Coprinoscolex, Oldest Known Echiuran from Mazon Creek

The Mazon Creek fossil shown here is colloquially called a “leech”, but is actually a member of the obscure Phylum Echiura, and are close relatives of the annelid worms. It is thought that the segmentation onserved in some fossils (as here) is the result of contraction of the skin due to the shock of burial. Members of the Phylum are commonly called “spoon worms”. Echiura were initially regarded as annelids, were then excluded, but newer evidence suggests they are, in fact, annelids.