Geological Background

Abstract

Epibionts seem to be more common in siliciclastic units than in carbonate units. To evaluate this difference, the paleontology of the Middle Devonian Dundee Formation has been explored. A total of 245 brachiopod specimens were collected from a fossiliferous horizon of the Dundee Formation exposed at Whitehouse Quarry and identified to the generic level. Brachiopod genera identified were Strophodonta, Rhipidomella, Rhynchotrema, Atrypa, and Mucrospirifer. All the brachiopod shells were examined under a stereomicroscope for evidence of epibionts, and preferred host taxa were determined. Epibionts are absent on all the brachiopod shells except some Rhipidomella shells. Further examination of these Rhipidomella shells under 100x magnification showed evidence of biotic interactions in 21 out of 48 specimens. Large boreholes were produced by worm borers, scars were left on a few specimens by worms, branching grooves were the traces of soft-bodied ctenostome bryozoans, and sheet-like encrustation was produced by an indeterminate group of bryozoans. Ctenostome bryozoans had a commensal relationship with their host while a few worms had a parasitic relation with the host. While one might expect encrustation on hardgrounds within this carbonate unit, field work has determined that much of the Dundee Formation was extensively bioturbated, implying a soft substrate. It may be that bioturbation mixed shells down into the substrate before epibionts could attach.s