First armoured dinosaur hatchling discovered in China

Paradox in name and in nature, fossils belonging to a dinosaur called Liaoningosaurus paradoxus have puzzled scientists for more than two decades.

First described in 2001, the dinosaur was first identified as a type of armoured dinosaur known as an ankylosaur.

Since then, multiple Liaoningosaurus fossils have been discovered, but strangely none are more than 40 centimetres long. This is remarkably small compared to other fully grown ankylosaurs, which normally reach lengths of three metres or more.

As no remains belonging to a larger individuals have been discovered, some scientists considered the possibility that the dinosaur could be the first example of a miniature ankylosaur. Others have even hypothesised that they were semi-aquatic.

But new research published in the Journal of Vertebrate Paleontology has overturned these theories. Instead, the tiny reptiles are probably baby ankylosaurs. One of the fossils even showed signs that it had recently hatched, which would make this the youngest ankylosaur ever discovered.

Professor Paul Barrett, one of our dinosaur experts and a coauthor of the study, says, “Liaoningosaurus has caused a lot of debate because there is a lot we don’t know about this species and we haven’t managed to identify an adult.”

“But our research confirms that these are baby dinosaurs rather than small adults. Fossils of young ankylosaurs are rare, so there is a lot that these remains can tell us about the early development of armoured dinosaurs.”

How do we know the fossils are young ankylosaurs?

Because the remains of Liaoningosaurus were all similar in size, it was hard to rule out the possibility that they were miniature adult ankylosaurs based on their body size alone. To get an idea of how old these dinosaurs were at the time of death, researchers had to look in detail at their bone structure.

Bone tissue contains growth lines that can be read in a similar way to growth rings in trees, with each line representing a year in the animal’s life. The number of lines and the distance of the gap between them can tell us how old an individual was and how fast it was growing.

Researchers took samples from the bones of two Liaoningosaurus fossils to test this. One of them was the largest specimen discovered so far, while the other was one of the smallest.

The team found no growth lines in either individual, suggesting that both were less than a year old. The bone microstructure of the smaller specimen was also similar to that of other dinosaurs in an early stage of development.

“The smaller fossil showed characteristics that we can see in other newborn dinosaurs, such as the presence of a hatching line,” says Paul.

“This is a small, ring-like feature in the bone that is laid down at the time the animal hatches from the egg. So, we can say that this individual had very recently hatched at the time of its death, which would make it the first hatchling ankylosaur we’ve ever discovered.”

What do we know about these dinosaurs?

All Liaoningosaurus specimens come from Liaoning Province in northeastern China. Many remarkable fossils from the Cretaceous Period, between 145 and 66 million years ago, have been unearthed in this area, including feathered dinosaurs such as Microraptor and Sinornithosaurus.

Like their feathery contemporaries, the Liaoningosaurus fossils were preserved after the remains of the animals sunk to the bottom of a shallow lake. With lots of active volcanoes in the region, the lake beds were exceptionally well preserved under the ashfall. This has given us an incredibly detailed snapshot of life during this time.

But while the lack of adult Liaoningosaurus specimens means we know very little about what these dinosaurs were like in later life, we can still learn a lot about how ankylosaurs in general developed.

Fossils of young ankylosaurs are rare, and those that do exist don’t have the armour that is a prominent feature in the adults. This has led some researchers to suggest that armour is not something they are born with but develops as they age. Liaoningosaurus, however, shows that they had some armour from early in their life.

“As we have found so few fossil babies, Liaoningosaurus is really the only good window we have into what ankylosaurs are like just after they hatch,” says Paul.

“The Liaoningosaurus fossils had already developed some armour. Now that we know they are babies and not miniature adults, we can say that these kinds of features came in quite early during the animal’s growth.”

“But what would give us even bigger insights is if we also found an adult. Then we can find the differences between the adults and babies of the same species and see how these features are developing.”