New research led by Flinders University argues thick tooth enamel helped kangaroos chart an unconventional evolution story, compared to the animals of other continents.
A 50-million-year natural 'experiment' among Australia's marsupials suggests that the outcomes of evolution are far from certain.
Based on a detailed analysis of fossil teeth, a new study in Science led by palaeontologists at Flinders University shows that kangaroos – the continent's most diverse and common herbivores – evolved an unusual solution to the challenge of feeding on grasses with molars reinforced by thickened enamel.
Lead author Dr Aidan Couzens, a research associate at Flinders University, says that the evolutionary history of kangaroos led them to adapt in a very different way to the rise of grasslands compared with hoofed mammals like deer and horses which dominate most other continents.
"Feeding on grasses wears down teeth more rapidly than other kinds of plants do, because they're often covered in dust and their blades contain thousands of tiny silica particles. This is a big problem for herbivores because missing or damaged teeth means death," he says.
"Based on their diet, we might expect grazing kangaroos to have tall teeth with lots of complex crests, as do many grazing hoofed mammals.
"But they don't. In fact, the teeth and jaws of kangaroos are like those of herbivores which feed on softer leaves – manatees, monkeys, lemurs, and some extinct relatives of elephants – or animals that you would think have little in common with kangaroos.
"Most of these groups, except for kangaroos, declined in diversity over time or went extinct."
The Flinders researchers ask: Why are kangaroos the exception?
They solved this riddle by peering beneath tooth surfaces with X-ray scanning, which allowed them to measure the thickness of tooth enamel.
It revealed that kangaroos had a hidden trick; a conveyer belt of cheek teeth with thick enamel.
"Because kangaroos slice their food up vertically, the most straightforward way for them to adapt to grazing was by making their enamel thicker. This is unlike hoofed mammals, which chew from side-to-side," says Dr Couzens, from the College of Science and Engineering.
Things got weirder when the researchers compared kangaroo enamel thickness with other mammals.
Coauthor Professor Gavin Prideaux says: "We found that grazing kangaroos invest in thick enamel about as much as famously thickly-enamelled human ancestors like Paranthropus, the so-called 'Nutcracker man'.
"It's been debated whether human ancestors had thick enamel to eat grasses or sedges, or to protect teeth from breaking when chewing nuts. Our results support the idea that it was dietary abrasion driving their evolution."
Although thick enamel allowed kangaroos to become efficient grazers, it probably wasn't the only factor.
Early on, several groups of marsupial related to wombats, and with teeth more like those of hoofed mammals, dominated herbivore niches in Australia. These groups seemed well suited to dominating the grazing niche as well, but with the exception of wombats, they died out before grasslands arrived.
It's unknown what caused their decline, but competition with arid-adapted kangaroos might have been a factor.
Whatever its cause, their extinction left the playing field open to kangaroos, who took full advantage.
"In some ways the evolutionary history of herbivorous mammals in Australia is upside down because the vertical chewing herbivores (i.e., kangaroos) win out whereas the reverse occurred on the northern continents," adds Dr Couzens.
The different outcome of herbivore evolution in Australia versus other continents suggests that evolution is not necessarily predictable.
"Just because you have the right adaptations doesn't guarantee success. Other things need to go in your favour, including a certain degree of luck," he says.
"It should make us think twice about the certainty of our own evolution."
The article, 'Contingent evolution of thick enamel by kangaroos to resist dietary abrasion' (2026) by Aidan MC Couzens, Benedict King (Max Planck Institute for Evolutionary Anthropology, Germany) and Gavin J Prideaux and has been published in journal Science DOI: https://doi.org/10.1126/science.aeb2502 .
Photos and videos at the link: https://drive.google.com/drive/folders/1FY2LX8Ty7SOamsSiU_QVp7mEqmHzWCsm?usp=sharing
Funding: This work was supported by Deutscher Akademischer Austauschdienst and Betty Mayne Memorial fund from the Linnean Society of New South Wales to Dr Couzens and the Australian Research Council (grants DP110100726, FT130101728, DP190103636 and DP210100508 to Prof Prideaux.).
