A review of crural types, their relationships to shell microstructure, and significance among post-Palaeozoic Rhynchonellida

Abstract

This overview deals with a few key morphological features that appear crucial to the advancement of knowledge about relationships between macromorphology and micromorphology of the rhynchonellide brachiopod shell. Relevant aspects that may hinder description, comparison and ⁄ or interpretation of morphological skeletal structures of rhynchonellides when studied by means of serial grinding techniques are briefly reviewed. First, the types of crura, traditionally regarded of paramount taxonomic importance, are currently placed into four main cognate groups: septifal, arcual, raducal and ensimergal. Constituent types are characterized, and likely transformation series among them are also outlined. The value of crural types and their groupings within the systematic and evolutionary framework currently applied to post-Palaeozoic Rhynchonellida is corroborated, as is the significant role of heterochronic processes in the development of evolutionary novelties or in the recovery from a severe biotic crisis. Therefore, further studies of ontogenetic development of crura should be encouraged. Second, the main kinds of microstructural patterns of the secondary shell layer as seen in cross section are summarized and illustrated. The leptinoid pattern, typically displaying finer, anisometric fibres with anvil-like or halberd-like outline is contrasted with the eurinoid type, characteristically having coarser, isometric fibres with predominantly diamond-shaped cross section. A close correspondence established between leptinoid shell microstructure and superfamilies with crura of the raducal and ensimergal groups (such as hemithiridoids, rhynchonelloids, rhynchotrematoids, dimerelloids) on the one hand, and between eurinoid shell pattern and superfamilies with crura of the septifal and arcual groups (e.g. pugnacoids, wellerelloids, norelloids) on the other vindicates the prevailing classificatory scheme. Finally, some broad variations of the basic configurations in mantle canal patterns are outlined, confirming the merit of jointly analysing multiple morphological characters and of applying a variety of techniques. All these may lead towards a stronger, more stable and predictive classification of Rhynchonellida in which there is potential for reconsidering the suitability of proper subordinal divisions.