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Skeleton of a mesonychian. |
Scientists have long known that cetaceans (whales, dolphins, and porpoises) descended from four-footed land mammals. Cetaceans still have some features of land mammals; they use lungs to breathe air and give birth to young that are nursed by milk produced by the mother. Modern cetaceans cannot live on land, and look very different from land mammals in most respects.
Cetaceans evolved rapidly, and the entire transition from land mammal to obligate marine whale took less than 8 million years. These Eocene cetaceans are often called archaeocetes, and they can be divided into six families: Pakicetidae, Ambulocetidae, Remingtonocetidae, Protocetidae, Dorudontidae, and Basilosauridae. It is only since the 1990’s that relatively complete skeletons of the archaeocetes have been found in abundance and that the transition from land-to-water could be studied in detail.
An important new find concerns two skeletons of pakicetids. These skeletons changed our opinions on what the closest land relatives of cetaceans were. Until the recovery of these skeletons, most paleontologists thought that cetaceans were most closely related to mesonychians, Mesonychians are an extinct (Paleocene-Oligocene) group of hoofed mammals from the Northern Hemisphere. They varied in size from that of a weasel to a grizzly bear, and may have eaten carrion or meat.
Scientists studying DNA were of a different opinion, they consider hippopotamids as the closest relatives to cetaceans. Hippopotamids (including the recent Hippo and the Pygmy Hippo) are included in a group of mammals called even-toed ungulates or artiodactyls. Other artiodactyls are: pigs, peccaries, camels, llamas, giraffes, deer, goats, sheep, cattle, and antelopes.
Data gathered from the new pakicetid skeletons show that mesonychians are not the closest relatives of cetaceans (sister groups in scientific lingo). However, these new data also disagree, less strongly, with the sister group relation between hippos and cetaceans.
To understand the differences, let’s compare the relations of these groups of animals to those of a number of female members of a human family. Let’s say that we are trying to determine the relation of Celia to her relatives Heidi, Arlene, and Megan, and we know that all are from the same generation.
Substitute Cetacea for Celia, Hippos for Heidi, other artiodactyls for Arlene, and mesonychians for Megan.
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MESONYCHIAN
HYPOTHESIS
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| It could also be that Celia is a sister to Heidi, and that Arlene is their first cousin. Megan is more distantly related. |
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Finally, it could be that Heidi and Arlene are sisters, Celia is their first cousin, and Megan more distantly related. |
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Now these diagrams explain the essence of the differences between the three competing hypotheses for cetacean relations. The new pakicetid data indicate that the third branching diagram, the artiodactyl hypothesis, describes the relationships of cetaceans best.
All pictures on these pages are public access, although the source must be identified in publication.
Ambulocetidae
| Basilosaurids and
Dorudontids | Bibliography | Hearing
| India |
Locomotion | Mysticetes
| Odontocetes | Osmoregulation
| Pakicetidae |
Pakistan | Protocetidae
| Remingtonocetidae | Whale
| Whale Origins!