AN ECDYSOZOAN FROM THE EDIACARAN OF SOUTH AUSTRALIA

Molecular clocks and the presence of diverse animal phyla in the lower Cambrian strongly suggest the emergence of many modern metazoan clades prior to the Phanerozoic. Although rare representatives of complex animals have been described from the Ediacaran period, most modern animal groups lack representation among Precambrian assemblages. Included among these conspicuously absent lineages are ecdysozoans, which is notable given the extensive fossil record of this group, beginning at the base of the Cambrian. This may result from a diminished preservation potential among stem Ecdysozoa or the inherent difficulty of confidently identifying an early-branching taxon based exclusively on fossil morphology. However, modern phylogenies and more recent fossil data can provide a search image for characters likely present in basal members of extant clades.

A recently discovered taxon from the Ediacara Member at Nilpena Ediacara National Park (South Australia), Uncus dzaugisi (nov. gen and sp.), has several distinct traits consistent with stem ecdysozoans based on phylogenetic and fossil constraints. Uncus was a sinusoidal, unsegmented vermiform organism ranging from approximately 5 to 35 mm in length and averaging 1 mm in diameter with unusually high relief for such a small organism. This suggests a ridged outer membrane or cuticle. Associated trace fossils recorded in the preserved organic mat are consistent in morphology with those of modern ecdysozoans and similar to ichnofossils from the lower Cambrian attributed to putative nematoids. We interpret this discovery to represent the oldest ecdysozoan in the fossil record, bridging the significant gap between the fossil record and molecular clock data and providing an additional calibration point for future studies. More broadly, Uncus demonstrates that an additional major metazoan group was present during the Ediacaran, predating the Cambrian Radiation by several tens of millions of years.