Crinoids and blastoids,platyceratid gastropods and time: A taphonomic progression

Taphonomy aims to identify the history of preservation of a fossil, such as the order in which events occurred after death of the organism. For example, why are crinoid and blastoid thecae of Mississippian (Chadian) age never preserved in close association with parasitic/coprophagic platyceratid snails at Salthill Quarry, Clitheroe, Lancashire? They do occur in close juxtaposition at other localities globally where Upper Paleozoic strata crop out. We suggest that the intimate blastoid or crinoid/platyceratid association may occur at one of three identifiable levels. Level 1 includes those rare specimens of crinoids (not known in blastoids so far) that retain all arms and an attached platyceratid, a pattern of preservation indicating rapid burial causing death. Level 2 includes those thecae that have lost their brachioles (blastoids) or arms (crinoids), but still have an attached platyceratid. That is, the echinoderm has started to disarticulate following death, but the platyceratid continues to feed on the theca. In other words, and strangely, the snail is more intimately linked to the theca than certain of the echinoderm's appendages. Level 3 includes sites where crinoid/blastoid thecae and platyceratid conchs are preserved separately; determining if they were previously closely associated may be problematic unless attachment scars of snails are apparent on crinoid thecae. Suggestions that the pelmatozoan/platyceratid association involved aerosol filtration by the snail seem unlikely, because the intimate association is maintained after loss of the arms.