Small fossils of Liaoningosaurus paradoxus, once thought to represent a miniature species of armored dinosaur, have been confirmed as juveniles, including some less than a year old and a possible hatchling. Researchers analyzed bone growth patterns to reach this conclusion, challenging earlier ideas about the animal's size and lifestyle. The discovery provides insights into early development of ankylosaurs.
For two decades, dozens of unusually small fossils from Liaoningosaurus paradoxus puzzled paleontologists. Measuring no more than 40 centimeters long, these specimens stood in contrast to typical adult ankylosaurs, which reached three meters or more. Some scientists had speculated they belonged to a rare dwarf species, while others proposed a partly aquatic existence due to the lack of larger finds since the species' description in 2001. Professor Paul Barrett of the Natural History Museum, a coauthor on the study, noted the ongoing debate over the species, as no adults had been identified. New research published in the Journal of Vertebrate Paleontology examined the microscopic bone structure of two specimens—one of the largest and one of the smallest known. Like tree rings, the bones lacked growth lines, indicating both dinosaurs were under a year old at death. The tiniest showed a hatching line, a ring-like feature marking emergence from the egg, making it the youngest ankylosaur fossil ever found. Barrett explained, 'The smaller fossil showed characteristics that we can see in other newborn dinosaurs, such as the presence of a hatching line.' All fossils originate from Liaoning Province in northeastern China, a Cretaceous site (145 to 66 million years ago) renowned for detailed preservation from volcanic ash-covered lake beds. These juveniles already displayed some armor, suggesting it developed early in life. Barrett added, 'Liaoningosaurus is really the only good window we have into what ankylosaurs are like just after they hatch. Now that we know they are babies and not miniature adults, we can say that these kinds of features came in quite early.' The absence of adult specimens leaves questions about full growth, but these finds clarify juvenile stages of armored dinosaurs.