Ancient possums were a social bunch

Monday, 9 May 2011

by Catherine de Lange

Cosmos Online

Two intermingled and fragmentary skeletons of Pucadelphys andinus. Click to next image to view a reconstitution.

Credit: Lemzaouda/MNHN

LONDON: A rare fossil discovery made by an international team of scientists suggests for the first time that some early mammals - the ancient relatives of modern marsupials - lived in social groups.
The finding, published in the current issue of Nature, challenges the current understanding of mammalian evolution, and reveals the intricate details of how these opossum-like marsupials lived.
"This study is very exciting because we discovered 35 almost complete skeletons and skulls of marsupials, which is very rare. In the history of mammals we have very few clues as to how they lived because we only have access to a few fossil specimens," said lead author Sandrine Ladevèze, from the Royal Belgian Institute of Natural Sciences in Brussels, Belgium.
Modern marsupials weren't always loners
Living marsupials, with the exception of larger herbivores like kangaroos, live highly solitary lives. Until now, it was thought that this is how these animals evolved, but the evidence was based on a very limited fossil record, such as isolated fragments of teeth or crushed skulls.
This new, bumper find provides an unprecedented amount of fossil evidence for scientists to work with. What's more, it is the first time such a large number of specimens have been found all together, suggesting that a whole population was wiped out by a single catastrophic event around 65 million years ago - probably a land slide.
The mass grave included a mixture of males, females and young, all in close proximity, and their bodies were intermingled. This historical snapshot led Ladevèze and her team to think that these ancient marsupials lived in large social groups - unlike modern marsupials such as opossums.
Instead of being an ancient trait, the solitary lifestyle we see in marsupials today must have evolved much more recently, the study suggests.
Females not territorial
Ladevèze and her team studied the skulls and teeth of these ancient marsupials, called Pucadelphys andinus, to determine their sex. They concluded that there were 20 adult females and 6 males, as well as young adults and one infant.
The fact that the males and females were living together and the fossils were intermingled means that the females were not territorial, unlike the marsupials we are familiar with today. The skulls of the males were also longer and wider than those belonging to females, and had larger canine teeth.
These differences normally indicate that there would have been strong competition between males, and that each male would have had more than one mate.