Radiation of the Early Polygnathids of the Pragian/emsian Boundary Interval

The Pragian / Emsian boundary is substantial for the development of conodonts. This interval marks the emergence of genus Polygnathus - the most significant for the Lower Devonian biostratigraphy. The biodiversity occurred at the initial stage of development of this group is clearly demonstrated in the Lower Devonian sections of the Zeravshan-Gissar Folded Area (Uzbekistan). Great similarities in the development of early polygnathids have been found from the distant Perigondwana regions: Zerafshan-Gissar Folded Area, Uzbekistan and Guangxi, South China. Radiation of early polygnathids was distinguished for the Pragian interval of the Zinzilban Section (Uzbekistan). The main morphological trends which affected Emsian conodont biodiversity were established at this stratigraphic level. At least three successive lineages, corresponding to the main variations in the morphostructure of the species, could be distinguished. First main lineage - Polygnathus triliniaris, Po. pireneae, Po.kitabicus, Po. excavatus and Po. nothoperbonus, is characterized by development of adcarinal troughs. For the second lineage - Polygnathus pannonicus, Po. foveolatus,Po. dehiscens, development of the long transverse ridges on the flat platform is common. And third, included Polygnathus sokolovi, Po. hindeiand Po. tamarae, is characterized by development single to multi nodes on the flat platform. Two former lineages are considered long-lived, and the latter – short-lived. The same trends in the development of early polygnathids were found in the South China (Guangxi) sections. It should be noted that various morphotypes of Po. pireneae were identified in these sections, which could define additional trends in development of early polygnathids. Abrupt changes in the morphology of Polygnathus platform elements during the short late Pragian interval may be associated with a change in nutrients. The complication in morphology of the platform upper surface due to the appearance of ridges or nodes contributes to a better grinding of food. Changes in the food resource may be the reason for the appearance of a large number of morphotypes characteristic for the early polygnathids. On the other hand, another criterion for the appearance of new taxa may be an increase of radioactivity in the paleobasin waters, which leads to the formation of pathologies in conodonts.