Giant kangaroos died out when rainforests disappeared

Long before humans roamed the Australian continent, the land teemed with giants. Towering reptiles, thunderous birds, and hulking marsupials defined a wild and ancient world of sprawling rainforest ecosystems. Among these formidable creatures was Protemnodon, a now-extinct genus of giant kangaroo.

Despite its size – some individuals tipped the scales at 170 kilograms (375 pounds) – it wasn’t a long-range wanderer.

Instead, Protemnodon lived a curiously local life. New research reveals that these megafaunal marsupials stayed close to home, navigating only a small stretch of rainforest terrain. That fateful trait may have doomed them when the land began to dry.

A study led by Christopher Laurikainen Gaete at the University of Wollongong uncovers how restricted mobility, climate change, and the loss of rainforest habitats converged to erase these kangaroos from the map.

Giant kangaroos stayed close to rainforests

In the animal kingdom, large herbivores tend to move over vast areas to meet their dietary needs. Scientists use this trend to predict how extinct animals might have behaved.

Based on this logic, Protemnodon, with its substantial mass, should have had a home range exceeding 11 square kilometers. The research team, using a regression model anchored in data from living macropods, estimated a predicted home range of 11.6 ± 5.8 km² (4.5 ± 2.2 square miles).

But when they examined fossilized teeth from eight individual Protemnodon collected from the limestone caves of Mt Etna in central Queensland, they found something surprising. The chemical signatures etched into these ancient teeth told a different story.

Strontium isotopes, which reflect the geology of the area where an animal foraged, revealed the kangaroos fed within a narrow band of territory. Their enamel contained isotopic ratios matching only the local limestone of the Mount Alma Formation.

Fossil teeth prove they lived locally

The findings relied on a cutting-edge approach. The researchers conducted strontium isotope analysis using laser ablation mass spectrometry. This technique allowed them to trace where each individual likely lived and ate during enamel formation.

Surprisingly, the vast majority of samples matched strontium levels from the immediate area surrounding the Mt Etna caves.

Only one individual showed signs of a slightly wider movement pattern. This tooth, labeled WIGL8550, held isotope signatures more consistent with nearby formations like Alton Downs Basalt, suggesting a movement of about 1.5 km (0.9 miles) to the east or south.

However, the researchers can’t confirm whether this movement occurred in life or after death. One theory suggests scavengers, such as Thylacoleo carnifex or Sarcophilus, may have dragged the body to the cave after the animal died elsewhere.

In most other specimens, the teeth recorded a remarkably stable pattern. These kangaroos didn’t roam widely. They seemed to live and die within the same small patch of forest.

Rainforests gave kangaroos food and shelter

The study explored three main reasons why Protemnodon might have avoided long-distance travel. The first was environmental.

For over 200,000 years, the Mt Etna region maintained a stable rainforest ecosystem. This lush environment offered abundant food and shelter, meaning the kangaroos didn’t need to search far for survival.

Second, the species’ feeding habits likely played a role. While modern kangaroos like Macropus are grazers, feeding on grasses across open landscapes, Protemnodon probably browsed.

Its low-crowned teeth and long forelimbs suggest a diet of leaves, fruits, and soft vegetation, more easily found in dense forest environments. Browsers often have smaller foraging ranges compared to grazers of similar size, and this pattern seems to hold true here.

Third, locomotion appears to have been a limiting factor. Unlike their hopping cousins, Protemnodon may have moved on all fours. Its anatomy – including elongated forelimbs and robust humeri – points toward a quadrupedal gait.

This form of movement, although efficient in forests, restricts the speed and distance an animal can cover. Without the ability to hop long distances, these kangaroos were naturally more sedentary within rainforests.

Rainforests disappear, so do the giants

These limited movement patterns became a fatal flaw when the climate began to shift. Around 280,000 years ago, the once-stable rainforest began to decline. Increasing aridity transformed the landscape into dry woodlands. The thick vegetation Protemnodon relied upon began to vanish.

Because these kangaroos were so tightly tied to their habitat, they couldn’t adjust. They couldn’t migrate to find new rainforests. The kangaroos lacked both the physical capability and behavioral tendency to move long distances. In effect, they became stranded in an ecological trap. The resources they needed evaporated around them.

New dating methods, including thermally transferred optically stimulated luminescence (TT-OSL) and uranium-thorium dating, confirmed that these individuals lived between 280,000 and 330,000 years ago. Their extinction coincided with the local collapse of rainforest ecosystems.

Over time, the rainforest fauna of Mt Etna gave way to animals adapted to dry, open conditions.

New ideas about extinct species

The implications of this research stretch beyond a single species, challenging the assumption that body size alone predicts home range in extinct animals.

Instead, the study suggests that environment, diet, and movement style must also be considered. The idea that Australia’s megafauna had flexible ranges similar to African elephants or North American bison may not apply across the board.

“Using data from modern kangaroos, we predicted these giant extinct kangaroos would have much larger home ranges. We were astounded to find that they didn’t move far at all, with ranges mirroring smaller modern kangaroo species,” said Laurikainen Gaete.

“These new isotopic techniques have blown our field wide open. Imagine ancient GPS trackers – we can use the fossils to track individuals and know where they moved, what they ate, who they lived with and how they died. It’s Palaeo Big Brother,” added Dr. Scott Hocknull.

The fate of giant kangaroos

The Protemnodon case reveals how extinction can creep up slowly. The kangaroos didn’t die out in a dramatic, singular event – the loss of rainforests sealed their fate.

Rather, they were slowly cornered by environmental changes they couldn’t outrun. Their highly specialized biology, once perfectly suited to rainforest life, became a burden.

These findings offer a cautionary tale. Species adapted to narrow niches are often the first to go when habitats shrink. For scientists studying extinction, understanding those ancient patterns helps anticipate what modern species might face as climate shifts intensify.

The study is published in the journal PLOS One.

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