Tracing the evolutionary origins of the Hemichordata (Enteropneusta and Pterobranchia)

The early evolution of the Hemichordata (Enteropneusta and Pterobranchia) based on the available fossil record has its challenges and highlights. Even though the pterobranchs (Cephalodiscida and Graptolithina) secrete a highly durable housing construction or domicile (the tubarium), their fossil record is largely restricted to the tubaria of the planktic Graptoloidea, whereas the fossil record of their sister-group, the enteropneusts, is close to non-existent. Pterobranchs are present until today in a few inconspicuous taxa (Atubaria, Cephalodiscus, Rhabdopleura), but during the Ordovician Biodiversification Interval the planktic Graptoloidea was among the most highly evolved and diverse colonial organisms in the marine realm. The oldest definitive pterobranch fossils include Sokoloviina costata from the Rovno Horizon of Ukraine of possible basal Cambrian (Fortunian) age. Better preserved material originates from the Cambrian Series 2, Stage 4 and may be identified under the genus name Sphenoecium. Whereas Sphenoecium definitely represents a colonial pterobranch and can be referred to the Graptolithina, older material may be identified as 'pterobranch' only, as the fragmentary specimens cannot demonstrate coloniality unequivocally. These older 'pterobranchs' may alternately be referred to the non-colonial Cephalodiscida. According to the fossil record, the pterobranchs may have been the earliest group of animals to evolve coloniality during the Cambrian Explosion and, thus, differentiate between sexual and asexual reproduction in a single taxon. Coloniality also led to their evolutionary success in the early Palaeozoic. (C) 2018 Published by Elsevier B.V. on behalf of Nanjing Institute of Geology and Palaeontology, CAS.