Hemichordata (Pterobranchia, Enteropneusta) and the fossil record

The Hemichordata are generally interpreted as early deuterostomes, closely related to the chordates, a notion important for modern analyses of the origin of the deuterostomes. Because their fossil record is quite scanty, modern phylogenetic interpretations largely rely on the analysis of DNA of the available extant taxa. The tripartite body plan of the group of worm-like hemichordates, the Enteropneusta, may be traced back in deep time to a few poorly known Middle Cambrian (Series 3, Stage 5) taxa from the Burgess Shale biota. The derived small, colonial or pseudocolonial Pterobranchia (Cephalodiscida and Graptolithina) have a more complete fossil record due to their preservable housing construction, the tubarium. The relationships of fossil taxa, putatively identified as early deuterostomes and possible hemichordates or even as pterobranchs of Lower to Middle Cambrian age (e.g. Galeaplumosus, Herpetogaster), cannot be substantiated. The Pterobranchia and their housing construction are first seen in the Middle Cambrian Series 3, Stage 5 but a clonal, colonial organization of the tubaria can only be recognized in the basal Drumian. The fossil enteropneust Mazoglossus ramsdelli Bardack, 1997 from the Carboniferous Mazon Creek Biota is re-described, its lectotype designated and illustrated for the first time.