THE GRAVICALYMENE CELEBRA TRILOBITE ASSOCIATION FROM THE SILURIAN (WENLOCK) OF INDIANA, KENTUCKY, AND OHIO

The Gravicalymene celebra Trilobite Association is found in Silurian (Wenlock) rocks at widespread localities throughout the central United States. In the Ohio River Valley it occurs in stratigraphically equivalent units, including the upper Laurel Limestone of southeastern Indiana and north-central Kentucky, the upper Salamonie Dolomite in east-central Indiana, and the Springfield Dolomite in southwestern Ohio. It is absent from strata identified as the Laurel Limestone in Ohio, which is older than the Springfield Dolomite and is lower in the Silurian section there. All of these units exhibit the same general biota and are characterized by similar depositional and taphonomic features, having been deposited in quiet-water, shallow marine environments.

The trilobite Gravicalymene celebra is the most conspicuous fossil in these beds and occurs predominantly as semi-articulated exuviae. A high diversity of other trilobite taxa is present, but, with the exception of cheirurids, these trilobites usually are found as isolated skeletal elements. Regionally, the trilobite composition of this association exhibits some variation, but its basic composition remains the same.

Certain cephalopods, bryozoans, rugosid corals, and gastropods also occur with this association, although they exhibit a low diversity. Isolated skeletal elements of echinoderms are prominent bioclasts in these strata; only pisocrinid crinoid calyxes commonly are found as larger specimens. Brachiopods representing Benthic Assemblage 4 are diverse and locally common. In southwestern Ohio, however, some portions of the Springfield Dolomite also contain large specimens of Pentamerus, representing Benthic Assemblage 3. In these occurrences, this brachiopod is found in shell beds and, more rarely, as individuals in living position, along with Gravicalymene celebra.

Since the 1830s, the occurrence of Gravicalymene celebra has been used as a stratigraphic marker in Silurian rocks of this region. In addition, specimens of this trilobite, collected primarily by quarry workers, have long formed the basis of a commercial fossil trade, which has continued to the present day.