. Bulletins of American paleontology. Ordovician-Silmrian Rugose Corals: McAuley and Elias Cracken and Barnes, 1981, p. 66, fig. 12), and the Ordovician-Silurian boundary defined as the base of the Farakidograplus aciiniiiwtus Graptolite Zone (Cocks, 1985) may be situated within the nathani Zone (Lesperance, 1985, figs. 3, 4; McCracken and Nowlan, 1988, p. 77). Beneath the Keel Formation, the Sylvan Shale con- tains Late Ordovician (Maysvillian-Richmondian) graptoliles of the Dicellograptus complanatus Zone in the lower to middle portion of the unit (Decker, 1935, pp. 698-700), conodonts t
. Bulletins of American paleontology. Ordovician-Silmrian Rugose Corals: McAuley and Elias Cracken and Barnes, 1981, p. 66, fig. 12), and the Ordovician-Silurian boundary defined as the base of the Farakidograplus aciiniiiwtus Graptolite Zone (Cocks, 1985) may be situated within the nathani Zone (Lesperance, 1985, figs. 3, 4; McCracken and Nowlan, 1988, p. 77). Beneath the Keel Formation, the Sylvan Shale con- tains Late Ordovician (Maysvillian-Richmondian) graptoliles of the Dicellograptus complanatus Zone in the lower to middle portion of the unit (Decker, 1935, pp. 698-700), conodonts that probably represent the Amorphognathus ordovicicus Zone (Sweet and Berg- strom, 1976, p. 146), and Late Ordovician chitino- zoans throughout (Jenkins, 1970, pp. 284, 285). Above the Keel, the brachiopod Triplesia alata Ulrich and Cooper, 1936, apparently ranges through most of the Cochrane Formation (Amsden. 1971a, p. 145). Ams- den (1986, p. 6) considered it to be Early Silurian (early Late Llandovery, C,,,)- but noted that it could be youn- ger or older. The conodont fauna of the basal Cochrane was thought to be Llandovery (position uncertain) by Barrick (1986, pp. 57, 64, 67). Late Late Llandovery (Cj) species of the Ptewspathodus celloni Conodont Zone were identified in the uppermost Cochrane by Barrick and Klapper (1976, p. 66). Age assignments we follow are shown in Text-figure 2. Paleoenvironment Amsden (1960, pp. 41, 42, 160) considered the Keel oolite to have formed in warm, shallow, agitated but not strongly turbulent water within the zone of effective light penetration. It is inferred from the random ori- entation of solitary rugosan coralla in the Brevilam- nulella beds and upper Keel oolite at Section 23 (Law- rence Quarry) that the directions of fluid motion may have been variable (Elias, McAuley, and Mattison, 1987,p. 810). Amsden (1986, pp. 10, 11, 42, 43) noted that fossils in the Keel are not in growth position and do not show excessive breakage, although t
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