Systematics, phylogeny and homeomorphy of the Engonoceratidae Hyatt, 1900 (Ammonoidea, Cretaceous) and revision of Engonoceras duboisi Latil, 1989

摘要

The Engonoceratidae may well have originated on the shallow marine carbonate platforms of the southern margin of the Mediterranean Province of the Tethyan Realm during the earliest Albian (?latest Aptian). In the entire Tethyan Realm and beyond adaptive radiation of the group was rapid and successful during the early Albian as indicated by endemic centres in the Peruvian Basin and the Western Interior Sea (USA). Later the group successfully enlarged its distribution, and invaded some provinces (Mowry Sea, Canada) of the Boreal Realm. The mode of life of engonoceratids seems to have been nektoplanktonic, epipelagic, and stenohaline, restricted to shallow water and platform or in some cases extremely shallow (littoral and lagoonal) facies which may have helped their radiation. The group is distinctive and consists of nine genera and up to a hundred species, although its origin is still obscure. Their appearance may have been triggered by the oceanic anoxic event (OAE 1b) and their rapid rise may have been helped by their shallow water mode of life and the global mid-Cretaceous warming and rise in sea level. The fall of the engonoceratids coincides with the end-Cenomanian rapid transgression, which may have changed their shallow water habitats. Today the group is considered to have been a successful colonizer, a reliable stratigraphical indicator for shallow marine environments and a pioneer taxon for recognition of transgressive phases. Based on new material and sutural analysis, Engonoceras duboisi is revised and assigned to Parengonoceras. A set of features consisting of a simplified ceratitid suture, lanceolate and compressed oxycone shell together comprising a shallow marine ecotype is pointed out as a successful and repeated morphotype among Mesozoic Ammonoidea, for it was repeated three times during ammonoid evolution.