THE INFLUENCE OF SPIRIFERIDE MICRO-ORNAMENT ON DEVONIAN EPIZOANS

Shell and skeletal topography - specifically, ornament – of hosts has great effect on encrustation, whether through deterrence of fouling organisms or for attraction of camouflaging epibionts. Paleozoic brachiopod surface textures had similar effects on epizoans, with significant, potentially preferential encrustation of spiriferide brachiopods. Spiriferides bear fine micro-ornament which might influence settling larvae of encrusting organisms greatly. Micro-ornament can be grouped into two categories (Gourvennec, 1989): (a) radial ornament running perpendicular to growth, and which essentially are miniature ribs; and (b) pseudoradial ornament running subperpendicular to growth, and which terminates on the crests of costae. Pseudoradial ornament may develop tiny spines or nodes. We examined the encrustation preferences on spiriferides from the Devonian Cedar Valley Group (Givetian) and Lime Creek Formation (Frasnian) of Iowa, and the Silica Shale (Givetian) of Ohio and Michigan. In all units and localities, brachiopods with radial micro-ornament are significantly more likely to be encrusted than pseudoradial taxa (chi-square, p << 0.01). This result is driven largely by the common epizoans on spiriferides (bryozoans, Hederella, and spirorbids), but rarer epizoans (e.g. craniid brachiopods and Cornulites) may have preferred spiriferides with non-radial micro-ornament.

If common epizoans genuinely preferred the radial micro-ornament of spiriferide brachiopods, then the larvae selected spiriferide substrates that are ornate, but which bear less micro-topography than is found on pseudoradial spiriferide brachiopods. It is possible that rarer epizoans avoided competition by choosing surface ornament different from those preferred by common encrusters. These results are comparable to results at higher scales of ornament - Devonian encrusters generally avoided smooth or spinose brachiopods (terebratulides, productides) in favor of atrypides, large orthides, stropheodonts, and spiriferides. As the diversity of radial micro-ornamented spiriferides increases during the Mid-Paleozoic, the preference of epizoans for this micro-ornament raises the possibility of coevloution between Mid-Paleozoic encrusters and their spiriferide hosts.