Cetartiodactyla - A Clade Apart

A Clade Apart

The morphology and paleontology-based opinion of the relationship of whales to other mammals has also undergone considerable changes since the early 1990s. Fossils, such as Rodhocetus, have been discovered that refute the notion that whales are derived from or are closely related to the mesonychids. Many morphologists and paleontologists support the notion of a clade called Cetartiodactyla that unites Cetacea with Artiodactyla. Many are not, however, in support of the hypothesis that Cetacea evolved from within the Artiodactyla. Under this definition, Artiodactyla remains a valid clade since artiodactyls evolved from a common ancestor separate from whales. Cetartiodactyla would represent a grandorder or superorder uniting the two orders.

The vast majority of phylogenetic analyses based on morphological characters have not uncovered a whale/hippo clade, but show Cetacea and Artiodactyla as distinct from one another. Features of the bones of the astragalus in the ankle region are cited as particular evidence for a monophyletic Artiodactyla. In any case, an origin of the Cetacea from within the Ungulata was proposed as early as 1900 by Frank Evers Beddard in A book of whales. But, Beddard found the anatomical and fossil record at that time insufficient to rule out other possibilities, such as the notion that whales are a basal lineage among placental mammals, or the popular theory relating them to terrestrial carnivores. Because Beddard himself was not fully convinced, his bold proposal was largely forgotten.

Hippo fossils are not observed until the Miocene, but whale ancestors have been dated to the Eocene. The whale/hippo hypothesis imposes a gap of nearly 30 million years where no hippo ancestors are present. Certain anthracotheres have been proposed as hippo ancestors, but this concept has not garnered wide support among paleontologists. However, the most recent hypothesis into the origins of hippopotamids suggests that hippos and whales shared a common semi-aquatic ancestor that branched off from other Artiodactyls around 60 million years ago. This hypothesized ancestral group likely split into two branches around 54 million years ago. One branch would evolve into cetaceans, possibly beginning with the proto-whale Pakicetus from 52 million years ago with other early whale ancestors collectively known as Archaeoceti, which eventually underwent aquatic adaptation into the completely aquatic cetaceans. Thus, Cetartiodactyla is supported as a monophyletic group.