Ecdysozoa (moulting animals) comprises the protostome Phyla Arthropoda, Kinorhyncha, Loricifera, Nematoda, Nematomorpha, Onychophora, Priapulida and Tardigrada, but our precise understanding of the phylogenetic relationships between these is disputed. Ecdysozoa is an extremely ancient clade that originated in the oceans, and ecdysozoans remain major components of modern marine and terrestrial ecosystems, including the most diverse and abundant of all animal Phyla (Arthropoda and Nematoda respectively). In this thesis, I explore the phylogenetic relationships and divergence times of fossil and extant ecdysozoans in order to address several outstanding issues in the early evolutionary history and palaeobiology of Ecdysozoa, employing data from fossils and molecular sequences.
First, the phylogenetic relationships of the eight ecdysozoan phyla was tested using Bayesian models from a molecular matrix containing newly sequenced taxa from Nematomorpha, Priapulida and Tardigrada. Analyses retrieved a monophyletic Scalidophora (Kinorhyncha, Loricifera and Priapulida) which in turn is sister-group to a clade comprising Nematoida (Nematoda and Nematomorpha) and Panarthropoda (Arthropoda, Onychophora and Tardigrada) – this is named Cryptovermes nov. An improved set of fossil calibrations was compiled and used to infer the divergence times of ecdysozoans under a range of alternative parameters. Crown-group Ecdysozoa diverged in the Ediacaran Period between 636 – 578 Ma, at least 23 million years before the oldest potential fossil evidence of ecdysozoans in the late Ediacaran (