REEVALUATION OF THE HIRNANTIA FAUNA DIVERSITY BASED ON NEW FINDS FROM SPAIN

The Hirnantia Fauna represents a widespread assemblage of opportunistic species of brachiopods and trilobites with almost global distribution that developed at the time of the latest Ordovician Extinction Event. This was caused by the drastic climate change of the Hirnantian glaciation on the Gondwana continent. It is widely accepted that in Ordovician polar and subpolar paleolatitudes, the Hirnantia Fauna is of extremely low diversity (the Bani province), but diversity increases in slightly more temperate paleolatitudes beyond the circumpolar region (the Kosov province), and it reaches a maximum in the tropical and subtropical paleolatitudes (the Edgewood province). During the Hirnantian the Iberian Peninsula was located approximately at 60ºS latitude, close to the North margin of Gondwana. To date, the occurrences of Hirnantia Fauna in this region were represented by two occurrences of brachiopods in the Central Iberian Zone and by one in the Cantabrian Zone, plus two single records of a typical Hirnantian trilobite from glaciomarine sediments in the Iberian Cordillera and the Ossa-Morena Zone. All these assemblages, of low diversity, fit perfectly within the Bani province. Recent field studies in the Cantabrian Zone of NW Spain resulted in the discovery of a new locality with a surprising high diversity assemblage of rhynchonelliform brachiopods belonging to 11 genera, occurring in association with diverse trilobites, bryozoans, gastropods, pelmatozoans, poriferans, machaeridians and scolecodonts. This fits, in terms of diversity, within the Kosov province that is atypical for such palaeolatitude.

The co-occurrence of Bani and Kosov faunas in the same palaeogeographic area reopens the question of whether temperature or paleolatitude was the only limiting factor for the geographical distribution of the Hirnantia Fauna, or alternatively, if other environmental factors such as substrate or depth, also played a significant role.

To answer this question, the Beta diversity of all known localities from each faunistic province has been analyzed taking into account to which Benthic Assemblage (BA) they belong.