Direction of evolution within Annelida and the definition of Pleistoannelida

Abstract

The direction of evolution within Annelida has been a long and contentious debate since the middle of the last century. Based on functional morphology two hypotheses were predominantly discussed. The first hypothesis proposed an earthworm-like ancestor and the evolution of a segmented coelom as an aid for peristaltic burrowing, whereas the second one postulated that the ancestral annelid resembled more an errant polychaete and the segmented coelom evolved as a requirement for blood transportation to the parapodia. A recent phylogenomic study of annelids as well as new fossil records of early annelids provide new insights to this debate, which will be discussed in this paper. Based on these data the ancestral annelid is most likely a mixture of sedentary and errant annelid features indicating an evolutionary origin of Annelida as a microphagous surface deposit-feeder crawling upon and through soft-bottom habitats. Hence, the evolution of iterative parapodia as locomotive devices most likely drove the evolution of segmentation in Annelida. Moreover, as Struck et al. (2011) did not provide a proper definition and name for their clade 1 this clade shall be named Pleistoannelida and is defined by the last common ancestor of Sedentaria and Errantia sensu Struck et al. (2011), and all the descendants of that ancestor.