An artist's impression of Bashanosaurus primitivus

Bashanosaurus primitivus has distinctive grooves on its armoured plates, which helped researchers identity if as a new species. Image © Banana Art Studio

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New stegosaur dinosaur could be the oldest ever discovered

One of the earliest stegosaurs has been uncovered in China. 

The new species, Bashanosaurus primitivus, dates to the Middle Jurassic and could be a close relative of the ancestor of all stegosaurs that followed. 

The origin of all stegosaurs could lie in Asia after what could be the oldest member of the group has been unearthed in China.

The description of a new species of early stegosaur, Bashanosaurus primitivus, suggests that the dinosaurs may have emerged in what was then the supercontinent of Laurasia over 165 million years ago.

The group then rapidly spread across large areas of the world, giving rise to many species including Stegosaurus, which gives the group of dinosaurs their name, before vanishing in the Late Cretaceous.

Bashanosaurus primitivus supplants the previous oldest known definitive stegosaur, Adratiklit boulahfa, which was discovered in Morocco in 2019.

Dr Susannah Maidment, a Principal Researcher at the Museum, was involved in the description of both these species. She says, 'The discovery of this stegosaur adds to an increasing body of evidence that the group evolved in the early Middle Jurassic, or perhaps even in the Early Jurassic, and as such represent some of the earliest known bird-hipped dinosaurs.

'China seems to have been a hotspot for stegosaur diversity, with numerous species now known from the Middle Jurassic right the way through until the end of the Early Cretaceous Period.'

The description of the species, led by Chinese researchers, was published in the Journal of Vertebrate Palaeontology.

Watch Sophie the Stegosaurus walk around as Sir David Attenborough explains how this dinosaur would have moved.

What are stegosaurs?

The stegosaurs were an iconic group of armoured dinosaurs with distinctive bony plates which ran along their back. 

The first stegosaur ever discovered, Dacentrurus, was unearthed in Swindon, UK, in 1874 and was described by Richard Owen, the scientist who coined the term dinosaur.

Three years later, in 1877, Othniel Charles Marsh unearthed Stegosaurus armatus, which gives the group their name, in the Rocky Mountains, USA. At first, it was believed that the plates lay flat along the back as armour, until additional specimens with the plates preserved upright were found.

But this group of dinosaurs has caused a lot of confusion. Many fossils initially named as different Stegosaurus species are now considered to be the same species or from a completely different genus. Even Stegosaurus armatus has been replaced as the representative of its genus by Stegosaurus stenops due to concerns about it being difficult to identify.

All this reflects the difficulty palaeontologists have had in deciphering the group's family tree. Fossils of these dinosaurs are 'surprisingly rare', according to Susannah, and can often consist of only a few bones which make attempts to compare species difficult.

For instance, Isaberrysaura mollensis, was originally classed as an entirely different type of dinosaur before new analysis suggested it as a candidate for one of the earliest stegosaurs. More recently, early stegosaurs from Africa and China have also helped to clarify the origins of the group somewhat.

Bashanosaurus primitivus may be older still than these dinosaurs, and is currently the earliest stegosaur to be described. 

A reconstruction of Huayangosaurus

Huayangosaurus is one of the earliest stegosaurs known, and, alongside relatives, provides evidence the group could have emerged in Asia. Image © AdInfinitum, licensed under CC BY-SA 4.0 via Wikimedia Commons.

Could stegosaurs have emerged in Asia?

The site the new stegosaur was found in was discovered in 2016 near the village of Laojun in Chongqing Municipality, China. The site has been rich in new species, with excavations uncovering the remains of sauropod and other herbivorous dinosaurs in the years since its discovery.

Three incomplete skeletons of the new species were uncovered, alongside bones from a fourth unidentified stegosaur. The most complete of these skeletons comprised of ribs, vertebrae, and bones of the left leg, ankle and the shoulder. It also had three pieces of dermal armour, including a plate and spines, which help confirm its identity as a stegosaur.

The base of these plates have distinct grooves, which along with a number of other features allowed scientists to confirm its identity as a new species.

Its generic name, Bashanosaurus, refers to the ancient name of Chongqing, while its specific name refers to its position as one of the earliest-diverging stegosaurs. Analysis of its characteristics suggests its closest relative is Chungkingosaurus, another stegosaur found elsewhere in the same rock formation.

This pair of dinosaurs, alongside Huayangosaurus, are some of the earliest stegosaurs known to have evolved. This may mean that the group could have originated in what is now Asia, although a lack of early specimens makes this difficult to confirm.

Some inferences can be made about what Bashanosaurus primitivus would have been like. Similar to other stegosaurs, it would have been a herbivore with defensive tail spikes, which are believed to have been used to ward off attacks from predators. 

The most complete individual is estimated to have been around three metres in length when it died, which may have meant it was a juvenile. 

Though it may have been small, early stegosaurs like Bashanosaurus primitivus are a big help to palaeontologists trying to piece together the puzzle of the stegosaurs.