Article: New evidence of a spiriferide ancestor for the Thecideidina (Brachiopoda)

Investigation of the microstructure of the ventral interarea of a juvenile denticulate spiriferacean assignable to Unispirifer reveals rod-like structures which, apart from a difference in size, are structurally almost identical with the tubercle cores of a recently discovered Aalenian thecideidine species Mimikonstantia sculpta Baker and Elston, 1984. The coincidence of cyrtomatodont teeth, shell resorption, and secondary fibrous shell, together with rod-like granular calcite structures ensheathed in secondary fibres, links the thecideidines with denticulate spiriferaceans. Comparison of the thecideidine shell microstructure with that of a stropheodontid strophomenide Amphistrophia has failed to reveal comparable microstructural elements. The new evidence indicates that the spiriferacean denticle is a structural homologue of the thecideidine tubercle and, from a systematic point of view, removes any remaining objection to the formal assignment of the Thecideidina as a suborder of the Spiriferida. The morphological similarity between thecideidines, suessiacean spiriferides, and certain davidsoniacean and productidine strophomenides is now regarded as an expression of homoeomorphy.