THE MEDITERRANEAN LOWER CRETACEOUS

A similar endemism in the Albian ammonites is found on the western coast
of Africa and in Brasil (Elobiceras, Neokentroceras, etc.) (R a w s о n, 1980).
3.2.3. Main Factors of the Ammonite Ecology ^
Since ammonites are a predominantly nektonic group, many researchers assume
even now that they do not depend on the facies. Indeed, most ammonites are relatively
independent of the facies, but as shown by the studies of S с о 1 1 (1940),
Z i e g 1 e r (1967) and others, many ecological factors control the distribution and
the way of life of ammonites. Their wide adaptive radiation has resulted in great
systematic and morphological variety, though definite morphological types manifest
affinity to concrete biotopes. This demonstrates the influence of the ecological
factors.
1. Depth. Such important factors as light, pressure, temperature and nutrient
resources are connected with the depth. Usually the smooth involute forms, e. g.
Phylloceras, Sitesites, Subsaynella, etc., are capable of considerable vertical movements,
therefore they can be found in both neritic and pelagic sediments. However,
some groups, such as the large Ancyloceratina, large Deshayesitidae, Douvilleiceras,
etc., do not transcend the boundaries of the neritic zone.
Bathymétrie control does not influence essentially the ammonite differentiation,
but in combination with other factors it affects ammonite distribution. This
may explain the difference in the ammonite spectra in the Barremian of Northeastern
and Northwestern Bulgaria. A neritic biotope with clayey-calcareous bottom
and extremely rich trophic resources is developed in Northeastern Bulgaria.
This is the reason for the exceptional flourishing of ammonite faunas, including large
vagile-benthos forms. Such a biotope also exists in Southeastern France, in Montagne
de Lure, as well as in Ardèche, the Crimea and Georgia, where the ammonite
association is the same, whereas bathyal biotopes with sludge bottom
and more scanty food resources are developed in Northwestern Bulgaria and in the
Vocontian trough.
It should be pointed out that ammonites occur most frequently in the neritic
zone at a depth exceeding 40 m, and in pelagic sediments mainly from the bathyal
zone. In the Mediterranean Region no ammonites have been found in abyssal sediments,
which is probably connected with the influence of the calcium carbonate
compensation.
2. Temperature. It is a basic factor for the ammonite differentiation. Different
i e
definitions of p O indicate an average of 20-22°C for the European-Caucasian Province,
and 27.7-28.1°C for the Alpine Province (according to data from Albian belemnite
rostrums from the High Alps) (В о w e n, 1961 ; T e i s et al., 1975). Irrespective
of the temperature maximum during the Albian Age, which is doubtful,
the Early Cretaceous was always with a warm climate. Islands in the Alpine Province
were with markedly humid climate, an indication for which is the formation
of a number of bauxite deposits.
Temperature influenced the variety of the faunas, which decreased from south
to north. The amounts of limestones also decreased in the same direction.
3. Character of the sea bottom and degree of activity of the water. To a considerable
extent these factors have a determining influence on ammonite morphology.
Among terrigenous or shallow-sea carbonate sediments formed in a high-energy
environment, ammonites are usually with coarse ribbing, being in most cases with
depressed cross-section. An interesting example in this respect is the Berriasian genus
Pomeliceras whose nominate subgenus, inhabiting sublittoral biotopes with terrigenous
sediments, has a coarse morphology but with depressed section, whereas
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