Complex sentence: Sporadic elk sightings were again reported near the headwaters of the Tygart and Greenbrier Rivers as late as 1875, but they were certainly gone by 1880 when the subspecies as a whole is thought to have been completely exterminated.
Split into simpler sentences: Sporadic elk sightings were again reported near the headwaters of the Tygart and Greenbrier Rivers, however, as late as 1875. If this is accurate, they were nonetheless certainly gone by 1880 when the subspecies as a whole is considered to have been completely exterminated.