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Earth can be ruthlessly temperamental. Archaeologists found that out when the entire city of Pompeii emerged from volcanic ash, but millions of years before the fateful Mt. Vesuvius eruption, could the same thing have happened to dinosaurs on the other side of the planet?
Psittacosaurus lujiatunensis fossils that were unearthed from the Lujitan outcropping of China’s Yixian Formation in Liaoning Province, seem to suggest that. These creatures are basically what you would get if you Frankensteined the head of a parrot onto the body of an oversized lizard. Many dinosaurs that perished in this area were found to have met their end from pyroclastic flows or lahar, which (as if lava wasn’t bad enough) is a deadly gloop of volcano vomit and mud. While volcanic tantrums in this “Cretaceous Pompeii” froze many dinosaurs in time, it wasn’t the case for the two impeccably preserved psittacosaurs.
“Famous for 3D Early Cretaceous dinosaurs and mammals, [Lujitan] has unresolved depositional, taphonomic and stratigraphic settings,” said Elaine Chen, who led a study recently presented at the annual meeting of the American Geophysical union. “LA- ICP-MS dating of zircons extracted from matrix of two Lujiatun blocks with articulated Psittacosaurus provide tests of competing hypotheses of taphonomy.”
What made this prehistoric mystery even more confusing was the level at which the skeletons were preserved. Dinosaurs drowned in lava have sometimes revealed fragments of feathers and soft tissue, some of which even retained coloration. Not unlike Changmaiania lianingensis, which was discovered earlier this year and believed to be a victim of lava or lahar, they appear to have just gone to sleep and never woken up.
The problem is that zircon in the sediment grains surrounding and on the skeleton showed they were 250 million to 2.5 billion years old. The petrified volcanic deposits of Lujitan are only around 150 million years old. Psittacosaurs were not roaming the Earth billions of years ago, but the older rock particles are indicative of mud rather than lava, and the mud could be a sign of something else. It is possible that the one thing P. lujiatunensis and C. lianingensis have in common is that they were both burrowing dinosaurs. Until now, there has been no evidence that any type of psittacosaur dug burrows, and while this theory has not been proven conclusively, it is plausible.
Burrowing could also explain how a mudslide engulfed the sleeping dinosaurs that ended up being fossilized in what appears to be an eternal sleep.
“Given the complete articulation of Psittacosaurus and most other dinosaurs and mammals, the simplest hypothesis for their burial is attrition in burrows,” Chen said.
So how did sediment even older than the dinosaurs even get there? It could have been carried by the flow of a river. If that river flooded and the burrow collapsed into a mud pit, that would explain why these skeletons are so incredibly articulated.
C. lianingensis was found to be a burrower because of certain morphological characteristics. Its short but sturdy neck and forearms, along with a shovel-shaped snout and shoulder blades that were not too different from extant burrowing vertebrates like moles and rabbits, suggested it liked to hide out underground. Psittacosaur snouts were shaped more like a parrot’s beak than a shovel. While most parrots do not burrow — with the exception of one bizarre species from Chile and Argentina — it is not completely out of the question for a psitaccosaur. While some paleontologists argue that their forelimbs were too short for this behavior, there is also an argument that they could have used their hindlimbs.
There are some parallels between the preservation of Pompeii and this “Cretaceous Pompeii.” What most tourists don’t know is that the human shapes in the ancient ruins are an eerie throwback to what happened that day; what they don’t know is that most preserved remains were skeletons. When archaeologists took a closer look at the city when it was still buried, they noticed human-shaped voids in the ash. The voids were perfect 3-D impressions of the people who had once lived in the ancient Roman city. Once bones were removed for study, plaster was poured into them, producing identical casts of the bodies that would be displayed in museums as well as the petrified disaster area.
Dinosaurs that were covered by pyroclasic flows of lava, ash, and volcanic gas are examples of a similar phenomenon. Their bones are then studied and displayed since there is no perceived ethical dilemma about showing off dinosaur remains in a museum as opposed to human remains.
Whether this or any other species of psittacosaur actually hid out from predators underground remains a mystery. Maybe a burrow frozen in time will someday come to the surface.