MAZON-CREEK-LIKE SOFT-BODIED PRESERVATION IN NORTHWESTERN PENNSYLVANIA -- THE FIRST-KNOWN FLESHY-BRANCHED CTENOSTOME BRYOZOAN IN THE FOSSIL RECORD (MID-PENNSYLVANIAN)
This fossil came from southeastern Elk County, 22 km (14 mi) N120°E of Saint Marys and 4.4 km (2.7 mi) N42°E of Benezette, on the south slope of Winslow Hill. The specimen is contained in a siderite nodule from the dark shale equivalent laterally to the marine Vanport Limestone, low in the Allegheny Formation (mid-Pennsylvanian, roughly 310 million years old).
This specimen, somewhat flattened, measures 9 by 11 cm and is 1 cm thick. It has a plate-like base, from which arise at least 28 upright elongate cylinders or branches, a few of which bifurcate. The branch surfaces are covered with minute low mounds, ~0.3 mm across, the remains of the soft-bodied zooids or polyps. These are arranged in places as rows obliquely spiraling upward around the branch, and elsewhere as rows parallel to branch length. The fossil resembles, but is not identical to, modern gelatinous Alcyonidium ctenostome species found along eastern American and western European shores.
The United States during the Middle Pennsylvanian lay under a shallow epicontinental sea, extending from Nebraska, Kansas, and Oklahoma eastward to end in a wide bay covering western Pennsylvania. Its northern shore was deltaic, and extended from near the Winslow Hill locality westward all the way to northern Illinois’ Mazon Creek area, well known for its siderite nodules containing exceptionally preserved soft-bodied fossils like jellyfish and worms.
Both Pennsylvania and Illinois back then were complex mosaics of coastal, estuarine, and marine shallows. Animals living on the bottoms there would have been smothered and rapidly buried by periodic sudden influxes of mud (obrution) supplied from the adjacent land. Immediately, bacteria would have begun interacting among the organic carcasses, interstitial pore waters, and minerals in the sediment, stimulating diagenetic geochemical processes (involving especially sulfur, methane, iron, and organics) to precipitate siderite around the bodies very soon (only days, weeks, or months).