Youngibelus tubularis and
Y. levis are two species from
Toarcian aged rocks in
Yorkshire. It has been suggested that these two species are actually morphs of the same species. The divergence in
morphology occurs in the gerontic stage of development, characterised by differences in the rostrum. In
Y. levis, the
rostrum thickens, whilst in
Y. tubularis, an epirostrum develops. Peter Doyle proposed that the epirostrum developed as either a counterbalance for a sexual adaptation, or for display in courtship. ==References==