Corynotrypa from the Ordovician of North America: Colony growth in a primitive Stenolaemate

Colonies of the runner-like bryozoans Corynotrypa delicatula (James) and C. inflata (Hall) are common encrusters of Middle and Upper Ordovician shells and hardgrounds, especially in Cincinnatian deposits. The simplicity of their zooids contrasts with complexities in the dynamics of colonial organization. Both species have uniserial branches that bifurcate at intervals and, in addition, give rise periodically to lateral ramifications. Although angles of bifurcation and lateral ramification each average about 80°, bifurcations and lateral ramifications are fundamentally different modes of branch multiplication. In C. delicatula new lateral branches have conspicuous secondary zones of astogenetic change distinguished by elongation of successively budded zooids. Unlike bifurcations, the first zooids in lateral branches in Corynotrypa are not linked to the parent branch by a narrow basal canal, and each new lateral branch can be regarded as a distinct subcolonial unit. The ancestrula, described here for the first time in an Ordovician species of Corynotrypa, has a poorly differentiated protoecium and initiates a primary zone of astogenetic change. Colony growth in Corynotrypa was more plastic than in many other bryozoan runners. The systematics of C. delicatula and C. inflata are revised, and a lectotype is chosen for the former species.