It has been a question that has vexed paleontologists for more than 40 years. Dinosaurs were debated in the 70s about whether they were at their peak or in decline before their extinction. The number of species on the scene seemed to decrease during the last million years of the Cretaceous, as scientists noted at that time. Some researchers think the asteroid that hit the Gulf of Mexico was the final blow for a vulnerable group of animals. Some argue that the decrease in dinosaur diversity may be an artifact of how hard it is to accurately count them. Paleontologists can't rely on the accessibility of outcrops to find the types of fossils they need. What was happening to dinosaur diversity at that moment is what our work is about. A close examination of the specimen we thought was a juvenile, revealed that it was actually part of an adult from a completely new species. The life stage of our specimen shows that dinosaur diversity may not have gone down before the asteroid hit, but that there are more species from this time period yet to be discovered.
There are clues inside the dinosaur's bones.
The four hindlimb bones were discovered in South Dakota in the Hell Creek Formation. We identified the bones as belonging to a group of dinosaurs known as the caenagnathids, which had toothless beaks, long legs and short tails. The dinosaurs were covered in feathers similar to modern birds. Anzu, also known as the "chicken from Hell," was a species of caenagnathid that was between 450 and 750 pounds. It's a matter of debate whether it was an omnivore or just a juvenile. The animal would have changed had it continued to grow, because of the differences we noticed to its juvenile status and smaller size. We were excited to learn more about how Anzu grew and changed throughout its life, as it's rare and no definite juvenile has been published in the scientific literature. They would tell us how old the animal was, and how fast it was growing. We were able to see the internal structure of the cross-sections after cutting through the middle of three of the bones.
In a juvenile, we would expect the lines of growth in the bone to be uniformly distributed, with even spacing between the lines from the inside to the outside. This animal was almost at its adult size and its growth had slowed, we saw that the later lines were closer together. It was an adult of a new species we named Eoneophron infernalis. The unique features of the smaller Eoneophron include fused ankle bones and a ridge on one of the foot bones. The caenagnathid family tree is expanding.
We began to make comparisons with other members of the family to see where Eoneophron fit in. It inspired us to reexamine other bones that were thought to be Anzu, as we now know that more caenagnathid dinosaurs lived in western North America during that time. A partial foot bone smaller than our new specimen appeared to be distinct from Anzu and Eoneophron. There are two chickens from Hell, one large and one medium, and one small and yet unnamed, all close in size. A man named Zubin Erik Dutta.
The Dinosaur Park Formation of Alberta preserves dinosaurs that lived between 76.5 million and 74.4 million years ago, and Hell Creek has the same number of caenagnathid species. We found evidence for three species there, including Caenagnathus, which is similar to Anzu, Eoneophron, and Citipes. The new discovery suggests that the dinosaur group was not declining in diversity at the end of the Cretaceous. The fossils show that there are still new species to be found, and support the idea that at least part of the pattern of decreasing diversity is due to sampling and preservation biases. There are still a lot of unanswered questions in the extinction debate, but Eoneophron adds evidence that the caenagnathids were doing well before the asteroid.