A fossil preserves a dinosaur and a mammal locked in combat

Mesozoic mammals are often imagined as small, nocturnal animals living in the shadows, doing their best to avoid becoming food for large reptiles. That picture is not without fossil support: small mammalian remains have previously been found in the gut contents of dinosaur fossils. Yet one fossil from west of Lujiatun Village, Beipiao City, Liaoning Province, China, captures a very different moment: a Psittacosaurus lujiatunensis and a Repenomamus robustus entangled with one another. The specimen comes from the Lujiatun Member of the Yixian Formation and dates to roughly 125.7 million years ago. The posture of the two animals looks as though the final moment of their lives was suddenly frozen in place, leaving behind a direct record of a struggle from the Early Cretaceous.

Life restoration of Psittacosaurus（Image source：TotalDino， CC BY 4.0 ）

Life restoration of Repenomamus robustus（Image source：Nix Illustration， CC BY-NC 4.0 ）

The Lujiatun area has long yielded abundant vertebrate fossils, especially those of Psittacosaurus lujiatunensis. The sediment surrounding this fossil consists mainly of medium- to coarse-grained brecciated tuff, containing abundant volcanic lithic fragments, feldspar, quartz, and minerals formed from altered fine volcanic ash. The clasts are poorly sorted and highly angular, and the rock lacks clear stratification or signs of bioturbation. These clues point to a rapid burial event, most likely a sudden volcanic debris flow that entombed the two animals close to the place where they died.

Fossil of Psittacosaurus lujiatunensis and Repenomamus robustus locked in combat (specimen number: WZSSM VF000011)（Image source：Han G et al. (2023)， CC BY 4.0 ）

The Psittacosaurus skeleton in the specimen is complete. The animal lies prone, with its hindlimbs folded on either side of the body and its neck and tail curled to the left. Based on femur length and known growth data, it was at least about 6.5 years old at death, and more likely close to 10 years old. Its estimated body mass was about 10.6 kg. The skeleton of Repenomamus is also nearly complete, missing only the distal end of the tail. Its body mass is estimated at about 3.43 kg, and the individual was probably a subadult. In other words, the dinosaur was roughly three times heavier than the mammal.

The body of Repenomamus lies atop the left side of Psittacosaurus. Its left forelimb grips the dinosaur's lower jaw, while its left hindlimb is trapped within the folded left leg of the dinosaur. The mammal's left hind foot grips the dinosaur's left shin, immediately below the knee. Even more strikingly, the lower jaw of Repenomamus is biting the left anterior dorsal ribs of Psittacosaurus, with its teeth positioned against the dinosaur's bones.

Life restoration of Psittacosaurus lujiatunensis and Repenomamus robustus locked in combat（Image source：Han G et al. (2023)， CC BY 4.0 ）

A fossil like this naturally raises the question of whether it could have been forged. Composite fossils have been reported before from the Jehol Group of China, so the researchers addressed this possibility directly. However, typical forgeries tend to involve the simple juxtaposition of two independent fossils. They do not easily reproduce such a complex association of entangled limbs, grasping extremities, and biting jaws. In addition, both skeletons are highly complete, with displacement limited mostly to smaller distal phalanges, distal tail vertebrae, and the dinosaur's lower jaw. This indicates that the carcasses were not transported over any significant distance. Had they been moved by geological processes, the skeletons would likely be more broken and scattered. These lines of evidence support the interpretation that the association is authentic, and that death and burial occurred very close together in both time and place.

So was Repenomamus hunting Psittacosaurus, or was it scavenging a carcass? Scavenging is possible: the dinosaur was much larger than the mammal, and ribs would generally be easier to access on a carcass than on a living animal. But that explanation faces several problems. First, the Psittacosaurus skeleton lacks the additional tooth marks that would usually be expected if a carnivorous mammal had been feeding on a carcass. Second, if the dinosaur had already been dead, it is difficult to explain how the two animals became so tightly entangled. Third, Repenomamus is positioned on top of the dinosaur, as though subduing weakened prey, rather than feeding from the ground beside a carcass. For these reasons, the researchers considered predation the better-supported explanation: Repenomamus was attacking a weakened or collapsed Psittacosaurus when a volcanic debris flow suddenly arrived and buried them both.

Small predators attacking prey many times their own body mass is not unusual. Wolverines (Gulo gulo), for example, are known to attack much larger animals such as reindeer (Rangifer tarandus) and moose (Alces alces). Least weasels (Mustela nivalis) have also been reported attacking birds or hares far larger than themselves. Seen in the context of the Early Cretaceous Lujiatun ecosystem, the Mesozoic was not a world ruled by dinosaurs alone. This Repenomamus fossil shows that some Cretaceous mammals were already capable of posing a threat even to near-adult small dinosaurs.

Author: Shui-Ye You

Reference:

Han G et al. (2023). An extraordinary fossil captures the struggle for existence during the Mesozoic. Scientific Reports.