The most commonly suggested reason is that a herd of animals was suddenly killed by a natural disaster, like a volcanic eruption or flood.
Their bodies were buried and remained in proximity to each other as they preserved, and today excavations uncover the remains of the unfortunate herd. Multiple other monospecific bonebeds have been found for other species of horned dinosaurs, such as Achelousaurus, causing researchers to suggest some groups of horned dinosaurs did exhibit herding behaviour— and that sometimes they met sudden unfortunate ends. But is sudden mass death from a natural disaster the only reason for monospecific bonebeds?
Researchers say no. While the monospecific nature is still largely argued to represent herding in many cases, natural disaster is not always the cause of death. Sometimes large numbers of animals die from disease or starvation. Their carcasses could later be pushed together and buried by an event like a mudflow unrelated to their deaths. Their bones could also sit on the surface for years before an event that buries them.
To understand the cause of a bonebed, researchers look at the bones themselves and the sediment that surrounds them. Bonebeds can tell us a lot about how these animals were living— but there is a lot to be learned from trying to figure out how they died, too.
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Two new horned dinosaurs from the Upper Cretaceous Two Medicine Formation of Montana; with a phylogenetic analysis of the Centrosaurinae (Ornithischia: Ceratopsidae). Journal of Vertebrate Paleontology, 15(4), 743-760. • Schmitt, J. G., Jackson, F. D., & Hanna, R. R. (2014). Debris flow origin of an unusual late Cretaceous hadrosaur bonebed in the Two Medicine Formation of western Montana. Hadrosaurs. Indiana Press, Bloomington, 486-501.
