The evolutionary origins of the Hemichordata (Enteropneusta & Pterobranchia) - A review based on fossil evidence and interpretations.

Few fossils demonstrate the presence of the Hemichordata (Enteropneusta, Pterobranchia - excluding benthic and planktic Graptolithina) in the early Palaeozoic. Many of these early records are questionable and are not supported by a reliable fossil record. First definitive fossils are found in the Fortunian (early Cambrian) with the record of Sokoloviina representing fragments of the tubarium of the Pterobranchia. Possible Enteropneusta have been described from the Miaolingian (Wuliuan) and the slightly older Cambrian Series 2, Stage 3, but the record is largely based on poor inconclusive material. However, the Fortunian pterobranchs clearly indicate that a differentiation of the Enteropneusta and Pterobranchia must have happened earlier, even though a definitive fossil record is not available. The evolutionary origin of the Hemichordata (Enteropneusta & Pterobranchia), thus is likely to be found in the latest Ediacaran. Fossil burrows of Gyrolithes spp. at the Ediacaran-Cambrian GSSP may represent the activity of Enteropneusta, but cannot demonstrate without doubt their presence. Difficult is also the interpretation of meandering deep-water feeding traces or faecal traces in the Cambrian (cf. Nereites ichnofacies) that are similar to those produced by extant Enteropneusta (Torquaratoridae). The possible erect housing construction identified as Margaretia is unlikely to be built by an enteropneust and is not comparable in any way with the organic-walled tubarium constructions of the Pterobranchia.