Scrat, the saber-toothed squirrel in the movie Ice Age, is not just the imagined creation of some computer animator. Paleontologists from the University of Louisville and Argentina’s Universidad Maimónides have found two skulls belonging to the first known mammal of the early Late Cretaceous period of South America, 144 to 65 million years ago when dinosaurs were in their heyday. The skulls are about 100 million years old and were found embedded in rock in a remote area of northern Patagonia; it took several years of lab work to remove the skulls. The creature has been named Cronopio dentiacutus and is a dryolestoid, which is an extinct group of mammals who are distantly related to today’s marsupials and placentals:
Cronopio was shrew-sized, about 4-6 inches in length, and was an insectivore with a diet of the insects, grubs and other bugs of the time. It lived when giant dinosaurs roamed Earth — more than 100 million years ago — and made its home in a vegetated river plain.
The skulls reveal that Cronopio had extremely long canine teeth, a narrow muzzle and a short, rounded skull. “These first fossil remains of dryolestoids … give us a complete picture of the skull for the group,” John R. Wible, Ph.D., curator of mammals at the Carnegie Museum of Natural History, said. “The new dryolestoid, Cronopio, is without a doubt one of the most unusual mammals that I have seen, extinct or living, with its elongate, compressed snout and oversized canine teeth. What it did with that unusual morphology perhaps may come to light with additional discoveries… .”
The discovery of the skulls — which are very rare due to their small size and fragility — bridges a previous 60-million-gap in the fossil record that had been “somewhat of a blank slate” and also confirms that mammals were endemic in southern continents at that time.
It is usually teeth or bone fragments, rather than an entire skull, that are found dating back to the Late Cretaceous period, so the two skulls are a major find. Discovering the skulls enables paleontologists to have a much more complete view of the genealogy of mammals at such a very early stage in their — in our — evolution. Says lead author Guillermo Rougier, Ph.D., professor of anatomical sciences and neurobiology at the University of Louisville:
“This tells us a little more of the full history of our lineage, a very resilient lineage. Cronopio lived in a completely different world than ours, dominated by dinosaurs and with a different geography; these new fossils give us information on how transient and ever-evolving our world is.”
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