Paläobiogeographie jurassischer Muschelfaunen: Beziehung zwischen Süd- und Nordrand der Tethys

Abstract

Similarities of mid-Jurassic bivalve faunas between the European and the Ethiopian faunal province are very high at the genus-level. At the species-level, however, it is shown that during the Bathonian and Callovian 35% of the bivalves occurring in the Ethiopian faunal province are restricted to this province. In the region of Kachchh (W-India) in the same time-interval 25% of all bivalves are endemic. In the Ethiopian faunal province a clear tendency of increasing endemism from the Bathonian to the Tithonian/Lower Cretaceous at the genus-level and, even more obviously, at the species-level exists. Endemism and provincialism are most marked within the orders Arcoida, Trigonioida, and Nuculoida. The degree of endemism is lower within the Veneroida, but still very high. The orders Mytiloida, Pterioida, and Pholadomyoida hold the largest portion of cosmopolitan species. The rise of endemism and provincialism in Kachchh and m the Ethiopian faunal province from the Bathonian onwards can be explained only partly by the increasing broadening of the Tethys and its effect as an oceanic barrier. The steep increase of endemism in the Upper Jurassic of Kachchh is essentially caused by a radiation within the astartids and trigoniids, accompanied by a reduction of facies-types, due to a regional regression. The very southerly palaeogeographic position of India, the opening of the ‘South African Seaway’, and a change in the marine current system in the uppermost Jurassic led to an increasing differentiation of the Ethiopian faunal province in an ‘Ethiopian-Tethyan’ subprovince to the north and an ‘Ethiopian-Austral’ subprovince to the south.

A migration of bivalves in mid-Jurassic times can be reconstructed along the southern margin of the Tethys mainly from east to west. On the other hand, an easternward migration of bivalves along the northern margin of the Tethys from Europe to China and Japan can be documented especially in the Upper Jurassic. This corroberates the existence of a clock-wise marine current system in the northern hemisphere in the Jurassic. The distribution patterns of bivalves in Kachchh and the Ethiopian faunal province are essentially characterized by ‘migration’ of bivalves. The opening of the ‘Hispanic Corridor’ in the Pliensbachian gave way to the immigration of East Pacific bivalves via the western Tethys as far as Kachchh and Madagascar. The dispersal ofPisotrigonia, Seebachia, Tendagurium, andMegacucullaea in the uppermost Jurassic/lowermost Cretaceous from Kachchh and East-Africa respectively to South-Africa and South-America documents the establishment of a ‘South-African Seaway’ and favours migration. However, ‘migration’ and ‘vicariance’ do not exclude each other. On the contrary, both are important mechanisms for creating distributional patterns of bivalves, although within different geological dimensions. Vicariance events produce faunal provinces which last for a long time and within this time-interval, migration seems to be the more important mechanism affecting palaeobiogeographic distribution of bivalves.

There is no evidence that the distribution patterns of bivalves in Kachchh and in the Ethiopian faunal province are governed by eustatic sea-level changes. The dominating factors have been a change of the palaeogeographic constellation as a consequence of the break-up of Gondwana, and the local facies distribution.

The number of bivalve species known from Europe is much larger than the number of species of the Ethiopian faunal province. A comparison of rarefaction curves of associations, however, shows that this is not a primary feature, but is a consequence of a greater number of different facies types and is due to a far more intense collecting activity in Europe.

The bivalves of the Spiti Shales are unequivocal Ethiopian-Tethyan in character. The composition of the fauna indicates the deposition on the deeper shelf of the southern margin of the Tethys. All occurring ‘European’ faunal elements are not significant because of their more or less cosmopolitan distribution.

Kurzfassung

Jurassische Muschelfaunen der europäischen und der äthiopischen Faunenprovinz werden miteinander verglichen. Im Mitteljura sind ca. 35% aller Muscheln der äthiopischen Faunenprovinz endemisch, mit stark zunehmender Tendenz (vor allem bei Arcoida, Trigonioida und Nuculoida) im Oberjura, die im wesentlichen auf einer Radiation innerhalb der Astartiden und Trigoniiden und einer deutlichen Faziesverarmung beruht. Die weit südliche Lage Indiens und die Öffnung des „South African Seaway“ im Tithon, sowie eine Änderung im marinen Zirkulationssystem führten zu einer zunehmenden Differenzierung der äthiopischen Faunenprovinz. Das Ausbreitungsmuster der Muscheln zeichnet das im Uhrzeigersinn zirkulierende Strömungssystem der Tethys nach und wird entscheidend von „aktiver Ausbreitung“ von Arten geprägt, besonders im Gefolge von Öffnungen mariner „Korridore“. Es wird weniger von eustatischen Meeresspiegel-Schwankungen, als vielmehr von den Folgen des Zerbrechens von Pangäa und der lokalen Faziesausbildung gesteuert. Die sehr viel größere Zahl der aus Europa bekannten Muschelarten ist in der größeren Faziesvielfalt und sehr viel intensiveren Sammelaktivität in Europa begründet. Die Benthosfauna der Spiti-Shales zeigt eindeutig äthiopisch-tethyschen Charakter und spricht für eine Ablagerung auf dem tieferen Schelf des Tethys-Südrandes.