A new tube-builder hydractinian, symbiotic with hermit crabs, from the Cretaceous of Antarctica

Abstract

An Upper Cretaceous (early Maastrichtian) tube-building hydractinian (Psammoactinia antarctica n. gen. and sp.) from Sanctuary Cliffs, Snow Hill Island, believed to live in association with hermit crabs, is described for the first time from Antarctica. Psammoactinia forms thick, concentric, globular colonies that encrust gastropod shells and extend the shell aperture by forming an open spiral tube. The colony consists of concentric layers with chambers and pillars made of silt and very fine sand grains agglutinated by collophane, which is interpreted as a diagenetic modification of an original chitinous phosphatic material. On the basis of the additional finding of isolated claws of pagurid crabs, assigned to Paguristes sp., the functional analysis of the hydractinian structure, and a comparison with modern and fossil analogous structures it is concluded that the peculiar hydractinian tube is a carcinoecium that housed a symbiotic hermit crab.Paleoenvironmental and paleoecological inferences suggest that the Psammoactinia–Paguristes association is mainly controlled by a fine-grained substrate and by a lack of abundant gastropod shells of different sizes. These factors do not necessarily correlate with absolute water depth, and favorable environments could include either offshore, relatively deep water or shallow restricted depositional settings.