Insights into the lifestyle and preservation of Arenactinia ipuensis n. gen. et n. sp. (Anthozoa, Actiniaria) from the Early Silurian (Ipu Formation, Parnaíba Basin, Brazil)

Soft-bodied cnidarians are scarce in the fossil record and only a few sea anemones are documented in Paleozoic, mainly in Konservat Lagerstätten . We present a new cnidarian taxon exhibiting wide intraspecific morphological diversity: Arenactinia ipuensis n. gen. et n. sp. occurs in coarse/conglomeratic sandstones from the Early Silurian Ipu Formation, Parnaíba Basin (Northeast Brazil). Individuals ascribed to this species may possess an oral disc with discrete tentacles, an inner cavity or coelenteron, a column, and a physa in the aboral region. Body shapes may be discoidal, conical, cylindrical, flattened, or bipartite with the body divided into a cylindrical column with expanded base. The morphological variation observed in the Ipu assemblage differs from that of previously described fossil sea anemones but is very similar to shapes generated by some modern anemones that burrow in soft substrates. Preservation of the sea anemones likely resulted from multiple factors acting together, including microbial mat growth, rapid deposition of coarse-grained sediments, infaunal mode of life, and covering behavior, which significantly increased the chances of preserving this biota. The appearance of this new species may be related to a special moment of adaptive irradiation in marine ecosystems following the Late Ordovician Mass Extinction. • New sea anemone Arenactinia ipuensis n. gen. et n. sp. was recorded in Silurian. • Sea anemone soft body casts preserved in coarse sandstone. • Fossils of sea anemone Arenactinia ipuensis n. gen. et n. sp. reveal its lifestyle. • New sea anemone of Silurian shows enigmatic preservation.