pdf: http://www.ville-ge.ch/mhng/paleo/paleo-pdf/27-2/pal-27-2-15.pdf

Abstract
Ammonoids are traditionally considered the best Jurassic guide fossils, and the biozones based upon them have the formal value of chronostratigraphic units. However, the taxonomic and phylogenetic knowledge of this extremely diversified cephalopod group is still incomplete. Here new data and hypotheses on the relationships of various Early Jurassic major taxa are presented, with particular attention to the forms of Pliensbachian and Toarcian stages. These relations are showed in a phylogenetic tree expressing close connections between taxonomy, phylogeny, biostratigraphy, environmental crises sensu lato and palaeo(bio)geography. Its differences compared to the classical schemes are suggested by the examination of the morphological and structural characters of the shell joined with the stratigraphic and geographic distribution. Families Polymorphitidae, Tropidoceratidae, Hammatoceratidae and Hildoceratidae are particularly well-documented in the Apennine area, and for this reason they are discussed with detail. For their origin, new hypotheses are advanced basing on Mediterranean Tethys data, an area from which come the oldest representatives of these taxa. Some of their traditionally-accepted relations are questioned : for instance in our opinion Hildoceratoidea do not have direct ties with the Eoderoceratoidea, a group that according to the Apennine documentation would have originated the Hammatoceratoidea. A remarkable addition in our phylogenetic tree is the presence of the Sinuiceratidae : this recently-established Pliensbachian family represents a higher rank taxon, until now found only within the Mediterranean area. Its affinities and derivation are still uncertain, and this denotes a wide gap in the knowledge of the Hettangian and Sinemurian. The origin of the taxa here more thoroughly discussed with new data is linked to two Early Jurassic critical moments : one is at the Sinemurian/Pliensbachian boundary, while the other corresponds to the early Toarcian OAE (Oceanic Anoxic Event). For the mentioned groups, the appearance of new organizational plans (macroevolution) cannot be directly interpreted by analysing the inner and outer whorls of their shell, as instead is often possible in transitions between genera and species. This leads to hypothesize a ‘breakdown’ in the normal evolution, with development ‘regression’ to small smooth forms. These phenomena, although still hard to understand for the lack of data on ammonite soft body and biology, preceded in time the adaptive radiations, which are well-represented in the Apennine documentation. Functional morphology is an important evolutive element for the ‘control’ of the phyletic lineages, as can be seen in the iterative tendency to achieve discoidal shells, interpretable as greater and greater hydrodynamic adaptations (e.g. Polymorphitidae, Fanninoceratidae and Hildoceratidae). From a more general point of view, the present study can also lead to well-founded chrono- and biostratigraphic subdivisions for a better definition of the boundaries, as well as to palaeogeographic checking. This in turn provides a fundamental reference basis for other kinds of researches as magnetostratigraphy, geochemistry of the environmental changes, and so on.