the discovery of Ambulocetus, the only evidence of this in the fossil record was the 52-million-year-old (fully terrestrial) Pakicetus and the Paleocene mesonychids (as there was a hypothesised link between cetaceans and mesonychids). The limbs of more aquatic Eocene cetaceans did not preserve very well. Ambulocetus demonstrated that cetaceans swam by flexing the spine up and down (undulation) before they had evolved the tail fluke, forelimb propulsion evolved relatively late, and that cetaceans went through an otter-like phase with spinal undulation and hindlimb propulsion. These had already been hypothesised to have occurred in the earliest aquatic cetaceans, but were impossible to test without more complete remains. The describers noted that, "Ambulocetus represents a critical intermediate between land mammals and marine cetaceans." Classification Modern cetaceans (Neoceti) are grouped into either the parvorders Mysticeti (baleen whales) or Odontoceti (toothed whales). Neoceti are descended from the ancient Archaeoceti, whose members span the transition from terrestrial to fully aquatic. Archaeoceti are thus paraphyletic (it is a non-natural group which does not comprise both a common ancestor and all of its descendants). Ambulocetus was an archaeocete. By the time Ambulocetus was discovered, archaeocetes were classified into the families Protocetidae (which included what are now the terrestrial Pakicetidae, and the rest were amphibious), Remingtonocetidae (amphibious), Basilosauridae (aquatic), and Dorudontidae (aquatic, now a subfamily of Basilosauridae). The earliest cetaceans were thought to be the mesonychids, proposed before any firm early cetacean fossils were identified. In the original description, Ambulocetus was preliminarily placed into Protocetidae, until the further description of the holotype prompted Thewissen and colleagues to move it into its own family Ambulocetidae in 1996. At the same time, they also erected the family Pakicetidae. They also proposed that some members of Pakicetidae, Protocetidae, and Ambulocetidae were the other two archaeocete families' ancestors. They suggested that mesonychids gave rise to pakicetids, which gave rise to ambulocetids, which gave rise to both protocetids and remingtonocetids. Though middle-to-late-Eocene archaeocetes are also known from North America, Europe, and Africa, most of these are found only on the Indian subcontinent. Therefore, it is thought cetaceans originally evolved in that region. Based on molecular data, cetaceans are most closely allied with hippos (Whippomorpha), and they split approximately 54.9 million years ago. They are all placed in the order Cetartiodactyla alongside terrestrial even-toed ungulates (hoofed mammals). This puts mesonychids as a distant relative of cetaceans rather than an ancestor, and their somewhat similar