Image: MARK A. KLINGER/CMNH
Key features in the mammalian ear and brain evolved far earlier than previously thought, paleontologists say. According to a report published today in the journal Science, the fossilized remains of a diminutive creature that researchers have dubbed Hadrocodium wui push the origins of these characteristic innovations back by some 45 million years.
Zhe-Xi Luo of the Carnegie Museum of Natural History and his colleagues discovered the 195-million-year-old fossil, which comes from the Lufeng Basin in southwestern China's Yunnan Province. Comparing Hadrocodium to other known fossil mammals, the team concluded that it is the closest known relative to living mammals. Characteristics of the braincase and middle ear bones were particularly revealing. The transition from mammal-like reptiles to true mammals involved increasing brain size and detaching the middle ear bones from the lower jaw, but identifying the origins of these features had proved problematic. That Hadrocodium resembles a modern mammal in both regards suggests that these features may have evolved together.
The new fossil also reveals that there must have been considerable diversity among early mammals: Hadrocodium, which weighed no more than a paper clip, is far smaller than other mammals from the early Jurassic period. "The welcome discovery of the tiny but crucial fossil of Hadrocodium," Andr¿ Wyss of the University of California at Santa Barbara writes in a commentary accompanying the Science report, "demonstrates yet again the continued handsome scientific payoffs that emerge from the time-honored practice of hunting for ancient bones."