Guest Post by: Kyle Atkins-Weltman, Gregory F. Funston, Eric Snively
Palaeontologists argue: were dinosaurs already on their way out when an asteroid hit Earth 66 million years ago and ended the Cretaceous Period, or did it take them at the peak of their dynasty? New evidence from an unexpected discovery has given us a bit more support for the second hypothesis.
The dinosaur extinction debate started in the late 1970s, when some palaeontologists found that there were fewer species known from the final 10 million years of the Cretaceous (145–66 million years ago; Sloan et al., 1986; Van Valen and Sloan, 1977) . Reading this pattern directly, other researchers have interpreted this to mean that the asteroid that struck the Gulf of Mexico was simply the finishing blow for an already precarious lineage (Condamine et al., 2021; Sakamoto et al., 2016).
However, others have pointed to the difficulty of truly counting the diversity of dinosaurs, because the rocks that preserve their fossils have inherent biases in what gets trapped and turned to stone (Bonsor et al., 2020; Russell, 1984; Wang and Dodson, 2006). Their models suggest that the rocks right before the extinction are less likely to preserve some dinosaurs, especially smaller species. This is a problem, because without taking into account changing likelihoods of fossils being preserved, we cannot get the evidence we need to conclusively answer this question.
Our new study may help choosing between these competing hypotheses. We discovered a new species hiding in plain sight, hinting that the diversity we are missing in the latest part of the Cretaceous might be unveiled if we look inside the bones. We used methods that let us reconstruct the age of a fossil from growth lines in its bones. From this, we discovered that what we assumed to be a juvenile of an already-existing species was actually an adult of a completely new species.
When we first examined the bones, we identified them as a rare kind of dinosaur called a caenagnathid – a group of bird-like dinosaurs with feathers, toothless beaks, and hollow bones (Funston and Currie, 2020; Qiang et al., 1998; Xu et al., 2010; Zhou et al., 2000) (Barsbold et al., 2000a, 2000b; Kurzanov, 1982; Lamanna et al., 2014; Pittman et al., 2020a, 2020b). The bones were unearthed in South Dakota, in rocks known as the Hell Creek Formation, dating to the final 2 million years of the Cretaceous. We already know a species of caenagnathid from the Hell Creek, called Anzu (Lamanna et al., 2014), sometimes called the “Chicken from Hell” (Sample, 2014). Because our specimen was significantly smaller than Anzu, and this group is rare, we assumed that our bones were a juvenile Anzu—an exciting chance to learn more about its growth. We noted some differences in the bones, but we chalked these up to features that changed as Anzu grew up.
To confirm the age and maturity of our new specimen, we cut through three of the bones and created microscope slides to examine the internal tissues, a common technique nowadays. As bone grows, it lays down marks each year, like tree rings, so looking at these slides we could see how old this animal was, and how fast it was growing. The results completely uprooted our initial assumptions.
In a juvenile Anzu, we would expect growth marks to be distant from each other, showing that it was adding a lot of new bone each year. But in our bones, we saw that the final growth marks before death were progressively closer spaced, indicating that this animal had slowed down its growth. This was no juvenile – this was an adult of an entirely new but much smaller species, which we dubbed Eoneophron infernalis. The name pays homage not only to where the bones are from (“infernalis” means “Hell” like Hell Creek), but also tribute to a special Nile Monitor named Pharoah (the whole name means “Pharoah’s dawn chicken from Hell”). The differences from Anzu we initially thought might have changed through growth actually help us identify Eoneophron: some ankle bones are fused together and it has longer legs.
When we compared these features to other caenagnathids to figure out its relationships, another specimen from the Hell Creek, a partial foot bone much smaller than even our new specimen, appeared distinct from both Anzu and Eoneophron. Where once there was one “chicken from Hell,” we now had solid evidence for two, and maybe even a third – one large (Anzu), weighing as much as a grizzly bear, one medium (Eoneophron; human-like in weight), and one small , close in size to a German shepherd.
So how does this help us untangle the dinosaur extinction? This new discovery tells us that there may be undiscovered species hiding in plain sight, and so we might be underestimating diversity in these critical rocks, creating an artificial decline. Indeed, by comparing our new picture of diversity in the Hell Creek to earlier ecosystems, we find no decline in diversity in at least one group—caenagnathids—at the very end of the Cretaceous. It seems that caenagnathids like Anzu and Eoneophron were doing quite well for themselves before the asteroid ruined everything.
An unintended analogy for extinction, Hemingway quipped about going broke: slowly, then all at once. Was this the fate of the dinosaurs? There are still many more questions to answer before we know for sure, but we can at least add a caenagnathid-shaped puzzle piece to our emerging picture of the end of the dinosaur era.
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Greg Funston Palaeontology 