For over two decades, paleontologists have puzzled over a set of unusually small dinosaur fossils from northeastern China. Every Liaoningosaurus paradoxus specimen discovered measured less than 40 centimeters, much smaller than the three-meter length typical of adult ankylosaurs. The absence of larger fossils led to debate about whether these animals were a rare dwarf species or represented something else entirely.
Recent research has solved this mystery. The fossils are not miniature adults but juveniles, some less than a year old. One specimen is now recognized as the first ankylosaur hatchling identified in the fossil record.
The study, published in the Journal of Vertebrate Paleontology, was led by researchers from the Zhejiang Museum of Natural History and the Chinese Academy of Sciences, with co-author Professor Paul Barrett from the Natural History Museum in London. It includes the first bone histology analysis of Liaoningosaurus specimens, providing new information about this species.
The Trouble With Size Alone
Since all Liaoningosaurus fossils are similar in size, researchers couldn’t rely on body length to distinguish between small adults and young animals. This uncertainty led to years of debate. Some scientists argued the species might represent a genuine case of miniaturization in ankylosaurs, while others pointed to its unusual anatomy as possible evidence for a semi-aquatic lifestyle not seen in other ankylosaurs.
To address the size question, the researchers examined bone microstructure. Dinosaur bones preserve growth lines, similar to tree rings, which form annually. Counting these lines and measuring their spacing allows scientists to estimate the age of the animal and its growth rate at the time of death. The team analyzed bones from both the largest and one of the smallest Liaoningosaurus specimens. Neither specimen showed any growth lines.
“Our research confirms that these are baby dinosaurs rather than small adults,” Barrett said. “Fossils of young ankylosaurs are rare, so there is a lot that these remains can tell us about the early development of armored dinosaurs.”
A Hatchling in the Record
The smaller specimen provided additional evidence. Its bone microstructure matched patterns found in newborn dinosaurs of other species, and it preserved a hatching line, a ring-like feature in bone tissue that forms when an animal emerges from its egg.
“The smaller fossil showed characteristics that we can see in other newborn dinosaurs, such as the presence of a hatching line,” Barrett said. “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.”
This specimen, cataloged as ELDM V1121, is currently the only confirmed ankylosaur hatchling in the global fossil record.
Armored From the Start
This discovery also revises previous assumptions about the development of ankylosaur armor. Most juvenile ankylosaurs lack the protective plates seen in adults, leading to the theory that armor develops later in life. The Liaoningosaurus fossils tell a different story.
The fossils indicate that at least some armor was present at a very early stage. This finding moves the origin of ankylosaur armor earlier in their development than previously thought.
“The Liaoningosaurus fossils had already developed some armor,” Barrett said. “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.”
No Adult in Sight
All known Liaoningosaurus fossils have been found in Liaoning Province, northeastern China, a region known for exceptionally well-preserved Cretaceous fossils, including Microraptor and Sinornithosaurus. These animals were preserved when their remains settled in shallow lakes and were buried by volcanic ash in fine sediment.
Although many specimens have been found, no adult Liaoningosaurus fossils have been found. This absence limits understanding of the species’ full growth, but doesn’t diminish what the juveniles reveal about ankylosaur development.
“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,” Barrett said. “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.”
While questions remain about Liaoningosaurus, researchers now have a clearer understanding of these fossils and their place in ankylosaur evolution.
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.