Giant armadillo-like creatures that lumbered across the plains of South America until around 10,000 years ago have some close living relatives, to scientists' surprise.
A new study by an international team of researchers used DNA evidence to figure out exactly what those living relatives are — and they include the smallest living armadillo.
Glyptodonts were giant, plant-eating mammals with domed, armoured shells that grew to be as large as a Volkswagen Beetle and weighed up to two tonnes. They lived in the grasslands and possibly some forested areas in Central and South America.
While most people think they look like super-sized versions of modern armadillos, paleontologists have been preoccupied by some of their more unusual traits.
"Most paleontologists thought because of its weird shape and size, that glyptodonts emerged almost as a separate lineage," said Hendrik Poinar, an evolutionary geneticist at McMaster University who co-authored the new study.
They have a lot of strange characteristics, said French researcher Frederic Delsuc, lead author of the new paper published in the journal Current Biology. Those include:
- Peculiar shells were completely fused together into a rigid dome, unlike the hinged shells that let modern armadillos curl into a ball.
- Very short faces like those of modern sloths, without the long snouts on modern armadillos.
- Lobed teeth, unlike the very simple teeth of modern armadillos.
They also "also went fairly crazy" with "external ossifications" – bone structures on the outsides of their bodies, said Ross MacPhee, a paleontologist at American Museum of Natural History who also co-authored the study.
For example, males of Doedicurus, the particular glyptodont with the DNA that was analyzed in this study, had spiked, club-like tails thought to have been used to fight other males and possibly predators, MacPhee said in an email from Antarctica, where he is currently doing field work.
But despite all those unusual traits, the DNA analysis shows that glyptodonts weren't cousins of armadillos who shared a common ancestor. They actually belong to the armadillo branch of their family tree.
One of their closest living relatives is the squirrel-sized pink fairy armadillo — the smallest living armadillo. They're also closely related to giant armadillos, who despite their name, only grow as large as a medium-sized dog.
Poinar said that's as surprising as it would be to discover that monkeys belong in the same branch of the primate family tree as humans.
It's also "very suggestive that body size and shape are of little value" when reconstructing evolutionary family trees, he added.
Delsuc, a researcher at France's centre national de la recherche scientifique and the University of Montpellier, said there has been one previous analysis that suggested glyptodonts were part of the armadillo branch of the evolutionary tree based on a detailed analysis of their skulls.
But it wasn't easy to do a genetic analysis to corroborate that.
DNA doesn't preserve well in open grasslands where glyptodonts once lived, Poinar said.
He and his colleagues tried to tease out DNA from more than 60 glyptodont fossils, including many at Toronto's Royal Ontario Museum. The one used in the study, from the shell of a Doedicurus at a museum in Argentina, "was the only one that had just a few molecules buried in there."
DNA is made up of two strands that stick together if their sequences line up properly. Poinar and his colleagues used single strands of DNA that he thought glyptodont DNA might stick to in order to fish it out. They tested sequences from modern armadillos and their cousins the sloths and anteaters. They also deduced what sequences a common ancestor of some of those animals might have and tried fishing with those as well.
The results didn't just show that glyptodonts are close relatives of modern armadillos. They also showed that the glyptodonts evolved later than thought, about 30 million years ago. That means their strange and unusual traits evolved quite quickly.
Delsuc, who has been studying the family tree of anteaters, sloths and armadillos for years, said he was "kind of surprised" by the results. But this isn't the first surprise of its kind in evolutionary biology. "It happens a lot in the tree of life."