The origin of whales Taking into consideration all other animals, both living and fossil, these are the closest known taxa to modern and prehistoric Odontoceti (toothed) whales. Baleen whales (Mysticeti), likeBalaenoptera, have another origin, with Paleoparadoxia and kin.
Ichthyolestes pinfoldi (Dehm and Oettingen-Spielberg 1958), Middle Eocene 45 mya, half the size of Pakicetus with a longer, lower skull and relatively shorter limbs.
Ambulocetus natans (Thewissen, Madar and Hussain 1996) early Eocene ~50 mya, 3 m in length, was an amphibious archaeocete. It probably swam by undulating the back vertically, as do otters and whales. It did not have external ears and hunted like a crocodile. It was able to swallow underwater. The teeth were similar to those of early whales. Distinct from Ichthyolestes, Ambulocetus was larger, lower and longer. The head was relatively larger and longer. The tail was more robust and the sacral spines were taller.
Rodhocetus kasrani (Gingerich 2001, R. balochistanensis) early Eocene ~50 mya, 2.6 m in length, was preserved with its ankles, which had a double-spooled trochlea on the astragalus, a trait shared only with living artiodactyls, such as hippos. Distinct from Ambulocetus, Rodhocetus had taller dorsal spines and a shorter, more robust tail. The eye sockets were no longer on the top of the skull, but had moved lower, to the sides.
Maiacetus inuus (Gingerich et al. 2009) Eocene ~47 mya, 2.6 m in length, was discovered with an apparent embryo inside. Distinct from Rodhocetus, Maiacetus had a straighter ventral margin of the mandible, a shorter neck and a longer, more robust tail.