Chemico-structural phylogeny of the discinoid brachiopod shell

Abstract

Stratiform shells of living discinids are composed of membranous laminae and variously aggregated, protein-coated granules of apatitic francolite supported by proteinaceous and chitinous nets in glycosaminoglycans (GAGs) to form laminae in rhythmic sets. The succession is like that of living lingulids but differs significantly in the structure of the periostracum, the nature of baculate sets and in its organic composition. In particular, discinids have a higher level of amino acids although with relatively lower acidic and higher basic concentrations; and their overall lower organic content is owing to lower levels of hydrophilic components, like GAGs and chitin. The organic constituents are not completely degraded during fossilization; but data are presently too meagre to distinguish between linguloid and discinoid ancestries. Many differences among three of the four described extant genera emanate from transformations with a long geological history. Pelagodiscus is characterized by regular, concentric rheomorphic folding (fila) of the flexible periostracum and the plastic primary layer and by sporadically developed hemispherical imprints of periostracal vesicles. Both features are more strikingly developed in Palaeozoic discinids. In the oldest discinid, the Ordovician Schizotreta, and the younger Orbiculoidea and related genera, vesicles were persistent, hexagonal close-packed arrays fading out over fila. They must have differed in composition, however, as the larger vesicles of Schizotreta were simple (possibly mucinous), whereas the smaller vesicles of Orbiculoidea and younger genera were composites of thickly coated spheroids, possibly of lipoproteins, which survive as disaggregated relicts in Pelagodiscus. Baculate sets within the secondary layer are also less well developed in living discinids, being incipient in Pelagodiscus and restricted to the dorsal valve of Discinisca. The trellised rods (baculi) with proteinaceous cores are composed of pinacoids or prisms of apatite, depending on whether they are supported by chitinous nets or proteinaceous strands in GAGs. This differentiation occurred in Schizotreta but in that stock (and Trematis) the baculate set is symmetrical with baculi subtended between compact laminae, whereas in younger and post-Palaeozoic species the outer bounding lamina(e) of the set is normally membranous and/or stratified. The most striking synapomorphy of living discinids is the intravesicular secretion of organsiliceous tablets with a crystalline habit within the larval outer epithelium and their exocytosis as a close- or open-packed, transient, biomineral cover for larvae. Canals, on the other hand, are homologous with those pervading lingulid shells. Both systems interconnect with chitinous and proteinaceous sets and have probably always served as vertical struts in an organic scaffolding supporting the stratiform successions. A phylogenetic analysis based mainly on shell structure confirms the discinoids as the sister group of the linguloids but, contrary to current taxonomic practice, also supports the inclusion of acrotretoids within a 'discinoid' clade as a sister group to the discinids.

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