OSTRACODS PRESERVED WITHIN CEPHALOPOD CONCHS FROM THE CARBONIFEROUS OF UK: THE RECORD OF CRYPTIC MOULTING BEHAVIOUR

Monospecific accumulations of two species of ostracods, a spine-bearing Janischewskya? sp. and a smooth-shelled Cavellina? sp., were found preserved within the deeper parts of the body chamber, siphuncle and camerae of two goniatites and one nautiloid specimen, all from the Mississippian (Asbian) Cracoean reefs of North Yorkshire, UK. The ostracod concentrations occur as well-preserved single valves of juvenile individuals, with delicate spines of Janischewskya? sp. intact on most specimens. The ostracod valves are not associated with other fossils, bioclasts or sediment grains, and rarely occur in the surrounding matrix outside the shells. This indicates that the concentrations of ostracods did not result from a sedimentary process. The occurrence of these ostracods in the deeper parts of the conchs, the absence of associated adult individuals, and the lack of morphological features indicative of scavengers, all negate any hypothesis that the ostracods represent mass mortality events following feeding and subsequent burial by obrution. Instead, their preservation suggests that the ostracods deliberately entered the empty conchs in order to moult in a sheltered environment. In the case of the smooth-shelled Cavellina? sp., the ostracod valves preserved within a camera of the nautiloid have similar size, indicating that a synchronized mass moulting may have taken place with this species.These accumulations thus represent the first examples of cryptic moulting in ostracods. They also imply that such behaviour in this group of arthropods has a long evolutionary history, at least since the Carboniferous, and potentially may still be persistent in present day ostracods.