GIVETIAN (MIDDLE DEVONIAN) OPSICONIDION (PHYLUM BRACHIOPODA, CLASS INARTICULATA, ORDER ACROTRETIDA) FROM THE GOVIALTAI FORMATION, SHINE JINST REGION, SOUTHERN MONGOLIA

Six whole and fragmentary valves of Opsiconidion c.f. Opsiconidion aldridgi (Cocks, 1979) were recovered from formic acid residues of carbonate samples collected from the Tentaculite Zone of the lower Govialtai Formation near Tsakhir Well in the Shine Jinst area of southern Mongolia. The specimens are characterized by a highly conical pedicle valve which flares outwards at the junction between the post-larval shell and the protegulum. The protegulum is ornamented by a variety of coarse pits that range in diameter from 4.29 µm to 7.24 µm. Major and minor growth lines on the post-larval shell were not preserved well enough to correlate with any time cycles. This occurrence marks the first record of Opsiconidion in Mongolia, extends the range of the species (?) from Eifelian to the Givetian, and indicates that the taxon was widespread within the Middle Devonian.