Article: The morphology and shell microstructure of the thecideidine brachiopod Ancorellina ageri from the Lower Jurassic of Argentina

Serial sectioning of complete shells of Ancorellina ageri enables the first description of dorsal valve internal morphology and shell microstructure. The diagnostic description by Mancenido and Damborenea (1990) is inadequate, as the supposedly diagnostic features of the Ancorellina hemispondylium are known to occur in the Aalenian thecidellinid Moorellina. However, the Ancorellina brachidium, consisting of a laterally supported bifurcating column, is currently unique in the Thecideidina. The early ontogeny conforms with the thecideid pattern. The adult brachidium is believed to have supported a ptycholophe and may be interpreted as a precursor of the ramulate condition of lacazellins. Accordingly, the genus is placed in a new subfamily, Ancorellininae, in the family Thecideidae. The brachidium-supporting pillars resemble the hamate skeletal structures of thecospirellids, introducing the possibility that thecospirellids are close to thecideid ancestral stock. The shell microstructure indicates a phylogenetic link with the Carnian Thecospira haidingeri and probable dispersal from Tethyan faunas early in the Early Jurassic.