Matskási István (szerk.): A Magyar Természettudományi Múzeum évkönyve 79. (Budapest 1987)

Conti, M. A. ; Szabó, J.: Comparison of Bajocian gastropod faunas from the Bakony Mts. (Hungary) and Umbria (Italy)

concern this plant-association). — In the remaining part of the photic zone (~ 50 ~ 150 m), archaeogastropods might have inhabited both the hard and soft bottom types. — The third depth zone ("3") contains the uppermost few hundred meters of the bathyal region. In Umbria two different faunal types are known but they will be treated here as one unit to reach a general picture. The ecological composition of the faunas from the two regions are quite diiferent. In Umbria, more than a half of the specimens (> 62%) belongs to the soft bottom dweller species (54%). This group is much smaller in the Bakony fauna : 29% of the specimens and 42% of the species. Within the group, preferring soft bottom, other important differences are also existing: the forms, living in vegetation, are practically missing from the Bakony fauna. At the same time this means the absence of the "biting" herbivorous gastropods, too. The image of this latter fauna is determined by hard bottom dwellers. Important difference is in the ratio of Pleurotomariidae. The family is frequent in the Bakony Mountains (6 species, 25 specimens) but it is just present in Umbria (1 species, 4 specimens). The possible explanation: the Umbria region had not as wide depth series of biotopes as the Somhegy region. The diversity of the hard bottom elements is higher in Umbria than in the Bakony (Williams' div. indices: Umbria — 8, Bakony — 6). This is the most conspicuous within Trochidae (div. indeces: T/Umbria — 4, T/Bakony — 1). The higher diversity suggests a less changeable environment for this group in Umbria than in the Bakony. The low number of patelliform gastropods is a character in common in the two faunas that may be explained by the lack of bottom parts, raising into the tidal zone. Rarity of in faunal gastropods is observable also in both faunas but the meaning of this fact is somewhat different for the two localities. In Umbria, there were soft bottom areas without vegetations only of reduced oxtent. In the Somhegy, the low number of the infaunal elements, together with the mentioned almost full lack of plant-assotiations, denotes a restricted presence of the soft bottom itself. Summing up, the most important écologie features of the two localities are: — in the Umbrian region, the soft bottom with vegetation must have been widespread; the characteristic depth interval was the region between the middle of the first and the upper third of the second depth-zone (~ 25 ~ 75 m), — in the Somhegy region, the hard bottom was predominant; the characteristic depth interval might have been the lower part of the first and the upper half of the second depth-zone (~ 50 ~ 100 m). The degree of taxonomical similarity is high in the ecological groups occurring in the two faunas. The significant differences correlate well with the different bottom types and their extent. This correlation exclude an explanation for the divergences which is based on palaeogeography; thus the two regions must have belonged to the same province. PALAEOBIOGEOGRAPHY Both localities belong to the classical "Mediterranean Province" (NEUMAYR 1882), based mainly on cephalopod faunas. Theoretically, the benthonic faunas make a more marked distinction and finer palaeogeographical subdivision possible. The Bajocian gastropod faunas seem to support this theory. They show very special evolutionary lines which are independent from those of the the surrounding areas. Naturally, the lowermost taxonomic level shows the greatest differences. In the two faunas, compared here, there are 98 identified species but only 5 of them are known to be present in NW-European faunas, too. Seven additional species, identified by using of the open nomenclature, may increase the group occurring