Mystery Mammal Fossil Found in India
for National Geographic News
|November 8, 2007|
A 66-million-year-old tooth found in India has paleontologists questioning their theories about the rise and spread of hoofed mammals.
The tooth may have belonged to one of the condylarths, according to a new study led by Guntupalli Prasad of the University of Jammu in India.
This group of primitive mammals included the ancestors of modern hoofed animals such as goats, horses, cows, sheep, and deer.
But the newfound molar is about three million years older than the earliest known condylarth specimen.
This places it on the Indian subcontinent at a time when the landmass was a drifting island that had just broken away from the supercontinent called Gondwana.
The find, described in this week's issue of the journal Science, may help fuel a controversial theory that what is now India was the prehistoric source of many plant and animal species.
"The fossil record shows [for example] that the Indian subcontinent was the center of origin for whales" 55 million years ago, Prasad said.
"Similarly, many researchers had postulated that mammalian groups may have originated from basal Gondwanan stocks during the northward flight of India and finally dispersed to Asia when India collided with the Asian mainland around 55 million years ago."
But the tooth's ultimate identification is far from certain.
J. David Archibald, an evolutionary biologist at San Diego State University, believes that the molar certainly belonged to an ancient mammal, but that we cannot be certain what type.
"It has interesting biogeographic implications, whatever it is," he said.
The now extinct condylarths were chipmunk-size to sheep-size mammals. Some had grinding teeth that show early evolutionary adaptations in mammals toward a vegetarian diet.
The current fossil record suggests that most condylarths lived during the Paleocene epoch, which lasted between 65.5 and 55.8 million years ago.
The newfound tooth was unearthed in sedimentary rock sandwiched between lava flows from the late part of the Cretaceous period, which spanned from 145.5 to 65.5 million years ago.
About 250 million years ago all Earth's landmasses were connected, forming a supercontinent called Pangaea.
But during the early Cretaceous tectonic drift forced a chunk of land to split away and become the supercontinent Gondwana. This landmass contained what are now South America, Africa, Antarctica, Australia, India, and Madagascar.
During the late Cretaceous, experts think, India probably broke away to become an island continent adrift in what is now the Indian Ocean.
(Related news: "Slimmer Indian Continent Drifted Ten Times Faster" [October 17, 2007].)
Mammal fossils found on the southern continents that date to the time of Gondwana remain quite rare.
"With a few minor exceptions, the only other place where there are really good records of mammals at that time is North America," Archibald said.
"So this tooth is in limbo both biogeographically and in time."
Kenneth Rose, a paleontologist at Johns Hopkins University's Center for Functional Anatomy and Evolution in Baltimore, Maryland, is also intrigued by the tooth's presence in ancient India.
"How did this animal get onto India, when [the landmass] was probably about in the middle of the Indian Ocean on a rapid drift northward?" he asked.
"That's one of the really intriguing questions here. If it is an ungulate [hoofed mammal], it provides an interesting scenario that some primitive stock got onto India and later evolved into ungulates.
"But that's just an interesting hypothesis."
Some researchers believe that good interspecies exchange existed between India and the Asian continent even during the Cretaceous.
If so, the early ungulates might not have originated in India but somehow moved there from northern locales, where their fossils are more commonly found.
There is also a chance that the tooth may not be that of an ungulate after all—more samples are needed to know for certain.
"Of course, if it turns out to be something else, it means nothing as far as the radiation of ungulates," Rose said.
"But it's still quite interesting to realize that there is some kind of placental mammal that was on India in the late Cretaceous.
"It looks much more like the [mammals] that were on the northern continents at that time."
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