Fossil of hairy, squirrel-sized creature sheds light on evolution of earliest mammals

It appears that a 165myr-old omnivore may have had an armadillo-like gait. A newly discovered fossil has revealed the evolutionary adaptations of a 165myr-old proto-mammal, providing evidence that traits such as hair and fur originated well before the rise of the first true mammals. University of Chicago scientists have described the biological features of this ancient mammalian relative, named Megaconus mammaliaformis, in the August 2013 issue of Nature.

“We finally have a glimpse of what may be the ancestral condition of all mammals, by looking at what is preserved in Megaconus. It allows us to piece together poorly understood details of the critical transition of modern mammals from pre-mammalian ancestors,” Zhe-Xi Luo, professor of organismal biology and anatomy told me.

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Fig. 1. A new Jurassic fossil, Megaconus mammaliaformis, was recovered in the Inner Mongolia region of China, at the famous Daohugou fossil site in the Tiaojishan Formation, which is dated to be 165myrs old. The site is northeast of Beijing. Megaconus comes from a group of primitive mammal relatives, predating modern mammal ancestors.

Luo shared the details of this discovery with me during the summer of 2103 at a meeting in his third-floor office in the Anatomy Building on the UChicago campus. Discovered in Inner Mongolia, China, Megaconus is one of the best-preserved fossils of the mammaliaform groups, which are long-extinct relatives of modern mammals. Dated to be about 165myrs-old, Megaconus co-existed with feathered dinosaurs in the Jurassic, nearly 100myrs before Tyrannosaurus rex roamed the earth.

A terrestrial animal about the size of a large ground squirrel, Megaconus was probably 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.

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Fig. 2. Megaconus mammaliaformis is preserved as a slab (left) and a counterpart (right) of shale deposited in a shallow lake. The preserved part of the skeleton, from head to rump, is about 21cm. By the length of the long bones, Megaconus is estimated to have weighed about 250g. The fossil assemblage from the Daohugou site includes several other mammals, such as the semi-aquatic swimmer (Castorocauda), a gliding mammal (Volaticotherium), feathered dinosaurs, amphibians, abundant arthropods and plants. Megaconus is the first skeletal fossil of a mammaliaform group otherwise only known by their teeth, but having a long history extending back to the Late Triassic and a wide distribution in the Jurassic. The holotype specimen is deposited at Palaeontological Museum of Liaoning, Shenyang, in China.

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. In addition, 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 hypothesise that it had a naked abdomen. On its heels, 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.

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Fig. 3. By its skeletal features, Megaconus was a terrestrial mammal with an ambulatory (‘walking’) gait and lived like a modern rock hyrax or armadillo. This new fossil species has many features that are ancestral to modern mammals, but its first cheek tooth has a large, cone-shaped cusp, resulting in its name Megaconus mammaliaformis (‘ancestral mammal with a big cusp’). It was an omnivore, living on ferns and cycad and gymnosperm plants, but could also hunt down worms and arthropods, both on land and in shallow water.

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 (Science, 331: 1123-1127, DOI:10.1126/science.1123026).

However, Luo and his team identified clear, 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.

“We cannot say that Megaconus is our direct ancestor, but it certainly looks like a great-great-grand uncle 165myrs removed. These features are evidence of what our mammalian ancestor looked like during the Triassic-Jurassic transition,” Luo told me.

Megaconus shows that many adaptations found in modern mammals were already tried by our distant, extinct relatives. In a sense, the three big branches of modern mammals are all accidental survivors among many other mammaliaform lineages that perished in extinction,” Luo added.

The fossil, now in the collections of the Palaeontological Museum of Liaoning in China, was discovered and studied by an international team of palaeontologists from the Palaeontological Museum of Liaoning, the University of Bonn in Germany and the University of Chicago.

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Fig. 4. Megaconus is an extinct relative of modern mammals and is classified as a ‘proto-mammal’ on the family tree. It has exquisitely preserved features, such as hair, which show that the origin of some mammal-like features can be traced further back to more distant mammalian ancestors and relatives.

Acknowledgements: The work was supported by the Key Lab for Palaeobiological Evolution of Northeastern Asia, the Ministry of Land Resources of China, Shenyang Normal University, the Paleontological Museum of Liaonig, the German Research Foundation (Deutsche Forschungsmeinschaft), the Alexander von Humbolt Foundation, the National Science Foundation and the University of Chicago.


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