Fossils show a mammal attacking a dinosaur

The fossils of Repenomamus robustus and Psittacosaurus lujiatunensis with insets showing some magnified details of their interactions that are considered among the evidence that they died during a fight
An article published in the journal “Scientific Reports” describes what appears to all intents and purposes a fight in which a mammal of the species Repenomamus robustus attacked a dinosaur of the species Psittacosaurus lujiatunensis. A team of researchers examined fossils discovered in today’s China dating back to about 125 million years ago, in the Jurassic period, which offer the first evidence of a mammal hunting a dinosaur. The two animals engaged in a mortal fight became entangled in a flow of volcanic mud similar to the one called lahar in jargon, leaving evidence of that fight.

The top image (Courtesy Gang Han. All rights reserved) shows the fossils of Repenomamus robustus and Psittacosaurus lujiatunensis with insets showing some magnified details of their interactions that are considered among the evidence that they died during a fight.

In the common conception of the dinosaur age, mammals were small, generally nocturnal animals that hid and posed no threat to dinosaurs. Some discoveries already indicated that the situation was somewhat different because Repenomamus robustus was already known as a predator and fossils are known that include bones of baby Psittacosaurus lujiatunensis in its stomach. However, a new study indicates that the mammal could have preyed on an adult dinosaur as well.

The fossils object of this study were discovered in the Yixian Formation, in the Chinese province of Liaoning. This fossil deposit is considered a sort of Chinese Pompeii because the ancient volcanic activity generated flows of muddy material that hit animals that were in the wrong place at the wrong time. Quickly covered by mud and ashes, these animals fossilized excellently and that’s also true for the Repenomamus robustus and Psittacosaurus lujiatunensis discovered in 2012.

Repenomamus robustus was among the largest mammals of the Mesozoic, surpassed perhaps only by its “cousin” Repenomamus giganticus. With a body almost half a meter long and a badger-like appearance, it may seem unlikely as a dinosaur predator but it seems a case where appearances can be deceiving.

Psittacosaurus lujiatunensis is considered the oldest species of a genus of herbivorous bipedal dinosaurs well known thanks to a significant amount of fossils available. The specimen under study was almost exactly 120 cm long.

Today there are mammals that hunt much larger prey and it would seem that this ability already existed in the Jurassic period. Eating dinosaur puppies and eggs was a behavior already known in Repenomamus robustus but attacking adult specimens much larger than them such as Psittacosaurus lujiatunensis is quite di different matter.

The researchers examined the very well-preserved fossils of the two animals to try to prove beyond a doubt that what they were seeing was a fight between a predator and its prey. The bones of Psittacosaurus lujiatunensis show no tooth marks that would lead one to think that the mammal wanted to feed on the carcass of an already dead animal. Repenomamus robustus is on top of the dinosaur in a position that indicates it was trying to hold it down to kill it. The mammal’s hind leg appears to be wedged under the dinosaur’s hind leg. It’s a position that indicates that the dinosaur had fallen on the mammal during a fight and therefore must have been alive and ready to fight for its life. Lastly, Repenomamus robustus has its teeth sunk into the rib cage of Psittacosaurus lujiatunensis.

In essence, the examination of the fossils indicates that the two animals were in a dynamic situation compatible with a fight when they were hit by a flow of muddy material emitted by a volcano that was active at the time. Today, that type of material is called lahar and has characteristics similar to those of concrete. Mudslides can be very fast and therefore take the unfortunate animals present in the area by surprise, in this case, while they were engaged in a fight, burying them in a short time and then solidifying quickly.

The result is a precious testimony of how even in the Jurassic, there were some mammal predators that attacked herbivorous dinosaurs. According to the researchers, the Yixian Formation deposit could still offer surprises by revealing interactions between different species that would otherwise be impossible to prove.

Illustration of a Repenomamus robustus mammal attacking a Psittacosaurus lujiatunensis dinosaur (Image courtesy Michael W. Skrepnick)
Illustration of a Repenomamus robustus mammal attacking a Psittacosaurus lujiatunensis dinosaur (Image courtesy Michael W. Skrepnick)

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