Giant glyptodont armadillos may have been hunted by early South Americans

A chink in the armour of giant South American armadillos may have left them vulnerable to human hunting.

Four skulls of these megaherbivores, known as glyptodonts, have been found with similar fractures on the upper surface where their skull's armour was at its thinnest. 

Researchers believe that these blows could have been inflicted by humans to stun the animals, allowing the hunters to then access their vulnerable underside.

Dr Jorge Carrillo‑Briceño, the co-author of new research detailing this phenomenon, says, 'Glyptodonts were built like a tank, and even the tail was covered with protective structures which may have allowed it to be used as a defensive weapon against predators.'

'The only unprotected section was its underside, but its armour was so close to the ground it was very difficult to get underneath.'

'While the head includes helmet-like protection in their skull, in the parietal region this is not as thick. Our hypothesis is that this was the armadillo's Achilles heel, and that striking this area with a stone or some other hard instrument could stun the animal and allow hunters time to kill it.'

The findings of the study were published in the Swiss Journal of Palaeontology. 

What are glyptodonts?

Glyptodonts were a group of species that along with the armadillos form the armoured mammals known as the Cingulata. Their name means 'grooved teeth', and they used these to feed on a diet of mainly grasses and other low-lying plants.

They evolved in South America during the Late Eocene and early Miocene over 30 million years ago. The forming of the isthmus of Panama around 2.7 million years ago then allowed these animals to spread into Central and North America.

Their dispersal was enabled by their adaptability, with different species able to live in a variety of habitats from low grasslands to mountainous areas over 1000 meters high. The largest species, Doedicurus clavicaudatus, was up to four metres long and could weigh as much as 2.4 tonnes, which is slightly heavier than a fully grown black rhino.

Much of this weight was due to the glyptodonts being covered in scale-like bony structures known as osteoderms, which built up to form a shell-like structure somewhat similar to a turtle carapace, but much thicker. Unlike turtles, however, the glyptodonts could not retract their head and so had additional armour on their skull.

At the other end of their body, their tail had additional rings of bone which strengthened it for use as a weapon. It is believed the animals could swing their tail in a similar way to ankylosaur dinosaurs, and some glyptodonts may have even had a spiked club on the end.

Despite these defences, the glyptodonts were driven to extinction along with many other large mammals as the last Ice Age came to an end. While climactic changes are thought to play a role, humans have also been implicated in the loss of these species.

Researchers hoping to understand how extinct South American animals such as giant ground sloths and the glyptodonts were affected by the arrival of humans have searched in Colombia and Venezuela for new evidence.

'Venezuela and Colombia are critical to understanding the arrival of the first humans in South America, as these countries are the gates to the southern continent,' Jorge says. 'However, the evidence of human and megafaunal interactions in the fossil record is not as common as in Europe and Asia.'

'This is partly due to a bias in the preservation and discovery of these sites compared to those in other areas of the world. We're trying to bridge this gap by performing new excavations and carrying out radiocarbon dating to get a better idea of what interactions there might have been.' 

What evidence is there of humans hunting glyptodonts?

The researchers focused on two archaeological sites known as Muaco and Taima-Taima in northern Venezuela, which represent some of the earliest sites in South America with evidence of human activity. Muaco has been dated at around 16,000 years ago, while Taima-Taima is slightly younger.

Three of the study's authors, including lead author Professor Alfredo Carlini from the Museo de La Plata in Argentina, uncovered a glyptodont skull attributed to the species Glyptotheriun cylindricum from Muaco in 2006 that was in danger of being eroded away. 

After preparation, and even though it was long dead, the specimen was then rushed to a local hospital.

'We didn't have CT scanning equipment in the local museum where the specimens were housed, so the only opportunity we had was to take it to a hospital in Coro City to be scanned instead,' Jorge explains. 'This data was used to make a digital model of the skull, which revealed that the bone was broken from the outside.'

The skull was compared to four other skulls from Taima-Taima, which were held in the Museo de Ciencias Naturales de Caracas, as well as photographs of a further specimen which disappeared from the site after being excavated in the 1990s. 

Three of the specimens also contained a similar fracture pattern in the same area of the skull as the one from Muaco. This pattern could suggest that the glyptodonts' defensive position had a blind spot on one side, but with only a few skulls the damage could also have been a simple coincidence.

But fragments of the skull preserved in the fracture suggest that these injuries were inflicted while the animal was alive, rather than after death, as the osteoderm and soft tissue of the living animals helped to keep it in place.

Jorge suggests that after striking the skull, the glyptodont would have been stunned long enough for hunters to finish killing it and then access the soft underside of the animal. 

This hypothesis could also support the earlier discovery of an inverted and empty glyptodont carapace at Taima-Taima, which suggests the animal was flipped over before its internal bones and flesh were scooped out.

'These animals were very heavy, and when they died they would have sunk down rather than turning over,' Jorge says. 'We estimate that one carcass could have provided hunters with up to 170 kilograms of flesh.'

'The head wasn't used though, and we're not sure why as the brain would have been high in protein and nutrients. It may be that the blow to the head destroyed the brain, but it's impossible to prove this without a time machine.'

The researchers hope to continue searching new and existing sites for further evidence of how humans and megafauna interacted in South America, giving us a better idea of how these animals were driven to extinction.