Fossil backs up DNA clock

Posted on September 13, 2011 by


A fossil discovery made by a Pittsburgh-led team in China has pushed back the date of divergence between marsupial and placental mammals by millions of years, and has confirmed predictions based on the DNA of living mammals.

I came to Pittsburgh from Australia, where animals like koalas, kangaroos, Tasmanian devils, and hairy-nosed wombats live. Most Australian mammals have pouches that protect jelly-bean-sized babies until they are fully-grown and ready for the outside world. These mammals are called marsupials, after the Latin for pouch (Marsupium).

In North America however, all mammal mums (except Virginian Opossums) keep their young inside their body for much longer, feeding and protecting the developing embryo with a placenta. These animals, including humans, are called placentals.

Despite their fundamental differences, placentals and marsupials are still mammals – they have hair and feed their young with milk – and they descended from a common ancestor that was also a mammal. So when did the evolutionary history of these two kinds of mammals diverge? Or to put it the way the New York Times did, when did the ancestors of humans and kangaroos split?

Evidence from the genes of living mammals puts the date at somewhere around 143 to 178 million years ago, during the Jurassic period. But until recently, this date seemed to be much too old, given that the first placental fossils were much younger than this, dated to only 125 million years ago.

Three weeks ago, a team led by Carnegie Museum of Natural History paleontologist Zhe-Xi Luo published their description of a fossil mammal discovered in the Liaoning Province of China. Juramaia sinensis (‘Jurassic mother from China’) is similar to early placentals, but was found in rocks that are 160 million years old, corroborating the dates derived from the earlier DNA studies.

This ancient placental was small, weighing about 15-17g, with teeth that suggest it ate mostly insects, and limbs that suggest it climbed trees. The authors of the study speculate that since most other mammals of the period lived on the ground, that the divergence of placentals was accompanied by their adaptation to living in trees.

A Jurassic eutherian mammal and divergence of marsupials and placentals.

Zhe-Xi Luo, Chong-Xi Yuan, Qing-Jin Meng & Qiang Ji

Nature 476, 442–445; 25 August 2011

Posted in: Recent finds