From here, according to Eisenberg, the ungulate split into 5 groups:
1.Eparctocyon (animals with cloven hooves or even toes)
2.Cete (whales and dolphins)
3.Phenacodonta (horses, tapirs and rhinos)
4.Meridungulata (an extinct species)
5.Paenungulata (super order elephant ancestors)
It is thought that the modern-day elephant evolved from an aquatic animal that used its trunk as a sort of snorkel while it spent extended periods of time beneath the surface of the water. Even today, elephants are able to swim in this way, staying underwater for hours at a time, with only the tip of their trunk exposed. According to these theories, elephants (Proboscidea) are closely related to manatees and dugong (Sirenia). It is deduced, then, that Proboscidea and Sirenia must share a common ancestor, from which each developed into different species. The Paenungulata group has been identified as the elephant ancestor by intensive DNA research, which revealed that the hyraxes branch off earlier, while the Proboscidea and Sirenia are still closely related in Tethytheria.