Article: Functional morphology and ontogenetic variation in the Callovian brachiopod Septirhynchia from Tunisia
The brachiopod Septirhynchia numidiensis sp. nov. is described from the Callovian (Jurassic) of southern Tunisia. Studies of the internal characters and their development reveal that only the adult forms possess the cardinal process and ventral median septum regarded as diagnostic of the genus. Inferred functional morphology suggests that these adult features represent direct or indirect adaptations to living partially buried in sediment, and their gradual development reflects a progressive change from an epifaunal to a semi-infaunal mode of life during ontogeny. Reports in the literature of synchronous homeomorphs with other species of Septirhynchia seemingly refer to juvenile forms. The implications for brachiopod taxonomy of the observed ontogenetic variation in Septirhynchia are discussed. The possible affinities and origins of the genus are re-examined and it is proposed that the present monotypic family based on Septirhynchia should be lowered to the level of a subfamily, the Septirhynchiinae within the Rhynchonellidae. The known stratigraphical and geographical distribution of the genus is reviewed, showing an apparent restriction to the Callovian of the south Tethyan region.