The ichnogenus Rhizocorallium: Classification, trace makers, palaeoenvironments and evolution

Rhizocorallium is one of the oldest known trace fossils, with wide distribution through the Phanerozoic and all over the world. Originally introduced from the epicontinental Triassic of central Germany, its high morphological lability gave reason for the subsequent erection of about twenty ichnospecies. The study of newly collected material from the type area and many specimens from various collections permits the conclusion that Rhizocorallium jenense and Rhizocorallium commune are the only valid ichnospecies of Rhizocorallium. The type ichnospecies, R. jenense, is a comparatively small, inclined and heavily scratched firmground burrow with passive fill, while R. commune consists of extensive, more or less horizontal burrows with occasionally scratched marginal tubes and an actively filled spreite between the tubes. The faecal pellets Coprulus oblongus are typically associated with R. commune. Morphological variations of R. commune are captured in ichnosubspecies and varieties of this ichnospecies and can aid a refined reconstruction of palaeoenvironments. A review of more than 180 records from the literature reveals the common confusion of both ichnospecies, which has consequences for the application of Rhizocorallium in facies interpretations. The end members of both ichnospecies may be linked by transitional forms, which suggests the same kind of trace maker. Polychaetes are the most likely producers of marine Rhizocorallium, based on their long-ranging occurrence, morphological features, appearance of faecal pellets, associated soft-body remains, and modern analogues. R. commune occurs from Early Cambrian to Holocene, while R. jenense just appears after the end-Permian mass extinction, probably as a consequence of an adapted firmground burrowing lifestyle of its producer. Fluvial R. jenense are probably produced by mayfly larvae in homology to marine polychaete burrows. A consequent application of the newly established classification scheme allows for a more rigorous application of Rhizocorallium in the reconstruction of palaeoenvironments. Thus, Palaeozoic and Mesozoic R. commune are restricted to the Cruziana ichnofacies of shallow-marine environments, while in Cenozoic time similar forms are also found in deep-marine deposits. R. jenense, on the other hand, is a constituent of the widespread Glossifungites ichnofacies and, aside from continental environments, occurs in peritidal to deep-marine deposits. Several studies have demonstrated the value of Rhizocorallium for interpreting sequence-stratigraphical surfaces, current directions, and fluctuations in salinity and oxygen.