SYDNEY — Scientists have discovered a cave filled with 15-million-year-old fossils of prehistoric marsupials in the Outback, a rare find that has revealed some surprising similarities between the creatures and modern-day kangaroos and koalas.

Researchers have unearthed a treasure trove of beautifully preserved fossils from the cave, including 26 skulls from an extinct, wombat-like marsupial called Nimbadon lavarackorum, an odd sheep-sized creature with giant claws. The findings were described in an article published this week in the Journal of Vertebrate Paleontology.

"It's extraordinarily exciting for us," said University of New South Wales paleontologist Mike Archer, co-author of the article. "It's given us a window into the past of Australia that we simply didn't even have a pigeonhole into before. It's an extra insight into some of the strangest animals you could possibly imagine."

Researchers have been digging at the site, in the Riversleigh World Heritage fossil field in northwest Queensland state, since 1990 and discovered the first of the Nimbadon skulls in 1993. The scientists were amazed at how well preserved the fossils were — and by how many they found.

Discovering such a large cluster suggests the animals may have traveled in mobs — or herds — like modern-day kangaroos, said paleontologist Karen Black, who led the research team.

How the animals all ended up there is a mystery. One theory is that they accidentally plunged into the cave through an opening obscured by vegetation and either died from the fall, or became trapped and later perished.

The Nimbadon skulls included those of babies still in their mothers' pouches, allowing the researchers to study how the animals developed. The skulls revealed that bones at the front of the face developed quite quickly, which would have allowed the baby to suckle from its mother at an extremely young age.

Those findings suggest the Nimbadon babies developed very similarly to how kangaroos develop today — likely being born after a month's gestation and crawling into their mother's pouch for the rest of development, Black said.

The Nimbadon also may have something in common with another marsupial. The fossils revealed the creatures had large claws, which may have been used to climb trees — as koalas do, Black said.

The discovery of the fossils is very significant, said paleontologist Liz Reed of Flinders University in South Australia.

"To find a complete specimen like that and so many from an age range is quite unique," said Reed, who was not affiliated with the study. "It allows us to say something about behavior and growth and a whole bunch of things that we wouldn't normally be able to do."