The last phylum: Occupation of Bryozoa morpho-ecospace (colony growth habits) during the early phase of the Great Ordovician Biodiversification Event

Abstract

Most major phyla, and all skeletonized phyla have their first appearance in the Cambrian. The exception is the Phylum Bryozoa, which first appear in the Early Ordovician (Tremadocian 1b). Bryozoans have an excellent fossil record, due in part to their benthic marine habitat with skeletal colonies composed largely of stable, low-Magnesium calcite. These factors provide an unrivaled opportunity to observe patterns and rates of radiation of a new phylum into disparate morpho- ecospace through the Great Ordovician Biodiversification Event (GOBE).In this study, the colonial growth habits of all known skeletonized Bryozoa (181 species) from the Early and Middle Ordovician are characterized in a new classification scheme based on processes of growth, rather than their end geometry as most traditional classifications of bryozoan growth habits. These fundamental categories are: orientation, dimensions of primary growth, width of colony unit, layers of zooecia, substrate relationships, space utilization, skeleton mineralization, plus sub-categories of orientation based on their geometry.In the Early and Middle Ordovician there are 85 unique growth habits defined by these eight growth habit characters. By the end of Middle Ordovician, about 4.8% of the morpho-ecospace defined by two subsets of the characters had been occupied by five bryozoan orders (represented by 34 families, 77 genera). When plotted by Ordovician stage time-slices (ca. 2.2 my each), a sharp increase in taxonomic and growth habit occurrence is observed at the Early-Middle Ordovician transition, earlier than in other groups. Individual growth habit character states also show significant changes at this transition. In the Early Ordovician, low bryozoan taxonomic and growth habit richness is due in part to sampling bias and modified search methods are need, however, the overall patterns (Early-Mid. Ordovician transition) observed in this study are robust and not expected to change fundamentally with additional data.