National Geographic News
An animal whose skull was found embedded in a 195 million-year-old fossil from China was as tiny as a paper clip. The discovery of it, however, has big implications for our understanding of how mammals have evolved.
Scientists say the new species, which they named Hadrocodium wui, is the closest known relative to existing mammals that has been found so far.
According to the Chinese and U.S. scientists who analyzed the fossil, the tiny creature had a skull only 12 millimeters (about half an inch) long, a total body weight of two grams (less than one tenth of an ounce), and teeth that suggest it ate small insects.
Most significant, its body structure was like that of living mammals, especially with a brain cavity that was unusually large in proportion to the animal's overall body size and middle-ear bones that were completely separated from the jaw.
This finding was a surprise because scientists previously thought mammal features such as these did not emerge in the evolutionary pathway until some 45 million years after the period from which the fossil dates, the Early Jurassic. This means the tiny creature lived in the shadow of dinosaurs, at a time when a class of mammal-like reptiles was thought, until now, to be the closest link with modern mammals.
"Hadrocodium is considered to be a sister taxon to living mammal groups, the closest relative to all extant mammals," said Zhe-Xi Luo, a vertebrate paleontologist at the Carnegie Museum of Natural History who headed the research team that studied the fossil.
"Its very small size," he added, "suggests much greater ecological differences among the earliest known mammals, besides the fact that it adds a new lineage to early mammal diversity."
Luo, whose research has been supported by the National Geographic Society, is co-author of a report in the May 25 issue of the journal Science describing the new species. The other team members were Alfred Compton of Harvard University's Museum of Comparative Zoology and Ai-lin Sun of the Chinese Academy of Sciences.
Importance Initially Overlooked
The researchers named the new mammal Hadrocodium wui for the Greek word hadro, meaning "large and full," and codium, or "head." Luo said the second part of the name, wui, is a Latinized version of the name of the Chinese scientist, Xiao-Chun Wu, who found the fossil in 1985 in the the Lufeng Basin in Yunnan Province, which has yielded many clues about vertebrate land-based creatures that lived during the Early Jurassic.
Luo said researchers initially failed to realize the significance of the discovery because they thought the fossil was simply a bone fragment of a much larger animal. Later, when the specimen was being prepared for detailed analysis, "it turned out to be a complete skull of extremely small size," said Luo.
From the size and features of the skull and knowledge of mammals, Luo explained, scientists pieced together a likely anatomical portrait of the animal. Based on the body proportions of modern mammals, in which the head is 35 to 40 percent of overall body length, the researchers estimated that the new animal was only about 32 millimeters (1.25 inches) long from the tip of the nose to the end of the rump, "the size of a regular paper clip," said Luo.
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