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(geology) Of a geologic epoch within the Neogene period from about 5.3 to 1.7 million years ago; marked by the appearance of humanity's first ancestors. quotations
[I]n a final sentence he expressed his conviction that his opponents "did not in truth represent the thought of the twentieth century, but might rather be regarded as mental fossils dug from some early Pliocene horizon ".
1925 July – 1926 May, A[rthur] Conan Doyle, “(please specify the chapter number)”, in The Land of Mist (eBook no. 0601351h.html), Australia: Project Gutenberg Australia, published April 2019
(geology) The Pliocene epoch.