image: The graph illustrates the reconstruction of Megaconus mammaliaformis. view more
Credit: April M. Isch, University of Chicago
For the longest time, all that was known about this long-extinct mammal was a few little teeth with striking cusps on their occlusal surfaces. "Paleontologists have been wondering for over a hundred years what the animal that went with these teeth might have looked like," said Prof. Dr. Thomas Martin from the Steinmann-Institut of the University of Bonn. The matter was elucidated when locals found a completely preserved skeleton of the enigmatic mammal in Northeast China, which was then aquired by the Paleontological Museum of Liaoning in Shenyang.
Together with Dr. Chang-Fu Zhou and Dr. Shaoyuan Wu, researchers from Shenyang Normal University, as well as Prof. Dr. Zhe-Xi Luo from the University of Chicago (USA), who was working as a visiting scholar at the University of Bonn, Prof. Martin studied the fossil dating back about 165 million years. The researchers used microscopes and a micro-CT scanner to learn more about the skeleton. As in other early forms of mammals, the mammalian middle ear bones were still firmly connected to the lower jaw in Megaconus.
Tooth cusps indicate that this was not a primitive mammal
The name "megaconus" means "large cusp." The anterior cheek teeth are equipped with strikingly large cusps. These animals used them to crush hard plant materials. The posterior molars have rows of cusps aligned longitudinally, indicating that Megaconus mammaliaformis ground up tough plants by moving its jaws in a longitudinal direction. This is an unusual specialization for teeth since early mammals fed primarily on insects.
"Based on this find, we were able to show that this early mammaliaform was not a primitive animal," said the paleontologist from the University of Bonn, adding that special adaptations are not a privilege of modern mammals. Molars with such rows of cusps as in the extinct Megaconus mammaliaformis have also been found in other mammals, such as in multituberculates, which are also extinct, but also in today's rodents. "This shows that complex structures can occur multiple times in evolution, independently of each other," said Prof. Martin.
Spurs with poisonous glands as protection from being eaten
This highly specialized mammal was about the size of a rat and had a soft fur. On the stone slab with the skeleton, the hairs can still clearly be seen. "So, before the last common ancestor of modern mammals, the early mammaliaforms already had fur, but are extinct today," said Prof. Martin. Megaconus mammaliaformis obviously had an ambulatory mode of locomotion on the ground, reported the paleontologist from the University of Bonn. But it was not capable of climbing and jumping from branch to branch like a squirrel–its claws did not have enough curvature, and in addition, its tibia and fibula were fused.
"In good climbers, the two lower leg bones must be flexible against each other," explained Prof. Martin. Since this furry, 250 gram-animal did not have this kind of motility in its legs, it was not able to flee up a tree from predators. And yet this cute early mammaliaform was not an easy prey for predators, because its hind legs were equipped with spurs that had poisonous glands that apparently served as a defense mechanism.
Publication: A Jurassic Mammaliaform and the Earliest Mammalian Evolutionary Adaptations, in: "Nature," DOI: 10.1038/nature12429
Journal
Nature