August 8, 2013
Fossils Found Of Earliest Squirrel-Like Proto-Mammal
Brett Smith for redOrbit.com - Your Universe Online
A newly discovered skeleton has evolutionary biologists adding new details to the story of mammalian evolution.
The fossilized skeleton belongs to a proto-mammal that lived 165 million years ago in northeast China and had hair well before the emergence of true mammals, according to a report of the find that appeared in the journal Nature. For years, paleontologists' only evidence of the species was a few fossilized teeth.
"Paleontologists have been wondering for over a hundred years what the animal that went with these teeth might have looked like," said study co-author Thomas Martin, a professor of paleontology at the University of Bonn.
Dubbed Megaconus mammaliaformis, the squirrel-like animal was found with a few hairs around its abdomen and a long keratinous spur, which may have been poisonous. Male platypuses have a similar poisonous spur, suggesting that the fossilized creature could be male.
Researchers suspect that Megaconus was likely an omnivore based on an analysis of its dental features and jaw hinge. While its molars were suited to chewing on plants, anterior teeth allowed Megaconus to eat insects, worms and possibly other small vertebrates. Some of its dental features are similar to unrelated modern-day rodents.
The animal's claws and hind legs indicated that it had an armadillo-like walk and probably did not climb up trees to escape predators.
"In good climbers, the two lower leg bones must be flexible against each other," Martin noted, which is not the case with Megaconus.
The long-extinct animal also had many features that set it apart from mammals. The animal's reptile-like primitive middle ear was attached it its jaw. It also had a skeletal structure similar to other mammal-like reptiles.
"We cannot say that megaconus is our direct ancestor, but it certainly looks like a great-great-grand uncle 165 million years removed," said co-author Zhe-Xi Luo, professor of organismal biology and anatomy at the University of Chicago. "These features are evidence of what our mammalian ancestor looked like during the Triassic-Jurassic transition."
"Megaconus shows that many adaptations found in modern mammals were already tried by our distant, extinct relatives," he added. "In a sense, the three big branches of modern mammals are all accidental survivors among many other mammaliaform lineages that perished in extinction."
"Based on this find, we were able to show that this early mammaliaform was not a primitive animal," Martin said.
Another recently published Nature report described a very different mammalia form that was also found in China. Said to be in the newly created genus Arboroharamiya, this animal lived around the same time as Megaconus, roughly 160 million years ago.
Unlike Megaconus, however, Arboroharamiya thrived in the trees, researchers say. Long digits probably allowed it to navigate branches, while tail bones indicate that it may have had a prehensile tail like those seen in modern spider monkeys or New World opossums which allow them to grasp and hang from tree branches or other horizontal objects. Arboroharamiya also has a mammal-like jaw made of only one bone. Its teeth indicate that it probably was an omnivore that fed mostly on seeds.
To determine Arboroharamiya's relationship to other similar animals, a research team compared over 400 anatomical features from 50 different species living between 250 million and 100 million years ago. The team concluded that Arboroharamiya was well within the mammalian family tree.