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Whale Origins as a Poster Child for Macroevolution


Fossils collected in the last decade document the ways in which Cetacea (whales, dolphins, and porpoises) became aquatic, a transition that is one of the best documented examples of macroevolution in mammals

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Whales indisputably are mammals, which is clear from their means of oxygen intake (they breathe with lungs), their care of newborns (mothers nurse their calves with milk), and a host of other features. This implies that whales evolved from other mammals and, because ancestral mammals were land animals, that whales had land ancestors. What happened in the transition to life in the ocean has been hard to imagine for scientists and laypeople alike. In the first edition of the Origin of Species (1859), Darwin suggested that a bearlike animal swimming with an open mouth might be a precursor of a filter-feeding baleen whale. This statement attracted much ridicule; in a letter, Darwin observed, “It is laughable how often I have been attacked and misrepresented about this bear” (Gould 1995). In later editions, Darwin deleted this reference to evolution entirely and merely noted that a bear sifting water for insects is “almost like a whale.”

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pakicetids, ambulocetids, remingtonocetids, protocetids, dorudontids, and basilosaurids (Figure 1). Taken together they describe the transition from a terrestrial quadruped to a fully aquatic marine mammal. The oldest known cetaceans are the pakicetids, which lived in the floodplains of present-day South Asia 50 million years ago. Pakicetids are followed by the semiaquatic ambulocetids, which inhabited the bays and estuaries of the Tethys Ocean in northern Pakistan. The slender-snouted remingtonocetids are also found in shallow marine deposits of Pakistan and India; however, they appear to have been more aquatic than the older ambulocetids. Contemporaries of the remingtonocetids, the protocetids are more aquatic, although they retain hind limbs, and are distributed globally with fossil remains uncovered in South Asia, Africa, and North America. By the middle to late Eocene, dorudontids and basilosaurids were swimming in an open marine environment and had nearly lost all vestiges of life on land.

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The pelvis (or hip girdle) is dramatically different in modern whales and land mammals (Figure 11). The pelvis in land mammals consists of sacrum and left and right innominate bones. The sacrum is a series of vertebrae (five in humans) that are fused to each other and connect to the innominates at the first (most anterior) of these vertebrae. The innominate is an elongated bone that bears the socket (acetabulum) for the femur, forming the hip joint, and has two branches posteriorly (ischium and pubis) that surround a foramen, or opening, and an anterior branch (ilium) that contacts the innominate from the other side. Sacrum and left and right innominates form a strong and rigid bony girdle that anchors the hind limbs and supports much of the body in locomotion. In modern whales, in contrast, the sacrum cannot be recognized, as there are no fused vertebrae and no vertebra has a joint for the innominate. In fact, the innominate in modern cetaceans is a tiny bar of bone lacking an acetabulum and distinct ischium, pubis, and ilium. The innominate is embedded in the ventral body wall where a few tiny muscles that are not involved in locomotion attach to it.

In Ambulocetus and Kutchicetus, the pelvis is much like that of a land mammal: The sacrum comprises four fused vertebrae, and these articulate with the innominate bones (Figure 11). Each innominate consists of ilium, ischium, and pubis and bears an acetabulum for the femur, as it does in land mammals. The femurs are long when compared with other whales, but shorter than most land mammals. Ambulocetus had large hind limbs and enormous feet. Feet are not known in remingtonocetids, but the sacral vertebrae of Rodhocetus are less strongly fused, and the femurs are shorter than in Ambulocetus. However, Rodhocetus innominates retain the deep acetabulum and the articulation with the sacrum. Georgiacetus is even more like modern whales, lacking a fused series of sacral vertebrae as well as a contact between sacral vertebrae and innominates. Unlike modern whales, the sacral vertebrae of Georgiacetus differ from caudal and lumbar vertebrae, and each innominate is large and consists of ilium, ischium, and pubis. The femur of Georgiacetus is not known. In basilosaurids and dorudontids, the innominates are small bones, with ilia that are not distinct. However, they retain an acetabulum and foramen. The innominates are not connected to the vertebral column, but these whales retain femurs that are longer than in modern cetaceans. The entire hind limb (Figure 6) is smaller than in other Eocene cetaceans and barely protrude from the body wall.

Together these pelvises form an excellent transitional series, in which ambulocetids and remingtonocetids retain all elements of land mammals, and protocetids lose the fused sacrum (Rodhocetus) and the iliosacral joints (Georgiacetus) and have short femurs (in known forms). Basilosaurids and dorudontids have greatly reduced hind limbs and reduced ilia, while still retaining the acetabulum and the foramen of the innominates. Only vestiges of these structures are present in modern whales.

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Originally posted by pleco

My fav is homo sapiens, because we don't know where it is going... :-)

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Originally posted by beskeptigal

I don't suppose bacteria passing genetic information around like a frat party spilling beer counts as a transitional fossil. Can I put it in as transitional genetic material?

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Ambulocetid cetaceans are slightly younger and more derived than pakicetid cetaceans. They were also much larger, similar in size to large sea lions. A nearly complete skeleton for Ambulocetus (Figure 3) shows that the animal had a large head, long muscular body, and a long tail. Its limbs were short but the feet long. In overall body shape, Ambulocetus looked somewhat like a crocodile, although its hind limbs and feet were considerably longer. It may have lived as an ambush predator of fish in shallow water. Ambulocetids lived in coastal environments such as bays and estuaries approximately 49 million years ago (mya) and are known only from India and Pakistan.

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Originally posted by pleco

I wonder if we'll ever see a link where these quotes are being pulled from....any bets?

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Originally posted by Bill scott

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Originally posted by pleco

I wonder if we'll ever see a link where these quotes are being pulled from....any bets?

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I did list the link, first thing...


http://en.wikipedia.org/wiki/Archaeoraptor

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