LATE DEVONIAN ABERRANT CONODONT MORPHOTYPES FROM LOWER KELLWASSER EXTINCTION AND POST-EXTINCTION INTERVAL OF THE SWEETLAND CREEK SHALE, IOWA BASIN-CENTRAL NORTH AMERICA

Offshore condensed hemipelagic shale and mudstone facies of the Sweetland Creek Shale at its type section in south- eastern Iowa preserve a complete Upper Frasnian to Lower Famennian (Late Devonian) conodont succession in an epeiric basin setting. Ninety-four new samples were collected from 2017-2021 from Sweetland Creek and overlying Grassy Creek shales. New biostratigraphic data clearly identify Lower Kellwasser Extinction (LKE) and Upper Kellwasser Extinction (UKE) intervals and the Frasnian-Famennian boundary position.

In the Sweetland Creek section, the LKE interval is the upper 20 cm interval of Frasnian Zone 12 immediately below the base of Frasnian Subzone 13a. The onset of the LKE interval is marked by the Last Appearance Datum (LAD) of Ancryognathus iowensis, and the upper part is identified by the LADs of Palmatolepis kireevae, Pa. amplifcata, Pa. foliacea and Pa. muelleri in the upper 5 cm of Zone 12, associated with an aberrant morphotype of Polygnathus decorosus? with a bifurcating posterior platform. The post-extinction recovery interval comprises the lower 15 cm of Frasnian Subzone 13a is marked by FADs of Pa. bogartensis, and Ancryognathus calvini with an aberrant growth form of Palmatolepis sp. with a malformed platform and lateral lobe, an malformed morphytype of Pa. bogartensis, and an unusual new species of Palmatolepis with a sculpted and upturned platform margin . The latter is restricted to the survivor recovery interval.

These aberrant and unusual morphotypes may represent ancient examples of teratological abnormalities during ontogeny within the LKE crisis interval that perhaps persisted into the lower part of Subzone 13a. If so, the environmental factor(s) resulting in malformation (teratology) of pelagic offshore conodonts appears to be directly associated with the environmental crises resulting in the Lower Kellwasser Extinction in the epiric Illinois and Iowa basins of central Laurussia during the Late Frasnian. Ongoing investigation of directly associated chitinozoans, bulk whole rock inorganic δC13 geochemistry, and isotopic and REE analyses of conodont apatites will likely provide additional evidence of the nature of the environmental perturbation during the LKE.