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The fossil remains of this small animal from the age of the dinosaurs show that it shares features of both modern mammals and their reptilian relatives,  and that it lived near a lake with a rich diversity of vertebrates,  insects,  and plants.

Discovering a Missing Link

Carnegie Scientist Identifies New Fossil as Connection Between Modern Mammals and Their Primitive Ancestors

by Kathryn M. Duda

The Beijing scientists knew they had something big on their hands. An amateur fossil collector had just unearthed the first complete fossil of a rare, long-extinct and very early mammal. He donated it to them for study at the Institute of Vertebrate Paleontology and Paleoanthropology, and now they wanted an expert to work with them in describing this extremely valuable specimen. Who to call?

The answer was simple: Zhexi Luo, assistant curator of Vertebrate Paleontology at Carnegie Museum of Natural History. Recognized as one of the world’s specialists on Mesozoic mammals, Luo joined the IVPP scientists in studying the fossil and reporting on it in the prestigious British scientific journal, Nature (November 13, 1997). The discovery sheds light on the biology and lifestyle of a major group of mammals, and it answers important questions about the evolutionary history of all living mammals, including humans.

The fossil was named Zhangheotherium quinquescuspidens (pronounced Jang-ha-theer-ee-um Kin-kay-kusp-i-dens) after its discoverer, Zhang He. The name means "Zhang He’s beast with five cusps on its teeth." It was discovered in 1992 at a fossil-rich site in northern China. Fossil fishes had been found there as far back as the 1920s, but it wasn’t until the 1990s that paleontologists began digging up other specimens, most notably fossil birds and dinosaurs. Zhangheotherium was the first fossil mammal to be found there, and it is the oldest complete skeleton of any mammal.

Zhangheotherium belongs to an extinct group of prehistoric mammals that has eluded scientists for decades—symmetrodonts (meaning "symmetrical tooth"). Until this discovery, scientists had been basing all their information about symmetrodonts on isolated teeth and jaw fragments. At five inches in length, the live Zhangheotherium looked somewhat like a rat, with razor-like, triangular-shaped teeth that fit together like scissors. It lived by the lake and ate insects, and apparently was itself food for dinosaurs.

"Fossil mammals from this period are rare and extremely difficult to find," Luo says. "We knew symmetrodonts existed but had no idea what they looked like or what ecological role they played in a world dominated by dinosaurs." When the research team* first looked at the new fossil, they recognized the jaw and knew it was a symmetrodont. "But," recalls Luo, "the issue was: What was the anatomical significance of the rest of the animal’s body? That’s what we spent a great deal of time researching. And this skeleton provides us with a remarkably clear picture."

Symmetrodonts are one of six ancient mammal groups. Besides symmetrodonts, two others are also extinct: triconodonts and multituberculates. Most of us are familiar with the others, which still exist and evolved in this order: egg-laying mammals, or monotremes, like the duck-billed platypus or spiny anteater; marsupials, like the kangaroo, which carry their young in pouches; and placentals, like humans, dogs and cows.

Scientists knew the lineage of contemporary placental and marsupial mammals went back to the late Triassic/early Jurassic era (175–210 million years ago). But the first symmetrodont jaw alone was not enough to show how current-day mammals were linked to the now-extinct groups. The discovery of the full skeleton, however, has shed light on this connection.

Luo and his colleagues found that symmetrodonts have both modern and primitive characteristics and thus link modern mammals to their prehistoric ancestors. Several of their primitive traits are common to reptiles—preserved from the time before reptiles and mammals split in their evolutionary paths. For instance, the symmetrodont’s front legs are splayed out much like a reptile’s legs are, yet its shoulders can move independently—a very modern trait. It resembles a monotreme, or egg-laying mammal, by the presence of a tiny spur on each rear foot that ejects a venom. And its ear structure places it after the most primitive animals, but before modern mammals: the inner ear is protected by a single bone, which is more modern than a reptile’s multi-bone inner-ear structure. But at the same time, it lacks the spiral structure that allows advanced mammals to hear high-pitched sounds. The cochlea, however, is straight like a reptile’s—nothing like the spiral one we advanced mammals have. These traits tie together what was known about primitive mammals and those we observe today.

"The skeleton of Zhangheotherium helps to pin down our own human ancestry among the diversity of Mesozoic mammals," says Luo. Based on all the evidence, the research team determined that symmetrodonts developed after the primitive monotremes, and before the modern marsupial and placental mammals, to which it appears more closely related.

"This is an important treasure," says mammal expert Michael Novacek of the American Museum of Natural History in New York, in a Boston Globe story on Zhangheotherium. "A huge amount of information is in that skeleton." With this valuable find, another important link is closed between humans and our earliest ancestors.

Kathryn M. Duda is associate editor of Carnegie Magazine.

* The research team included Y. Hu, Y. Wang and C-K Li of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, and Zhexi Luo of Carnegie Museum of Natural History. Research is supported by the National Science Foundation, National Geographic Society, and the Netting Fund of Carnegie Museum of Natural History. This first color illustration of the animal is by Mark Klingler of Carnegie Museum of Natural History.

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