Australian Discovery Provides Evidence of “Mob” Behaviour in Ancient Marsupials

By the late Eocene, Australia had become isolated from the rest of the world.  Here in splendid isolation, its primitive mammals, especially the marsupials were allowed to evolve unheeded without the frequent migration of other types of creature into Australian habitats.  In contrast to their South American cousins, which were usurped from many environmental niches by more modern placental mammals from North America, the Australian marsupials survived as the dominant group in this part of the world.

Scientists from the University of New South Wales have reported on the discovery of a cave which contains a substantial number of prehistoric marsupial fossils.  Their findings are reported in the scientific publication “The Journal of Vertebrate Palaeontology”, analysis of the fossils indicate that they are approximately 15 million years old (dating to the middle of the Miocene Epoch).

The cave is located in the northwest of Queensland, near to the famous Riversleigh site, an area famous for its prolific quantities of Cenozoic mammal fossils.  Riversleigh is a relatively remote area, with few townships and lots of ranches.  Evidence for Australian marsupials ranging from 23 million years ago to less than 20,000  years old have been found.  Fossils have been excavated from the Riversleigh area for more than 100 years but the real breakthrough which put this relatively obscure part of Australia firmly on the palaeontological map occurred in 1983 when a few weathered blocks of Riversleigh limestone revealed more than 30 new species of mammal.

For much of the Miocene and Pliocene Epochs this part of Australia was a lush lowland rain forest that was teeming with life.  One particular site at Riversleigh has been recorded as one of the most vertebrate fossil rich locations known.  In 1983, a two cubic metre block of limestone yielded 58 new mammal species.  The Riversleigh area is a limestone terrain which in the Miocene had a lot of surface water.  These were ideal conditions for bone preservation.

One of the Beautifully Preserved Nimbadon Fossil Skulls

Picture Credit: Associated Press

The cave system contains a number of complete prehistoric marsupials skeletons, their discovery has revealed some surprising similarities between these ancient creatures and extant species of modern-day kangaroos and koalas.

The cave has kept the fossils beautifully preserved.  The fossil find includes 26 skulls from an extinct, wombat-like marsupial called Nimbadon (N. lavarackorum).  The herbivorous Nimbadon was approximately the size of a sheep, but with giant claws.

Co-author of the study, University of New South Wales palaeontologist Mike Archer commented:

“It’s extraordinarily exciting for us.  It’s an extra insight into some of the strangest animals you could possibly imagine.”

The cave was found in the mid 1990s and has been extensively explored.  The scientists were amazed at how well preserved the fossils were and the large number of fossilised bones discovered at the site.  The number of skulls found together (twenty-six) suggests that these particular marsupials may have travelled as a group or to use an appropriate collective noun a “mob”.

The scientists remain unsure as to how all the animals ended up in the cave.  One theory put forward suggests that the mob of kangaroos fell into the cave through a whole in the roof that had been obscured by overgrowing vegetation.  These poor animals would have either been killed by the fall or become trapped and starved to death.

One of the Complete Fossilised Skeletons found at the Cave Site

Prehistoric Skeletons

Picture Credit: Associated Press

The fossil bones include the remains of joeys still in their mother’s pouches.  This gives the researchers an insight into how these ancient marsupials developed.  The skulls of the babies reveal that the bones at the front of the face developed quite quickly, this would have allowed the joey to suckle from its mother at an extremely young age.

Karen Black, the expedition’s leader, commented that the Nimbadon joeys developed in a similar way to extant kangaroos today, probably being born after a month’s gestation and crawling into their mother’s pouch for the remainder of their development.

The large claws indicate a potential arboreal existence, with Nimbadon climbing trees in a similar way to modern Koalas.  With large marsupial predators in the area, animals such as the carnivorous kangaroo Ekaltadeta and the marsupial “tiger” the Thylacine, being able to climb trees may have been a very effective strategy for defence.

Discussing this significant discovery, palaeontologist Liz Reed of Flinders University (South Australia) stated:

“To find a complete specimen like that and so many from an age range is quite unique.  It allows us to say something about the behaviour and growth and a whole bunch of things that we wouldn’t normally be able to do.”

Australia may be well known today for its unique fauna and flora but it seems back in the Miocene the unusual life in the Riversleigh area would have given today’s strange Australian natives a run for their money.
