SPRIGGINA IS A TRILOBITOID ECDYSOZOAN

Convincingly specific homologies between Ediacarans and members of recognized animal phyla have remained elusive, thus casting a shadow of doubt over the animal affinity interpretation of Ediacaran phylogeny. Spriggina floundersi, for example, has been described as a tomopterid annelid, an arthropod, and a frondose vendobiont. Reanalysis of Spriggina demonstrates the presence of genal spines (comparable to those of fallotaspidoid and paradoxidid trilobites), a cephalic region homologous to the effaced cephalons of agraulid and skehanid trilobites, and a dual cephalic region (also seen in Parvancorina) that compares to the parts of a trilobite cephalon anterior and posterior of the facial suture. Spriggina is thus a trilobitoid ecdysozoan, a conclusion in accord with Sven Jorgen Birket-Smith’s inference of an arthropod affinity for Spriggina. This result is among the first confident phylogenetic linkages between an Ediacaran and a Cambrian animal, and thus helps to demonstrate that Paleozoic animals could indeed be descended from Ediacarans. If Spriggina is a direct ancestor of trilobites, then a case can be made that Spriggina itself (or a direct descendant) served as the predator taxon that initiated the Cambrian ecotone transformation (McMenamin, M. A. S., 2003, Origin and early evolution of predators: The ecotone model and early evidence for macropredation. In: P. Kelley, M. Kowalewski and T. Hansen, eds., Predator-Prey Interactions in the Fossil Record). Such heterotrophy, however, does not preclude the possibility of photosymbiosis or other types of mixotrophy in Ediacarans with high surface area such as Marywadea and Dickinsonia.