THE DIVERSITY AND PHYLOGENY OF THE PATERINATE BRACHIOPODS

The chemico-structure and morphology of the shells of the earliest kno wn brachiopods, the paterinates, have many features consistent with th e antiquity of the group and its phylogenetic proximity to the ancestr al stock of the phylum. The organophosphatic shell is typically finely laminated and imprinted throughout with outwardly convex epithelial c asts in the older cryptotretids. The greatest concentration of amino a cids occurs in the shells of Ordovician Dictyonites but the higher lev el of aspartic acid/asparagine may not be related exclusively to the p ost-Cambrian age of the genus as the shell is uniquely 'perforate' thr ough periodic reductions in phosphatic secretion. The quadrilobate lar val dorsal valve, the interareas with delthyria and notothyria variabl y covered by homeodeltidia (or pseudodeltidia in some cryptotretids) a nd rarer homeochilidia suggest rhynchonelliform affinities as do mantl e canal impressions and a musculature which included diductors implant ed dorsally on the median plate within the notothyrium. Phylogenetic a nalysis with penecontemporaneous linguliforms and rhynchonelliforms as outgroups indicates that the paterinates are a sister group of the li ngulates and consist of two subclades, Paterinidae and Cryptotretidae. The latter were short lived but remarkably diverse and may well have evolved directly from the brachiopod stem group.