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

o nial pterobranch hemichordates mostly known by their tubes, preserved in the fossil record since the Cambrian Period. Graptolites differ from their sister group Cephalo­ discida, by the presence of a stolon system that supports a colonial lifestyle, the presence of a larval prosicula, and the anatomy of the zooids. Although zooids with preserved morphological details are essentially unknown among fossil graptolites, zooid anatomy is well known from the extant species Rhabdopleura (Mitchell et al. 2013, Maletz & Beli 2018). The subclass comprises the orders Dendroidea, which includes the benthic organisms with an encrusting to erect, bushy morphology formed by irregular branching, as well as the derived, planktic Graptoloidea (Maletz 2014b, Maletz & Cameron 2016). Due to poor fossil preservation, taphonomic processes, and similarities in morphology between taxonomic groups, identification of the specimens is difficult and sometimes mistakenly done, especially in Cambrian forms. The useful criteria to define a graptolite, when the soft­tissue material is not available, include an organic tubarium with fusellar structures surrounded by secondary cortical tissue, and the stolon system (Mitchell et al. 2013). Even when these characteristics are preserved, scanning electron microscopy is frequently used to obtain the most details from the specimens; otherwise, mostly outlines of organic­walled fossils are available for determination (Maletz et al. 2005, LoDuca et al. 2015a). An example of misidentified pterobranchs is the genus Yuknessia, which was originally regarded as an alga (Walcott 1919), but is now recognized as one of the earliest known pterobranchs from the Cambrian Series 3, based on the SEM identification of fuselli in two species (Steiner & Maletz 2012, LoDuca et al. 2015a). Like Yuknessia, a closer look at other taxa may establish a pterobranch affinity (e.g., Dalyia racemata and Malongitubus; Maletz & Steiner 2015, Hu et al. 2018. See Maletz & Beli 2018 for further discussion). We refer to these early forms simply as pterobranchs, based on their organic tubes with fusellar structures, because it is nearly impossible to classify them as cephalodiscids or graptolites. The pterobranch fossil record from the early and middle Cambrian is less complete compared to the Ordovician and Silurian periods (Rickards & Durman 2006), making difficult our understanding of the origin and early evolution of graptolites. It is known that early graptolites