The recessional history of the Late Cretaceous sea in

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northwest-central South Dakota is recorded in the offlap sequence beginning with the Elk Butte Member (offshore shelf) of the Pierre Shale followed by the Trail City (bar-influenced shelf), Timber Lake (offshore bar), and Iron Lightning (deltaic) Members of the Fox Hills Formation. Well-preserved molluscan assemblages from these facies permit comparison with assemblages from analogous recent environments.

Factors controlling the distribution of Late Cretaceous mollusk assemblages are related to (1) sediment organic content, (2) sedimentation rate, and (3) sediment-water interface stability. These limiting factors closely control feeding adaptations and are reflected, therefore, in the distribution of feeding groups.

Bottoms dominated by deposit feeders:
Deposit feeders are limited to bottoms containing an organic food source. Shelf areas receiving deltaic sediments (Iron Lightning) or deeper offshore areas receiving settling fines (Elk Butte) were dominated by this feeding group. Mud bottoms extensively reworked by deposit feeders have a high water content and suspended easily by weak bottom currents. High interface turbidity and instability may explain the low diversity of filter feeders from this bottom type as high concentrations of suspended silt-clay cause clogging of filtering mechanisms.

Bottoms dominated by filter feeders:
The clean-washed upper part of the Timber Lake sand bar and the distributary sands of the Iron Lightning were dominated by mobile filter-feeding bivalves burrowing into the shifting unstable sand bottoms. Interface instability excluded attached epifaunal forms.

Bottoms with mixed feeding groups:
Peak filter-feeding diversity was attained on bottoms during periods of low Trail City sedimentation. Physical stability of these bottoms provided a firm surface of attachment for epifaunal mollusks and interface turbidity was relatively low providing optimal conditions for filter feeders. Root structures and high gastropod diversity indicate that some parts of the bottom were covered with plants during Trail City deposition. The presence of small amounts of organic matter in the sediment also permitted the population of a few infaunal deposit feeders.

The distribution of the Fox Hills bivalves by feeding groups reflects the conditions of food source, sedimentation rate, and bottom stability. These relations are supported by independent evidence of lithologic and stratigraphic analysis. The recognition of feeding groups can provide a strong tool in environmental reconstruction and analysis of ancient community structure.

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