The Planktic Revolution

The planktic lifestyle has some advantages for the pterobranchs, especially the gain of new ecospace to explore. Anastomosis is another way of keeping strict distances between the stipes and stabilizing the graptolite tubaria. All benthic graptolites were attached to the substrate and fixed to a certain place for life. Benthic, dendroid graptolite colonies possess a relatively unordered, irregular colony shape based on environmental conditions. All derived benthic graptolites, the Dendroidea of Maletz, possess the typical triad budding system with alternating bithecae on the sides of the stipes associated with all thecae. One of the most obvious changes in graptolite colonies after the origination of a planktic lifestyle was the increase in diversity and disparity of graptoloid colony morphologies. The Anisograptidae are at the starting point of the explosive evolution of planktic graptolites. A clear differentiation of the thecal style is seen in Paradelograptus, a genus including numerous two-stiped to multiramous species of Upper Tremadocian age.