Image: Dinosaur Collector
Triceratops are among the most recognizable dinosaurs because of their distinct appearance. They had a large and elaborate bony shield around the back of their head, horns that jut out from the top of their head and nose like spears, and bony knobs on their cheeks. Because these large structures are energetically expensive to grow, they had to serve a purpose and this purpose was likely the establishment of social hierarchies. Thus, these ornaments provide circumstantial evidence that these dinosaurs lived in groups because such structures could have been used in visual displays. But which evolved first, social behavior or elaborate ornamentation?
But a recent fossil discovery in the Yixian Formation, an area in northeast China, is helping paleontologists to answer this question. Paul Barrett, a paleontologist at Britain’s Natural History Museum, and his colleagues studied a group of young Psittacosaurus that died together after being buried alive by a volcanic mudflow (below);
A herd of juvenile Psittacosaurus from the Lujiatun Beds of the Yixian Formation (lower Aptian: Lower Cretaceous), near Liu Tai village, Yixian, Liaoning Province, China (IVPP 14341). Letters refer to each of the six individuals present. Scale bar represents 100 mm.
Image: [larger view]
Based on their small size (approximately 50 centimeters (1.6 feet) long and weighing about one kilogram (2 lbs)), the six fossilized Psittacosaurus were estimated to be between 1.5 and 3 years old when they died. In comparison, adults of this species were about 2 meters long and weighed up to 30 kilograms.
“We don’t know very much about the early behavior of dinosaurs in general,” said Barrett. But because Psittacosaurus was an early relative of both Triceratops and Protoceratops, “[t]his discovery shows the early relatives were already social and living in groups.”
The fossil discovery consists of six animals of a variety of ages, so it is likely that they came from eggs laid by different parents, observed Barrett. The fossil remains formed a nursery with babies from at least two different parents, he added.
“These animals had left the nest and were already hanging out with each other,” Barrett explained. In short, they formed a mixed-age herd, just as modern herbivores do today.
This study indicates that horns and ornate frills only evolved after social behavior developed in these animals. Further, according to an earlier study that was published several years ago, Psittacosaurus cared for their offspring after they hatched [ref]. Thus, taken together, the evidence reveals that complex social behaviors preceded the development of flamboyant cranial ornaments that characterize this group of dinosaurs.
“It used to be thought that social behavior only occurred when these animals had their horns and frills,” Barrett concluded. “Now we know that they are incidental to it and that the social behavior comes before them.”
This research was published in the recent issue of the journal, Palaeontology.
“Social behavior and mass mortality in the basal Ceratopsian dinosaur Psittacosaurus (Early Cretaceous, People’s Republic of China)” by Zhao Qi, Paul M. Barrett & David A. Eberth. Palaeontology 50 (5), 1023-1029. doi: 10.1111/j.1475-4983.2007.00709.x [free PDF] (images, quotes)
“Palaeontology: Parental care in an ornithischian dinosaur” by Qingjin Meng, Jinyuan Liu, David J. Varricchio, Timothy Huang and Chunling Gao. Nature 431, 145-146 (9 September 2004) | doi:10.1038/431145a [abstract or free PDF]