A Eutherian Mammal from the Late Cretaceous of Kazakhstan.

Biologists studying modern mammals divide them into three groups, the egg-laying Monotremes, the pouched Marsupials, and the large-baby-producing Placentals. For palaeontologists the situation is not so clear, since it is usually impossible to tell how a fossil specimen reproduced. Palaeontologists consider all Mammals that fit within modern Placental groups, or which seem to have shared a common ancestor with them more recently than the common ancestor of all Placental groups, to be Placental Mammals, but have a wider term, the Eutheria, to describe the Placentals plus all Mammals more closely related to modern Placental Mammals than to the Marsupials. The Eutheria first appear in the fossil record in the Late Jurassic, 160 million years ago, and by the end of the Cretaceous appear to have become the dominant group of Mammals across modern Eurasia and North America. However for much of the Mesozoic they were overshadowed by the Multituberculates, an extinct group of Mammals more closely related to the Therians (Marsupials + Eutherians) than to the Monotremes. Exactly when the Eutherians took over as the dominant group is unclear, since Mammal fossils are fairly rare from the Mesozoic (this does not imply that Mammals were rare, simply that they were not following lifestyles likely to get them into the fossil record), and assemblages of fossil Mammals even more so.

In a paper published in the journal Acta Palaeontologica Polonica on 23 October 2012, Alexander Averianov of the Zoological Institute of the Russian Academy of Sciences, David Archibald of the Department of Biology at San Diego State University and Gareth Dyke of Ocean and Earth Sciences, National Oceanography Centre at the University of Southampton, describe a new Eutherian Mammal from the Late Cretaceous Bostobe Formation, exposed at the locality of Shakh Shakh in the northeast of the Aral Sea Region of Kazakhstan.

Map showing the location of the Shakh Shakh site in Kazakhstan. Averianov et al. (2012).

The new specimen is named Zhalmouzia bazhanovi, Zhalmouzia after the Zhalmouz Well, a nearby landmark, and bazhanovi after Valerian Semenovich Bazhanov, a Russian palaeontologist who worked on material from the region in the 1970s. The specimen is a fraction of a dentary (lower jaw) 15 mm in length, bearing two intact teeth.

The lower jaw of Zhalmouzia bazhanovi. Averianov et al. (2012).

See also A Eutherian Mammal from the Jurassic of China and Mammals on Sciency Thoughts YouTube.

Follow Sciency Thoughts on Facebook.

A Eutherian Mammal from the Jurassic of China.

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.

A reconstruction of Eomaia scansoria by Mark Klinger of the Carnegie Museum of Natural History.

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.

See also New 'oldest bird' found in China and Mammals on Sciency Thoughts YouTube.