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PALAEONTOLOGY
The finding of pharyngeal teeth and circumoral mouthparts in fossils of the Cambrian lobopodian animal Hallucigenia sparsa improves our understanding of the deep evolutionary links between moulting animals.
Fossils provide direct evidence of evolutionary history, and their unique morphological combinations can reveal crucial evolutionary links between extant taxa1. Most major animal phyla first appear in the fossil record during the Cambrian period, 541 million to 485 million years ago, and this early flowering of animal life has been termed the Cambrian explosion. Therefore, Cambrian fossils are particularly important for understanding the origin and early evolution of major animal groups. In this issue, Smith and Caron2 (page 75) redescribe one of the most celebrated Cambrian animals, Hallucigenia sparsa, and document several new features of this species, including its pharyngeal teeth and circumoral elements, which are suggested to be two of the few morphological characters uniting all groups within the Ecdysozoa.
The Ecdysozoa is far and away the richest animal group3. It is composed of eight extant phyla that shed their cuticle periodically to accommodate growth4 - nematode worms and crustaceans are familiar examples. The two commonly recognized subgroupings of ecdysozoans, Cycloneuralia and Panarthropoda, have distinctly different body plans (Fig. 1). Cycloneuralia unites wormlike organisms (Nematoda, Nematomorpha, Priapulida, Kinorhyncha and Loricifera) that have a non-segmented body terminating in a mouth that can turn inside out (eversible) and has a ring of nerves behind it - their brain. By contrast, panarthropods (Arthropoda, Onychophora and Tardigrada) are all segmented, with paired legs, and have a dorsal (upper side) brain in front of the mouth. These great morphological disparities have made it difficult to illuminate the...