PERSISTENT PARABOLIC PITS IN ECHINODERMS FROM THE MISSISSIPPIAN TO THE LATE CRETACEOUS

Morphologically simple trace fossils may be difficult or impossible to assign to a nominal group of producing organisms, but can indicate persistence of behavior. Thus, small round holes perforating shells are known to be predatory by observation of extant groups of shelly organisms; similar associations are inferred from trace fossil evidence from throughout the Phanerozoic. The activity is persistent, but the interacting organisms change over time.

A crinoid pluricolumnal from Salthill Quarry, Clitheroe, Lancashire, England (Mississippian), Pentagonocyclicus? (col.) sp., preserves multiple pits, Sedilichnus paraboloides (Bromley). Pits on one side of the specimen sit atop cyst-like structures and infested the crinoid when it was alive; the protuberances represent a growth reaction. At this time the column was either upright, pits concentrating on one side presumably in response to current direction, or recumbent (and distal). Pits on the other side engendered no growth reactions; the crinoid was dead and lying on the sea floor when infested. The infestation of the live crinoid must be considered ectoparasitic.

Morphologically similar pits are also known in the apical region of Late Cretaceous irregular echinoids of northern Europe. Echinocorys scutata Leske from the Chalk in southeast England were more or less infested by this trace. Pits commonly surround the apical system and, less commonly, occur within it; they occur preferentially anteriorly. Pits occur within plates, not along margins or sutures. Crosscutting of pits indicates that multiple spatfalls probably occurred. The host echinoid added new test plates adjacent to the apical system; thus, plates bearing S. paraboloides were moved abapically. The reduction in number of pits away from the apex, including those with echinoid tubercles re-established on the base, indicates that, following death of an infester, the echinoid ‘reclaimed’ and infilled them with calcite.