SHELL STACKS AT CALVERT CLIFFS: EVIDENCE FOR CEPHALOPOD MIDDEN CONSTRUCTION

Miocene strata of the Calvert Formation, Plum Point Marl Member, Bed 14, Kenwood Beach, Port Republic, Maryland host a 40-50 cm shell bed that contains Mercenaria spp., Glossus spp. and similar bivalves (65%), Chesapecten spp. (33%), and gastropods (3%; n = 138). Most of the bivalves are disarticulated, and of the disarticulated valves, 58% are concave-up and 42% are concave-down (n = 155). Silty fine sand matrix support occurs throughout most of the bed. Valves are mostly unbroken and are predominantly concave-up (58%). Unusual stacks of shells, with shells mostly concave-up, occur at isolated points throughout the bed. The stacks seem to be paucispecific (typically either Mercenaria or Chesapecten, respectively, but one stack shows an abrupt transition from non-scallop bivalves to scallops halfway up the stack). These odd stacks might be attributed to sediment compaction, dewatering, burrower disturbance, self-organized stack formation, or other factors. However, such factors cannot explain the stand-alone stacks of shells at Kenwood Beach, particularly considering that the Calvert Formation shell stacks are segregated according to bivalve type. We propose here that the stacks of shells are formed by octopodes. The cephalopods created middens that included stacks of shells placed like stacked plates or bowls. Such stacking behaviors (involving flame scallops [Lima scabra], coconut half-shells, etc.) have been documented in modern octopodes. This observation provides an explanation for the otherwise puzzling stacks of shells in the Plum Point Marl Member shell bed at Kenwood Beach, and implies that there is a cephalopod midden component in the Calvert Formation. The unique clam-to-scallop transition stack noted above may record a change in octopod prey selectivity or feeding preference, as the stack changes abruptly from a paucispecific stack of clams to a paucispecific stack of scallops. Bonebeds and shell beds interpreted as intrinsic biogenic concentrations (resulting from the behavior of the creatures preserved in the bed) are in some cases better interpreted as extrinsic biogenic concentrations (resulting from collecting activities of other creatures). Octopodes likely served as agents of extrinsic biogenic concentration at Kenwood Beach.