| Vertebrate Paleontology
Unearthing the Roots of the Family Tree
Among the most rare and challenging discoveries for any paleontologist are the early mammals that arose in the shadows of the great beasts of the Mesozoic. Few fossil mammals have survived the ravages of geologic history, and those that have survived are supremely difficult to find and collect. Most of what we know of the earliest mammals comes from a few fragmentary teeth and jaws, which, because of their durable mineral composition, tend to remain intact when other bones have crumbled. Despite these challenges, Zhe-Xi Luo, PhD, is forging ahead with some of the most important research in the field. Luo has been involved in two of the most significant discoveries in mammalian evolution, and his research on Jeholodens jenkinsi, published in the March 25, 1999, issue of Nature is helping to piece together a dark era in evolutionary history, shedding new light on the origin and interrelationships of early mammals.
The first mammals appeared on Earth together with the earliest dinosaurs about 220 million years ago, during the Mesozoic Era. These early mammals were not abundant, but they were quite diverse, and a number of distinct lineages appear early in their history. The animals from one of these lineages, the triconodonts, are distinguished by the three main projections on their back teeth. The triconodonts have a longand diverse fossil record that can be traced back 220 million years but ends abruptly in the late Cretaceous. Despite their significant position in early mammalian evolution, most fossils of triconodonts found to date are merely isolated teeth and fragmentary bones. Although the teeth of these early mammals indicate to paleontologists that these creatures were insect eaters, many other aspects of their biology and behavior have not been well understood. Filling in the Gaps Because very few skeletons of early mammals have been found, scientists have had only vague ideas about the lifestyle of these animals. The completeness of the Jeholodens jenkinsi skeleton, however, has allowed Luo and his collaborators to reconstruct some of the complexities of mammalian evolution. Studies of Jeholodens jenkinsi have shown that this early mammal had a very advanced These features reveal a more complex early evolution of the mammalian skeleton than previously imagined. The enigmatic combination of advanced features with primitive features in Jeholodens jenkinsi suggests that different parts of organisms evolve at different evolutionary rates. Beyond Liaoning
In 1997, Luo's collaboration with scientists from Beijing yielded what was at that time the most complete symmetrodont mammal ever found. Zhangheotherium quinquecuspidens, which lived 125 million years ago, filled an important gap in the understanding of how later mammals, including humans, evolved from their ancient ancestors. Considered one of the world's foremost specialists in Mesozoic mammals, Luo is continuing his research in Liaoning which will undoubtedly lead to more important discoveries and to an increased understanding of the roots of the mammalian family tree. Graphics, Illustrations, and Photos by Mark A. Klingler More Information |
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An exceptionally well-preserved 125-million-year-old fossil mammal,
described by former Carnegie Museum of Natural History curator Zhe-Xi
Luo and his collaborator, Professor Ji Qiang of the Chinese Academy of
Geological Sciences, offers new insight on the origin and evolution of
mammals. 
Early Mammals
The discovery of Jeholodens by Ji Qiang of the Chinese
Academy of Geological Sciences and Luo's comparative studies have
offered new insight into the lives of these early mammals. 
shoulder blade and collarbone. The flexible connections of these bones indicate that its forelimb was capable of an almost erect gait. It walked with an erect posture indicative of an active lifestyle, rather than with the sprawling gait of primitive mammals and reptiles.
Yet, in contrast to the advanced shoulder and forelimb, Jeholodens had avery primitive pelvis, sprawling hind limbs, and splayed hind feet. Many of its hind limb features are comparable to those of reptiles. 
Jeholodens jenkinsi was unearthed in 1994 near
the village of Si-He-Tun, northeast of Chao-Yang in China's Liaoning
Province. This site has gotten much attention recently because of a
number of spectacular fossil discoveries. In addition to the world's
most important early mammals, this site has produced the world's most
primitive birds and the famous Chinese feathered dinosaurs that stunned
the scientific community last year by offering concrete proof of the
evolutionary connections between dinosaurs and birds.