Meeting Abstract
Annelida is one of the most speciose (~17,000 species) and ecologically successful phyla. Key to this success is their flexible body plan with metameric trunk segments and bipartite heads consisting of a prostomium bearing most sensory structures and a peristomium containing the mouth. The flexibility of this body plan has traditionally been problematic for reconstructing the evolutionary relationships within the Annelida. While recent phylogenetic analyses, combining molecular and morphological data, retrieve two major clades within the crown-group, many questions remain regarding the early evolution of the annelid bodyplan itself, including the origin of the head, in large part due to the paucity of unequivocal annelid body fossils near their evolutionary origins. Here we describe a new Cambrian fossil polychaete based on abundant material from the 508 million year-old Burgess Shale Marble Canyon locality (British Columbia, Canada). Up to 2.8 cm in length, this new species possesses highly elongate chaetae, and its exceptionally preserved internal anatomy includes putative neural and vascular tissue. Most crucially, the head morphology includes a medial antenna (previously unknown from Cambrian forms) as well as a peristomium bearing uniramous parapodia and chaetae in addition to the mouth. A critical reappraisal of the Cambrian fossil annelids record lead us to propose a new hypothesis for the evolution of the modern annelid head condition which invokes developmental mechanisms found in extant taxa, namely the loss of juvenile parapodia during ontogeny in Magelona and peristomium+juvenile chaetiger fusion in Nereiidids. The medial antenna is likely autapomorphic and further suggests that a currently un-recognized morphological disparity may have existed amongst the early annelids.
