The ancestry of the Rhenish Middle Siegenian brachiopod fauna in the Iberian Chains and its palaeozoogeography (Early Devonian)

Abstract

At Nigüella (Eastern Iberian Chain), a spatially restricted brachiopod fauna is slightly older than the traditional Middle Siegenian. It is composed of taxa most of which invaded the Rhenohercynicum shortly later, when facies there changed from deltaic to shallow marine. Occurring in typically Rhenish lithofacies, the Nigüella fauna lacks the taxa that are most common in coeval faunas of lbero-Armorica, but its own taxa have earlier Ibero-­Armorican ancestors and origins, except Multispirifer cf. solitarius and Arbizustrophia n. sp. N. Hitherto, M. solitarius seemed to be an endemic of the Rhenohercynicum; Arbizustrophia was known from the Late Emsian of Asturias; the ancestors of both are unknown in Ibero-Armorica, which suggests viable zoogeographic connections with unknown further regions. Contrary to current opinions, in the late Silurian and early Early Devonian, Ibero-Armorica and Rhenohercynicum do have common taxa, and faunal differences are largely due to facies, especially deltaic facies in the Rhenohercynicum during 7 Ma of late Lochkovian and Pragian time. Effective oceanic faunal barriers lacked. Contradictory plate tectonic scenarios, which had been founded initially on defective faunistic concepts and supposed subduction of various broad oceans, are revised according to faunistic criteria and turn out to be unwarranted also in other aspects. The meanings of the traditional Siegenian substages are elucidated, whereas the actual standard stages cannot be applied to the interval concerned.