Modern mammals are divided into three groups by biologists: the monotremes which lay eggs, the marsupials which give birth to underdeveloped live young then raise these young in pouches, and the placental mammals which give birth to large, well developed young. Palaeontologists rarely get the opportunity to observe animals giving birth, so they use a different system of classification, based largely on dentition. This is complex, and includes many extinct groups, but the relevant bit here is that modern placental mammals are included in a group called the Eutheria, and modern marsupials are included in a group called the Metatheria.
It is not possible to determine how a fossil species carried its young, but palaeontologists are able to say that they believe any specimen they label a eutherian to be more closely related to modern placental mammals than it is to marsupials (the reverse is not true, since the eutherians are thought to have evolved from metatherians).
DNA evidence suggests that the eutherian and metatherian lines split around 160 million years ago, in the Middle-to-Late Jurassic, although fossil evidence of this has proved hard to find, since Mesozoic mammals were largely small, and tended to live in trees, giving them poor preservational potential.
Up until this year the oldest known eutherian mammal was Eomaia scansoria, a 10 cm long shrew-like mammal from the Early Cretaceous, described by a team lead by Qiang Ji of the Chinese Academy of Geological Sciences in a paper in the 15 March 2002 edition of the journal Nature.
The 25 August edition of Nature contained a paper by a team lead by Zhe-Xi Luo of the Carnegie Museum, which describes a new eutherian mammal, Juramia sinensis, from the Middle Jurassic of Liaoning in northeastern China. At 160 million years old this fossil is pretty much at the predicted origin of the group. The specimen is about 40 mm long, which includes the skull and torso but no tail. Preservation of the specimen is excellent, showing the dentition clearly, and traces of fur around the body.
Like most other Mesozoic mammals Juramaia appears to have been arboreal (tree dwelling) and omnivorous. It was found in the Tiaojishan Formation which is known largely for plant fossils, but which has produced a number of other well preserved vertebrate fossils, including small dinosaurs and pterosaurs. It appears to have been a subtropical woodland environment with a rich, volcanic soil. The formation is contains a number of pyroclastic (volcanic) deposits which allow for good isotopic dating. The earliest levels of the Tiaojisha appear to be about 160 million years old, with the youngest being no more recent than 153 million years.
Reconstruction of Juramaia sinensis, again by Mark Klinger.
The fossil has been deposited at the Beijing Museum of Natural History.