Palaeobiogeographical affinity of the early Pliensbachian (Early Jurassic) brachiopod assemblage of the Northern Caucasus (Russia): A new evidence

Abstract

Brachiopods are useful in detecting the Mesozoic palaeobiogeographical affinity of European terranes. Numerous representatives of this fossil group known from the lower Pliensbachian deposits of the Northern Caucasus (southwestern Russia) allow quantitative assessment of their palaeobiogeographical affinity. A total of 175 brachiopod species from the Northern Caucasus and several other regions (Bakony, Germany without the Alps, Hallstatt, the Pyrenees, Switzerland without the Southern Alps, the Swiss Southern Alps, and western Algeria) are considered for the purposes of the present study. The relationships between the regional assemblages were examined with three similarity indices, namely the Jaccard Index, the Czekanowski's Quantified Coefficient, and the Gower index. The results indicate the relatively strong similarity of the early Pliensbachian brachiopods of the Northern Caucasus to those of Germany, the Pyrenees, Switzerland, and western Algeria and the strong dissimilarity from the assemblages of the Bakony, Hallstatt, and the Swiss Southern Alps. These results reveal the affinity of the early Pliensbachian brachiopod assemblage of the Northern Caucasus to the Boreal biochore. However, there are Tethyan taxa in this assemblage, and, thus, the latter is of transitional nature. Palaeogeographical and plate tectonic configurations, eustatic fluctuations, differences between the regional depositional environments, etc. may explain the palaeobiogeographical patterns registered in the present study. The regional factors might have been especially important because of the moderate-to-weak global palaeobiogeographical differentiation in the early Pliensbachian. The concept of the Mediterranean microcontinent seems to be also useful for explanation of the early Pliensbachian brachiopod provinciality.