DISCOVERY OF FISH MORTALITY HORIZON AT THE K-T BOUNDARY ON SEYMOUR ISLAND - REEVALUATION OF EVENTS AT THE END OF THE CRETACEOUS

The discovery of a fish bone layer immediately overlying the K-T iridi um anomaly on Seymour Island, Antarctic Peninsula, which may represent the first documented mass kill associated with the impact event, toge ther with new faunal data across the boundary has provided new insight into events at the end of the Cretaceous. The utilization of a geogra phical approach and a new graphical representation of range data has r evealed that events at the end of the Cretaceous were not instantaneou s, but occurred over a finite period of time. Although the fish bone l ayer may contain victims of the impact event, the absence of ammonites in either the iridium-bearing layer or the overlying fish layer sugge sts that the extinction event at the end of the Cretaceous was the cul mination of several processes beginning in the late Campanian. The imp act was the proverbial ''straw that broke the camel's back,'' leading to the extinction of many others forms of life that might have survive d the period of global biotic stress during the waning stages of the M esozoic if then had not been an impact. The absence of mass extinction following catastrophic geologic events in a biotic robust world, such as the Middle Ordovician Millbrig-Big Bentonite volcanic event sugges ts that the biosphere is remarkably resilient to major geologic catast rophes with mass extinction events occurring only when there is a conj unction of geologic events none of which might be capable of producing a global mass extinction by itself.