THE MEDITERRANEAN LOWER CRETACEOUS

to them a possibility for rapid and wide migration by means of the ocean currents.
This is why, many Early Cretaceous benthonic organisms (e. g. a number of gastropods,
bivalvs, echinids and Orbitolinidae) may be found from one end to the other
of the Mediterranean Region. Consequently, the stratigraphie significance of the
species is determined not by the rate of their migration, which is considerable, but
by the rate of their evolution, which depends on many genetic and ecological factors.
3.2. Ammonite Provincialism during the Early Cretaceous
Today it is considered an unanimous opinion that the biogeographic boundaries
of the different groups of organisms do not coincide. Moreover, the lower the rank
of the palaeobiogeographic category, the greater the differences in the boundaries
according to the various groups.
Therefore, we shall discuss only the ammonite distribution during the Early
Cretaceous, which traces the basic lines of Ea'rly Cretaceous palaeobiogeography.
Naturally, it is possible to add to this many interesting data about the belemnites,
foraminifera and rudists, which introduce remarkable features in the panorama of
Early Cretaceous life in the Mediterranean Region.
The ammonite distribution at the beginning of the Early Cretaceous bears the
main features of the Tithonian ammonite provincialism. Together with this new features
are imposed, changing the configuration of the areas and especially of the provinces.
Fig. 23. Faunistic provinces during the Late Jurassic and Early Cretaceous (after U h 1 i g, 1911)
/ — Boreal Realm; la — Northern-Andean Province; 2—Mediterranean-Caucasian Realm; 2a — neritic marginal zonc;
3— Himalayan .Uealm; 3a — Ethiopian Subprovince; 3b — Maori Subprovince; 4—Japanese Province; 5 — Southern-
Andean Realm
11 The Mediterranean Lower Cretaceous 161