Sousei no Taiga

A story of a young man who lacks a sense of reality after being dumped by his lover. He was visiting a seminar on cultural anthropology with his fellow classmates when he discovers an ancient wall painting, but afterward, a cave-in occurs cutting them off from the the path they have come from. When they find the second exit, they find themselves in a world where mammoths and other ancient giant mammals have dominated it.

(Source: MU, edited)