STRATIGRAPHIC DISTRIBUTION OF THE AMMONOIDS SHUMARDITES AND VIDRIOCERAS AND IMPLICATIONS ON THE DEFINITION AND CORRELATION OF THE GLOBAL GZHELIAN STAGE, UPPER PENNSYLVANIAN SERIES

Historically, the Gzhelian Stage of the Moscow Basin and Russian Platform and the regional Virgilian Stage of North America have been considered equivalent and characterized by the appearance of the Shumardites-Vidrioceras ammonoid genozone. However, although the lower Gzhelian C3JZ Horizon in the Ural Mountains does not contain representatives of Vidrioceras Böse or Shumardites Smith, the fusulinaceans Rauserites rossicus Schellwien, R. paraarcticus Rauzer-Chernousova, and R. stuckenbergi Rauzer-Chernousova are present in the ammonoid-bearing horizon thus permitting correlation to the Moscow Basin. The stratotype for the lowermost Gzhelian Rusavkino Formation of the Moscow Basin contains non-diagnostic ammonoids along with the conodont Streptognathodus zethus Chernykh near the base and Idiognathodus simulator (Ellison) near the top of the section. Idiognathodus simulator occurs in numerous sections globally and is now widely recognized as an excellent FAD candidate for defining the Gzhelian Stage.

The basal Virgilian Haskell-Cass interval of the North American Midcontinent contains Streptognathodus zethus as well as the ammonods Eovidrioceras conlini (Miller & Downs) and Pseudaktubites stainbrooki (Plummer & Scott). The overlying Oread Limestone (Heebner Shale Member) contains the first appearance of Idiognathodus simulator together with the first appearance of most morphologically primitive species ofVidrioceras (V. uddeni Böse).

The Colony Creek Shale of north-central Texas contains the ammonoids Pseudaktubites stainbrooki and Eovidrioceras conlini. The overlying Finis Shale contains the first appearance of Idiognathodus simulator together with the morphologically most primitive species of Shumarditesand Vidrioceras, S. cuyleri Miller & Downs andV. uddeni Böse.

In conclusion, the use of the first appearance of Idiognathodus simulator to define the global Gzhelian Stage coincides with the appearance of Vidrioceras and Shumardites and provides continuity with historical usage of the Vidrioceras-Shumardites genozone as characterizing the Gzhelian Stage.