RARE BRYOZOANS IN THE SHRIVER FORMATION (LOWER DEVONIAN) AT BALD EAGLE STATE PARK (CENTRAL PENNSYLVANIA)

Bryozoans are extremely rare in the Shriver Siltstone/Tripoli/Chert (mid-Lower Devonian) in central Pennsylvania. However, faunal analysis based upon the lake-shore outcrop 0.7 mi (1.1 km) due south of the entrance and office of Bald Eagle State Park (Howard 7.5’ quad.) affords a glimpse into this phylum then. That Shriver exposure yields many silicified brachiopods and gastropods, plus occasional bryozoans and other taxa.

Diagenesis has leached away the original calcareous zooecial walls, but silicified the mud filling the zooecial tubes, leaving spectacularly unusual, inwardly radiating, spiny-ball-like, fibrous-appearing hand specimens found by K.L. Andrus. These “fibers” are actually internal molds of the zooecia.

These specimens were originally large (60mm x 15 mm), low-domed colonies, composed of long thin hollow prisms radiating upward and outward from a point (ancestrula) in the center of the colony base. Comparison with previously described bryozoan species from the Appalachian Siluro-Devonian indicates that such colonies represent the trepostome Cyphotrypa expanda. Medium-sized (25 mm x 12 mm) fibrous masses appear to be the fistuliporoid Cyclotrypa mutabilis, with large vesicles (cystopores) separating the mud-filled zooecial tubes. Small (13 mm square, 1 mm thick) lattice-like fragments match the character states of the fenestrate Fenestella cumberlandica; the preserved colony “imprints” represent external molds of the branches and dissepiments. This Shriver bryofaunule (1 trepostome, 1 fistuliporoid, 1 fenestrate) is greatly reduced compared to the slightly older Keyser (9 species of trepostomes, 1 ceramoporoid, 2 fistuliporoids, 1 rhabdomesid, 2 bifoliates, 5 fenestrates; Miller ‘79). Previous studies reconstruct the Shriver paleoenvironment as a level flat bottom of siliceous-calcareous-argillaceous mud, well off-shore, marine, tropical, quiet, in the middle of a shallow (~50 m or 150 ft) elongate embayment roughly coinciding with the present Appalachian trend from Virginia into Pennsylvania. The Shriver bryozoans would have been widely scattered across that bottom, inconspicuous among the much more numerous brachiopods and gastropods.