THE MEDITERRANEAN LOWER CRETACEOUS

2. MAIN FEATURES IN THE EVOLUTION
OF THE EARLY CRETACEOUS MARINE ORGANISMS
Irrespective of the considerable regression which occurred over considerable areas
in the Mediterranean Region at the end of the Jurassic, there is a gradual transition
from the Jurassic to the Cretaceous world of organisms. An essential renovation
is observed only among the molluscs, characterized by the disappearance of
many Jurassic species and genera, and by the appearance of new taxa.
2.1. Microflora
Marine algae, which were such a flourishing group^^earlier epochs, demonstrate
a certain decline in the Early Cretaceous seas. Nev^Bieless, several groups play a
considerable role in the Early Cretaceous marne eco^stems, namely: Charophyta,
Chlorophyta, Cyanophyta and Rhodophyta.
Of particular significance for the stratigraphy of the Lower Cretaceous carbonate
complexes are the representatives of the green algae: Dasycladacea and Codiacea,
as well as some other algal groups (M а с л о в, 1956; Radojcic,
1960; P r a t u r i о n, 1964; E 1 fi o"t, 1968; Б a к a л о в a, 1971-1976; Conrad,
Peybernes, 1978; Jaffrezo, 1981).
Widespread Dasycladacea are: Clypeina, Cylindroporella, Salpingoporella,
Triploporella, AngioporeHa, Likanella, Acroporella, Siippiluliumaella, Acicularia,
Neomeris, Heteroporella, Actinoporella, etc.
Prominent among the Codiacea arc: Gayeuxia, Boueina, Arabicadium, Bacinella,
Coptocampylodon, Thaumatoporclla, Carpathoporclla, Lithocodium, Marinella,
Ehelia, Diversocalis, etc., whose systematic status is not yet clear, demonstrate
considerable development.
Charophyta are not of essential significance for the stratigraphy of the marine
Lower Cretaceous, but with their help it is possible to differentiate well Wealdian
facies.
Moreover, plant organisms connected with the algae participate in the structure
of the carbonate rocks. These are above all different coccolithophorids and the
nannoplankton associated with them, i. e. remains of organisms whose systematic
status is not yet clear. They participate considerably in the Early Cretaceous plankton
and are particularly varied in the pelagic facies of the Lower Cretaceous.T hi e rstein
(1973) has generalized the main regularities in the nannofossils successions
of the Lower Cretaceous.
Dinoflagellr.tcs are an important group in the Early Cretaceous microplankton,
being at the boundary between the plant and animal kingdoms (Defrandre,
1935-1937; E i s e n а с k, 1958; G о с h t, 1958-1959; see references in A 1 b e rt
i, 1961). Dinoflagellate cysts occur in different types of rocks: limestones, claystones,
flint concretions, outlining clear successions through rich associations.
2.2. Protozoa
2.2.1. Radiolaria
The radiolaria have a more limited distribution than during the Late Jurassic, though
in the Early Cretaceous plankton there are numerous species of Spumellaria and
Nasseilaria. In most cases they participate in the deposition of deep-sea radiolarites,
but sometimes their development is connected with submarine volcanic activity and
in such cases they may propagate to different bathymétrie zones, including in the
sublittoral.
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