Ancient roo evolution aided by their uniquely thick tooth enamel
Australia’s nickname as the land Down Under may be closer to the truth than anyone knew, as new research suggests that the ancient kangaroo’s story of survival was a surprising mouthful
READING LEVEL: ORANGE
A 50-million-year natural “experiment” among Australia’s marsupials* suggests that the outcomes of evolution are far from certain.
New research led by Flinders University suggests an unlikely source helped kangaroos hop down an unconventional evolutionary* path compared to animals on other continents.
Based on a detailed analysis of fossil teeth, a new study in Science led by palaeontologists* at Flinders University shows that kangaroos – Australia’s most diverse and common herbivores* – overcame the challenge of grass feeding thanks to molars* reinforced by thickened tooth enamel*.
Flinders University College of Science and Engineering research associate and lead author Dr Aidan Couzens said that the evolutionary history of kangaroos led them to adapt very differently when grasslands arrived, compared with hoofed mammals like deer and horses that dominated most other continents.
“Feeding on grasses wears down teeth more rapidly than other kinds of plants do, because (grasses are) often covered in dust and their blades contain thousands of tiny silica* particles,” Dr Couzens said. “This is a big problem for herbivores, because missing or damaged teeth means death.”
Based on their diet, Dr Couzens said, we might expect grazing kangaroos to have tall teeth with lots of complex crests like 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,” Dr Couzens said.
“Most of these groups, except for kangaroos, declined in diversity over time or went extinct.”
So why were kangaroos the exception?
The Flinders researchers solved this riddle by peering beneath tooth surfaces with X-ray scanning, which allowed them to measure the thickness of tooth enamel.
The scans showed that kangaroos had a hidden trick: a conveyor 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,” said Dr Couzens. “This is unlike hoofed mammals, which chew from side-to-side.”
Researchers next compared kangaroo enamel thickness with other mammals, including early humans.
“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,” study co-author Professor Gavin Prideaux said.
Although thick enamel allowed kangaroos to become efficient grazers, it probably wasn’t the only feature working in favour of their survival.
Early on, several groups of marsupials related to wombats, and with teeth more like those of hoofed mammals, dominated herbivore niches* in Australia. These groups seemed well suited to owning 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, the extinction of these distant wombat ancestors left the feeding fields 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 (kangaroos) win out, whereas the reverse occurred on the northern continents,” Dr Couzens said.
Our iconic* herbivore’s triumphant evolution in Australia versus that of herbivores on other continents suggests that evolution can still deliver surprising survivors.
“Just because you have the right adaptations doesn’t guarantee success,” said Dr Couzens. “Other things need to go in your favour, including a certain degree of luck. It should make us think twice about the certainty of our own evolution.”
POLL
GLOSSARY
- marsupials: type of mammal from Australasia or South and Central America whose females carry their babies in a pouch on their stomach
- evolutionary: relating to the origins, evolution and development of species
- palaeontologists: a person specialising in the study of fossils to determine the structure and evolution of extinct animals and plants
- herbivores: animals that only eat plants
- enamel: outermost layer on the tooth and is responsible for protecting against tooth decay
- silica: ​a chemical containing silicon found in sand and in rocks such as quartz, used in making glass and cement
- extinct: animal or plant species that has no living representatives, having died out
- sedges: plants like grass that grows on wet ground, often next to rivers.
- abrasion: a place where the surface of something, such as skin, has been rubbed away
- arid: having little or no rain, very dry, parched with heat
- iconic: very famous or popular, especially being considered to represent particular opinions or a particular time
- niches: small pockets or examples of something that appeal or are suited to a small, limited or specialised group of animals or people
EXTRA READING
Small change gets design change
Protecting a native Aussie icon
Giant extinct kangaroos identified
QUICK QUIZ
- What wears down teeth faster than other types of plant food?
- What are at least three animals with teeth and jaws more like the kangaroo’s than one would expect?
- Why early humans needed thick enamel has been debated between which two main food theories?
- What kind of technology was used to solve the mystery of why kangaroos were an evolutionary exception?
- What do grass blades often contain?
LISTEN TO THIS STORY
CLASSROOM ACTIVITIES
1. Big evolution questions
This Kids News article shows that the “best”, “strongest” or “biggest” animals don’t always survive.
Explain in your own words how kangaroos managed to outlive other species as the Earth evolved?
Time: allow 20 minutes to complete this activity
Curriculum Links: English, Science, Personal and Social, Critical and Creative Thinking
2. Extension
Create a comic strip outlining Earth before grass, then grass spreading, and how the animals coped to where we are today. Add in some humour and facts from the article.
Time: allow 20 minutes to complete this activity
Curriculum Links: English, Science, Visual Arts, Personal and Social, Critical and Creative Thinking
VCOP ACTIVITY
Stretch your sentence
Find a “who” in the cartoon – a person or an animal. Write it down.
Add three adjectives to describe them better.
Now add a verb to your list. What are they doing?
Add an adverb about how they are doing the action.
Using all the words listed, create one descriptive sentence.