Whales ancestors' uncovered
Raoellids, whales' ancestor, would have looked like small deer
Thursday, 20, Dec 2007 10:16
The missing link between whales and their four-footed ancestors has been uncovered by scientists examining fossil records.
Since Darwin scientists have known that marine animals known as cetaceans – whales, dolphins and porpoises – originated about 50 million years ago in south Asia, but it has not been known who their terrestrial ancestor was.
A report published in the journal Nature says it is likely to have been a racoon-sized animal from the Kashmir region of India known as a raoellid.
It is thought this creature, which would have looked like a miniature deer, took to water in times of danger.
Dr Hans Thewissen and colleagues from the Northeastern Ohio Universities Colleges of Medicine and Pharmacy made the discovery after studying a layer of mudstone and hundreds of bones from raoellids (Indohyus).
Although whales and raoellids look very different, they share key similarities including structures in the skull and ear.
The bones of raoellids had a thick outside layer, much thicker than other mammals of this size; a characteristic often seen in mammals that are slow aquatic waders, such as the hippopotamus.
Further clues linking whales with raoellids include the chemical composition of the creature's teeth.
This includes oxygen isotope ratios similar to those of aquatic animals.
The researchers propose that the raoellids' bones and teeth structure suggest it spent much of its time in water.
Some scientists have proposed that whales descended from carnivorous terrestrial ancestors, with some speculating that whales became aquatic to feed on ocean-dwelling fish.
"Clearly this is not the case, Indohyus is a plant-eater and already is aquatic," said Dr Thewissen.
"Apparently the dietary shift to hunting animals (as modern whales do) came later than the habitat shift to the water."