CAN THE REWORKED CONODONTS OF THE NORTH EVANS LIMESTONE (CONODONT BED OF HINDE, 1879) BE BROUGHT TO LIFE TO HELP SOLVE THE GEOLOGICAL PUZZLE OF DEVONIAN EXTINCTIONS?

At the Appalachian Basin margin in western New York, the change from the Middle Devonian neritic Hamilton Group deposits to the Middle to Upper Devonian dark basin facies of the Genesee Group is abrupt and profound. This is the interval of the diachronous Taghanic Unconformity, marked by the erosion of the upper Hamilton and most of the Tully Limestone; the post-Tully lower Genesee succession (with Leicester Pyrite at its base), onlaps westward, becomes internally condensed, and is also beveled to a feather edge below a still younger erosion surface beneath the mid-Genesee Genundewa Limestone. This strongly telescoped succession is also the interval of the Taghanic Extinction, a major global extinction.

Sandwiched between the Taghanic and sub-Genundewa erosion surfaces is the North Evans Limestone (Conodont Bed of Hinde, 1879) a thin crinoidal lag facies long recognized as a concentrate of conodonts and fish debris. The North Evans spans an interval of some six conodont zones whose representative taxa, known from other horizons in the region, are here missing or present in a range of preservations. The taphonomic age of the North Evans is early Frasnian but the unit is full of Givetian elements, particularly the long-ranging, cosmopolitan Polygnathus linguiformis.

Polygnathus linguiformis should be a prime candidate for testing the new biogeochemical techniques reported at the 2006 Pander Symposium. The growth lamella of conodont denticles show fine-scale patterns of trace elements (like ‘tree-rings') and oxygen isotopes that appear to be primary (and not modified by diagenesis), thus reflecting biotic response to environmental conditions. If biologically coeval specimens (geochemical groupings) can be recognized in the North Evans by their trace-element patterns, it may be possible to track environmental changes through their counterparts in the Hamilton, Tully, and Leicester.

Oxygen depletion and related chemical changes associated with eustatic sea-level rise are believed to have been the principal trigger of Devonian extinctions. Detecting fluctuations in ocean conditions in the apatite of conodonts will be a challenge for a new generation of paleobiologists. For the North Evans, the new methods could bring life to an old fauna in the effort to solve the puzzle of Devonian extinctions.