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?
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.