ROLE OF SHELL STRUCTURE IN THE CLASSIFICATION OF THE ORTHOTETIDINE BRACHIOPODS

The secondary shell of the spire-bearing Davidsonia is fibrous, wherea s in all true orthotetidine brachiopods it is laminar. For this reason , Davidsonia and related genera, which constitute the Davidsoniidae, a re transferred to the spire-bearing brachiopods, the Atrypidina. The o ldest 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 a ppropriate for orthotetoid pseudopunctae as a whole, which have not ye t been found to contain undoubted taleolae. Among schuchertellids, inw ardly projecting pseudopunctae were replaced by outwardly pointing ext ropunctae which could have evolved by changes in the rate of shell sec retion relative to a different kind of organic holdfast. Koskinoid per forations also penetrate most orthotetidine shells, but they do so wit hout deflecting lamination and were probably drilled mechanically by b oring organisms. Assuming shell structure and the loss of a functional pedicle foramen each to have the same taxonomic weight as all the mor phological features developed for articulation and muscle support, phy logenetic analysis confirms that the orthotetidines belong to two supe rfamilies: an older paraphyletic Chilidiopsoidea, and a younger monoph yletic Orthotetoidea. Both groups were affected by homeomorphic trends resulting from cementation and conical deepening of the ventral valve s of many independent stocks. They can, however, be distinguished by p hylogenetic analysis which provides cladograms consistent with their s tratigraphic distribution.