Article: The classification, origin and phylogeny of thecideidine brachiopods

Recent studies on ontogeny and shell microstructurc enable recognition of the inherent weakness of basing thecideidine taxonomy and phylogeny on morphological characteristics of separated brachial valves. Some previous assumptions are invalidated. The more robust components of lophophore supports are useful in determining relationships, but the form of the brachial lobes, because of their fragility, is less easy to establish, and, without the supporting evidence from sectioned complete shells, their value as taxonomic indicators is questionable. General evolutionary trends may be established through increasing complexity of lophophore supports, but for a clear understanding of thecideidine phylogony detailed investigation of ontogeny and shell microstructure is required. Neotenous origin and the masking effects of convergent evolution have combined to obscure the line of thecideidine descent. However, shell microstructural evidence now points clearly to a spiriferacean ancestor. The high degree of external morphological similarity makes it essential to consider evidence compiled from studies of morphology, ontogeny and shell microstructure. A revised taxonomy assigning the Thecideidina, Thecospiracea and Bactryniidae to the Spiriferida is proposed.