BRYOZOAN SPECIES AND ROLES IN THE STOBO CRINOID-BANK BIOHERM (LOWER MISSISSIPPIAN, SOUTH-CENTRAL INDIANA)

Mississippian bioherms vary from abundantly crinoidal grainstones to sparsely bryozoan micstones. A well-developed example of the former was examined to compare its bryozoans and their roles with those in several of the latter previously studied.

The crinoid bank studied (“Stobo”) is exposed along Ind. Hwy. 46 3.3 mi (5.3 km) east of its junction with Ind. 446 on the east edge of Bloomington; (center NW1/4 NE1/4 sec. 4, T8N, R1E, Unionville 7.5’ quad., Monroe Co.). Bryozoans are scattered among crinoid debris in the lower half of that roadcut. The build-up is upper Osagean, high in the Borden Siltstone, at the horizon of the Floyds Knob or basal Edwardsville members. The roadcut exposes thin-bedded but mounded crinoidal limestones interlayered with sparsely fossiliferous silty shales. It has been studied previously, but not for bryozoans until now. Over 100 specimens were identified, from both old and recent monographs (especially Ulrich 1890 and Snyder 1991).

The Stobo bank was originally a pile of loose crinoid debris, lying on terrigenous mud bottoms on the shallow-marine topset of the Borden Delta on the edge of the intracratonic Illinois Basin.

Most of the Stobo bryozoans are fallen, horizontal to bedding; a few are encrusting on crinoid debris; none are upright or in-place. They functioned as accessory or hidden-encrusting community participants, subordinate to crinoids, forming some skeletal sediment, and perhaps locally stabilizing or baffling sediments.

Among the bryozoans, 2 species are common, delicate fenestrate “Fenestella” regalis with large fenestrules, thin branches, and low-angle bifurcations, and bifoliate fistuliporoid Cystodictya lineata with longitudinally striated branches and oblong zooecial apertures. Two species are uncommon, finer-mesh fenestrate Banastella delicata and smooth-surfaced bifoliate Cystodictya ocellata. In addition, 25 more species are rare at Stobo: 17 fenestrates, 5 rhomboporoids, 2 trepostomes, and 1 fistuliporoid. Eight Stobo species do not occur in the previously studied Mid-Western Mississippian “Waulsortian-like” mud-mounds; the other 21 do.