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 mammalsancestors 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.
SOURCES AND RELATED WEB SITES