A pivotal question in paleontology centers on the state of dinosaurs before the cataclysmic asteroid impact 66 million years ago. Were they flourishing,or were they already on a path to extinction,with the asteroid delivering the final blow?

Unearthing the past: A North american Fossil Study

To investigate this debate,a team of researchers meticulously examined the fossil record of North America,concentrating on the 18 million years leading up to the Cretaceous-Paleogene extinction event. Their findings, published in Current Biology, suggest that dinosaurs were, in fact, thriving before the asteroid’s impact. This conclusion bolsters a growing body of evidence supporting the dinosaurs’ pre-impact vitality.

the Fossil Record: A Matter of Interpretation

Initially, the available fossil data—over 8,000 specimens—suggested a peak in dinosaur species diversity around 75 million years ago, followed by a decline in the subsequent 9 million years before the asteroid impact. Though, interpreting the fossil record requires careful consideration of its inherent biases.

Everything is reduced to the fossil record and its fidelity, or its quality. And there has been an awareness since the 1970s that the fossil record is not precise, but is a biased reflection of the past.
Chris Dean, Paleontology Researcher, University College London

Dean further elaborated on the magnitude of this bias, stating, Only in very recent years we have begun to see the total magnitude (of the problem of bias), when using these large fossil occurrences databases.

Occupation Modeling: A Novel Approach

To gain a clearer understanding of the dinosaur extinction, Dean and his colleagues employed a statistical technique known as occupation modeling.This method, commonly used in modern ecology and conservation, accounts for the possibility that a species may be present in a particular area but go undetected during surveys. This study marks the first request of occupation modeling to the study of dinosaurs on a large scale.

Applying a new technique is really arduous. I don’t think it’s the last word. I’m sure there’s much more to say.
Chris Dean, Paleontology Researcher, University College London

Analyzing Dinosaur Families

The research focused on four major dinosaur families:

• Ankylosauridae: Armored herbivores, such as Ankylosaurus.
• Ceratopsidae: Large, horned herbivores, including Triceratops.
• Hadrosauridae: duck-billed dinosaurs.
• Tyrannosauridae: carnivores, such as Tyrannosaurus rex.

Dean explained the selection process, We observe these bigger groups to have more data. We divide North America into a large space grid (and determine) the places where we can find fossils, (the places where) we have found physically fossil and how many times people have gone to look for fossils (in these places). This data was than fed into a computer model, allowing the researchers to compare the physical fossil record with the model’s predictions, revealing notable discrepancies.

Filling the Gaps in the fossil Record

The occupation model suggested that the proportion of land occupied by these four dinosaur clades remained relatively constant during the 18-million-year period. This indicates that their potential habitat area was stable,and the risk of extinction was low. A clade is defined as a group of organisms believed to have descended from a common ancestor.

Geological Factors and Biodiversity

One factor that may have obscured the true diversity patterns of dinosaurs is the limited availability of exposed rock surfaces from that period. These exposed rocks are crucial for fossil finding.

In this study, we show that this apparent decline is more likely to the result of a reduced sampling window, caused by geological changes in these terminal fossil layers of the Mesozoic – impulsed by processes such as tectonic, mountainous lifting and the backward of sea level – instead of genuine fluctuations in biodiversity.
Alfio Allessandro Chiarenza, Newton International Scholarship of the Royal Society, University College London

Chiarenza further speculated, Dinosaurs were probably not inevitably condemned to extinction at the end of the Mesozoic. if it weren’t for that asteroid, they could continue sharing this planet with mammals, lizards and their surviving descendants: birds.

Expert Perspectives

Zelenitsky, a paleontologist at the University of Calgary, who was not involved in the study, noted that the research highlights the biases that can influence scientists’ understanding of dinosaur diversity leading up to the extinction event. Due to the nature of the rock record,(paleontologists have) found that it was more difficult to detect dinosaurs and,therefore,understand their diversity patterns in that time space just before mass extinction.

She added, It certainly makes sense, since we know that there are biases related to the registration of rocks that can cover real biological patterns. The more rock is exposed on the surface (today), the better it will be our possibility to find dinosaurs in that rock, which in turn leads to a better understanding of their diversity patterns.

A Contrasting View

Mike Benton,a professor of vertebrate paleontology at the University of Bristol,while describing the article as “exhaustive and detailed,” argued that it does not definitively prove that there was no reduction in dinosaur diversity before the extinction event. Benton’s previous work has suggested a decline in dinosaur populations prior to the asteroid impact. He stated, The current article suggests that ‘reduction’ can be explained as a statistical artifact.What it shows is (…) simply that the reduction could be real or could be explained by reduced sampling,from my personal perspective.