Ancient amphibians as big as alligators died in mass mortality event in Triassic Wyoming

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Dozens of amphibians perished together on an ancient floodplain around 230 million years ago, according to a study published in the open-access journal PLOS One by Aaron M. Kufner of the University of Wisconsin-Madison, U.S., and colleagues.

Early in the Age of Dinosaurs, alligator-sized amphibians called metoposaurid temnospondyls were common in freshwater habitats. Several fossil sites contain large concentrations of temnospondyls buried together, but determining how these bonebeds formed is often difficult due to a lack of context or detailed examination.

In this study, Kufner and colleagues performed an in-depth assessment of a site called Nobby Knob in Dubois, Wyoming, where fossil remains of a temnospondyl called Buettnererpeton bakeri were found in rocks dating to roughly 230 million years ago.

Fine-grained ancient soils and finely layered sediments indicate that these fossils were buried in a floodplain setting. Unlike similar sites, the water was evidently very calm, since even very delicate parts of the skeletons are preserved intact, and the bones aren't arranged in any pattern that would suggest strong currents.

The authors conclude that these animals may have concentrated in this area as a breeding colony or because of restricted waterways due to drought and were ultimately buried together where they died.

This assemblage represents more than 50% of the known fossils of Buettnererpeton, providing an excellent opportunity for future study of the species.

Whether or not this kind of die-off was common among temnospondyls is difficult to assess since few other sites have received this level of detailed examination and there is limited data regarding how temnospondyl burial varies with different environments. The authors suggest these as directions for future research.

"This assemblage is a snapshot of a single population rather than an accumulation over time, that more than doubles the number of known Buettnererpeton bakeri individuals," said Kufner.

Whether or not this kind of die-off was common among temnospondyls is difficult to assess, considering most bonebeds of this group represent accumulations that were transported from the site of death.

Few other metoposaurid sites have received this level of detailed examination, highlighting the importance of mapping and systematic data collection during the excavation and preparation of mass mortality beds. The Nobby Knob fossil assemblage provides an excellent opportunity for future research into Late Triassic ecosystems and metoposaurid biology.

More information: Kufner AM, et al. A new metoposaurid (Temnospondyli) bonebed from the lower Popo Agie Formation (Carnian, Triassic) and an assessment of skeletal sorting, PLOS One (2025). DOI: 10.1371/journal.pone.0317325

Journal information: PLoS ONE

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