Putative Ordovician green alga Krejciella reinterpreted as enteropneust hemichordate tube (Czech Republic)

Fossil and extant representatives of Enteropneusta play an important role in the interpretation of early evolution of animals, such as echinoderms, hemichordates and chordates. However, remains of fossil Enteropneusta are rare. Re-examination of available specimens of organic tubes of the Ordovician putative green alga Krejciella putzkeri Obrhel 1968 does not show any morphological difference from the Cambrian Margaretia dorus Walcott, 1931. The latter species has been recently interpreted as an organic tube produced and inhabited by the worm-like enteropneust hemichordate Oesia disjuncta Walcott, 1911. However, the absence of the subterranean lateral extension in Ordovician specimens excludes the synonymy of Krejciella and Margaretia . Geographic distribution of Cambrian organic tubes classified as Margaretia Walcott, 1931 indicates a possible latitudinal control, as all occurrences are apparently restricted to tropical and subtropical belts when plotted in Cambrian palaeogeographic maps. In comparison, the occurrence of the herein studied specimens of Krejciella is restricted to cold-water localities of West Gondwana. The micropalaeontological analy-sis of a rock sample bearing one specimen of Krejciella shows the presence of moder-ately preserved chitinozoans, including the zonal species Linochitina pissotensis . This taxon is, for the first time, documented from the Prague Basin and determines the Mid-dle/Late Ordovician boundary interval of the analysed sample. The herein studied specimens of Krejciella extend the record of organic tubes produced by enteropneust hemichordates both stratigraphically and palaeogeographically to the Middle/Late Ordovician cold-water area.