Back in 2004, a rock slab containing the skeletons of 24 baby dinosaurs and one older individual was discovered in the Lujiatun beds of the Yixian Formation, northeastern China. The 120 million year old specimens were very briefly described in a one-page paper which came to the conclusion that the older individual was probably a parent that was looking after its young.
Although an interesting find, it was hardly remarkable, so nothing more was said for some time. However, University of Pennsylvania researchers were convinced that there was more to the story, so they decided to re-examine the slab, which yielded some rather fascinating results. Rather than a parent, the researchers believe the larger specimen may have been a caretaker. Their findings have been described in the journal Cretaceous Research.
All of the dinosaurs were a small plant-eating species called Psittacosaurus lujiatunensis. Psittacosaurus, the “parrot dinosaur,” is the most species-rich dinosaur genus known with 15 named species so far. They were all gazelle-shaped bipedal dinosaurs, easily recognizable from their high, powerful beak on the upper jaw.
After examining the material in which the specimens were preserved, the researchers found that they were preserved in volcanic rock, suggesting that the animals were buried in flowing material from an eruption. There was no evidence that the bones were affected by intense heat which indicated that were probably smothered by lahar, a slurry of water, mud and rock, as opposed to lava. Furthermore, all of their spines were pointing in the same direction, further supporting the idea that they were caught up in flowing material.
The 24 younger animals were all of a similar size and while it was initially contemplated that they could have been embryos inside their eggs, the researchers couldn’t find any evidence to support this. For instance, they failed to find any eggshell material, and even smaller specimens have been discovered previously. Furthermore, according to lead author Brandon Hedrick, the bones were well developed, suggesting they were capable of moving around. The researchers therefore concluded that the dinosaurs were indeed post-hatchlings.
Judging by the size of the larger specimen’s skull, the researchers believe that it was a 4-5 year old juvenile. Previous research has suggested that this species probably didn’t reproduce until it was at least 8 years old, indicating that it was likely not the parent. The team proposes that it could have been an older sibling that was taking care of its relatives. Some of the younger specimens were actually found intertwined with the juvenile, suggesting a close association when they died.
Taken together, these data suggest that the specimen is not an example of parental care, but post-hatching cooperation, a behavior seen today in some extant bird species. While the assemblage has tell-tale signs of a nest, the researchers caution that they cannot definitively call it a nest as it fails to meet some of the necessary criteria. However, the researchers would like to take this work forward by further examining the bones in order determine whether they were all at the same developmental stage. If this turns out to be the case, then it is likely that they were indeed one clutch of animals.