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.
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