THE MEDITERRANEAN LOWER CRETACEOUS

calcareous bottom. Most characteristic among them are Spatangoidea, genus Toxaster
being the most frequent and represented by many species from the Valanginian
to the Aptian.
A number of genera belonging to Disasteridae (Cardiopelta, Collyrilcs, Disaster,
Tithonia, Corthya, Dialaster and Acrolusia) and to Toxasteridae (Toxaster,
Epiaster, Heteraster and Hemiaster) outline successions in the Lower Cretaceous
and are used for stratigraphie purposes. The most important change which the echinids
undergo during the Early Cretaceous is the evolution of their apical apparatus,
which is markedly elongated in the Berriasian to become gradually almost quadrangular
during the Albian (D e v r i e s, 1965).
2.3.4. Brachiopoda
Irrespective of their extensive distribution in the Lower Cretaceous, brachiopods
are less studied in this series than in the Jurassic. Indeed, their Early Cretaceous
evolution manifests a certain decline, but careful investigations indicate that brachiopods
form well-outlined successions which, although comprising greater intervals,
may be used successfully in the Lower Cretaceous stratigraphy. In a generalization
Aimeras (1965) has demonstrated clear brachiopod successions in the
Lower Cretaceous of France and the neighbouring countries.
Pygope is a particularly characteristic genus for the deep-water facies. Rhynchonella
and Terebratula, which define the appearance of the Early Cretaceous brachiopod
ensembles, occur very frequently, as well as Terebratulina, Magelanila and
Terebralella (K i 1 i a n, 1907-1913; Смирнова, 1960; Aimeras, 1965).
2.3.5. Mollusca
Gastropoda occur frequently among Lower Cretaceous sediments, but few of their
representatives are of interest to stratigraphy. Their evolution is characterized by
slow rates and usually forms specialized species which are adapted to definite facies,
most often carbonate or terrigenous-carbonate.
Most widespread are Natica, Pleurotomaria, Nerinea, Bathrotomaria, Purpuroidea,
Ampullina, Conotomaria, Tylostoma and Ampullospira, which are tied to neritic
facies (K i 1 i a n, 1907-1913; Димитрова, 1974).
Bivalvia are one of the most widespread groups during the Early Cretaceous,
with considerable systematic and morphological variety which competes with the
cephalopods. However, they have slower rates of evolution, most of them are stenotopic
and they are adapted mainly to neritic environment. Only genus Nucula
occurs almost exclusively among the deep-sea Mediterranean sediments, whereas
Area and Leda are found less frequently.
Cucullaea, Arcomytilus, Lithophaga, Pinna, Propeaniussium, Camponectes,
Chlamys, Prohinnites, Hinnites, Pholadomya, Gonyomya, Arctica, Glossus, Pronoella,
Veniella and Venicardia have considerable development. Plicatula occurs frequently
among marl sediments.
Ostreidea are extremely developed: Ostrea, Exogyra and Alectryonia are frequent
forms among the Early Cretaceous neritic faunas. In a number of cases they
build entire calcareous packets (oyster banks), though they also occur among marl
sediments in the peripheral part of the sublittoral zone and very rarely in deep-sea
sediments.
Of the Bivalvia Family Trigonnidae flourished during the Early Cretaceous,
moreover including both Mediterranean and cosmopolitan forms. They rapidly
declined after the Cenomanian. Owing to its relatively faster evolution, this family
forms characteristic succesions of great stratigraphie significance (Gil let, 1965).
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