For more than 230 million years, dinosaurs and mammals have coexisted on Earth, shaping the course of evolution and leaving a profound impact on the planet’s biodiversity. While both groups have thrived throughout history, direct evidence of their interactions has been scarce, making fossil finds that shed light on their ancient interactions even more extraordinary.
In a groundbreaking discovery that has captured the attention of paleontologists around the world, a new fossil from the Lujiatun Member of China’s Early Cretaceous Yixian Formation has revealed a unique insight into the past. The fossil, designated WZSSM VF000011, features an incredibly rare scene of interlocking skeletons, depicting a fierce fight between a gobiconodontid mammal and a psittacosaurid dinosaur.
The Lujiatun Member of the Yixian Formation is known for its abundance of vertebrate fossils, particularly Psittacosaurus lujiatunensis, a small ceratopsian dinosaur that made up almost 90% of the local fossil assemblages. The discovery site near Lujiatun village in Liaoning province has provided invaluable information about the ancient ecosystem that thrived during the Early Cretaceous.
The dinosaur’s skeleton is complete, allowing paleontologists to confidently identify it as Psittacosaurus Lujiatunensis. Key diagnostic features, such as the width of the prefrontal bone in relation to the nasal bone and the contact between the quadratojugal and squamosal bones along the anterior margin of the quadrate diaphysis, confirm their identity. The position of the dinosaur’s body in the fossil indicates that it lay face down, with the hind limbs bent to either side of the body, while the neck and tail were curved to the left.
The mammal’s skeleton is also remarkably well preserved, although the distal end of the tail is missing. The preservation of key skeletal elements, such as the jaw and teeth, allowed experts to identify it as Repenomamus Robustus. While the mammal’s mandibular and dental features were not fully visible in the fossil, its small size and unique features, such as the faint sagittal and lambdoid crests and zygomatic arches, strongly suggest that it is R. Robustus and not the largest R. Giganticus.
The most surprising aspect of the fossil is the intimate intertwining of the two individuals. The mammal’s left hand firmly grasps the dinosaur’s lower jaw, and its left hind paw becomes trapped inside the dinosaur’s bent left leg, with its hind paw grasping the dinosaur’s shin just below the knee. The mammal’s bite focuses on two of the dinosaur’s left anterior dorsal ribs, which appear to be broken, suggesting a forceful and fatal attack.
The positioning and grasping behavior of the mammal suggest that it was the aggressor in this ancient battle, while the dinosaur was apparently in a defensive posture, attempting to protect itself from the attack. This interpretation aligns with previous research indicating that Repenomamus Robustus was a carnivorous mammal that likely preyed on smaller animals.
Role of volcanic debris flow in fossil preservation
The scientific team that carried out the investigation affirms that the two animals perished in the middle of a conflict, trapped by a flow of volcanic mud that buried them. Since Psittacosaurus showed no signs of biting anywhere else, the researchers firmly concluded that the mammal was actively feeding on a live dinosaur rather than scavenging on a carcass.
An alternative and more convincing explanation is that the mammal was actively feeding on the weakened dinosaur when they were both buried. The mammal’s position on top of the dinosaur, grasping its lower jaw and biting its ribs, suggests attempted predation rather than scavenging behavior. The sudden flow of volcanic debris likely interrupted this fatal encounter, preserving it for millions of years and giving scientists a unique window into this ancient drama.
The preservation and meaning of the fossil comes from the characteristics of the rock matrix in which it was found. Analysis of the sedimentary material revealed that it was a medium-thick brecciated tuff, supported by a matrix, indicative of a volcanic origin and the presence of a volcanic debris flow. This lahar-type volcanic debris flow, which occurred suddenly and violently, likely resulted in the burial of the two combatants and played a crucial role in preserving this ancient battle scene.
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Comparison of the two species.
The two intertwined individuals depicted in the fossil are a small dinosaur, Psittacosaurus lujiatunensis, and an even smaller mammal, Repenomamus Robustus. Psittacosaurus was a relatively small ceratopsian dinosaur, known for its beak-shaped mouth and leaf-shaped molars, indicating a herbivorous diet. Repenomamus, on the other hand, was a gobiconodontid mammal, a group of primitive mammals that displayed a mix of reptilian and mammalian characteristics.
Another fascinating aspect of the fossil find is the estimated body mass of the two individuals. The dinosaur, Psittacosaurus lujiatunensis, had approximately three times the mass of the mammal, Repenomamus Robustus. Despite the significant difference in size, the fossil association is within the 95% prediction intervals for a linear model of maximum prey body mass versus predator body mass among terrestrial carnivores. This analysis indicates that the size difference between the two combatants was not an insurmountable obstacle for the mammal, challenging the notion that predators typically target prey close to their size.
Implications for understanding Mesozoic ecosystems
In fact, current examples of small carnivorous mammals feeding on much larger animals support the plausibility of such an encounter. For example, wolverines have been known to occasionally attack prey many times larger than themselves, such as moose and caribou. This analogy suggests that the small mammal, Repenomamus Robustus, may have had similar predatory abilities and could have been a formidable hunter of even relatively large dinosaurs.
The discovery of this ancient battle scene not only provides insight into predator-prey interactions during the Mesozoic era, but also highlights the exceptional preservation potential of clogging deposits resulting from catastrophic events such as volcanic debris flows. The Lujiatun Member of the Yixian Formation, often referred to as the “Chinese Pompeii,” has played a pivotal role in revealing the diverse fauna of small-bodied dinosaurs and other creatures from this period. As research in the area continues, paleontologists anticipate more fossil discoveries that could shed even more light on dinosaur-mammal interactions and the ecological dynamics of this ancient world.
Despite their smaller size, the mammal’s attempted predation against the dinosaur challenges the common assumption that Mesozoic mammals were simply subordinate to their larger dinosaur counterparts. The fossil provides evidence that Mesozoic mammals were more than mere spectators and could pose a genuine threat even to adult dinosaurs. These insights into the dynamics of ancient ecosystems are invaluable for understanding the complex interactions between species and the factors that shaped their evolutionary trajectories.
Jordan Mallon, a paleobiologist at the Canadian Museum of Nature in Ottawa and co-author of the study, says:
“As productive as the fossil beds in the Lujiatun area are, there are still many unanswered questions about this ancient ecosystem. There is a tendency in those fossil sites to preserve only the small animals that existed at the time. Presumably the larger dinosaurs were able to escape the volcanic landslides that the smaller animals fell victim to, so I would love to learn more about the large animals living in that area, which will require continued prospecting.”
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