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Complete skeleton provides new insights of the biological evolution of the earliest mammals
Famous Chinese feathered dinosaur site yields another important discovery

Pittsburgh, Pennsylvania USA. . . An unusually well-preserved skeleton of a newly discovered mouse-sized, insect-eating triconodont mammal named Jeholodens jenkinsi is yielding very important insights into the evolution of the earliest mammals. These insights are reported in a paper co-written by Dr. Luo Zhexi, assistant curator in the section of Vertebrate Paleontology at Carnegie Museum of Natural History, Pittsburgh, with two of his Chinese colleagues, and are featured in the latest issue of the prestigious British journal Nature.

The new specimen of Jeholodens is the earliest known complete skeleton of any mammal and an extremely rare find for Mesozoic mammals. This unprecedented find has shed much insight into the biological adaptations of these otherwise enigmatic triconodont mammals.

Jeholodens lived during the time of the dinosaurs some 145 million years ago and is one of the last common ancestors of modern mammals. The new mammal was discovered in late 1997 from the famous feathered dinosaur site in Liaoning Province, China.

"Before this discovery, it was not known how early mammals walked," said Dr. Luo. "Now we know their front legs were capable of almost erect gaits."

The earliest mammals appeared on Earth together with the earliest dinosaurs about 220 million years ago. The Mesozoic mammals, although not abundant, were quite diverse. One of several lineages of early mammals is the triconodonts. They have a long and diverse fossil record that can be traced back 220 million years, but they went extinct in the late Cretaceous and have no living descendants.

Like other triconodont mammals, Jeholodens has three-cusped teeth that were specialized for eating insects. Despite their significant position in early mammalian evolution, most fossils of triconodonts found to date are merely isolated teeth and fragmentary bones. Although paleontologists have determined that these earliest mammals were insect eaters by their teeth, many other aspects of their biology have been poorly known.

All living therian mammals have a mobile shoulder girdle that allows the front legs or arms to move in many dexterous ways. The ability to rotate the shoulder joint (known as the "rotator's cuff") is crucial to the locomotion of mammals and many other aspects of their ways of life. Without a mobile shoulder, a squirrel would not be able to hold up an acorn, a bat would not be able to fly, a monkey would not be able to climb and a baseball pitcher would not be able to throw a ball. Once again, because very few skeletons of early mammals have been found, scientists have only vague ideas about their lifestyles.

The discovery and comparative studies of the new mammal have changed much of that. Jeholodens has a very advanced shoulder blade (scapula) and collarbone (clavicle). The flexible connections of these bones indicate that its forelimb was capable of an almost erect gait. It walked like many therian mammals, rather than with the sprawling gait of living monotremes and reptiles. Living therian mammals, such as opossum and monkeys, have an erect posture related to their more active style of walking and running. In this regard, Jeholodens is clearly more similar to the living therian mammals than to living monotremes.

In contrast to the advanced shoulder and forelimb, Jeholodens has a very primitive pelvis, a sprawling hind limb, and a splayed hind foot. Many of its hind limb features are more primitive than those of monotremes, and comparable to those of reptiles. The co-existence of advanced features with primitive features in the same organism is known as mosaic evolution. It suggests that different parts of organisms evolved at different evolutionary rates. This enigmatic combination of advanced forelimbs and shoulder and the primitive pelvis and hind limbs in the newly discovered Jeholodens, shows a more complex early evolution of the mammalian skeleton than previously imagined.

The feet of Jeholodens suggests that it lived on the ground in a terrestrial niche, rather than on/in trees, indicating that mammals probably originated on the ground. The fossil is preserved with a rich fauna, including fish, amphibians, small dinosaurs, birds and insects. The shale in which the fossil is preserved indicates it was buried in a lake.

The famous feathered dinosaur site in the western Liaoning Province has not only produced the feathered dinosaurs and the world's most primitive birds, it has also produced the world's most important early mammals. Dr. Luo and his Chinese collaborators are continuing their work on the spectacular fossils from this world-famous site.

The Nature article was co-written by Dr. Luo and his colleagues Dr. Ji Qiang and Dr. Ji Shu-an of the National Geological Museum of China.

Dr. Luo is considered one of the world's foremost specialists in Mesozoic animals. In 1997, a collaboration with scientists from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing yielded what was at that time the most complete mammal ever found. A small symmetrodont mammal, which lived 120 to 140 million years ago, filled an important gap in the understanding of how humans evolved from ancient mammals. This discovery was also featured in Nature. He has also published numerous articles in many other prestigious scientific publications and journals.

"This is Dr. Luo's second important discovery from this location," said Dr. Jay Apt, director of Carnegie Museum of Natural History. "His past and continuing research strengthens the museum's international reputation as a leading research center for the evolution of early mammals. It also demonstrates the museum's continued commitment to scientific research."

Dr. Luo has been at Carnegie Museum of Natural History since 1996.

• Comments on the Importance of this Discovery by Dr. Timothy Rowe
• Images to Accompany this Article