Directional macroevolution of the diplograptacean graptolites: a product of astogenetic heterochrony and directed speciation

Graptolite macroevolutionary history comprised a succession of rapid evolutionary radiations initiated by structural innovations in colony design. Transitions from one astogenetic pattern to another during the evolution of the Diplograptacea invariably proceeded from a complex pattern to a less complex pattern. Thirteen such transitions are known and took place as a consequence of abrupt heterochronic rearrangements of colonial development. Clades characterized by colonies with complex astogenies waned while, in some cases, possessors of the newly simplified astogenies proliferated. By the Ashgill graptoloid faunas were almost exclusively composed of colonies with a highly simplified astogeny. Accordingly, the directional character of diplograptacean macroevolution was dominantly a consequence of directed speciation. There is little evidence that anagenetic progress was a significant agent in the evolution of diplograptacean colonial organization. -from Author