Dinosaur fossil discovery may be the oldest stegosaur ever found

Fossils have led researchers to a previously unknown species of armoured dinosaur that lived 168 million years ago, and it's unlike anything they've ever seen before.

The ancient fossil belongs to a stegosaur, an herbivorous dinosaur with a tiny head and bony plates marching down its back, ending in a spiked tail. The specimen also had some unique characteristics that can't be traced in other stegosaurs. It's the oldest one recovered from Asia and may belong to one of the earliest stegosaurs ever found, according to new research.

The study published Thursday in the Journal of Vertebrate Paleontology.

Stegosauria is one of a larger group of armored dinosaurs known as thyreophorans that lived during parts of the Jurassic period as well as the early Cretaceous period, between 100.5 million and 201 million years ago. Their fossils, belonging to 14 different species, have been found nearly everywhere except for Antarctica and Australia.

Researchers uncovered dinosaur fossils in 2016 at a well-known site called the Shaximiao Formation in the Chongqing municipality of China. Part of this site dates to the Middle Jurassic period, which lasted between 163.5 million and 174.1 million years ago.

Located between layers of purplish-red mudstone and grayish-yellow sandstone were several armor plates and the shoulder, thigh, feet, back and rib bones of a stegosaur. But stegosaur fossils from the Middle Jurassic are rare, making this discovery unique since the dinosaur lived much earlier than most of its known relatives.

The researchers named the dinosaur Bashanosaurus primitivus, with "Bashan" in reference to the ancient name of Chongqing and "primitivus" signifying first in Latin, given the dinosaur's age.

The fossil is helping scientists better understand how stegosaurs evolved -- something of which they know precious little.

RARE EARLY STEGOSAUR

Some stegosaurs were known for having giant shoulder spines, while others had a long neck, but it's been difficult for researchers to determine how they related to one another.

Bashanosaurus had smaller and less developed shoulder blades, thicker yet narrower bases for its armor plates and other eccentricities that make it stand out from other stegosaurs. It also shared similarities with some of the first armored dinosaurs that lived 20 million years earlier.

The newly discovered species is "a really weird mix of features," usually seen in more primitive armored dinosaurs as well as hallmarks of stegosaurs, said study coauthor Susannah Maidment, a paleontologist at London's Natural History Museum.

Some of the primitive features of Bashanosaurus include longer tail vertebrae and narrower shoulder blades that flared out. Unlike other known stegosaurs, the bases of its armor plates curved outward.

"It's exciting because it helps us understand how stegosaurs gradually acquired the features of their iconic body plan," Maidment said. "Bashanosaurus didn't have the giant plates extending along its back like Stegosaurus had; instead it had much smaller, spine-like (armor), that arguably would have been more useful for defending itself against predators than the very thin but very flashy (armor) of Stegosaurus, which might have been more about display."

Maidment was part of a team that discovered Adratiklit boulahfa, a 168 million-year-old stegosaur fossil found in Morocco in 2019.

"Adratiklit is roughly the same age, or perhaps a couple of million years younger than Bashanosaurus, but Adratiklit has features typical of younger stegosaurs," Maidment said.

NEED FOR MORE FOSSIL SPECIMENS

The differences between these two stegosaurs, as well as other known species, shows the level of diversity the dinosaurs had reached by the Middle Jurassic, she said.

By comparison, Bashanosaurus was relatively small, only reaching 9 feet (2.7 meters) from nose to tail, causing the team to question if the fossil belonged to an adult or juvenile dinosaur.

The answer to that question will likely remain a mystery. The most reliable way to determine the age of a dinosaur when it died is to cut into the bone and analyze the bone tissue, which can reveal the animal's growth, Maidment said. Young dinosaurs had to grow quickly to reach their large body sizes after hatching, but once they neared that size, the growth would slow down.

"As we only have one definitive specimen of Bashanosaurus, we don't really want to destroy part of it, and many of the bones are preserved in a huge rock wall, so they aren't appropriate for sampling," Maidment said.

Bashanosaurus is just the latest stegosaur discovery at the Shaximiao Formation, something that may suggest these dinosaurs originated in Asia, said study coauthor Ning Li, a researcher at the Chongqing Laboratory of Geoheritage Protection and Research.

"By using the data that we do have to reconstruct their family tree, we can start to work out the probability that stegosaurs originated in various places," Maidment said, "At the moment, the most primitive stegosaurs we have are from Asia and Argentina. Finding more early stegosaurs will help us understand this problem."