Article: Phylogeny and evolution of 'pentameride' brachiopods

Despite their importance in articulate brachiopod evolutionary history, relatively little is known in detail about the phylogenetic relationships among 'pentameride' taxa, and of ' pentamerides' to other articulates. Phylogenetic relationships among all named 'pentameride' families and rhynchonellide superfamilies were reanalysed using outgroup methods of polarity determination. A detailed working hypothesis of 'pentameride' phylogeny and the supporting evidence on character distribution is presented. As currently diagnosed, Pentamerida and Syntrophiidina are paraphyletic, while Rhynchonellida and Pentameridina are monophyletic. Generally acknowledged patterns of morphological change may now be examined in detail, as they are expressed in a comprehensive pattern of relationship. In the evolutionary history of these taxa, strophic hinge line length decreased steadily and astrophic hinge lines evolved twice. Interlocking hinge structures arose twice from the non-interlocking condition, and muscle platforms in the dorsal and ventral valves evolved several times independently. These phylogenetic results have significant implications for several issues relevant to the study of brachiopod systematics. Agreement between the stratigraphical first appearance of 'pentameride' families and their cladistic rank is quite good, suggesting that both outgroup and palaeontological methods indicate the same direction of character polarity in the evolution of' pentamerides'. The paraphyletic 'syntrophiidines' suffer pseudoextinction in transforming to the monophyletic rhynchonellides (extant) and the monophyletic pentameridines (extinct), which possess a combination of characters (very strong biconvexity, large adult size, lack of pedicle, non-interlocking dentition) that apparently rendered them less able to adapt over time to changes in their habitat. A highly corroborated phylogenetic hypothesis provides an explicit framework within which causal hypotheses of macroevolutionary phenomena may be generated and tested.