Middle Cambrian pterobranchs and the Question: What is a graptolite?



The presence of distinct fusellar structure is taken as evidence to include a number of fossils from the Middle Cambrian to the Lower Ordovician of North America and Europe with the Pterobranchia. The dome of the pterobranchs and the prosicula of the planktic graptolites are contrasted and evidence is given for the re-assignment of a number of well known dendroid graptolites to the pterobranchs. A non-destructive method is described to reveal fusellar development of delicate hemichordate exoskeletons from shales. Rhabdotubus robustus n. sp. from the Czech Republic and ? Cephalodiscus sp. from the Wheeler Shale of North America are described as new Middle Cambrian pterobranchs.