Whale Evolution Missing Link?

[Northern Sharks]

Here's another interesting article clipped from today's Toronto Star newspaper

[Gatorman]

Interesting but I don't see a resemblance

[Northern Sharks]

Interesting but I don't see a resemblance

Like they say with babies, he has dad's eyes

[Harry Pristis]

Like they say with babies, he has dad's eyes

Here's the story in an easier to read format:

Whales Evolved From Tiny Deerlike Mammals, Study Says

*IMAGE*

James Owen

for National Geographic News

December 19, 2007

The nearest ancestors of Earth's largest-ever animals were tiny deerlike

creatures that jumped into rivers to flee prehistoric predators, a new study

suggests.

These semiaquatic, raccoon-size mammals dubbed Indohyus lived in southern

Asia some 48 million years ago. (See more pictures of the possible whale

ancestor.)

Indohyus is part of a large group of mammals known as artiodactyls, which

includes pigs, sheep, hippos, and giraffes.

Several recent fossil studies suggest that artiodactyls gave rise to whales,

and that the hippopotamus is their closest living relative.

But hippos don't appear in the fossil record until about 35 million years

after whales diverged from their land-dwelling ancestors, leaving a gap in

the evolutionary chain.

For the new study, a team led by paleontologist Hans Thewissen examined

hundreds of Indohyus fossils found in mudstone in Indian-controlled Kashmir.

"We think that Indohyus was living there in little herds and that a whole

bunch of these animals died," said Thewissen, of Northeastern Ohio

Universities Colleges of Medicine and Pharmacy.

"Their bones were then washed into this river and they were all buried

together."

The fossils show distinct features that suggest the ancient ungulates, or

hoofed mammals, are the long-sought "missing links" in the evolution of

whales, the scientists report in tomorrow's issue of the journal Nature.

"Ballast" Bones

In particular, the structure of the animal's skull and ears show that

Indohyus was closely related to whales, the study team said.

These findings were "very surprising," given Indohyus' deerlike appearance,

Thewissen said.

"It was only the size of a raccoon, but if you saw it in a zoo, you would

think it was a miniaturized deer," he added.

Despite this, the structure of its bones suggests that the animal spent much

of its time in the water.

The limb bones had a thick, dense outer layer-a feature often seen in modern

aquatic mammals, such as hippos, that require extra ballast, the study team

noted.

"It allows them to walk on the bottom of the river without their buoyancy

pushing them up and making them float," Thewissen said. "So [indohyus] was

also a wader in water."

In addition, the ratios of chemical clues called isotopes in the creature's

teeth are characteristic of animals that ingest water while feeding, the

team said.

The fossil teeth's chemistry also indicates the creature was an herbivore,

even though it's widely thought that the earliest whales took to the water

in pursuit of fish.

"Clearly, this is not the case," Thewissen commented.

"Indohyus is a plant-eater and already aquatic. Apparently the dietary shift

to hunting animals [as modern whales do] came later than the habitat shift

to water."

The study proposes that Indohyus may have been drawn to water to escape land

predators.

The team says similar behavior is seen today in the African mouse deer, a

tropical forest dweller that feeds on land but flees into rivers when in

danger.

Hippolike Candidate

But this latest attempt to solve the long-running conundrum of how whales

first evolved doesn't satisfy University of Michigan paleontologist Philip

D. Gingerich, a past grantee of the National Geographic Society's Committee

for Research and Exploration. (National Geographic News is part of the

National Geographic Society.)

More convincing information is needed than is provided in the Nature paper,

he said.

The crucial piece of evidence that would link Indohyus to whales is a full

analysis of the ear bone covering the animal's middle ear, Gingerich

commented.

"[Thewissen and colleagues] say one side is much thicker than the other, and

that that's a whale characteristic, which it is," he said.

"But it's so surprising to see that in an animal that otherwise looks

completely terrestrial."

Since this ear bone is "absolutely the key feature," Gingerich said, "I

cannot understand why they wouldn't show us some kind of cross-section,

computerized tomography scan, or anything that would convince a person that

they hadn't just measured [a fossil ear bone] that was broken."

Various scientists have speculated that Indohyus-type mammals might have

been the artiodactyls' closest relatives to whales, Gingerich said.

But other researchers favor another, hippolike candidate: anthracotheres, or

"coal beasts," which are fossil animals first discovered in swampy coal-peat

deposits in Europe.

Anthracotheres are known to have to been at least partially aquatic and are

believed to be the relatives of modern-day hippos, Gingerich said.

DNA studies "tell us that hippopotami have some special relationship with

whales, exclusive of all other artiodactyls," he said.

The fossil bones of anthracotheres are also consistent with them being the

sister group of whales, Gingerich added.

Kenneth D. Rose, from the Center for Functional Anatomy and Evolution at the

Johns Hopkins University School of Medicine in Baltimore, also said he has

initial reservations about the new research.

"Studies in recent years have shown that whales and artiodactyls are closely

related, and some evidence suggests that whales are nested within

artiodactyls," Rose said.

Given this previous work, Indohyus could indeed be the ancestor of whales,

he noted.

But "I do not believe the evidence presented here demonstrates that with

confidence," he said in an email.

"It is an interesting hypothesis to be tested as more complete [indohyus]

fossils are discovered."

http://news.nationalgeographic.com/news/20...es-evolved.html

more photos at:

http://news.nationalgeographic.com/news/20...ures/index.html