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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