Article: Evaluation of a thecideidine brachiopod from the Middle Jurassic of the Cotswolds, England
P. G. Baker
The discovery of sources of material of Aalenian (Middle Jurassic) age has enabled restudy of Moorellina dundriensis (Rollier). Although in general morphology the species resembles other monoseptate moorellinins, the absence of brachial lobes, together with the characteristically thickened shell and distinctive skeletal shell fabric, clearly separates it from all other contemporaneous representatives of Moorellina. On this basis it is assigned to a new genus Pachymoorellina. The shell microstructure, with its suppressed secondary fibrous mosaic, resembles that of the polyseptate Mimikonstantia sculpta Baker and Elston with which it is associated. The microstructure of the pedicle valve indicates a phylogenetic link with the Lower Cretaceous Thecidiopsis tetragona (Roemer). Furthermore, correlation of the microstructure of certain tubercles in the brachial valve of Pachymoorellina dundriensis with almost identical structures in the Recent Thecidellina barretti (Davidson) suggests a more rectilinear evolutionary pattern for the Thecidellina group than has previously been supposed.