The oldest cetaceans date back to the early Eocene (53 million years ago), whereas the oldest known hippopotamus dates back only to the Miocene (15 million years ago). Some doubts have arisen regarding the relationship between the two, as there is a 40-million-year gap between their first appearances in the fossil record. It seems unlikely that there were ancestral hippos that left no remains, given the high number of even-toed ungulate fossils. Some studies proposed the late emergence of hippos is because they are relatives of peccaries and split recently, but molecular findings contradict this. Research is therefore focused on anthracortheres (family Anthracotheriidae); one dating from the Eocene to Miocene was declared to be "hippo-like" upon discovery in the 19th century. A study from 2005 showed that the anthracotheres and hippopotamuses have very similar skulls, but differed in the adaptations of their teeth. It was nevertheless believed that cetaceans and anthracothereres descended from a common ancestor, and that hippopotamuses developed from anthracotheres. A study published in 2015 was able to confirm this, but also revealed that hippopotamuses were derived from older anthracotheriens. The newly introduced genus "Epirigenys" from eastern Africa is thus the sister group of hippos.