CHARACTER STATE EVOLUTION IN A NEW PHYLUM: RADIATION OF BRYOZOA DURING THE EARLY TO MIDDLE ORDOVICIAN, WITH A DAPINGIAN TO DARRIWILIAN TRANSITION

Bryozoa have their first appearance in the Early Ordovician (Tremadocian Stage Slice 1B). Five orders of stenolaemate bryozoans are found in the Tremadocian, without clear higher-level relationships, supporting the idea of the existence and radiation of a soft-bodied ancestor in the Cambrian. However, Bryozoa are the only major phylum without evidence in the classic Cambrian radiation of phyla and classes. Unlike most other major groups, bryozoans do have an excellent record during their initial radiation as a result of their stable skeletons of low magnesium calcite and global abundance of shallow marine limestones during this interval. Early to Middle Ordovician Bryozoa are well documented taxonomically across Europe and North America and are known from elsewhere. This, combined with a well-established, sub-stage level biostratigraphy allows for correlation of bryozoan species occurrences, globally at a temporal resolution of ~2 My. Combined, these properties allow for a unique opportunity to document the appearance of fundamental character states associated with the colony form of bryozoans during their initial radiation.

Species-level occurrences of seven fundamental parameters that control colony growth and form (orientation, dimensions of growth, feeding currents, layers of zooecia, substrate relationships, space utilization) were coded for all known Early to Middle Ordovician taxa (179 species, 78 genera, 34 families) through 11 time slices. Bryozoan taxonomic and growth habit richness increases slowly through the early Ordovician (Tremadocian 1b to Dapingian 2c). But there is a marked change in both richness and the relative importance of fundamental growth characters, beginning at the transition from the Early to Middle Ordovician (Dapingian 2c to Darriwilian 3a). Many of these trends continue to the end of the Middle Ordovician. The timing of this change predates similar trends in geochemistry (∂C13, atmospheric oxygen) and diversity of other groups (brachiopods and blastozoans). Changes among bryozoan growth habit features and fundamental changes in features of carbonate substrates at the Early to Middle Ordovician transition, may be related to an increase in availability of firm and hard substrates substrates.