CALCITE ON CALCITE: BRYOZOANS ENCRUSTING ON, AND BARNACLE BORINGS IN, MYALINID PELECYPODS (UPPER PENNSYLVANIAN - LOWER PERMIAN; KANSAS, OKLAHOMA, AND TEXAS)

Is preservation of encrusting fossils affected by their mineralogy compared to that of their substrate? Paleozoic bryozoans (calcitic) frequent calcite brachiopods, so will they also appear in numbers on myalinids (also calcitic, unlike most pelecypods)?

Several thousand myalinid shells, accumulated over a century, were examined in the University of Kansas Natural History Museum and later fieldwork, from 28 horizons from topmost Mid-Pennsylvanian (DesMoinesian) through Upper Pennsylvanian (Missourian, Virgilian) into Lower Permian (Wolfcampian), from 44 localities in 19 counties from northeast Kansas down through south-central Kansas and into Oklahoma and Texas.

Bryozoans are surprisingly few but moderately diverse and widely distributed.

Encrusting sheets are very rare: trepostomes Tabulipora carbonaria and Leptotrypella?, and fistuliporoid Triphyllotrypa?, in central Kansas and northern Oklahoma, in mid- and upper Virgilian, all on Orthomyalina slocomi.

More numerous are encrusting threads: ctenostomes, mostly Condranema parvula plus occasional Ropalonaria?, from northern to southern Kansas, in uppermost Missourian, Virgilian, and mid-Wolfcampian, on Orthomyalina slocomi and four other species.

Originally mistaken for ctenostome bryozoan traces, hollow dents in the shells are acrothoracic barnacle borings, Rogerella (Bascomella) gigantea. On six myalinid species including Orthomyalina slocomi, these occur more southerly (nearer shore; southern Kansas, Oklahoma, Texas), but longer ranging (topmost DesMoinesian to mid-Wolfcampian).

Localities with common myalinids mostly lack stenohaline fossils, thus matching cyclothemic molluscan limestones, deposited under brackish salinity on mud bottoms. Such habitats would have been unsuitable for well-calcified bryozoan sheets, but somewhat more hospitable to ctenostomes and barnacles, exactly as observed.

The negative impact of those environmental stressors on bryozoans apparently outweighed any potential positive preservational effect from them and myalinids both being calcitic.