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Tiny Fossil From Early Jurassic Fills New Niche in Mammal Evolution

D.L. Parsell
National Geographic News
May 24, 2001
 
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.

The tiny animal's relatively big brain is not just a matter of increased volume, he added. A CT scan of the fossil revealed an enlargement in specific areas of the brain, such as the olfactory lobes that are connected with the sense of smell.

Earlier research has shown that other insect-eating, mammal-like species from the Early Jurassic ranged widely in size. But Hadrocodium wui's probable body weight of two grams makes it one of the smallest mammals ever found.

The new finding and other recent discoveries around the world "suggest that there's more diversity among early mammals than we thought," said Compton.

Key Brain and Ear Traits

The scientists compared the features of Hadrocodium with those found in other early mammal fossils and in living mammals. How the lower jaw and middle ear are constructed and connected is one of the most telling features in determining whether an animal is part of the group officially classified as mammals.

"The new species has a large brain and a middle ear of modern mammals, and it suggests that these two features may have evolved together," said Luo.

The creature's expanded brain size, he added, may have pushed the ear bones away from the jaw, leading eventually to the complete detachment seen in modern mammals. It is possible, Luo suggested, that "Hadrocodium represents the final step in the separation between the middle ear and the mandible."

The discovery is significant because "it has been a challenge for scientists to trace the origins of these important mammalian features in the fossil record," Luo said. Before this latest discovery, animals showing these key mammalian traits had been traced only as far back as the Late Jurassic period, about 150 million years ago.

Exact Relationship Unknown

Writing in a companion article in Science, André Wyss of the University of California-Santa Barbara notes that the last 65 million years of the Earth's history have been widely regarded as the "age of mammals." Yet it was known, he points out, that "the major branches of the mammalian evolutionary tree diverged millions of years earlier." Sketchy fossil evidence, however, made it difficult to piece together the larger picture.

In recent decades, a large trove of important fossil discoveries and the development of more sophisticated methods of analyzing and comparing them have rapidly improved scientific understanding of developmental pathways and physiological relationships.

"As the nearest securely identified relative of mammals, Hadrocodium has proved uniquely valuable for documenting the sequence of morphological changes that led to the emergence of mammalian ancestors," Wyss writes.

Hadrocodium represents a new lineage on the developmental spectrum extending from advanced mammal-like reptiles (cynodonts) to true, living mammals.

The mammal-like reptiles, like those whose remains were found among the fossils from the Lufeng Basin in China, had jaw and middle ear features similar to those of later mammals but with the pieces still unconnected.

At the other end of the transitional evolutionary leap were the early mammals—ancestors of modern egg-laying mammals, or monotremes (such as the platypus and echidna) and mammals that give birth to live young (marsupials and placentals such as kangaroos and humans).

Wyss said it's widely assumed that these two groups once shared a common ancestor and later diverged. The analysis of Hadrocodium, he points out, reveals that it branched off before the appearance of that common ancestor of living mammals.

While Hadrocodium appears to be more closely related to living mammals than the mammal-like cynodonts are, the exact nature of that relationship can't be determined from the present fossil evidence, said Luo.

"Hadrocodium could be our distant cousin, an early mammal that existed alongside the ancestor of living mammals," he said. "Or it could be our great-great grand uncle, closely related to living mammals but not in our direct lineage. Or Hadrocodium could be the direct ancestor of living mammals."

"The fossil evidence can't distinguish between these three possibilities," he added. "But we are satisfied to know that Hadrocodium is the sister taxon to all living mammals."

More information about this story will air on National Geographic Today in the United States on May 24 at 7 p.m. ET/PT.
 

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