Mammillichnis monstrum isp. nov., a new sea anemone trace fossil from the Carboniferous of the Donets Basin, Ukraine

A new ichnospecies Mammillichnis monstrum isp. nov. (Domichnia) is described from turbidite sandstone in the upper part of the Mospyne Formation (late Bashkirian, Early Pennsylvanian) of southern Luhansk Oblast, eastern Ukraine. Mammillichnis monstrum isp. nov. is represented by hypichnial, hemispherical, slightly flattened passive fillings of a large burrow; the apex of burrow casts has concentric depression or a small rounded tubercle. Mammillichnis monstrum isp. nov. differs from Mammillichnis aggeris Chamberlain, 1971 (Vendian–Oligocene) and Mammillichnis jakubi Pokorny et al., 2017 (Late Miocene, Neogene) by its much larger size. The depth of burrows varies greatly, which seems to be related to the different preservation of burrow casts, as well as, apparently, different maturity of burrow producers. In addition, such an important diagnostic feature as a rounded tubercle at the apex of the burrow casts is not observed in all specimens, which is probably also due to the different types of preservation of the trace fossils. Perhaps, the differences in the morphology of the apex of some examined specimens are due to the activity of organisms that lived in the burrows after the death of the trace fossil producer. The producers of the trace fossils Mammillichnis are apparently sea anemones (Actiniaria). The fossil record of Actiniaria has significant gaps because these animals lack a mineral skeleton that can be preserved as a fossil. The oldest sea anemones are known from the Vendian; there is no data on post-Carboniferous fossil Actiniaria. It seems that the only way to study the evolution of this group of animals is studying of their trace fossils, such as burrows (e.g. ichnogenera Astropolichnus, Bergaueria, Conostichus, and Kulindrichnus), which demonstrate some details of the body morphology of their producers. The trace fossils Mammillichnis monstrum apparently marks insignificant pauses in sedimentation preceding the accumulation of turbidite sandstones. In the coarse sandy sediments are preserved mainly large depressions left by sessile coelenterates. Trace fossils are well preserved in some sandstone lenses and poorly preserved in others. This circumstance is apparently related to the different intensity of erosive activity of turbidity currents.