PERMIAN BIOSTRATIGRAPHY OF THE SHALLOW-WATER TAXON SWEETOGNATHUS

A preponderance of shallow-water successions during the Permian, including very restricted conditions during the Kungurian, has resulted in the shallow-water taxon Sweetognathus and its descendants, being used extensively for biostratigraphic zonation. In the Uralian succession of Russia, Streptognathodus becomes extinct during the early Sakmarian at the same time that Sweetognathus diversifies. Sweetognathus represents the ecologic replacement of Late Carboniferous to earliest Permian Streptognathodus, and as such, typically occupies relatively shallow carbonate platform environments. This ecologic replacement occurs during the meltdown of the Gondwanan ice-sheet, allowing these taxa to migrate widely during the accompanying transgression. In most regions this replacement is abrupt and coincides with the termination of high-frequency cyclothems in response to the end of P1 Permian glaciation; providing a distinct, nearly global, sequence biostratigraphic signature. The apparent long overlap of Streptognathodus and Sweetognathus in cyclothems of the midcontinent USA and Bolivia may be explained by distinguishing the development of an early lineage of Sweetognathus at its evolutionary geographic centre. Like many shallow-water taxa, these species exhibit a high degree of morphologic plasticity, exemplified by the development of Sweetognathus bucaramangus with numerous pustulose accessory nodes. The Sweetognathus lineage appears to have evolved early in the midcontinent USA as well as in Bolivia. The correlation of the Sweetognathus whitei Zone thus becomes problematic. Sweetognathus whitei first appears in the Florence Member of the Barneston Limestone in Kansas. Astronomical tuning of cyclothems suggests an age of 295 Ma for this species appearance in Kansas, five million years prior to the appearance of an early Artinskian homeomorph in the type Uralian succession. This means that co-occurrences of S. whitei and S. bucaramangus in the Elm Creek Limestone in the Midland Basin, W. Texas and in the Winfield Limestone of the Chase Group in Kansas are early Sakmarian and not early Artinskian as previously reported. Sweetognathus migrated to its distributional acme during the post-glacial transgression and later, during the Middle and Upper Permian, became restricted to the Tethys region.