SCLEROBIONT-DEFINED DEPTH GRADIENT IN THE MIDDLE DEVONIAN OF THE MICHIGAN BASIN, USA

Sclerobiont encrustation patterns on a variety of brachiopod host taxa successfully record differences in paleodepth in M. Devonian strata of the Michigan Basin. Assemblages were documented using a series of metrics: areal percent cover, relative abundance, richness, encrustation frequency, and taxonomic composition. The resulting patterns performed in predictable ways. A range of brachiopod host taxa, Athyris, Pseudoatrypa, Strophodonta, Mucrospirifer, and Devonochonetes, exhibited similar encrustation in shallow facies, but sclerobionts avoided Mucrospirifer in shallow water facies. Declines in all measured encrustation metrics were observed with depth. Areal % cover declined from 10.5% in shallow facies to 3% in deep; richness decreased from 21 sclerobiont taxa to 15, and frequency of encrustation decreased from 67% to 41%. Moderate and deep facies were difficult to distinguish when preferentially avoided hosts were included in the analysis. Moderate facies showed sparse sclerobiont assemblages and patterns consistent with deep water samples, but notable taxonomic differences between moderate and deep samples remained.

Shallow facies were dominated by encrusting foraminifera and forms of Ascodictyon and Allonema. Moderately deep facies were characterized by cheilostome and trepostome bryozoans. Trepostomes alone dominated deep environments; they accounted for over twice the host area occupied by any other sclerobiont in deep facies. Cheilostomes were nearly absent in deep water sclerobiont assemblages. Multiple brachiopod species introduced additional variables into successfully establishing sclerobiont encrustation patterns along a depth gradient. Yet in deeper water facies, host preference does not seem to impact encrustation patterns. Thus, even when multiple host species are used, sclerobiont assemblages can be successfully applied to establish a relative depth zones in the M. Devonian strata of the Michigan Basin.