Invertebrate trace fossils and ichnodiversity in shallow marine carbonates of the German Middle Triassic (Muschelkalk)

The fossil record (including trace fossils) is remarkably reduced after the end-Permian mass extinction. Faunal recovery increased continuously during the Mesozoic, and a marked restructuring of shallow marine benthic communities during Jurassic and Cretaceous time is known as the Mesozoic Marine Revolution. While middle and late Mesozoic trace-fossil associations are diverse and well studied around the world, those of the Triassic are minimally documented. Especially the bioerosional aspects are poorly understood compared to other periods. An abundant and diverse ichnofauna in the Middle Triassic of the Germanic Muschelkalk Basin provides insights into the ichnologic record at the beginning of the Mesozoic. For more than two centuries, this basin has been the subject of numerous studies, and several ichnogenera were established from the German Triassic (e.g., Rhizocorallium, Trypanites, Balanoglossites, Pholeus). A first overview allows the estimation of about forty invertebrate ichnotaxa, which can be grouped into three categories in terms of their appearance: (1) Burrow trace fossils-besides well-known ichnotaxa such as Bergaueria, Cochlichnus, Conichnus, Curvolithus, Lockeia, Phycodes, Protovirgularia, Rhizocorallium, and Thalassinoides, this group also contains ichnotaxa that are poorly known elsewhere, e.g., Archaeonassa, Arachnostega, and Pholeus. Occurrences of the oldest Mesozoic nearshore Zoophycos from the Muschelkalk are important for the interpretation of the general evolutionary trends of the tracemakers and their behavioral convergence. Of special interest is the interpretation herein of complex trace fossils, such as Mixoteichichnus coniungus and Balanoglossites triadicus. (2) Bioerosion trace fossils- many horizons of the Muschelkalk succession are characterized by omission surfaces and allow the study of bioeroded firmgrounds and hardgrounds with well-established ichnotaxa (e.g., Trypanites weisei). Most bioerosion trace fossils are recognized in the German Triassic for the first time, including ichnospecies of Gastrochaeonolites, Caulostrepsis, Maeandropolydora, and Palaeosabella. (3) Meiobenthic trace fossils-micritic bedding planes exhibit a diverse trace-fossil association with burrows and trails, where in many instances the producer itself is preserved at the end of the trace. The tracemakers, preserved by recrystallization, are mainly in the size range of meiofaunal species and commonly appear as worm-like (nematoid) or arthropod-like organisms. Cochlichnus, Helminthopsis, and Helminthoidichnites are the most common ichnotaxa of this group.