by Were dinosaurs already on the way out when an asteroid hit Earth 66 million years ago, ending the Cretaceous, the geological period that began about 145 million years ago? It’s a question that has plagued paleontologists like us for over 40 years.
In the late 1970s, a debate began about whether the dinosaurs were at their peak or in decline before their extinction. Scientists noted at the time that while dinosaur diversity appeared to have increased in the geological phase that lasted from 83.6 million to 71.2 million years ago, the number of species on the scene appeared to have declined in the last few million years of the Cretaceous. Some researchers have interpreted this pattern to mean that the asteroid that hit the Gulf of Mexico was just the final blow to an already vulnerable group of animals.
However, others have argued that the apparent decline in dinosaur diversity may be an artifact of how difficult it is to count them accurately. Fossil formations may preserve different dinosaurs more or less often based on factors such as their preferred environment and how easily their bodies fossilized there. The accessibility of different outcrops may influence the types of fossils researchers have found so far. These biases are a problem because the fossils are what paleontologists have to rely on to make a positive answer about how healthy dinosaur populations were when the asteroid hit.
At that crucial moment, what was really happening to a variety of dinosaurs? The discovery, identification and description of new dinosaurs provide vital clues. This is where our work comes in. Close examination of what we thought was a juvenile specimen of a dinosaur species already known from this time period revealed that it was part of an adult from an entirely new species.
Our work focusing on the lifespan of our specimen shows that dinosaur diversity may not have been declining before the asteroid, but that there are still more species from this time period to come – perhaps even through the reclassification of fossils in collections the museums already. .
Clues inside bird dinosaur bones
Our new study focused on four spinal bones – a femur, a tibia and two metatarsals. They were discovered in South Dakota, in the rocks of the Hell Creek Formation, and date back to the last 2 million years of the Cretaceous.
When we first examined the bones, we identified them as members of a family of dinosaurs known as the ceanognatids – a group of bird-like dinosaurs with toothless beaks, long legs and short tails. Direct fossil evidence and inference suggest that these dinosaurs were covered in complex feathers, like modern birds.
It was the only caenagnathid species known from this time and region Anzu, sometimes called the “chicken from hell.” Covered in feathers and sporting wings and a toothless beak, Anzu were approximately 450 to 750 pounds (200 to 340 kilograms). Despite his fearsome nickname, however, his diet is a matter of debate. It was probably an omnivore, eating both plant matter and small animals.
Because our specimen was much smaller than Anzu, we just assumed he was a teenager. We looked at the anatomical differences we noticed in its juvenile stature and smaller size – and we considered that the animal would change if it continued to grow. Anzu specimens are rare, and no definitive juvenile has been published in the scientific literature, so we were excited to learn more about how it grew and changed throughout its life by looking inside its bones.
Just like tree rings, bone records rings called trapped growth lines. Each annual line represents part of a year in which the animal’s growth slowed. They will 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 dispelled our initial assumptions.
In an adolescent, we would expect the lines of growth caught 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. Here, we saw that the later lines were gradually spread closer together, indicating that this animal had slowed down and was almost at adult size.
This was not young. Instead, it was an adult of an entirely new species, which we named Eoneophron infernalis. The name means “Pharaoh’s chicken from Hell,” referring to the nickname of his larger cousin Anzu. Characteristics unique to this species include ankle bones fused to the tibia, and a well-developed ridge on one of its leg bones. These elements were not young Anzu There would be more progress, but unique features at least Eoneophron.
Expanding the caenagnathid family tree
With this new evidence, we began to make thorough comparisons with other family members to find out where Eoneophron infernalis fit within the group.
It also prompted us to re-examine other previously believed bones Anzu, because we now knew that there were more caenagnathid dinosaurs living in western North America during that time. One specimen, part of a leg bone smaller than our new specimen, appeared different from both Anzu and Eoneophron. Where there was once one “chicken from Hell”, now there were two, and evidence of a third: a big one (Anzu), which weighs as much as a grizzly bear, one average (Eoneophron), humanlike in weight, and a small but still nameless head, close in size to a German shepherd.
Comparing Hell Creek with older fossil formations such as Alberta’s famous Dinosaur Park Formation that preserves dinosaurs that lived between 76.5 million and 74.4 million years ago, we see not only the same number of caenagnathid species, but also the same size classes. Yes, we must Caenagnathuscomparable to Anzu, Christ’s notescomparable to Eoneophronand cities, comparable to the third species for which we found evidence. These similarities in both species counts and relative sizes provide compelling evidence that caenagnathids remained stable throughout the late Cretaceous.
Our new discovery suggests that this group of dinosaurs was not declining in diversity at the end of the Cretaceous. These fossils indicate that new species are yet to appear, and support the idea that at least part of the pattern of reduced diversity is the result of sampling and preservation biases.
Did large dinosaurs become extinct as Hemingway’s character managed to break: “gradually, then suddenly”? Although there are still many issues to be resolved in this deportation debate, Eoneophron this adds to evidence that caenagnathids were doing well for themselves before the asteroid destroyed everything.
This article is republished from The Conversation, a non-profit, independent news organization that brings you facts and analysis to help you make sense of our complex world.
It was written by: Kyle Atkins-Weltman, Oklahoma State University and Eric Snively, Oklahoma State University.
Read more:
The authors do not work for, consult with, or own shares in, or receive funding from, any company or organization that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.
