The diversity and phylogeny of the paterinate brachiopods

Abstract

The chemico-structure and morphology of the shells of the earliest known brachiopods, the paterinates, have many features consistent with the antiquity of the group and its phylogenetic proximity to the ancestral stock of the phylum. The organophosphatic shell is typically finely laminated and imprinted throughout with outwardly convex epithelial casts in the older cryptotretids. The greatest concentration of amino acids occurs in the shells of Ordovician Dictyonites but the higher level of aspartic acid/asparagine may not be related exclusively to the post-Cambrian age of the genus as the shell is uniquely 'perforate' through periodic reductions in phosphatic secretion. The quadrilobate larval dorsal valve, the interareas with delthyria and notothyria variably covered by homeodeltidia (or pseudodeltidia in some cryptotretids) and rarer homeochilidia suggest rhynchonelliform affinities as do mantle canal impressions and a musculature which included diductors implanted dorsally on the median plate within the notothyrium. Phylogenetic analysis with penecontemporaneous linguliforms and rhynchonelliforms as outgroups indicates that the paterinates are a sister group of the lingulates 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.