The ancient fossil belonged to a stegosaurus, a herbivorous dinosaur with a small head and bony plates running on its back, ending in a spiky tail. The specimen also had some unique characteristics that could not be traced back to other stegosaurus. According to new research, it’s the oldest recovered from Asia and may belong to one of the oldest stegosaurs ever found.
Stegosauria is one of the largest group of armored dinosaurs known as therophorans that lived during parts of the Jurassic as well as the Early Cretaceous period, between 100.5 million and 201 million years ago. Their fossils, belonging to 14 different species, are found almost everywhere except Antarctica and Australia.
Researchers discovered dinosaur fossils in 2016 at a well-known site called Shaximiao Formation in Chongqing Municipality in China. Part of this site dates back to the Middle Jurassic period, which lasted between 163.5 million and 174.1 million years ago.
Among the layers of purplish-red mudstone and grayish-yellow sandstone were several plates of armor and the bones of the shoulder, thigh, feet, back and ribs of a stegosaurus. But fossils of stegosaurs from the Middle Jurassic period 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 the word “Bashan” referring to the ancient name of Chongqing and “primitivus” denoting first in Latin, given the age of the dinosaurs.
The fossil is helping scientists better understand how stegosaurs evolved—something they know very little about.
Rare early stegosaurus
Some species of stegosaurus have been known to have giant spines in the shoulder, while others have a long neck, but it has been difficult for researchers to determine how they were related to each other.
Bashanosaurus had smaller and less developed shoulder blades, thicker but narrower bases for its armor plates and other deviations that make it stand out from other stegosaurus. They also share similarities with some of the first armored dinosaurs that lived 20 million years ago.
The newly discovered species is a “really strange mix of features,” study co-author Susanna Maidment said, typically seen in more primitive armored dinosaurs as well as the hallmarks of stegosaurus. Paleontologist at the Natural History Museum in London.
Some of the primitive features of Baschanosaurus include longer tail vertebrae and narrower shoulder blades that extend outward. Unlike other known stegosaurs, the bases of their armored plates are curved outward.
“It’s exciting because it helps us understand how stegosaurus gradually acquired the features of their iconic body plan,” Maidment said. Bashanosaurus did not have the giant plates running along its back like Stegosaurus; instead, it had a much smaller, spine-like (armor), which, arguably, would have been more useful for defending itself against predators than the skinny one. A very but very flashy (shield) of Stegosaurus, which probably had more to do with the show.”
Maidment was part of the team that discovered Adratiklit boulahfaa 168 million-year-old stegosaur fossil found in Morocco in 2019.
“Adratiklit is about the same age, or perhaps 2 million years younger than Bashanosaurus, but Adratiklit has features typical of younger stegosaurus,” Maidment said.
The need for more fossil specimens
She said the differences between these two types of stegosaurs, as well as other known species, show the level of diversity that dinosaurs reached in the Middle Jurassic.
By comparison, Bashanosaurus was relatively small, measuring just 9 feet (2.7 meters) from nose to tail, which led the team to wonder whether the fossil belonged to an adult or a small dinosaur.
The answer to this question is likely to remain a mystery. Maidment said the most reliable way to determine the age of a dinosaur when it died is to cut the bones and analyze the bone tissue, which can reveal the animal’s growth. Small dinosaurs had to grow quickly to reach their large body sizes after hatching, but once they got close to that size, growth would slow down.
“Because we only have one specific specimen of Pacinosaurus, we don’t really want to destroy part of it, and many of the bones are preserved in a huge rock wall, so they are not suitable for sampling,” Maidment said.
The Bachanosaurus is the latest stegosaurus discovery in the Shaximiao Formation, which may indicate that these dinosaurs originated in Asia, said study co-author Ning Li, a researcher with the Chongqing Geographical Heritage Conservation and Research Laboratory.
“By using the data we have to reconstruct their family tree, we can begin to work out the likelihood that stegosaurs originated in different places,” Maidment said. Finding more early stegosaurs will help us understand this problem.”
