Pittsburgh -- An international team of researchers led by Carnegie Museum of Natural History Vertebrate Paleontologist Dr. Zhe-Xi Luo has discovered a 195-million-year-old fossil mammal. The new mammal is the smallest known for the Mesozoic Era and represents a new branch on the mammalian family tree.
In an article published today in the prestigious journal Science, the team of American and Chinese scientists described this new mammal as having a precociously large brain and the middle ear of modern mammals. It suggests that these two features may have evolved together.
Previously, these important mammalian traits could only be traced to the late Jurassic (approximately 150 million years ago). This discovery pushes back their origins by some 45 million years to the Early Jurassic.
The new species is named Hadrocodium wui for its exceptionally large brain ([hadro] – Greek for "large and full" and [codium] – Greek for head). The fossil has widespread implications to scientists piecing together the earliest mammalian evolutionary history.
"Mammals differ from non-mammalian vertebrates by possessing a very large brain and an advanced ear structure," said Dr. Luo. "It has been a challenge for scientists to trace the origins of these important mammalian features in the fossil record."
The newest addition to the mammalian family tree also happens to be the tiniest mammal known from the Mesozoic Era and one of the smallest mammals ever. Based on the size of its well-preserved skull, it is estimated that the whole animal weighed only two grams, less than the weight of a paper clip. With such a tiny body, its diet was likely limited to very small insects and small worms. Its enlarged brain and very small body also tell scientists that the animal had a very high metabolism, forcing it to continuously eat.
Co-existing with the extremely small Hadrocodium in the Early Jurassic were several other primitive mammals with much larger body size. "This tiny creature greatly stretches the range of body size for the earliest known mammals," added Dr. Luo.
Hadrocodium is a distant and extinct relative of living mammals such as the platypus, kangaroos and primates. It is more closely related to mammals that exist today than the primitive cynodonts or "mammal-like reptiles."
Hadrocodium was discovered in the famous Lufeng Basin in Yunnan Province, southwestern China. It is one of the most prolific sites for early Jurassic land vertebrates. The Mesozoic Redbeds of the Lufeng Basin have yielded many vertebrate fossils. Among the fossils that have been unearthed are the carnivorous dinosaur Dilosphosaurus, the prosauropod Lufengosaurus, crocodiles, and lizard-like animals, herbivorous mammal-like reptiles, and some of the earliest mammals ever discovered.
Dr. Luo's research team includes Alfred W. Crompton of Harvard University and Ai-Lin Sun of the Chinese Academy of Sciences.
Funding for this research was provided by the National Science Foundation, National Geographic Society, Carnegie Museum of Natural History's Putnam Funds and Museum of Comparative Zoology at Harvard University.
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