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Archaeocetes: The Oldest Whales

Dorudon atrox skeleton

The oldest fossil whales are often grouped together, largely for convenience, in a taxon known as the archaeocetes. Archaeocetes show several features that modern whales lack. Their teeth, like those of most land mammals, still show differentiation into several types. (Modern whales either lack teeth, or have teeth that are all virtually identical in shape and size). Archaeocetes also had nostrils near the tip of the nose, like land mammals, rather than a blowhole on the top of the head. Some retained substantial hind limbs that would have been visible outside the animal's body; in the earliest archaeocetes, these limbs and the pelvis were attached to the vertebrae by a sacral joint, but in later ones the limbs and pelvis were not attached to the rest of the skeleton.
Pakicetus, shown above right, is a Middle Eocene archaeocete from the Kuldana Formation of Pakistan; it is currently the earliest known well-preserved cetacean, and the archaeocete features are clearly visible in this replica skull from UCMP's collections. Pakicetus is so far known only from its skull, but recent finds in Pakistan have produced other whale species that show very primitive characters in both the skull and the rest of the skeleton. These animals had relatively well developed limbs, but were aquatic. Below right is a drawing of the known skeleton of Rodhocetus, a recently discovered archaeocete from Pakistan, a few million years younger than Packicetus. Rodhocetus had well-developed hind limbs (although only the thighbone, or femur, has been preserved), but unlike land mammals, Rodhocetus did not have its vertebrae in the pelvic region fused together into a sacrum. Early whales such as Rodhocetus show many similarities with an early group of land mammals known as mesonychids, which are also close to the root of the hoofed mammals. In fact, some fossil teeth that were once identified as mesonychids are now known to have come from archaeocetes.
By the late Eocene, archaeocete whales had spread to many parts of the world. Zygorhiza, shown above left, is fairly common in the Gulf Coastal region of the southeastern United States. Although the body is very whale-like (though somewhat less compact than a modern whale), Zygorhiza shows differentiated teeth and nostrils near the end of the nose.
Basilosaurus, below left, is another Eocene archaeocete from the Gulf Coast, with a rather unfortunate name: Dr. Richard Harlan, the 19th-century American physician who described and named it, believed it to be a giant reptile, and in 1843 named it "king lizard": Basilosaurus in Greek. Scientists who later examined Basilosaurus, notably Richard Owen, realized the error. Owen renamed the fossil Zeuglodon, meaning "yoked tooth" -- a more appropriate description of the double-rooted molars of this animal. However, under the rules that govern naming species, the earliest name for a species has priority, even if that name is not an accurate description. Its name aside, Basilosaurus is significant because it is known to have retained small but well-developed hind limbs that projected from the body, although there was no joint between the pelvic bones and the vertebrae. Living whales retain only tiny splint-like bones as remnants of the pelvis and hind limbs. Basilosaurus has been found in Eocene rocks of the southeastern United States, notably in Alabama, Mississippi, and Louisiana -- in fact it is the state fossil of Alabama, and the "state co-fossil" of Mississippi along with Zygorhiza. Both specimens shown here were found in Mississippi, and are on display at the Mississippi Museum of Natural Science in Jackson. Basilosaurus has also been found in Eocene rocks of Australia and Egypt.