Temporal change as an aspect of biogenic shell utilization and damage, Pleistocene of North Carolina, U.S.A

Abstract

Sediments and fossils of the Smith Gut and Beard Creek members, middle Pleistocene Flanner Beach Formation, record the development of an interglacial, predominantly softbottom, estuarine basin that consisted first of open bay, then more restricted bay, closed lagoonal, and finally open lagoonal settings as barrier islands formed to the east and partitioned this segment of the North Carolina coast. Within this series of beds were found the remains of a sequence of benthic paleocommunities that changed through time in step with gradually changing properties of the surrounding environments, such as salinity levels, seasonality, water depth and substrate textures. As such, this faunal transition is an example of community replacement, the gradual to abrupt substitution of one community for another owing to directional habitat alterations. This long-term, subevolutionary pattern of faunal change also was resolvable in a study of macroscopic epi- and endozoans, and signs of predation, in autochthonous fossil shells. Open bay fossil associations contained a rich variety of encrusters, borers, and evidences of predator—prey interactions. The suceeding restricted bay associations contained fewer types of epi- and endozoans. At slightly higher stratigraphy levels, bay associations were replaced by comparatively species-poor, high-dominance, closed lagoonal associations containing still fewer encrusters and no boring organisms. And in the highest fossiliferous levels of the Flanner Beach, open lagoonal associations became enriched in epi- and endozoans but never reattained the diversity of forms seen at lower levels in open bay associations. Predation signs vary throughout the sequence in frequency of occurrence, not in kind. The sequential change in shell utilization by encrusters and borers described here does not resemble succession-like sequences observed over short time spans involving hard-substrates faunas, but rather is a conformational transition controlled by long-term environmental changes.