Sinodelphys szalayi

Sinodelphys szalayi Illustration: Mark A. Klingler

The most primitive and oldest known relative of all marsupial mammals

A collaborative team of Chinese and American scientists including John Wible of Carnegie Museum of Natural History and former Carnegie curator Zhe-Xi Luo discovered Sinodelphys, a 125-million-year-old fossil animal that is the most primitive and oldest known relative of all marsupial mammals.

Found in China’s Liaoning Province, the nearly complete skeleton of Sinodelphys was surrounded by well-preserved impressions of fur and some soft tissues. About 6 inches (15 centimeters) in length, Sinodelphys weighed approximately one ounce (about 30 grams). Features that define the animal as a marsupial are in its wrist, ankle, and front teeth. The cusps of the back teeth indicate that Sinodelphys ate insects and worms. The shoulder, limbs, and feet suggest that it was quite capable of climbing. Sinodelphys lived in woods or shrubs on lakeshores or riverbanks.

Marsupial Mammals
All living mammals are divided into three groups primarily based on their reproductive systems: monotremes (those who lay eggs), placentals (those who give birth to live and more mature young), and marsupials (those who give birth to live, less mature young that they then nurse in their pouches).

Zhe-Xi Luo and John R. Wible holding Sinodelphys szalayi fossil. Photo: Mark A. Klingler

With more than 270 living species, including the kangaroo and koala, marsupials are the second most diverse mammal group. Today marsupials live primarily in Australia, New Zealand, New Guinea, and South America. Only one species of marsupial exists in North America—the Virginia opossum. However, in the Age of Dinosaurs (250–65 million years ago), fossil relatives of marsupials evolved in Asia and North America, before spreading to the rest of the world after the extinction of the dinosaurs.

Modern marsupials and their extinct relatives make up an important mammalian lineage known as metatherian, consisting of mammals that are more closely related to modern marsupial mammals (such as opossum, kangaroo, and koala) than to placentals (such as human, rodent, and whale). Modern marsupials are a significant part of the larger metatherian mammal group, and are the descendants of the extinct metatherians that lived during the Age of Dinosaurs, known as the Mesozoic Era.

Sinodelphys szalayi
Sinodelphys shared its world with the feathered theropod and giant sauropod dinosaurs. There were also pterosaurs, primitive birds, amphibians, reptiles, fish, insects, snails, and many diverse plants. Sinodelphys was one of several mammals in the Yixian biota, including the earliest-known placental relative, Eomaia, the symmetrodonts Zhangheotherium and Maotherium, the eutriconodonts Jeholodens and Repenomamus, and the multituberculate Sinobaatar.

Prior to the discovery of Sinodelphys, the previous earliest metatherian fossils were some isolated teeth from 110-million-year-old sediments of North America. The oldest jaw fragments of metatherians were from deposits of Uzbekistan 90 million years in age. The previous oldest skeletal fossil is from Mongolia and is 75 million years in age.

The name Sinodelphys szalayi is derived from [Sino]—Latin for China, [delphys]—Greek term used for basal marsupial species, and [szalayi]—in honor of Professor F.S. Szalay, a leading expert on mammalian skeletal evolution.

For more information

• Museum Press Release
• Available Press Images
• Science magazine online
• John R. Wible
• Mark A. Klingler