165-mn-yr-old mammal ancestor fossil found in Mongolia
Washington: A newly discovered fossil has shown the evolutionary adaptations of a 165-million-year-old proto-mammal, which has provided proofs that traits like hair and fur developed well before the first true mammals started roaming the Earth.
The ancient mammalian relative has been named Megaconus mammaliaformis.
Zhe-Xi Luo, professor of organismal biology and anatomy at the University of Chicago, said that they finally have a glimpse of what may be the ancestral condition of all mammals, by looking at what is preserved in Megaconus.
He said that it allowed them to piece together poorly understood details of the critical transition of modern mammals from pre-mammalian ancestors.
Discovered in Inner Mongolia, China, Megaconus is one of the best-preserved fossils of the mammaliaform groups, which are long-extinct relatives to modern mammals.
Dated to be around 165 million years old, Megaconus co-existed with feathered dinosaurs in the Jurassic era, nearly 100 million years before Tyrannosaurus Rex roamed Earth.
Preserved in the fossil is a clear halo of guard hairs and underfur residue, making Megaconus only the second known pre-mammalian fossil with fur.
It was found with sparse hairs around its abdomen, leading the team to hypothesize that it had a naked abdomen.
On its heel, Megaconus possessed a long keratinous spur, which was possibly poisonous. Similar to spurs found on modern egg-laying mammals, such as male platypuses, the spur is evidence that this fossil was most likely a male member of its species.
"Megaconus confirms that many modern mammalian biological functions related to skin and integument had already evolved before the rise of modern mammals," said Luo, who was also part of the team that first discovered evidence of hair in pre-mammalian species in 2006.
A terrestrial animal about the size of a large ground squirrel, Megaconus was likely an omnivore, possessing clearly mammalian dental features and jaw hinge.
Its molars had elaborate rows of cusps for chewing on plants, and some of its anterior teeth possessed large cusps that allowed it to eat insects and worms, perhaps even other small vertebrates.
It had teeth with high crowns and fused roots similar to more modern, but unrelated, mammalian species such as rodents. Its high-crowned teeth also appeared to be slow growing like modern placental mammals.
The skeleton of Megaconus, especially its hind-leg bones and finger claws, likely gave it a gait similar to modern armadillos, a previously unknown type of locomotion in mammaliaforms.
Luo and his team identified clearly non-mammalian characteristics as well. Its primitive middle ear, still attached to the jaw, was reptile-like. Its anklebones and vertebral column are also similar to the anatomy of previously known mammal-like reptiles.
The findings have been published in Nature.
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