Newly Discovered, Parrot-Like Dinosaur Roamed North America Alongside T. Rex

While larger dinosaurs are comparatively well-known, finding smaller species paints a more complete picture of life before the mass extinction

Eoneophron Infernalis
Paleontologists have recognized three related, parrot-like dinosaurs in the Hell Creek Formation. Eoneophron infernalis (top left) walks by MOR 752 (bottom left) and Anzu wyliei (right). Illustration by Zubin Erik Dutta / Atkins-Weltman et al., 2024, PLOS One, CC-BY 4.0

For more than a century, paleontologists have been scouring western North America’s Hell Creek Formation for new dinosaurs. The rocks preserve some of the last non-avian dinosaurs before Earth’s fifth mass extinction, including icons such as Tyrannosaurus and Triceratops. Now, experts have uncovered a new dinosaur species from the Cretaceous layers, a turkey-sized creature that was originally thought to be a young example of a different dinosaur.

Named Eoneophron infernalis and described Wednesday in PLOS One, the creature is a member of a mysterious group of roughly parrot-like dinosaurs called caenagnathids. Its discovery helps paint a fuller picture of life on Earth in the days before the infamous asteroid strike 66 million years ago.

Scientists principally identified E. infernalis by closely examining its hindlimb from the thigh bones to the base of the foot. The precise anatomical details of the fossils, as well as the dinosaur’s age at death, help differentiate it from other Cretaceous species and hint at a previously hidden array of beaked dinosaurs that lived alongside iconic Hell Creek species such as Edmontosaurus and Pachycephalosaurus.

The toothless caenagnathids were related to dinosaurs like Oviraptor from Mongolia, although their fossil record in North America is much sparser and often represented only by a few skeletal parts. But a fossil described a decade ago—affectionately nicknamed the “chicken from hell”—finally provided paleontologists with a more complete view of the family. Named Anzu, this dinosaur was big for a caenagnathid, about the size of an ostrich with a long tail, and was the most complete dinosaur of its kind found within the Hell Creek Formation. When paleontologists found smaller caenagnathid bones, it at first seemed likely that they could be from a young Anzu.

When Royal Ontario Museum paleontologist Gregory Funston was contacted about some caenagnathid bones found in the Hell Creek, it appeared that the leg belonged to the famous hell chicken. “Initially, my colleagues and I thought the bones must be from a juvenile Anzu because they were smaller than other specimens but bigger than another species named in 2001,” Funston says. When Funston and co-authors looked at the microscopic details inside the bone, however, they found evidence that the leg represents a novel species.

Just as you can learn about the life of a tree by examining its rings, details of a dinosaur’s life can be seen in the structure of its bones through a science called histology. Different bone tissues represent fast, slow or ceased growth, for example, and many dinosaur specimens show rings from when their growth slowed during harsh seasons. The details inside dinosaur bones help experts estimate how old the animal was when it died and what stage of life it was in, which was the key to realizing Eoneophron is a new species.

If the fossil leg had come from a young dinosaur like a juvenile Anzu, then the bone tissue inside would have shown signs that the dinosaur was still growing and had not yet reached skeletal maturity. Instead, Funston says, “we found closely spaced marks at the bone surface that told us that this animal had drastically slowed its growth in the last few years of its life, or that it was just reaching adulthood.” The leg had come from a dinosaur larger than the smallest caenagnathid but not so large as Anzu, a distinct species. In life, Eoneophron would have likely resembled other caenagnathids in having a toothless beak and some sort of crest.

“The new study provides a strongly argued case for the presence of multiple caenagnathid species in the Hell Creek Formation,” says University of Calgary paleontologist Jared Voris, who was not involved in the new study. The histological data is compelling evidence that Eoneophron is truly a new dinosaur and not just the younger form of another.

Along with another, as-yet-unnamed species, at least three caenagnathids have been found in the Hell Creek Formation. The pattern fits what paleontologists have uncovered elsewhere. “Although this was a bit of a surprise, because we thought we were looking at a juvenile rather than a smaller species,” Funston says, “the presence of a third species makes a lot of sense, because another well-known ecosystem with caenagnathids in Dinosaur Provincial Park, Alberta, has three species of different sizes.”

The existence of three similar dinosaurs at different sizes hints that they were somehow splitting up the ancient habitats to avoid competing for food and other resources. It may be that caenagnathids were primarily plant-eaters, Funston says, just as many small species today are herbivores that dine on different botanical foods in the same habitat.

A century ago, museums in America’s northeast and elsewhere were hoping to acquire stunning dinosaur skeletons for their fossil halls. The focus was on large and relatively complete skeletons. Fossils of smaller species were not only rare, but also often overlooked in favor of reptilian showstoppers. Now that the larger dinosaur species in places such as the Hell Creek Formation are well known, experts have been making a new effort to find, piece together and recognize the menagerie of smaller species that lived alongside the iconic giants.

Caenagnathids are among the most elusive of these dinosaurs. They were small and had relatively delicate bones compared with bigger dinosaurs in the same habitat. Caenagnathids were not only rare but also incompletely preserved in most cases. “It is always a welcome sight to see more research being done on them,” Voris says, “and goes to show that we still have much to learn about the Hell Creek ecosystem.”

Even when paleontologists are setting out to find small dinosaurs, it’s a challenging task. “When you’re out in the hills searching, you are more likely to find a big bone sticking out than a small one,” Funston says, with smaller fossils often collected together at sites full of teeth, scales and bones. Even then, experts need enough of a skeleton to compare with other dinosaurs. “To truly uncover the diversity of small-bodied and elusive animals in any package of rocks, we really have to find relatively complete skeletons, which are doubly rare,” Funston says.

Individual finds such as Eoneophron may change what paleontologists expect of how dinosaurs were faring in the days leading up to the asteroid strike that brought about their demise. The Hell Creek Formation documents this last part of the Cretaceous before impact, and for decades paleontologists have been debating whether the number of dinosaur species was in decline during that time. The available evidence, says Royal Ontario Museum paleontologist and study co-author Jade Simon, is that caenagnathids were holding steady or even increasing in the number of species toward the end of the Cretaceous. “With caenagnathids, it seems that each time we take a closer look we find that their evolutionary story is richer and more complex than previously thought,” she says.

The fact that new small species are being found in Hell Creek Formation rocks indicates that the full count of dinosaurs is not in yet. “Our understanding of the debated dinosaur decline prior to the end-Cretaceous extinction could be based on spotty data,” Funston says. As paleontologists fill in the smaller members of the dinosaur menagerie, experts will assemble a new image of the last days of the dinosaurs.

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