Article: Sexual dimorphism in the belemnite Youngibelus from the Lower Jurassic of Yorkshire
Two belemnite species, Youngibelus tubularis (Young and Bird) and Y. levis (Simpson), are described from one 7-6 m bituminous shale horizon in the falciferum Subzone of the Yorkshire Toarcian. Ontogenetic studies show a common identity for the two species in their initial growth stages (nepionic, neanic). Divergence in morphology occurs in late growth (gerontic stage) by the development of an epirostrum in Y. tubularis and rostral thickening in Y. levis. It is concluded that Y. tubularis and Y. levis are dimorphs of a single biological species. Further, it is postulated that the epirostrum was used for counterbalancing a sexual adaptation or for courtship display purposes. Both forms are described as separate morphospecies.