Fossils from hilltop caves tell tales of a flooded land bridge

by Matthew McDowell | Jun 30, 2024 | Once was a grassy plain

As a palaeontologist, I’m fascinated by the animals, especially the small mammals, that lived on the land bridge we call the Bassian Plain and how they adapted or disappeared as the land bridge transformed back into a shallow sea, leaving a scattering of islands in its wake.

One remarkable source of information for the impact of this change on native animals can be found in the regurgitated meals of ancient owls.

Disc-faced owls, like this Eastern Barn Owl (Tyto javanica) frequently roost in caves. Their regurgitated pellets, which build up over time, can become a valuable window to the past. Photograph by Dave Curtis. CC BY-NC-ND 2.0.

About the Author

Matthew McDowell

Dr Matt McDowell is a palaeontologist at Monash University and the University of Tasmania. Matt has been interested in natural history all his life but has always had a particular soft spot for fossils. He specialises in the study of small mammal remains, usually concentrated in ancient owl roost deposits in caves.

With thanks to

Monash University, University of Tasmania and Australian Research Council Centre of Excellence for Australian Biodiversity and Heritage (CABAH).

During the last ice age, as ocean water became locked up as snow and ice at the poles, sea levels fell revealing the continental shelf between Victoria and Lutruwita/Tasmania.

The exposed land became the Bassian Plain. It was a cold, windswept grassland dotted with shrubs, heath and mallee trees that provided little shelter from the wind. This might sound like an awful place to us, but Australia’s cold tolerant native mammals would disagree.

The broad-toothed rat (Mastacomys fuscus), which still lives in Lutruwita/Tasmania and Victoria, lived on the land bridge. This endearing chubby-cheeked mammal thrives in cold conditions and can live anywhere between sea level and alpine regions.

Possums, pademelons, wallabies, kangaroos, even quolls, are also quite happy in the cold. They have adapted by growing thicker fur. This is why animal skins from chilly Lutruwita/Tasmania were highly prized by nineteenth century European fur traders, because their fur was much thicker, and therefore commanded a higher price than mainland varieties. These fluffy creatures were also at home on the Bassian Plain.

In fact, many cold-adapted mammals which today live in Lutruwita/Tasmania lived on the Bassian Plain from around 40,000 to 14,000 years ago. Grey kangaroos, emus, red-necked wallabies, and wombats roamed the lowlands. Small marsupials of the plain included southern brown bandicoots (Isoodon obsulus) and eastern barred bandicoots (Perameles gunnii), rufous-bellied pademelon (Thylogale billardierii) – a wallaby-like night-dweller with a distinctive coughing call, mouse-sized carnivores swamp antechinus (Antechinus minimus) and dusky antechinus (Antechinus swainsonii), the spotty, cat-sized, grassy woodland lover the eastern quoll (Dasyurus viverrinus), and the long-nosed potoroo (Potorous tridactylus) – a cute little hopper which has, as its name suggests, a long nose. There was also a variety of small rodents including the long-tailed mouse (Pseudomys higginsi), the New Holland mouse (Pseudomys novaehollandiae), the swamp rat (Rattus lutreolus), and our friend the broad-toothed rat, plus a range of birds, snakes and lizards.

How do we know which animals lived on the Bassian Plain? It’s a difficult place to study because about 14,000 years ago global warming began to melt polar ice causing sea level to rise and re-flood the plain. Consequently, much of the evidence now lies underwater.

Fortunately, there are areas of land we can still research. As the Southern Ocean slowly reclaimed the plain, the peaks of more than 150 hilltops and promontories became small islands. A handful of these islands harbour caves, rock shelters and small lakes, which hold past environmental records. These special places preserve fossils, lake sediments, and evidence from the Old People and avian hunters.

Understanding deep past environments and the animal and plant life on the plain from these records requires different teams of experts to work together. Separate branches of science add different pieces to the puzzle. And because we have only a few clues from very few places, many things remain a mystery.

Figuring out the ancient plant life involves studying pollen extracted from lake sediments by driving a plastic pipe deep into lake beds. When the tube is pulled out it reveals a long cylindrical sample of the lake bed’s sediment, including pollen that records thousands of years of environmental change. By extracting sediment cores from these few remaining lakes, scientists have been able to reconstruct the vegetation – the cold windswept grasslands – that grew here and observe how it changed as the climate shifted.

Some scientists study the archaeological deposits left in caves and rock shelters by people who lived on the land bridge. Archaeological artifacts provide evidence of the Old People’s daily life which build up over time during day-to-day activities such as preparing and cooking food, tidying up and making tools. Among these artifacts we find animal remains that indicate they hunted a range of diurnal (daytime) prey, ranging from medium to large animals such as kangaroos, wallabies, and wombats to small animals such as bandicoots, reptiles, birds and fish.

The broad-toothed rat (Mastacomys fuscus), which doesn’t mind the cold, lived on the ice age Bassian Plain. Photograph by David Paul. ©Museums Victoria. CC BY 4.0.

Many cold-adapted mammals which live in Lutruwita/Tasmania today lived on the Bassian Plain, such as the rufous-bellied pademelon (Thylogale billardierii). Photograph by Leone Moffat. CC BY-NC-ND 2.0.

Eastern quoll (Dasyurus viverrinus). Photograph by Charles Sharp. CC BY-SA 4.0.

After sea levels rose, the peaks of more than 150 hilltops and promontories became small islands, some of which hold evidence from the time of the Bassian Plain. View from Tayaritja/the Furneaux Group of Islands in the eastern Bass Strait. Photograph by Simon Haberle. CC BY-ND-NC 4.0.

The long-tailed mouse (Pseudomys higginsi) was ideal prey for owls of the Bassian Plain. Photograph by Bruce Deagle. CC BY 2.0.

The New Holland mouse (Pseudomys novaehollandiae) appears to have been the first species driven to local extinction after sea level rise. Photograph by Doug Beckers. CC BY-SA 2.0.

My research adds a different piece to the puzzle. I study the animal remains left by owls. In contrast to people, owls are nocturnal (nighttime) predators that eat small mammals such as rodents, small marsupials (including quolls), birds, lizards and frogs. Two types of native owls hunt in Australia, and both lived on the Bassian Plain. Disc-faced owls (Tyto species) frequently roost in caves, whereas hawk-faced owls (Ninox species) prefer to roost in trees. Both rest during the day and spend the night hunting.

Like all birds of prey, owls do not have a gizzard – a muscular fore-stomach – that many birds use to grind up their food, frequently swallowing stones to help in the process. Instead, an owl’s prey goes directly into its stomach where it gets digested by stomach acid. Before going out to hunt each night, the indigestible parts of their prey – the bones and fur – are pressed into a pellet, covered in mucous and vomited up to make room for more. The cave roosts of disc-faced owls become littered with pellets. Over time the fur breaks down leaving the bones of the owls’ prey behind.

Even if an owl only eats one or two animals a night, the bones of their prey build up quickly. In a good roost, this process can continue over many generations of owls. These bones become a record of the environment the owls hunted in. Owls aren’t fussy, they eat whatever’s available that they can safely subdue. As a result, if something in the environment changes and a prey species can longer thrive in that place, it won’t be available for owls to eat. This quickly shows up as a shift in the owl’s diet. These piles of very old owl vomit provide valuable windows into the past that we can use to learn about ancient environmental change.

The combined fossil evidence shows that as sea-level rose the animals that had once lived on the Bassian Plain found themselves in a changed environment competing for diminishing resources. Only the most adaptable – and maybe the luckiest – species survived. Some, like the wombats and the emus, coped by becoming smaller, a process known as dwarfing. Other animals persisted for a time but eventually disappeared.

The New Holland mouse, which lives in recently burnt heath and is today endangered, appears to have been the first species driven to local extinction (where animals go extinct in one area but still live on elsewhere) following sea-level rise. Other mammals followed, but the timing of other local extinctions remains unclear. It appears several species may have become locally extinct at the same time, probably in response to global warming, vegetation change, island isolation and overcrowding.

By the time whalers, sealers and fur traders arrived in the Bass Strait islands to exploit Lutruwita/Tasmania’s wildlife on a never-before-seen scale, only wombats, wallabies, pademelons, potoroos, southern brown bandicoots, water rats and swamp rats remained.

The rest – the kangaroos, dunnarts, quolls, antechinus, eastern barred bandicoots, the mice and the broad-toothed rat – had vanished, leaving behind only their bones.