New species of big-thumbed dinosaur discovered in Mongolia

Preserved in a resting pose for more than 70 million years, a new species of dinosaur has finally been brought to light.

Named Jaculinykus yaruui, or the ‘speedy clawed dragon’, the small, possibly feathered, dinosaur was well adapted to dashing around what is now Mongolia. With hands dominated by a large thumb, it could have dug into insect colonies in the search for food.

Sometime during the Late Cretaceous, however, the luck of one particular Jaculinykus ran out. Settling down to sleep in a pose similar to modern birds, it was rapidly buried under sand. This preserved it for millions of years until it was discovered by palaeontologists in 2016.

Kohta Kubo, a PhD student and lead author of new research describing the new species, says, ‘As most of Jaculinykus’ skeleton has remained in its original position, it could have been buried rapidly during a sandstorm or a catastrophic event.’

‘Alternatively, it’s possible that it was buried while in a burrow. Alvarezsaurids like Jaculinykus had tiny but powerful forelimbs that might possibly have allowed them to excavate underground tunnels, and if this collapsed, it would have preserved its life position.

‘However it came to be buried, being underground protected it from scavengers. This ensured it is one of the best preserved alvarezsaurids from this period, greatly improving our understanding of their anatomy and ecology.’

The findings of the study were published in the journal PLOS One. 

Digging dinosaurs

The new species was found in Mongolia’s Gobi Desert, which is one of the richest sources of dinosaur fossils in the world. The first recognised dinosaur eggs were discovered here a century ago, while many famous species such as Oviraptor, Protoceratops and Velociraptor have also been unearthed in the region.

Professor Paul Barrett, a dinosaur expert from the Natural History Museum, says, ‘Mongolia is rich in Late Cretaceous rocks which provide a really detailed view of the end of the dinosaur era.’

‘Fossils of dinosaurs, mammals, lizards and other animals are well-preserved, and the bones look almost modern when they come out of the ochre-coloured rocks.’

Among the many dinosaurs to be discovered in the region are the alvarezsaurids, a group of dinosaurs with many similarities to birds. These include characteristics such as a light skull, small body size and a structure known as a keeled sternum, which allows large arm muscles to attach.

The similarities are so pronounced that when they were first discovered, some scientists believed these dinosaurs might be the direct ancestors of some living birds. More recent research has instead revealed that they are relatives who adapted to similar challenges in a process known as convergent evolution.

Though they’re not directly related to birds, the alvarezsaurids continue to fascinate due to their unusual characteristics – most peculiarly of all, their hands.

‘Like all meat-eating dinosaurs, or theropods, the earliest alvarezsaurids had three-fingered hands,’ Paul explains. ‘Over time, however, the outer fingers get smaller, and eventually disappear altogether.’

‘Meanwhile, their thumbs develop a big claw and get much larger, while their arms become shorter but stronger. It’s thought they might be anteaters, and they also have flexible skulls which might have helped them to eat larger, struggling insects.’

Jaculinykus is midway through this process, still having one small finger alongside its large digging thumb. This helps to fill a gap in the evolution of these dinosaurs, which tend to be found with either one or three digits.

The secrets of sleep

As well as its arms, the posture of the new species is also revealing. At the time of its death, the dinosaur had its legs folded on either side of its body, its tail wrapped around them, and its head curled back.

This is similar to how birds sleep, with the pose thought to help the animals keep warm by reducing heat loss.

‘Previously, this posture has only been seen in a couple of dinosaurs, including a troodontid known as Mei,’ Paul explains. ‘As it comes later in the dinosaur lineages than other bird-like poses, such as brooding, it’s reasonable to assume that birds and their near relatives inherited their resting position from a common ancestor.’

Finding out what this common ancestor could be, however, is difficult, as the fossils of small dinosaurs like the alvarezsaurids are often badly preserved.

‘Small dinosaurs can have thinner and more fragile bones, which make them more vulnerable to being damaged,’ Paul says. ‘From the fragments that survive, we know that alvarezsaurids certainly lived in other areas of the world, but we may not be seeing the full diversity of the species that lived there.’

But in Mongolia at least, they appear to have been relatively common. With the discovery of Jaculinykus, nine genera of alvarezsaurid have now been found in the region.

While these dinosaurs wouldn’t have all been alive at the same time, there may have been three or four different species co-existing at different points in the Late Cretaceous.

‘This diversity might be explained by the environments of the past,’ Kohta says. ‘The Cretaceous sediments in the Nemegt Basin suggest there were once a variety of moist and dry habitats.’

‘As these environments changed over time, they could have promoted the adaptive radiation and diversification of the region’s terrestrial vertebrates, including the alvarezsaurids.’

It’s hoped that the completeness of Jaculinykus will contribute towards explorations of this diversity, and to help reconstruct its more fragmentary relatives around the globe.