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Thursday, December 27, 2007
Indohyus: Yet Another Piece of the Puzzle

Based on their anatomical features, scientists have long hypothesized that cetaceans (whales, dolphins, and porpoises) evolved from land-dwelling mammals. Like their land-based relatives, cetaceans are warm-blooded animals that breathe air, give live birth, feed their young with milk, and move by flexing their spines up and down. These distinctly mammalian traits are known to have evolved in land-dwelling mammals long before the earliest cetaceans first took to the water.

Despite the obvious connections, scientists were puzzled for most of the 20th century by a lack of intermediate fossils illustrating the transition of cetaceans from land to water. Then, beginning in the early 1990s, several amazing transitional forms were discovered that fit neatly into the existing gaps.

Hans Thewissen and Indohyus
Over the last ten years, Hans Thewissen and his colleagues at Northeastern Ohio Universities College of Medicine (NEOUCOM) have been busy locating the missing pieces of the puzzle. Recently, Thewissen announced the discovery of Indohyus, a 48-million-year-old hoofed mammal that may be the closest known fossil relative of cetaceans.

Indohyus was comparable in shape, size, and behavior to its modern analog, the water chevrotain:

Indohyus was clearly not a cetacean, but it may have been a close cousin to the ancestors of cetaceans. Indeed, as shown by the illustration below, there is very little resemblance between Indohyus and modern cetaceans.

Indohyus restoration by Carl Buell

The primary characteristics linking Indohyus to cetaceans are similarities found in the middle-ear region of the skull:

Indohyus shares with cetaceans several synapomorphies that are not present in other artiodactyls. Most significantly, Indohyus has a thickened medial lip of its auditory bulla, the involucrum, a feature previously thought to be present exclusively in cetaceans. Involucrum size varies among cetaceans, but the relative thickness of medial and lateral walls of the tympanic of Indohyus is clearly within the range of that of cetaceans and is well outside the range of other cetartiodactyls.

From the original article in Nature

Other paleontological evidence has suggested that the animal spent considerable time in the water:

Thewissen and colleagues also explored how Indohyus lived, and came up with some surprising results. They determined that the bones of the skeleton of Indohyus had a thick outside layer, much thicker than in other mammals of this size. This characteristic is often seen in mammals that are slow aquatic waders, such as the hippopotamus today. Indohyus' aquatic habits are further confirmed by the chemical composition of their teeth, which revealed oxygen isotope ratios similar to those of aquatic animals. All this implies that Indohyus spent much of its time in water.

From the NEOUCOM press release

After further analysis, the discovery of Indohyus may add one more piece of evidence to the puzzle that illustrates the connection between cetaceans and their land-based ancestors.

What is most intriguing about this piece of the puzzle is that it fits with the molecular evidence that has already identified hippos (members of a group known as "artiodactyls") as the closest living relatives of cetaceans. The existence of an artiodactyl group in the fossil record that is a sister group to cetaceans would confirm the evolutionary hypothesis that whales, dolphins and porpoises are derived from ancient artiodactyl ancestors.

Even with this latest find, there are still many unanswered questions about the evolutionary history of cetaceans. The puzzle will always be missing pieces, but each new discovery has consistently confirmed the hypothesis that cetaceans evolved from land-dwelling mammals.

Indeed, it may no longer be appropriate to refer to the evolutionary origin of cetaceans as a "hypothesis." The evidence collected so far certainly leaves little room for alternative explanations.

For more about the discovery of Indohyus, see this streaming video from Nature.

posted by Jeremy Mohn

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