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Charnia from the Ediacara biota is here examined in terms of its growth and development. The Ediacara biota comes from the critical period of evolution just before the Cambrian Explosion and is key to our understanding of the origin of animal life. We show that Charnia cannot be related to the modern cnidarian group the sea pens (Pennatulacea) with which it has for so long been compared, as generative zones cannot be homologized between these forms.
The Precambrian Ediacara biota (Ford 1958), which immediately predates the Cambrian Explosion, is critical to understanding the origin of animal life. The classic interpretation of this biota has been that it contains the precursors of cnidarians, arthropods and annelids, and possibly also echinoderms (Glaessner 1984; Gehling 1991) and molluscs (Fedonkin & Waggoner 1997). This paradigm was challenged by arguments that these fossils belong to an extinct 'phylum,' the Vendobionta (Seilacher 1984, 1985, 1989, 1992). The problem of distinguishing between these two main positions over the last 20 years is one reason why the Ediacara biota has developed into an icon within modern palaeontology. Arguably, until an approach is presented to resolve this stalemate, the Ediacara biota will remain one of the major pieces missing from the puzzle that is the history of life (Brasier & Antcliffe 2004).
The longest ranging genus of the newly defined Ediacaran Period (Knoll et al. 2004) is Charnia. This fern-like impression has usually been classified (along with its presumed close ally Charniodisciis) with the similar-looking sea pens (pennatulaceans, a derived group of octocorals) (Glaessner 1984; Gehling 1991). The latter, extant colonial cnidarians (Verrill 1865; von Kölliker 1880; Kükenthal 1915; Williams 1995) have, however, no undisputed fossil record until the Jurassic (see Williams 1997, who therefore questioned their affinities to the Ediacara biota), thus implying a minimum gap in the fossil record of 350 Ma, assuming faithful representatives of their group within the Ediacaran Period. Furthermore, their direct ancestors, the alcyonaceans, have no clear record before the Silurian (Lindstrom 1978; Bengtson 1981). Here we test the relationship between...