A relatively diverse biota of encrusting and boring organisms is present upon organic and inorganic hard substrates throughout rocks of the Cincinnatian Series (Upper Ordovician) of Ohio, Indiana, and Kentucky. Specimens of the enigmatic encruster Chaunograptus are relatively uncommon, but occur principally in Richmond Stage rocks, particularly in the Waynesville, Liberty, and Whitewater Formation. It is commonly associated with trepostome and cyclostome bryozoans, cornulitids, sphenothallids, and borings. Chaunograptus appears to be a colonial organism consisting of thin, black, irregularly branching strands that are commonly seen to encrust hardgrounds, cobbles, pebbles, and strophomenid brachiopodes. The strands are typically about 0.1 mm in diameter, and consist of almost pure carbon, possibly indicating an original chitinous or proteinaceous composition. Undoubted thecal-type or cyst-like structures have not been observed. Commonly, only the basal walls of strands of Chaunograptus are preserved, but collapsed or still-inflated hollow tubes also occur. Colony growth upon brachiopod shells is characterized as simple runner-type, with relatively few branching points. Colonies on inorganic hard substrates, however, are characterized by irregular and hightly contorted growth forms. Chaunograptus occurs in carbonated platform environments ranging from relatively low-energy, deeper water to high-energy, shallow water. Fossils referred to Chaunograptus have been interpreted as representing graptolites or hydroids. The absence of observed fusellar tissues in any available Cincinnatian specimens of Chaunograptus appears to preclude affinities with either graptolites or rhabdopleurid pterobranchs. The simple morphology of Chaunograptus makes assignement to a taxonomic group difficult, but it is tentatively considered here to represent a network of hydrozoan stolons.

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