The origin of whales Taking into consideration all other animals, both living and fossil, these are the closest known taxa to modern and prehistoric whales.
Ancodus americanus (Scott 1894, Bothriodon rostratus, Elomeryx) late Eocene, ~ 40 mya, ~1.5 m in length, is an anthrcothere artiodactyl (even-toed ungulate) even though it had five fingers it did have four toes. It was a sister to the modern Hippopotamus and was derived from a sister to Vulpavus. Long slender legs and a digitigrade stance gave few clues that this taxon was ancestral to whales. Diacodexis was similar, but rather more specialized in the extended length of the metacarpals and metatarsals.
Pakicetus inachus (Gingerich and Russell 1981) early Eocene ~50 mya, had eye sockets close together at the top of the skull, like hippos. Distinct from Ancodus, Pakicetus was smaller with a relatively lower skull and a longer, more robust tail. The ribcage was shallower. The feet were as long as the tibia.
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.
Dorudon serratus (Gibbes 1845) Eocene, 40 mya, ~ 5 m in length was a prehistoric whale. Distinct from Rodhocetus, the nostrils were further back on the skull. The neck was shorter and the lumbar area was longer due to additional vertebrae. The scapula was broader. The pelvis and hind limbs were vestiges.