The Jurassic ammonite Coelocerus: an atypical example of dimorphic progenesis elucidated by cladistics

Instances of convergence or parallelism among unrelated Lower Carixian ammonites are not uncommon. The case of polymorphitid-like ammonites is considered here, and the example of Coeloceras is investigated in detail. The small sexual dimorph (microconch) of Coeloceras pettos (Quensitedt) (Coeloceratidae), a progenetic species, provides a remarkable example of complex convergence with the microconch of Uptonia (Polymorphitidae). Homeomorphic morphology is here an amalgam of juvenile traits, of maturation-related transformations, and of completely new features bearing no obvious relation with either progenesis or maturation. A stratophenetic approach is hardly adequate for unravelling such an involved pattern of convergence. However, a cladistic analysis of ammonoid morphology suggests that Coeloceras was derived from the Tethyan Miltoceras. Indeed, greater biostratigraphic knowledge and the independent use of comparative shell morphology are both needed to improve ammonite phylogeny. □ Convergence, parallelism, progenesis, dimorphism, ammonites, Jurassic.