An extraordinary fossil captures the struggle for existence during the Mesozoic
By: Gang Han, Jordan C. Mallon, Aaron J. Lussier, Xiao-Chun Wu, Robert Mitchell, and Ling-Ji Li
Summarized by: Julia Zottola is a junior undergraduate student majoring in biology with a double minor in Spanish and geochemistry at Binghamton University. After graduation, she plans to attend graduate school for paleobiology and evolutionary biology. Once Julia earns her graduate degree, she plans to conduct research for a museum or to become a professor at a university. When not studying, Julia enjoys reading, knitting, and cheering for Scuderia Ferrari’s Formula One team.
What data were used?: The article is based on two fossils found in a Lagerstätte, or a place with unusually high levels of fossil preservation, in the Yixian Formation (a formation is a mappable rock layer with similar characteristics) in China. The fossils, a mammal and a dinosaur that were buried together, were determined to be from the Mesozoic Era, which lasted from 251 million years ago to 65.6 million years ago. To determine the specimens’ ages, the femur (upper leg bone) length was measured, along with the progress of bone fusion (as mammals age, the separate growth plates in the leg fuse to end growth) in the case of the mammal. Bone fusion was also used in an equation to establish body mass of Psittacosaurus lujiatunensis (the dinosaur) and Repenomamus robustus (the mammal). The body mass (in grams) was then used in an analysis of predation style and predator-prey body mass comparison. Further bone analysis of P. lujiatunensis and the positions the two specimens were discovered in were used to determine if mammal R. robustus was scavenging or was fighting with the intent to prey on dinosaur P. lujiatunensis.
What was the hypothesis of the article? This article focuses on predation relationships between dinosaurs and mammals, especially on how mammals may have actively preyed on dinosaurs, rather than solely scavenging. The fossils of the two animals that were discovered tackles the previously published idea that mammals stayed out of dinosaurs’ way during the Mesozoic and that mammals were mostly a source of prey for dinosaurs.
Methods: The dinosaur and mammal fossils that scientists identified as P. lujiatunensis and R. robustus, were found in Lujiatun Member in the Yixian Formation, central China. In order to establish the ages of the two specimens at the time of their death, femur lengths were used in a equation from another scientific article, Erikson et al. (2009), as the paleontologists who wrote this study were not able to take samples for the preferred method, skeletochronology (the process of aging a skeleton using the bones’ annual growth lines). The ages that were calculated were age ranges, not specific ages (e.g. teenager or adult). The dinosaur, P. lujiatunensis, walked on two legs when alive, so scientists used published equations for determining body mass from femur length for other two-legged non-avian (non-bird) dinosaurs. To take potential error from the estimations into account, the scientists performed statistical analyses to determine the range of error, and used the calculated body sizes with the least estimated errors. To determine if the mammal was a pack or solitary hunter, four statistical models/graphs were plotted comparing these calculated body masses in grams of both of the fossil species of other known predators (solitary and pack hunters) and their prey (Figure 1).
Results: Analysis of the rock in which the fossils were found indicate that it was created by a volcano; paleontologists currently hypothesize that the two animals were swept up in a hot mudslide of debris caused by a volcanic eruption, called a lahar. P. lujiatunensis’s complete skeleton was found intact, with it crouched on its legs and with the mammal on its left top side. It appears to have been an adult, between 6.5 to 10 years of age, and is estimated to have weighed approximately 10.6 kg at its death. R. robustus was a subadult, or a teenager, which was determined by the bone fusion methods mentioned in the previous section. The mammal weighed approximately 3.43 kg, at the time of its death. Scientists determined that R. robustus was preying on P. lujiatunensis at the time of their death. The mammal’s position on top of the dinosaur seems to have been used to subdue the prey; the mammal also died biting two of the dinosaur’s ribs, which is evidence of its role as predator. Due to the lack of bite marks on the dinosaur’s other bones and the entangled position of the two specimens, it seems that this wasn’t a scavenging attempt on a previously dead P. lujiatunensis, but an active attempt to prey on the animal.
Why is this study important?: This is the first known fossil example that shows active dinosaur-mammal predation interactions. While past specimens have included mammalian fossils with dinosaur bones in their abdominal cavity and vice versa, this fossil is the first discovered “fight to the death” between the two groups. This helps paleontologists learn more about species interactions during the Mesozoic Era (251 to 66.5 million years ago).
Broader implications: Paleontologists can learn more about species’ interactions during the Mesozoic. This fossil discovery also refutes a common misconception about species interactions during what is often called the “Age of Dinosaurs”: that mammals were just small, skittish prey for the dinosaurs to consume. While this is not the first evidence of mammals eating dinosaurs, this is seemingly the first snapshot of clear predation, rather than speculation or scavenging. Predation during that time seems to have gone both ways, rather than placing the role of predator entirely on dinosaurs and that of prey on our mammalian ancestors.
Citation: Han, G., Mallon, J.C., Lussier, A.J. et al. An extraordinary fossil captures the struggle for existence during the Mesozoic. Sci Rep 13, 11221 (2023). https://doi.org/10.1038/s41598-023-37545-8
Extra works cited:
- Campione, N. E., Evans, D. C., Brown, C. M. & Carrano, M. T. Body mass estimation in non-avian bipeds using a theoretical conversion to quadruped stylopodial proportions. Methods Ecol. Evol. 5, 913–923 (2014).
- Erickson, G. M., Makovicky, P. J., Inouye, B. D., Zhou, C.-F. & Gao, K.-Q. A life table for Psittacosaurus lujiatunensis: Initial insights into ornithischian dinosaur population biology. Anat. Rec. 292, 1514–1521 (2009).
- Myhrvold, N. P. Problems in Erickson et al. 2009. Anat. Rec. 298, 489–493 (2015).