The Brechin Lagerstätte of southern Ontario contains an exceptionally diverse and well-preserved Late Ordovician (Katian) crinoid fauna. Here, we describe four genera and eight species of camerate crinoids from the Brechin Lagerstätte, including six new species. Consequently, the total diversity of the fauna now stands at 27 genera and 39 nominal species, thereby making it the most taxonomically diverse Ordovician crinoid fauna known. Taxa described herein include the diplobathrid Pararchaeocrinus kiddi n. sp. and the monobathrids Glyptocrinus ramulosus, Periglyptocrinus priscus, Periglyptocrinus astricus n. sp., Periglyptocrinus kevinbretti n. sp., Periglyptocrinus mcdonaldi n. sp., Periglyptocrinus silvosus n. sp., and Abludoglyptocrinus steinheimerae n. sp. We summarize the taxonomic composition, diversity, and abundance distribution of all known crinoids from the Brechin Lagerstätte to better characterize the paleoecological structure and complexity of the community. We establish that the fauna is dominated by the subclass Pentacrinoidea, both in terms of abundance and species richness. In addition, we analyze species-level abundance data using relative abundance distribution (RAD) models to evaluate the ecological complexity of the paleocommunity. We find that community structure of the Brechin Lagerstätte is best explained by an ecologically “complex” RAD model, which suggests species partitioned niches along multiple resource axes and/or the presence of multiple ecological ways of life. These results indicate that the Brechin Lagerstätte is significant not only for being the most taxonomically diverse Katian crinoid assemblage, but also for being an early ecologically complex fauna that developed in the wake of the Great Ordovician Biodiversification Event.