Article: Role of the shell structure in the classification of the orthotetidine brachiopods

The secondary shell of the spire-bearing Davidsonia is fibrous, whereas in all true orthotetidine brachiopods it is laminar. For this reason, Davidsonia and related genera, which constitute the Davidsoniidae, are transferred to the spire-bearing brachiopods, the Atrypidina. The oldest known orthotetidines are impunctate, but the Ashgillian Fardenia scotica rarely bears incipient pseudopunctae, which apparently arise through spiral perpetuation of screw dislocations. This origin seems appropriate for orthotetoid pseudopunctae as a whole, which have not yet been found to contain undoubted taleolae. Among schuchertellids, inwardly projecting pseudopunctae were replaced by outwardly pointing extropunctae which could have evolved by changes in the rate of shell secretion relative to a different kind of organic holdfast. Koskinoid perforations also penetrate most orthotetidine shells, but they do so without deflecting lamination and were probably drilled mechanically by boring organisms.Assuming shell structure and the loss of a functional pedicle foramen each to have the same taxonomic weight as all the morphological features developed for articulation and muscle support, phylogenetic analysis confirms that the orthotetidines belong to two superfamilies: an older paraphyletic Chilidiopsoidea, and a younger monophyletic Orthotetoidea. Both groups were affected by homeomorphic trends resulting from cementation and conical deepening of the ventral valves of many independent stocks. They can, however, be distinguished 'by phylogenetic analysis which provides cladograms consistent with their stratigraphic distribution.