A team of researchers led by palaeontologist Guillermo Rougier from the University of Louisville (U of L), Kentucky, have found spectacular remains of a creature called Deltatheridium.

Guillermo Rougier's find provides missing information

The animal was first identified in the 1920s, but its precise family history has always been sketchy because of the poor quality of previous fossil discoveries - the best were some poorly preserved teeth found in Asia.

Now, new remains dug up in Ukhaa Tolgod, Mongolia, should give us a much clearer picture of Deltatheridium and its relatives.

Well preserved

The new find contained the remnants of several mammals in various growth and development stages, including one in the process of losing its baby teeth. Rougier and his team believe the fossils are 80 million years old which was in the Cretaceous period, when dinosaurs were reaching their peak.

A partial skull and jaws of Deltatheridium

The specimens are so well preserved that a jaw was found with fragments of a skull that the researchers think may be the remnants of the animal's last meal.

The discovery has allowed the ancient opossum-like Deltatheridium to be classified firmly as a marsupial. It also supports the theory that the mammal group, now most common in Australia, originated in Asia.

Marsupials rear their young in pouches. They are one of three mammalian groups. The other two are monotremes - egg-laying animals such as the duck-billed platypus and the anteater - and placentals, such as humans.

Teeth clues

The Deltatheridium has been confirmed as a marsupial because of information gleaned from its teeth. One of the fossils is of a young animal which died before its adult teeth had completely emerged, revealing the sequence in which they grew. Marsupials do not replace as many teeth as other mammals.

"Only the last three molars are replaced," said Michael Novacek from the American Museum of Natural History in New York, and a co-author on a paper about the find that appears in the science journal Nature.

"It's very specialised and that is the first clear evidence we have of this marsupial-like replacement in these very weird and ancient creatures."

Guillermo Rougier, an assistant professor at U of L, said teeth give especially reliable clues because their enamel is the body's hardest substance; therefore, they leave a good fossil record which registers important changes in the evolution of mammals.