Search results for "Polymorphitidae"

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

1994

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…