Given that the Earth has experienced no fewer than six mass extinctions over the last 500 million years, the list of species that once lived but have gone extinct over the ages is vast. As of 2014, the Center for Biological Diversity (CBD) reports that Earth is undergoing its sixth mass extinction, and that the rate of species extinctions stands at between 1,000 and 10,000 times the historical norm.
The list of animal species, to say nothing of life's other kingdoms, that have gone extinct is extensive. The last species of trilobite died out during the great Permian extinction, and various non-avian dinosaurs disappeared at the end of the Cretaceous, 65 million years ago. According to the CBD, the present rate of extinction as of 2014 surpasses that of any other time since the end of the Cretaceous, with fully 99 percent of threatened modern extinctions being caused by human activity.
The CBD reports that over 1,000 species known to science have gone extinct in just the last 500 years. Among these are West Virginia's woodland bison, Merriam’s elk and the Rocky Mountain grasshopper. Beyond those known to science, it is believed that tens of thousands more species have been lost without ever having been identified. Any list of extinct lifeforms, therefore, is bound to be largely speculative.