With its rodent-like appearance, the newly identified Megaconus certainly looks like a mammal. But this forest dweller lived over 165 million years ago, before the rise of the first true mammals.
Megaconus is what paleontologists consider to be a proto-mammal — a transitional line, called a haramiyid, that shared a common ancestor with modern mammals. Creatures like these emerged a remarkably long time ago. The 165 million year-old Megaconus predates the T-Rex by nearly 100 million years.
A well-preserved fossil of Megaconus was discovered in Inner Mongolia and it features a number of fascinating characteristics. It’s got a clear halo of guard hairs and underfur residue, indicating fur. Because it was found with sparse hairs around its abdomen, paleontologists suspect that its abdomen was bare.
It also had a long keratinous spur on its heel — which was possibly poisonous. Modern egg-laying mammals, like the platypus, feature similar spurs.
Megaconus was about the size of a large ground squirrel and walked around with a shambling gait (not unlike an armadillo). It was likely an omnivore that romped around on the forest floor. The hinge of its jaws and other dental features were very similar to modern mammals, including a row of molars for chewing on plants, and some anterior teeth which may have allowed it to feed on insects and worms — maybe even small vertebrates. Its teeth also had high crowns and fused roots, similar to rodents.
The common ancestor of all living mammals emerged about 180 million years ago. The haramiyids, including Megaconus, branched away about 40 million years before true mammals evolved.
"We cannot say that Megaconus is our direct ancestor, but it certainly looks like a great-great-grand uncle 165 million years removed,” noted Zhe-Xi Luo in a release. “These features are evidence of what our mammalian ancestor looked like during the Triassic-Jurassic transition.” Luo is a professor of organismal biology and anatomy at the University of Chicago.
But Megaconus is not the only new old kid in town. Paleontologists also recently unveiled Arboroharamiya, a small, tree-dwelling haramiyid that lived about 160 million years ago.
Nature News describes it:
It had relatively small hands and feet but exceptionally long digits...Proportionally, the creature’s digits were even longer than those of many modern tree-dwelling mammals, and features on some of its tail bones hint that it may have had a prehensile tail...
...The shape of the creature's teeth and the way they meshed while chewing suggest that Arboroharamiya either fed on seeds or was omnivorous...And each side of its lower jaw comprised only one bone, making it more like the jaw of modern mammals than of reptile jaws, which have three further bones that in mammals have evolved into the bones of the middle ears. This suggests that Arboroharamiya probably had a mammal-like ear structure. (No ear bones were found in the specimen, but these tiny bones are rarely preserved in mammalian fossils.)
Images: April Isch, University of Chicago/Zhe-Xi Luo et al.; Zhao Chuang/Jin Meng et al.