The unusual fossil from around 125 million years ago shows a dramatic moment in time when a species of badger-like carnivorous mammal called Repenomamus robustus attacked the bipedal plant-eating dinosaur Psittacosaurus lujiatunensi. The fossil challenges the view that dinosaurs had few threats from their mammal contemporaries during the Cretaceous period, when dinosaurs were the dominant animals.
Illustration showing Repenomamus robustus as it attacks Psittacosaurus lujiatunensis before a volcanic debris flow buries them both, approximately 125 million years ago. Image credit: Michael Skrepnick.
“The two animals are locked in mortal combat, intimately intertwined, and it’s among the first evidence to show actual predatory behavior by a mammal on a dinosaur,” said Dr. Jordan Mallon, a paleobiologist at the Canadian Museum of Nature.
The 125-million-year-old fossil was collected in 2012 from the Lower Cretaceous Yixian Formation in Liaoning Province, China.
The Repenomamus robustus specimen is almost complete — bar the tip of its tail — and measures 46.7 cm in length.
The complete skeleton of Psittacosaurus lujiatunensi measures 119.6 cm in length.
The dinosaur is lying prone on its front with its hindlimbs folded on either side of its body and its neck and tail curled to the left.
Repenomamus robustus lies on top of the dinosaur’s left side and curves to the right.
Psittacosaurus lujiatunensis–Repenomamus robustus pair locked in mortal combat. Insets depict (left to right): hand of the mammal wrapped around the lower jaw of the dinosaur, teeth of the mammal embedded in forearm of the dinosaur, hind foot of the mammal wrapped around lower hindlimb of the dinosaur. Scale bar – 10 cm. Image credit: Han et al., doi: 10.1038/s41598-023-37545-8.
The mammal’s left paw is gripping the lower jaw of the dinosaur, which has been slightly displaced forward.
Its left hindleg is trapped under the dinosaur’s folded left leg with its hind paw gripping the left shin.
The teeth of Repenomamus robustus were embedded in the dinosaur’s ribcage when the animals died. Both animals are thought to have been subadult individuals.
Dr. Mallon and colleagues hypothesize that Repenomamus robustus was attempting to prey on Psittacosaurus lujiatunensi when both animals were caught in a volcanic mudflow and buried.
The extent of their entanglement and lack of other bite marks on the dinosaur’s skeleton indicates that the mammal was not scavenging on the dinosaur’s carcass.
“The weight of the evidence suggests that an active attack was underway,” Dr. Mallon said.
Prior to the discovery, paleontologists knew that Repenomamus robustus preyed on dinosaurs including Psittacosaurus lujiatunensi because of fossilized baby bones of the herbivore found in the mammal’s stomach.
“The co-existence of these two animals is not new, but what’s new to science through this amazing fossil is the predatory behavior it shows,” Dr. Mallon said.
The discovery is described in a new paper in the journal Scientific Reports.
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G. Han et al. 2023. An extraordinary fossil captures the struggle for existence during the Mesozoic. Sci Rep 13, 11221; doi: 10.1038/s41598-023-37545-8
