The oldest fossils of even-toed ungulates date back to the early Eocene (about 53 million years ago). Since these findings almost simultaneously appeared in Europe, Asia, and North America, it is very difficult to accurately determine the origin of artiodactyls. The fossils are classified as belonging to the family Dichobunidae; their best-known and best-preserved member is "Diacodexis". These were small animals, some as small as a hare, with a slim build, lanky legs, and a long tail. Their hind legs were much longer than their front legs. The early to middle Eocene saw the emergence of the ancestors of most of today's mammals.