A LATE PLIOCENE PEA CRAB INFESTATION OF THE SLIPPER-SHELL CREPIDULA FORNICATA (LINNAEUS, 1758) FROM THE YORKTOWN FORMATION OF SOUTHEASTERN VIRGINIA

Initially discovered by Lyle D. Campbell, University of South Carolina, Spartanburg in the 1970's (Campbell, pers. comm.), fully articulated crabs herein tentatively identified as or closely allied to Zaops ostreum (Say, 1817) occur abundantly in strings of C. fornicata buried in apparent storm deposits of the Late Pliocene Yorktown Formation “Chuckatuck Bar” near the small village of Chuckatuck, Suffolk, Virginia. This bar trends N. 20̊ to 25̊ W., is about 2.4 km wide, more than 6.4 km long, has an axial thickness of about 20 meters, and is primarily composed of medium to coarse biofragmental sands (modified after G. H. Johnson and N. K. Coch, 1969).

The presence of this crab in fossil or extant C. fornicata has not been noted before, and the occurrence of pinnotherids in other crepiduloid gastropods are rare.

These minute, free-standing, exceedingly fragile fossil crabs occur in the interior of this fossil Crepidula, one or sometimes 2 per shell, with as many as four per string, in strings of up to 17 shells. Most appear to be immature or first crab-stage females along with a very low incidence of mature females and males. The first Crepidula of each string is often attached to the single valve of a small bivalve, like Spisula, suggesting that these strings were easily transported during severe storms before their apparent rapid burial and death by anoxia and/or starvation. These strings occur in concentrations of more than 50 per cubic decimeter. Preliminary findings suggest that this fossil relationship was much like that between living Z. ostreum and Crassostrea virginica (Gmelin, 1791) where the first crab-stage of this species invades this oyster in mid-summer, a time of potentially severe storms, thus also suggesting that such storms may have migrated along the Eastern United States during a “Pliocene summer” as well.