CONODONT SUCCESSION (POLYGNATHIDAE) AROUND THE PRAGIAN/EMSIAN (LOWER DEVONIAN) TRANSITION IN THE SPANISH CENTRAL PYRENEES AND ITS RELEVANCE FOR GLOBAL CORRELATIONS

The GSSP for the base of the Emsian is defined in Uzbekistan at the base of the Bed 9/5 in the Zinzil’ban Gorge section of the Kitab National Park. The index fossil used for the stage definition is the conodont Polygnathus kitabicus. Since its definition several workers have opposed this practice because the “kitabicus boundary” lies way below the traditional base of the Emsian in the Rhenish Slate Mountains of Germany and in a lower position within the classical Pragian (Praha Limestone) of the Prague Synform. After the required moratorium (10 years) the International Subcommission on Devonian Stratigraphy (SDS) decided to revise the base of the Emsian and a fieldtrip and business meeting was held in the Kitab Geological Reserve with visit to the section and discussions both in the field and in the official session. As a consequence, an agreement of seeking the new boundary around the entry of a specific taxon within the P. excavatus lineage was reached.

The Spanish Central Pyrenees has excellent outcrops and a rich conodont record spanning from the pireneae through the Middle excavatus zones and even higher; therefore, the basal Emsian boundary can be evaluated on both concepts (current and intended) in the aforementioned area. For this purpose six sections from the same stratigraphical unit have been analysed in detailed. The thorough sampling yielded a polygnathid sequence consisting of the globally distributed P. pireneae, P. kitabicus, P. excavatus excavatus, P. excavatus 114, two Asian taxa P. sokolovi, and P. pannonicus, and other four taxa of minor geographical distribution.

This Pyrenean conodont sequence allows recognition of the successive upper pireneae, kitabicus, Lower and Middle excavatus zones and therefore, these sections are critical for the revision of the Pragian/Emsian boundary according SDS intentions. Besides, they show the records of Asian taxa outside this region and therefore, these records reinforce the correlation with the Emsian stratotype. The great diversification of polygnathid taxa took place in the Pyrenees during the Lower excavatus Zone. Similar radiation, but with different taxa, is recorded slightly earlier in Zinzil’ban (around the pireneae and kitabicus zones). Therefore, the comparison of Asian and Pyrenean records suggests two pulses within the early radiation of the genus.