The Viola Group (Mohawkian – Cincinnatian) of south-central Oklahoma is a nearly pure carbonate sequence that was deposited on a carbonate ramp along a passive margin. This setting differs from siliciclastic-dominated environments forming in the Taconic Foreland Basin region during the same interval. The Viola Group, which consists of the lower Viola Springs and the upper Welling formations, contains four distinct lithofacies deposited along a lateral depth gradient. Each lithofacies represents a depositional environment ranging from deep subtidal to shallow shoal and preserves a unique trilobite assemblage.

Laminated to bioturbated carbonate mudstone was deposited below storm wave base in the deep subtidal environment of the distal ramp. The trilobites Cryptolithus, Pugilator and Ampyxina were the only benthic organisms preserved. Cryptolithus persisted as deposits shallowed and coarsened upward into an intermediate-depth subtidal environment, and Anataphrus, Isotelus and Flexicalymene were added to the assemblage. Shallow subtidal deposits formed in an area of intermediate energy on the proximal ramp where abundant carbonate mud was preserved between shell-rich, storm-winnowed beds. The trilobite assemblage found in this facies contains the highest diversity (approx. 20 species) and is dominated by Calyptaulax and an illaenid. High-energy shoal deposits on the ramp margins in the Viola Springs Formation are lithologically similar to shallow carbonate platform deposits of the Welling Formation and consist mainly of cross-bedded, well-washed rudstone. These environments supported slightly lower levels of diversity than the shallow subtidal habitat. Bumastoides, Thaleops and Ceraurus were dominant in shoal deposits of the Viola Springs, while large isotelines including Anataphrus, Isotelus and Ectenaspis were the most common elements in the younger Welling Formation.

The results of our study confirm that trilobites remained important and diverse components of paleocommunities at multiple depths in the Late Ordovician of southern Laurentia. Any argument postulating the displacement of trilobites by members of the Paleozoic Evolutionary Fauna must account for this persistence of trilobite diversity and abundance in shallow carbonate environments.