THE TWISTS AND TURNS OF STUDYING THE SHALLOW-WATER CONODONT SWEETOGNATHUS

The story begins near a long and winding gravel road close to Mayoworth Wyoming, with the discovery of a new species, Spathognathodus whitei Rhodes 1963. The species, recovered from a sandy dolomitic limestone immediately above Tensleep Formation eolian sandstone, represented only 5% of the recovered specimens – most were specimens of Streptognathodus. Terrestrial facies of red shale and sandstone dominate the overlying succession, making the recovery of this species an unlikely candidate for a biostratigraphic controversy. An interpreted Early Permian conodont crisis was marked in Nevada by the extinction of Carboniferous holdover genera, like Adetognathus and Streptognathodus, and a post-crisis fauna with Sweetognathus Clark 1972. However, Sweetognathus whitei is associated with abundant specimens of Streptognathodus in the Florence Limestone in cyclothems of Kansas. Rather than a crisis, the sharp demarcation in Nevada resulted from an unconformity with Sweetognathus occurring in overlying transgressive facies. Similarly, the original base of the Permian in China was associated with the first occurrence of Sweetognathus whitei in transgressive facies at the base of the Chihsia Formation, with Streptognathodus extinct below. There are two different types of occurrences of Sweetognathus whitei; the first in association with Streptognathodus in high frequency cyclic units that are correlated using astronomical tuning and radiometric ages to between 294-295 Ma (late Asselian). The second type is found in transgressive facies of longer duration 3^rd order sequences that lack Streptognathodus and are dated to 290.5 Ma (earliest Artinskian) at the GSSP Dalny Tulkas section in Russia. Qualitative characters and geometric morphometrics provide resolution of this biostratigraphic conundrum. Two lineages, distinguished by distributions of pustulose micro-ornamentation, incorporate recurrent morphologic characters as a product of parallel evolution. A phylogenetic study reveals that Sw. whitei evolved during the late Asselian and its younger ‘homeomorph’, Sw. asymmetricus, evolved from Sw. anceps during the late Sakmarian and early Artinskian. The widespread occurrence of Sw. asymmetricus at the base-Artinskian maximum flooding surface makes this species an excellent index fossil.