UNEXPECTED OPHIUROID DIVERSITY IN THE UPPER ORDOVICIAN CINCINNATIAN SERIES OF OHIO AND KENTUCKY

Ordovician ophiuroids (brittle stars) are often abundant in Lagerstätten, but we are typically left with a data gap in our knowledge where these extraordinary occurrences are absent. Only a handful of brittle stars (protasterids and possible stenuroids) have been recorded from the Cincinnatian Series, showing low diversity. Furthermore, the paucity of specimens makes it difficult to determine if any stratigraphic or biotic trends, such as the Richmondian Invasion signal, are recorded by these taxa. Recent work characterizing the morphology of Late Paleozoic ophiuroid lateral arm plates has greatly advanced our understanding of diversity and abundance patterns and species richness in this interval. Here we expand this work into the Cincinnatian Series of Ohio and Kentucky to determine if a hidden fauna is contained in these rocks.

Friable shale/mudstone samples were collected throughout the Cincinnatian Series and processed for microfossils. Generally, samples rich in echinoderm material with a diverse microfossil assemblage contain ophiuroid ossicles, but quality varies throughout the section. Edenian and Maysvillian age brittle star ossicles tend to be very sparse and generally poorly preserved, either eroded or somewhat pyritized. A low diversity of protasterids is present in our samples, but rare taxa may be missing given the paucity of material. Richmondian assemblages show a different pattern with samples from the Bull Fork, Waynesville and the Drakes formations each yielding richly abundant and diverse ophiuroids. Specimen abundance is dominated by five species of protasterids and one species each of cheiropterasterid, strenuroid and a possible stem lineage modern type ophiuroid. The latter is significant as it is likely the oldest occurrence of this lineage yet documented.

These results are very preliminary and subject to multiple interpretations. It is possible that the observed increase in both abundance and diversity within the ophiuroid fauna of upper Cincinnatian strata reflects diversification or faunal immigration of the Richmondian Invasion. Alternatively, the low specimen abundance and poor preservation in the lower part of the section may be a sampling artifact masking an as yet undocumented diversity. Additional sampling will be required to test these possibilities.