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


Authors: Ramírez-Guerrero GM, Cameron CB

Published in: Bulletin of Geosciences, volume 96, issue 1; pages: 1 - 18; Received 3 May 2020; Accepted in revised form 25 August 2020; Online 16 November 2020

Keywords: graptolites, Hemichordata, middle Cambrian, Burgess Shale,

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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.