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As early as 1945, the close relationship between whales and artiodactyls (through precipitin tests) was recognized. Of the artiodactyls, hippopotami seem to have the closest relationship to whales (Shimamura, 1997; Messenger, 1998; Thewissen, 1998). There are terrestrial Eocene species of artiodactyls (Icthyolestes, Gandakasia) which are close to the ancestry of whales. The skull of Artiocetus clavis, dated at 47 million years from the Eocene, possessed characters which are thought to be primitive features for the cetaceans and its foot bones link it to artiodactyls (Gingerich, 2001).
| The family Pakicetidae includes the earliest, most primitive
whales such as coyote-sized Pakicetus
and several smaller species known from incomplete fossils such as Ichthyolestes and Nalacetus. These earliest whales are known from the Eocene of India around
|The species of the family Ambulocetidae, such as Ambulocetus and Gandakasia, were larger reaching estimated weights of 141 to 235 kg. They would have swum in the same way that modern otters do. They had primitive tooth counts and all 4 types of teeth. Ambulocetus was a 7 foot animal with a large head and short legs (Zimmer, 1995b).|
The family Remingtonocetidae includes Remingtonocetus, Andrwsiphius, Dalanistes, and Attockicetus. Remingtonocetus possessed femurs which were not reduced in size.
The family Protocetidae includes Indocetus, Rhodhocetus, Pappocetus, Protocetus, Eocetus, Georgiacetus, and Takracetus (Thewissen, 1998). Rhodocetus had legs which were 1/3 shorter than those of Ambulocetus and would have had to walk in a way similar to the gait of crocodiles. Its tail was much more massive and it would have been a better swimmer. A second species of Rhodhocetus, R. balochistanensis, supplements the information known about R. kasrani. Rhodhocetus walked digitigrade on its hand and its short digits bore hooves. In contrast, the toe bones would not have been able to bear the weight of the body and Rhodhocetus would have walked on the plantar section of its foot. Its tail was robust. Characteristics of its hand and foot (such as the facet on the entocuneiform tarsal bone which bore a facet for the first metatarsal which Rhodhocetus lacked) link Rhodhocetus to early artiodactyls (Gingerich, 2001).
Indocetus was similar to Ambulocetus but its strong sacrum meant that it would have been a more powerful swimmer. Its longer legs suggests that it spent more time in shallow water than in deep water. In Protocetus, the hind limbs were not firmly attached to the sacrum and it is the first whale known that would have been fully aquatic.
Georgiacetus is intermediate between Rhodhocetus and Basilosaurus. Its limbs would probably not have been functional for movement. Its body was thinner than of earlier whales and it probably had some sort of fluke. Zygorhiza also was probably unable to use its legs in locomotion (Thewissen, 1998).
Advanced whales include the two modern
groups and the fossil basilosaurids and dorudontids (Thewissen, 1998).
Prozeuglodon had a 15 foot body and 6 inch legs. Basilosaurus (Zeuglodon) of the Upper Eocene was a large
predatory whale, which reached 60 feet but still retained a tiny hindlimb
with 3 toes. These limbs would have been useless in swimming and may have
been used only as claspers during reproduction.
Hundreds of specimens of Basilosaurus
are known, from .
Modern cetaceans do begin to develop hindlimbs as embryos although this growth is later arrested and the structure degenerates. It seems as if the lack of the gene Sonic Hedgehog, which is normally expressed in hindlimbs, is responsible for the failure of continued hindlimb development. Modern cetaceans develop hips and may possess homologs of the femur and tibia contained in the body wall. Mice embryos which are deprived of Shh expression show the same structural pattern. The development of additional phalanges in the front flippers of cetaceans seems to result from Sonic Hedgehog expression over longer time periods during development (Thewissen, 2006).
Extinct dolphins included species with a spear-like projection from the nasal area and walrus-like tusks in another species.
Genetic evidence suggests that modern sperm whales and dolphins share a common ancestry after the divergence of the baleen whale lineage (Nikaido, 2007).