COMPARISON OF THE STRUCTURE OF A MISSISSIPPIAN AND A HOLOCENE PEN SHELL ASSEMBLAGE

Benthic assemblages associated with pen shells (Pelecypoda: Pinnidae) in a Holocene back-barrier marine environment and in an exposure of Mi ssissippian rocks that are inferred to be back-barrier marine deposits , appear to have more structural similarities than differences, despit e their separation by over 300 million years and dominance by organism s that are not closely related taxonomically. Both of the studied asse mblages occur in fine-grained, calcareous siliciclastic deposits. Excl uding the pen shells, each assemblage is dominated by three species, a ll of which are epibenthic suspension feeders in the Mississippian ass ociation, while true of the three are epibenthic suspension feeders in the Holocene association. However, the Mississippian dominants are fr ee-lying or byssally attached, while one of the two epibenthic dominan ts in the Holocene association is cemented (Crassostrea) and the other is an intermittent swimmer (Aequipecten). The Holocene assemblage has a substantially higher proportion of shallow burrowers than did the M ississippian assemblage, which may be a contributing cause of the diff erences in life habits of the dominant species, inasmuch as mobile and cemented epibenthic organisms are less likely to be disturbed by chur ning of surficial sediment by shallow burrowers than are free-lying or ganisms. Within the Holocene study site, pen shells are found only in marine grass beds. The Mississippian pen shells are associated with a conspicuously increased abundance of erect bryozoan fragments and also with a higher species richness than associated beds within the measur ed stratigraphic section. It is inferred that the erect bryozoans baff led currents, thereby reducing flow velocity and increasing the availa bility of microenvironments, in much the same way as modern sea-grass patches that contain pen shells.