is made difficult to decipher by the ambiguous position of these vermiform taxa on the tree of life.

Indeed, morphological and molecular studies tend to disagree on the fundamental relationships between the ecdysozoan lineages. Morphological studies generally group worm-like taxa into a clade, ‘Cycloneuralia’, supported by the presence of a toothed pharynx and a circumpharyngeal brain—though recent work has suggested that this morphology characterised the ancestral ecdysozoan. This view resonates with the results of RNA and phylogenomic analyses, which typically place arthropods and their kin along with nematoid worms, hinting at a worm-like ancestry for Ecdysozoa.

This project will test this model through a comprehensive palaeontological analysis that incorporates all ecdysozoan phyla, with a complete representation of Cambrian fossils that, by virtue of their early age and unfamiliar combinations of morphological characteristics, hold the key to establishing the earliest evolutionary trajectory of the ecdysozoan clade.

The student will undertake detailed revisionist studies of palaeo­scol­ecid and ‘archaeo­priapulid’ macrofossils from Burgess Shale-type deposits. These problematic and widespread Cambrian–Silurian worms that exhibit annulations, button-like sclerites, an armoured proboscis, and in some cases tail hooks or serially repeated claws. Echoes of nematoid and lobopodian anatomy suggest that these worms have the capacity to inform the sequence and timing of body plan evolution in the nematoid and panarthropod lineages.

New morphological detail will be objectively evaluated through the production of a maximally instructive morphological dataset, constructed using a homology-driven approach to character definition informed by microstructural observations of fossil and extant ecdysozoans.

Phylogenetic characters will be defined atomistically, by decomposing complex variation into a series of individual, biologically independent characters. This approach maximises the information content of data (in the form of codable columns in a phylogenetic matrix) that can be extracted from fossil organisms, whilst minimising the distorting influence of missing data. Character correlation will be mitigated by defining hierarchies of reductively coded characters, which semantically acknowledges logical interdependence between characters and allows the appropriate treatment of inapplicable character states. Biological consistency will be maximised by adopting an explicitly defined framework of homology that will be tested and refined by iterative evaluation.

Phylogenetic analysis will employ modern analytical methods, including a new morphological model under development by Smith’s research group, and will integrate stratigraphic constraints inferred by establishing morphological links with stratigraphically-constrained macrofossil, small shelly fossil and small carbonaceous fossil datasets.