These tiny dinosaur fossils fooled scientists for 20 years

Specimens once believed to represent a miniature species of armored dinosaur have now been identified as young ankylosaurs. This discovery is giving researchers a clearer picture of how these heavily armored dinosaurs grew and developed.

Liaoningosaurus Fossils Confused Scientists for Decades

The fossils belong to a species called Liaoningosaurus paradoxus, a name that reflects the confusion it has caused. Since its initial description in 2001, it has been classified as an ankylosaur, a group known for their tough, armored bodies.

However, every known fossil of this species has been surprisingly small, measuring no more than 40 centimeters in length. That size is far below what scientists expect for adult ankylosaurs, which often grow to three meters or longer.

Because no larger specimens have ever been found, some researchers proposed that this might represent a rare case of a miniature ankylosaur. Others even suggested the animal may have lived partly in water.

New findings published in the Journal of Vertebrate Paleontologyopens in a new window challenge those ideas. Instead of being tiny adults, these fossils most likely represent juvenile dinosaurs. One specimen even shows signs that it had only recently hatched, making it the youngest known ankylosaur discovered so far.

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 armored dinosaurs."

Bone Growth Analysis Reveals Young Age

Because all known Liaoningosaurus fossils are similar in size, body length alone could not determine whether they were adults or juveniles. To answer that question, scientists examined the microscopic structure of the bones.

Like tree rings, bone tissue contains growth lines that mark each year of an animal's life. The number and spacing of these lines reveal both age and growth rate.

Researchers sampled bones from two specimens, one among the largest ever found and the other among the smallest.

Neither fossil showed any growth lines, indicating that both individuals were less than a year old. The smaller specimen also had bone features typical of very early developmental stages.

"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."

Fossil Site in China Preserves Ancient Life in Detail

All known Liaoningosaurus fossils come from Liaoning Province in northeastern China, a region famous for exceptionally well-preserved fossils from the Cretaceous Period, between 145 and 66 million years ago. This area has also produced feathered dinosaurs such as Microraptor and Sinornithosaurus.

The fossils formed when the animals' remains settled at the bottom of shallow lakes. Frequent volcanic activity covered the lake beds in ash, helping preserve fine details and offering a vivid snapshot of life from that era.

What Baby Ankylosaurs Reveal About Dinosaur Growth

Although adult Liaoningosaurus specimens have not yet been found, these young fossils still provide valuable clues about ankylosaur development.

Fossil evidence of juvenile ankylosaurs is rare, and most known examples lack the distinctive armor seen in adults. This led some scientists to believe that armor developed later in life.

However, Liaoningosaurus fossils show that at least some armor was already present at a very early stage.

"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 armor. 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."