In 2004, a specimen found in the Yixian Formation was claimed as evidence for parental care in dinosaurs. The specimen DNHM D2156 consists of 34 articulated juvenile "Psittacosaurus" skeletons, closely associated with the skull of an adult. The juveniles, all approximately the same age, are intertwined in a group underneath the adult, although all 34 skulls are positioned above the mass of bodies, as they would have been in life. This suggests that the animals were alive at the time of burial, which must have been extremely rapid, perhaps due to the collapse of a burrow. However, a 2013 paper pointed out that the adult specimen did not belong with the nest, its skull having no sedimentary connection to the main slab where the juveniles occurred, but had been glued onto it. This artificial association led to the inference that the skull belonged to an individual, possibly a "mother", that was providing parental care for the 34 juveniles—a claim that is unfounded. Furthermore, the adult was also shown to be six years old, whereas histological studies have shown "P. mongoliensis" was unable to breed until it reached ten years of age. It is also unlikely that a single female would have so many offspring at one time.