The Upper Ordovician Viola Group of south-central Oklahoma records deposition on a carbonate ramp that extended from platformal settings to a basin within the Southern Oklahoma aulacogen. Lithofacies analysis allows identification of inner, mid- and outer-ramp environments in the Viola Springs Formation, each of which hosted a distinct trilobite biofacies. There is a sharp break in both composition and species diversity of biofacies between the outer ramp within the aulacogen and shallower environments. The outer-ramp Cryptolithine Biofacies is characterized by low species diversity, whereas both the midramp Thaleops Biofacies and the inner-ramp Bumastoides Biofacies contain up to four times as many trilobite species and rare cryptolithines. Comparisons with Cambrian and older Ordovician shelf-to-basin trilobite distributions suggest that the pattern recorded in the Viola Springs is depth related. The overlying Welling Formation includes two biofacies in inner- to midramp environments, with the faunas of the aulacogen being less diverse than those of the platform. The high trilobite species diversity in shallow water environments of the Viola Group supports a dilution model for community reorganization during the Ordovician.