Upper Ordovician Conodonts From The Northwestern Margin Of North China Plate And Their Paleoceanology Implications

Ordovician conodonts from the North China Plate have been extensively reported,among others,conodont faunas of the Katian age are mostly distributed in the marginal areas and with shallow-warm water settings.In this contribution,we document 1215 conodont specimens recovered from the Wulanhudong and Baiyanhuashan formations in the Baiyanhuashan section of the Dashetai area on the northwestern margin of the North China Plate.Forty-three species belonging to 17 genera,including a new species,Tasmanognathus coronatustus n.sp.,with a stratigraphic range spanning the early to the middle Katian,are present.Three conodont biozones have been recognized based on the Dashetai material,i.e.,the Belodina confluens Biozone,the Yaoxianognathus neimengguensis Biozone and the Yaoxianognathus yaoxianensis Biozone.The Dashetai conodont fauna allowing a re-assessment of the correlations of the contemporaneous conodont zonal succession proposed for North China with those established for the shallow-water carbonate platforms at low latitudes.According to the previously published literature and Dashetai material,we reviewed the research history and present situation of the taxonomy and biostratigraphy about two widespread Late Ordovician genera Tasmanognathus Burrett,1979 and Yaoxianognathus An in An et al.,1985,and reassessed several species of them.The lineage Tasmanognathus sishuiensis-Tasmanognathus shichuanheensis-Yaoxianognathus borealis-Yaoxianognathus neimengguensis-Yaoxianognathus yaoxianensis shows that denticles of each element getting complex younger.Phylogenetic analyses demonstrate a clear evolutionary relationship of the two genera.In addition,it is postulated that these two genera might have originated from the genus Plectodina,and that Tasmanognathus shichuanheensis is a transitional taxon from Tasmanoganthus to Yaoxianognathus.Based on the dispersal pathways of both widely and restrictedly distributed conodont taxa Beldoina,Phragmodus,Taoqupognathus and Yaoxianognathus,Late Ordovician ocean surface circulation and continental position in the tropics are further discussed and some more detailes with respect to the paleogeography of this time interval is provided.This suggests that the migration of conodonts was accomplished via ocean currents;several genera living in off shore environments are able to have long-distance dispersalsThe present study enriched the recognition of Late Ordovician conodont faunas of North China,and also improved the precision of stratigraphic correlations with coeval successions.Conodont migration patterns provide new paleontological evidence for Ordovician palaeoceanography.