Hallucigenia's head

Abstract

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. See Letter p.75 Of all the strange creatures from the mid-Cambrian Burgess Shale of Canada, Hallucigenia is one of the strangest. In particular it has proved difficult to reconstruct the head area from the available fossils and its relationship to other creatures has therefore been enigmatic. Now Martin Smith and Jean-Bernard Caron present data on newly examined 508-million-year-old fossils of Hallucigenia sparsa that at last reveal the head in some detail. Hallucigenia's strangeness is given shape in the form of a mouth surrounded by a ring of toughened plates and its throat lined with needle-like teeth. Although a member of the group that includes modern velvet worms (onychophores), at the base of the arthropod radiation, the head and anterior gut of Hallicigenia indicate evolutionary links with more disparate phyla such as nematodes and kinorhynchs. Hallucigenia now provides concrete evidence of structures (that were previously just a matter of conjecture) that might have existed in the last common ancestor of the Ecdysozoa.