Paleontologists uncovered a 230 million-year-old mass grave of giant amphibians in Wyoming. The discovery, which included at least 19 individuals of the species Buettnererpeton bakeri, revealed that these creatures were as large as adult alligators.
"This assemblage is a snapshot of a single population rather than an accumulation over time," said Aaron Kufner, a paleontologist at the University of Wisconsin-Madison, according to Phys.org.
The researchers determined that the burial site was likely a floodplain. Based on ancient grains of sand and finely layered strata of sediment, they concluded that the fossils were buried under conditions of very calm water, indicating a floodplain setting.
Buettnererpeton bakeri belonged to an ancient group of amphibians called temnospondyls. These creatures were large, primitive four-legged amphibians that could resemble gigantic newts and were widespread in freshwater habitats during the early age of dinosaurs. The metoposaurid temnospondyls, known for their nearly triangular flat heads with eyes on top, could grow as large as modern alligators.
The closely grouped bones at the site suggest that the metoposaurid temnospondyls perished around the same time, as they were not carried to the site by currents.
Bone beds typically form when flowing water deposits bones in the same location over many years, but determining how these bone beds formed can be difficult due to a lack of context or detailed examination. However, the Nobby Knob fossil assemblage provides an opportunity for future research into Late Triassic ecosystems and metoposaurid biology.
The fossils feature preserved delicate small bones and parts of the creatures' overall skeletal structure, indicating that the water at Nobby Knob must have been very calm, helping preserve even very fragile parts of the animals' bodies. The evidence supports the hypothesis that the amphibians died and were buried in a floodplain setting under tranquil conditions.
The researchers found Buettnererpeton bakeri fossils of different sizes. These size variations could help explain how the animals grew and aged. Some of the skeletons were still connected in their original structure, while others were widely scattered.
Alongside the Buettnererpeton bakeri fossils, the research team also found fossilized plants, bivalves, and fossilized poop, called coprolites. These additional findings provide context for the environment in which these amphibians lived and died.
The cause of death of the metoposaurid temnospondyls remains elusive. It is unclear whether their die-off was a common occurrence or an isolated event. Researchers are uncertain if the mass die-off at Nobby Knob was a singular event or if it reflects a broader pattern among metoposaurids.
One theory suggests that the amphibians may have concentrated in the Nobby Knob area as a breeding colony or due to restricted waterways from drought. The researchers theorize that they may have died because they were somehow prevented from leaving a drying body of water they needed to survive, possibly due to a blocked waterway.
"Whether or not this kind of die-off was common among temnospondyls is difficult to assess, considering most bone beds of this group represent accumulations that were transported from the site of death," the scientists wrote in their study.
The Nobby Knob fossil assemblage doubles the previously known specimens of the species Buettnererpeton bakeri. This increase in the number of specimens provides an opportunity for future study and could help scientists date other metoposaurid fossils.
Buettnererpeton bakeri is the oldest known North American metoposaurid. This species lived during the Triassic period, which lasted from 252 million to 201 million years ago. The Triassic is the first of the three periods commonly referred to as the "Age of Dinosaurs."
Temnospondyls, including Buettnererpeton bakeri, were among the first vertebrate animals to conquer land about 350 million years ago. Most temnospondyls were semiaquatic, but some had adapted to become almost fully land-based, returning to water only to breed.
The discovery at Nobby Knob represents an opportunity to study the species Buettnererpeton bakeri and the causes of such mass mortality events among temnospondyls. The authors suggest these as directions for future research.
"This highlights the importance of mapping and systematic data collection during the excavation and preparation of mass mortality beds," the researchers noted.
The article was written with the assistance of a news analysis system.