Systematics of pterobranchs from the Cambrian Period Burgess Shales of Canada and the early evolution of graptolites

Pterobranchs originated in the basal Cambrian (Fortunian) and are mostly known by their tubes preserved in the fossil record. The earliest forms are represented by bushy erect growing colonies that are not widely studied due to their scarcity, preservation quality and species misidentification. For this reason, early phylogenetic relationships within the group are not clearly established. Middle Cambrian Burgess Shale graptolites were poorly known, based on the presence of Chaunograptus scandens, some debatable species of the genus Yuknessia, and other undetermined fragmented material. This study represents a complete description of C. scandens, a consensus for Yuknessia simplex and Y. stephenensis, and new reports of Protohalecium hallianum and Mastigograptus sp. from the Burgess Shale localities, which have also been found in other Burgess Shale type localities in Utah and Australia. Phylogenetic analyses of 34 discrete morphological traits from these Burgess Shale genera and some known benthic and planktic taxa (n = 22), place these Cambrian species as basal forms closer to the pseudocolonial pterobranch Cephalodiscus and the living graptolite Rhabdopleura.

Automated summary

Pterobranchs originated in the basal Cambrian and are mostly known by their preserved tubes in the fossil record. This study focused on poorly known Middle Cambrian Burgess Shale pterobranchs, providing a complete description of certain species and establishing their phylogenetic relationships within the group.