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Stromatoporoids

(Calcareous sponges)

Stromatoporoids, once thought to be related to the corals, are now recognized as being calcareous sponges.  Calcareous sponges similar to fossil stromatoporoids, are found in modern oceans today. Fossil stromatoporoids, called "stroms" for short, are common fossils in the Ordovician and Devonian of Kentucky. Stroms can be massive, chocolate-drop in shape, tabular, encrusting, cylindrical, or even arm-shaped ("ramose").

The Devonian Amphipora ramosa, above, is a genus of branching stromatoporoid common in the Jeffersonville Limestone. In fact, the branches are so common and distinctive that they are used to define a fossil zone within the Jeffersonville Limestone (Perkins, 1963; Conkin and Conkin, 1973; Greb and others, 1993). Tubes and branches in this specimen are a few mm across. In this specimen, they grew on, and almost completely cover a platy stromatoporoid. Because stromatoporoids are not a commonly recognized type of fossil, many amateur collectors may mistake these tubes for small corals or bryozoans. They can be differentiated from these by looking at the tubes under a magnifying glass. Tubular rugose corals like Aulocystus have a calyx or cup within the tube, which stromatoporoids lack. Tubular or branching tabulate corals are covered with many holes (calices) which stromatoporoids lack. Bryozoans are generally solid tubular shapes, rather than hollow, and are also covered with tiny holes. This specimen was collected by R. Todd Hendricks and donated to the Kentucky Geological Survey.

References for stroms:

Conkin, J.E., and Conkin, B.M., 1973, The Amphipora ramosa zone and its significance in Middle Devonian stratigraphy of east-central North America: Earth Research, v. 1, no. 1, p. 31-40.

Conkin, J.E., and Conkin, B.M.,1980, Handbook to strata and fossils at the Falls of the Ohio: University of Louisville Reproduction Services, 27 p.

Greb, S.F., Hendricks, R.T., Chesnut, D.R., Jr., 1993, Fossil beds of the Falls of the Ohio: Kentucky Geological Survey, Special Publication 19, 39 p.

Perkins, R.D., 1963, Petrology of the Jeffersonville Limestone (Middle Devonian) of southeastern Indiana: Geological Society of America Bulletin, v. 74, p. 157-174.

Powell, R.L., 1970, Geology of the Falls of the Ohio River: Indiana Geological Survey Circular 10, 45 p.