A new fossil suggests hair may have preceded the development of the mammal.
The Megaconus mammaliaformis is "one of the best-preserved fossils of the mammaliaform groups, which are long-extinct relatives to modern mammals," a University of Chicago press release reported.
The 165 million-year-old fossil was around the size of a modern day squirrel.
"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, said.
The "proto-mammal's" teeth indicated it dined on both plants and other animals. Staples of its diet may have been insects and worms, but Megaconus could have hunted other small animals too.
Its teeth and jaw resembled a mammal's features. The teeth possessed fused roots and a high crown, and were slow growing, similar to the rodent (which is not related to Megaconus).
The structure of the fossil's back legs suggest it had a similar gait to an armadillo, which has a distinctive walk.
Hairs found along the abdomen were "sparse," leading the researchers to believe the ancient animal had a naked body, also similar to and armadillo's.
The Megaconus had traits in common with the platypus. A "long keratinous spur" extended from the mammal-like creature's heel, and may have been poisonous. The presence of the spur suggests the specimen was male.
"Megaconus confirms that many modern mammalian biological functions related to skin and integument had already evolved before the rise of modern mammals," Luo said, according to the press release.
The team found evidence of non-mammalian features in the mammal-reptile hybrid. The inner ear was still attached to the jaw in a reptilian fashion. Its spinal column and bones in the ankles also resembled a reptile's anatomy
"We cannot say that Megaconus is our direct ancestor, but it certainly looks like a great-great-grand uncle 165 million years removed. These features are evidence of what our mammalian ancestor looked like during the Triassic-Jurassic transition," Luo said. "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."
