BEHAVIORAL VARIABILITY PRESERVED IN CONICHNUS AND MACARONICHNUS IN UPPER CRETACEOUS TOMBIGBEE SAND (EUTAW FORMATION), CENTRAL ALABAMA

Prior to its recent destruction, an isolated outcrop of Upper Cretaceous Tombigbee Sand Member (Eutaw Formation) in central Alabama provided an unusual glimpse of a high-energy tidal inlet/tidal delta sequence that included spectacular primary sedimentary structures linked mainly to tidal sandwave dynamics. Although only weakly bioturbated, this sequence of unconsolidated, fine- to medium-grained, cross-stratified sands included a moderately diverse suite of ichnofossils whose three-dimensional geometries could be readily assessed. Studies of two of the most commonly recurring ichnotaxa– Conichnus and Macaronichnus– demonstrate previously undocumented morphologic variability that reflects behavioral diversity.

Conichnus, likely produced by anemones, exhibits marginal and internal fabrics that reflect equilibrium behavioral responses to changes in sedimentation rate and erosion events associated with tidal sandwave migration. Specific fabrics can be linked to initial substrate penetration, subsequent retrusive movements in response to slow bottomset through rapid foreset sediment accumulation, protrusive movement in response to stoss-side erosion, and ultimate tracemaker escape. In addition to providing a sedimentation gauge, morphological observations provide minimum estimates of burrow longevity when viewed in the context of tidal cyclicity.

Eutaw specimens of Macaronichnus, which are unusually large (mean diameter=11 mm), exhibit axial changes in the disposition of mantle and core sediments that reflect variations in the relative timing of selective sediment feeding, defecation, and simple locomotion by deep burrowing opheliid(?) polychaetes. Textural and mineralogic comparisons of burrow cores (egested fecal strands), mantle (rejected sediment grains), and unprocessed host sediment provide information on the volume of sediment processed by the tracemakers and help to constrain the mechanisms/impetus involved in preferential sediment ingestion and consequent sediment segregation.