By Matt Kaplan of Nature magazine

Modern mammals often live in groups, but most marsupials are solitary. With no fossil evidence to suggest that the animals have ever behaved otherwise, paleontologists have long assumed that marsupials have been loners throughout their evolutionary history. This notion is now being overturned by the analysis of a fossil site containing many marsupials that seem to have been living together.

The site, in the Tiupampa locality of Bolivia, contains 35 specimens of Pucadelphys andinus, a primitive opossum from the early Palaeocene Epoch (64 million years ago).

Teeth are usually all that paleontologists can find of ancient mammals, because dentition is built to endure punishment, and fossilizes well. However, 22 of the 35 specimens at the Bolivian site consist of teeth, skulls and body skeletons in near-perfect shape.

Sandrine Ladevèze, a paleontologist at the Royal Belgian Institute of Natural Sciences in Brussels, and her colleagues publish an analysis of the specimens May 8 in Nature.

"To find a sample of this quality is almost unheard of," says Richard Cifelli, a paleontologist at the University of Oklahoma in Norman.

Full house

But it is not the condition, but the placement of the specimens at the fossil site that intrigued Ladevèze.

She and her team report that the marsupial specimens are fossilized in two clusters, neither covering an area larger than one square meter, with 12 nestled together at one location and 23 at another, just 3 meters away.

Natural forces such as the flow of river water could have brought these specimens together after they died, but Ladevèze considers this unlikely, because skeletons that are moved after death are rarely found in such good condition. Instead, Ladevèze proposes that the marsupials were living together in two burrows when some sort of disaster, such as a flash flood, buried them alive.

"Sociality of this sort has been seen in fossils of other groups before [such as dinosaurs], but this is the earliest example of social gathering in marsupials that we have ever seen," says Zhe-Xi Luo, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania.

The composition of the social groups inside the two burrows is very different from that seen in opossums today. Only a very few modern opossums will live in one territorial area, yet the morphology of the Bolivian specimens suggests that there were at least 6 adult males--identified by their large canines and long skulls--12 adult females, 4 sub-adults and one immature individual present at the site.

"That such a mix of marsupials lived together hints that their modern solitary lifestyle was not present 64 million years ago," says Ladevèze.

"The team's ability to identify these fossils as males, females and juveniles of the same species is just fabulous. Had I found these fossils as isolated specimens, I shudder to think how many species I would have classified them as," says Cifelli.

Dead end

However, Marcelo Sánchez, a paleontologist at the University of Zurich in Switzerland, questions whether the Bolivian find is indicative of general marsupial behavior during the Palaeocene.

"It is possible that the gregariousness seen in these early marsupials was an evolutionary 'experiment' on a line which is not ancestral to living marsupials, and solitary behavior was the ancient condition," he says.

Regardless of whether sociality was rampant among other species, it clearly applied to these Bolivian opossums. Yet what drove this urge to live in groups is a mystery.

Ladevèze speculates that a need for warmth might have drawn the animals together. Indeed, some typically solitary marsupials in South Australia snuggle together in groups of up to 40 to stay warm when temperatures fall in winter. But the climate of ancient Bolivia was mainly tropical, making this unlikely.

Christian de Muizon, a paleontologist at the Natural History Museum in Paris and a co-author of the paper, has another interpretation. "Marsupials were at this time radiating across South America. Gregarious behavior may have made settlement in new regions and in new environments easier, favoring reproduction and parental care. Once populations became established, the gregariousness no longer proved as useful and was lost over time. Of course, we need many more fossil finds to prove this correct."

This story was originally published May 8, 2011, in Nature.