A team of paleontologists from Oklahoma State University, the University of Toronto and Royal Ontario Museum has added another species of caenagnathid oviraptorosaurian dinosaur to the prehistoric catalog: Eoneophron infernalis.
An artist’s depiction of Eoneophron infernalis (top left), Anzu wyliei (right) and another caenagnathid dinosaur (bottom left) in the Hell Creek Formation. Image credit: Zubin Erik Dutta.
Eoneophron infernalis lived in what is now the United States during the Late Cretaceous epoch, between 68 and 66 million years ago.
The fossilized remains of the species were collected from exposures of the Hell Creek Formation in South Dakota.
These sediments resemble those of typical floodplain deposits such as siltstones with organic material as seen elsewhere in the Hell Creek Formation.
“When we first examined the bones, we identified them as belonging to a family of dinosaurs known as Caenagnathidae, a group of birdlike dinosaurs that had toothless beaks, long legs and short tails,” said Oklahoma State University paleontologist Kyle Atkins-Weltman and colleagues.
“Direct fossil and inferred evidence indicates these dinosaurs were covered in complex feathers, much like modern birds.”
“The only known species of caenagnathid from this time and region was Anzu. Covered in feathers and sporting wings and a toothless beak, Anzu was between 200 and 340 kg.”
“Despite its fearsome nickname, though, its diet is a matter of debate. It was likely an omnivore, eating both plant material and small animals.”
“Because our specimen was significantly smaller than Anzu, we simply assumed it was a juvenile.”
“We chalked up the anatomical differences we noticed to its juvenile status and smaller size — and figured the animal would have changed had it continued to grow.”
“Anzu specimens are rare, and no definite juveniles have been published in the scientific literature, so we were excited to potentially learn more about how it grew and changed throughout its lifetime by looking inside its bones,” the researchers said.
“Just like with a tree’s rings, bone records rings called lines of arrested growth. Each annual line represents part of a year when the animal’s growth slowed. They would tell us how old this animal was, and how fast or slow it was growing.”
“We cut through the middle of three of the bones so that we could microscopically examine the internal anatomy of the cross-sections. What we saw completely uprooted our initial assumptions.”
“In a juvenile, we would expect lines of arrested growth in the bone to be widely spaced, indicating rapid growth, with even spacing between the lines from the inside to the outside surface of the bone,” they explained.
“Here, we saw that the later lines were spaced progressively closer together, indicating that this animal’s growth had slowed and it was nearly at its adult size.”
“This was no juvenile. Instead, it was an adult of an entirely new species, which we dubbed Eoneophron infernalis.”
Eoneophron infernalis has implications for the ecology and diversity of caenagnathids in the latest Cretaceous period.
As many as three caenagnathid species of varying body size may have inhabited the ecosystems of the Hell Creek Formation, but like in other regions, a poor fossil record makes untangling the taxonomy of these species problematic.
The ecology of caenagnathid dinosaurs likewise remains poorly understood, but Eoneophron infernalis expands the range of variation within Caenagnathidae and suggests that their diversity in Laurasia remained largely stable between 84 and 66 million years ago, and these dinosaurs remained successful components of Laurasian ecosystems until the end-Cretaceous extinction.
“With this new evidence, we started making thorough comparisons with other members of the family to determine where Eoneophron infernalis fit within the group,” the scientists said.
“It also inspired us to reexamine other bones previously believed to be Anzu, as we now knew that more caenagnathid dinosaurs lived in western North America during that time.”
“Our new discovery suggests that this dinosaur group was not declining in diversity at the very end of the Cretaceous.”
“These fossils show that there are still new species to be discovered, and support the idea that at least part of the pattern of decreasing diversity is the result of sampling and preservation biases.”
The discovery of Eoneophron infernalis is reported in a paper in the journal PLoS ONE.
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K.L. Atkins-Weltman et al. 2024. A new oviraptorosaur (Dinosauria: Theropoda) from the end-Maastrichtian Hell Creek Formation of North America. PLoS ONE 19 (1): e0294901; doi: 10.1371/journal.pone.0294901
