THE JURASSIC AMMONITE COELOCERAS - 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 Coeloreras is investigated in d etail. The small sexual dimorph (microconch) of Coeloreras pettos (Que nstedt) (Coeloceratidae), a progenetic species, provides a remarkable example of complex convergence with the microconch of Uptonia(Polymorp hitidae). Homeomorphic morphology is here an amalgam of juvenile trait s, of maturation-related transformations, and of completely new featur es bearing no obvious relation with either progenesis or maturation. A stratophenetic approach is hardly adequate for unravelling such an in volved pattern of convergence. However, a cladistic analysis of ammono id morphology suggests that Coeloceras was derived from the Tethyan Mi ltoceras. Indeed, greater biostratigraphic knowledge and the independe nt use of comparative shell morphology are both needed to improve ammo nite phylogeny.