O R G A N I S M A L B I O L O G Y A N D A N AT O M Y
Midway News
Small, furry and 160 million years old The discovery of two new fossils helps fill in the blanks on how modern mammals evolved BY KEVIN JIANG
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f all the ancient life, dinosaurs seem to always get the glory. Gigantic and fearsome, with sharp teeth and claws, the terrible lizards are subjects of perpetual public fascination. But the story of ridged teeth, flexible ankles and tufts of hair holds far more relevance to the modern world. After all, these are the features that allowed the earliest mammals not only to thrive for more than 100 million years alongside the dinosaurs but also to outlive them all. Zhe-Xi Luo, PhD, professor of organismal biology and anatomy, has spent his career uncovering this story. Among his numerous discoveries is the earliest known placental mammal, which is most certainly our very distant ancestor; the earliest known fossil with fur, proof that hair evolved well before mammals did; and a 195-million-year-old creature, the size of a paper clip, that already had a modern mammal-like, large brain. With the recent discovery of two new fossils, described in August 2013 in the journals Nature and Science, Luo adds to his impressive body of work and our collective knowledge of how our ancestors came to be. About 165 million years ago, a furry animal about the size of a squirrel spent its life scurrying across the Jurassic ground. Discovered by Luo and his team in Inner Mongolia of China and detailed in Nature, the nearly complete skeleton of Megaconus mammaliaformis reveals the ancestral conditions that would give rise to features of today’s mammals. An extinct relative to mammals, its lineage was, until now, known from only scattered teeth and bone fragments.
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Life reconstruction of Megaconus mammaliaformis, top, by scientific illustrator April Isch of the University of Chicago. Rugosodon fossil photographed by Zhe-Xi Luo of the University of Chicago and Chongxi Yuan of the Chinese Academy of Geological Sciences.
With a halo of guard hairs and underfur residue, the Megaconus fossil is only the second-known premammalian fossil with fur. Its heel spur, which was a half inch long and possibly poisonous, was similar to the spur of a living platypus. Its teeth were mammal-like, with elaborate rows of cusps that allowed it to eat both plants and insects. However, Megaconus was not a mammal.
THE UNIVERSITY OF CHICAGO MEDICINE AND BIOLOGICAL SCIENCES DIVISION
Its primitive middle ear was still attached to its jaw — a reptilian feature. Its anklebones and vertebral column were similar to mammal-like reptiles. “We cannot say that Megaconus is our direct ancestor, but it certainly looks like a great-great-grand uncle 165 million years removed,” Luo said. “It allows us to piece together some enigmatic details of the