Whales, dolphins descended from deer?
WASHINGTON (AP) - December 19, 2007 What might be the missing evolutionary link between whales and
land animals is an odd animal that looks like a long-tailed deer
without antlers or an overgrown long-legged rat, fossils indicate.
The creature is called Indohyus, and recently unearthed fossils
reveal some crucial evolutionary similarities between it and
water-dwelling cetaceans, such as whales, dolphins and porpoises.
For years, the hippo has been the leading candidate for the
closest land relative because of its similar DNA and whale-like
features. So some scientists were skeptical of the new hypothesis
by an Ohio anatomy professor whose work was being published
Thursday in the journal Nature.
Still, some researchers have been troubled that hippos seem to
have lived in the wrong part of the world and popped up too
recently to be a whale ancestor.
Newer fossils point to the deer-like Indohyus. The animal is a
"missing link" to the sister species to ancient whales, said Hans
Thewissen, an anatomy professor at Northeastern Ohio Universities
College of Medicine.
"As a zoo animal, it looks nothing like a whale," Thewissen
said. But, he added, when it comes to anatomical features, the
Indohyus "is quite strikingly like one."
Thewissen, who earlier published papers on fossils of what he
called the first amphibious whale and the skeleton of the oldest
known whale, studied hundreds of Indohyus bones unearthed from
mudstone in the Kashmir region of India. From that cache of bones
he created a composite skeleton of a 48 million-year-old creature.
The key finding connecting Indohyus to the whale is its
thickened ear bone, something only seen in cetaceans. An
examination of its teeth showed that the land-dwelling creature
spent lots of time in the water and may have fed there, like hippos
and whales. Also, the specific positioning and shape of certain
molars connects Indohyus to the earliest whales, which are about 50
million years old, Thewissen said.
"The earliest whales didn't look like whales at all,"
Thewissen said. "It looked like a cross between a pig and a dog."
They lost their legs and ability to walk on land about 40 million
years ago, he said.
And the Indohyus? "A tiny little deer maybe the size of a
raccoon and no antlers," Thewissen said. He said it most resembles
the current African mousedeer, which has a rat-like nose and "when
danger approaches, it jumps in the water and hides."
India and Pakistan were the general region where early whales
lived. That matches with the Indohyus but not the early African
hippos, Thewissen said. While modern-day cetaceans are known to be
smart, early whales and Indohyus had small brains, the researcher
said.
Other scientists were intrigued, but far from convinced,
especially since the case for hippos has looked good, they said.
"While this new hypothesis for the origin of whales is
compelling, it will require further testing, especially since other
recent studies have suggested both hippos and Raoellids were
involved in whale ancestry," San Diego State University biology
professor Annalisa Berta said in an e-mail. Raoellids are the
larger grouping of species that include the Indohyus.
Kenneth Rose, a professor of functional anatomy and evolution at
Johns Hopkins University, said Thewissen didn't provide enough
evidence to merit his conclusions. He also questioned the use of
the composite skeleton. The ear bone thickness, the key trait that
Thewissen used, was difficult to judge and seemed based on a single
specimen, Rose said. Much of the work is based on teeth, and
overall the remains preserved from this family of species are
poorly preserved, he said.
Thewissen said there are problems with not enough well preserved
fossils, but he said what's left makes a strong case for Indohyus
as the closest land ancestor - with hippos as the closest living
land relative.
---
On the Net:
Nature: http://www.nature.com/nature
Hans Thewissen's whale origins research page:
http://www.neoucom.edu/DEPTS/ANAT/Thewissen/whale-origins/index.html
