SYSTEMATICS OF PTEROBRANCHS FROM THE BURGESS SHALES OF CANADA AND THE EARLY EVOLUTION OF GRAPTOLITES

Pterobranchs originated in the basal Cambrian (Fortunian) and are mostly known through their tubes preserved in the fossil record. The earliest forms are represented by bushy, erect-growing colonies that have not been widely studied due to their scarcity, poor preservation quality and species misidentification. As a result, early phylogenetic relationships within the group are not clearly established. Middle Cambrian Burgess Shale graptolites were also poorly known, primarly 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 were conducted using 34 discrete morphological traits from these Burgess Shale genera and some known benthic and planktic taxa. These results place these Cambrian species as basal forms closer to the pseudocolonial pterobranch Cephalodiscus and the living graptolite Rhabdopleura.