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The debate among paleontologists continues: Were dinosaurs flourishing when a massive asteroid struck Earth 66 million years ago during springtime, or were they already in decline, with the space rock delivering the final blow in a mass extinction event? A new study analyzing North American fossil records from the late Cretaceous period sheds light on dinosaur diversity before this cataclysmic event.
Dinosaur Diversity Before Asteroid Impact Examined
To delve into this long-standing question, a team of scientists scrutinized North America’s fossil record, concentrating on the 18 million years preceding the mass extinction that marked the end of the Cretaceous period. Their fresh evaluation, published in “Current Biology,” bolsters growing evidence suggesting dinosaurs were thriving prior to the asteroid’s devastating impact.
Initially, the available fossils from this era—over 8,000 specimens—appear to indicate that dinosaur species numbers peaked approximately 75 million years ago. This was followed by a reduction in the 9 million years leading up to the asteroid collision.
Fossil Record Fidelity and Bias
“The crux of the matter lies in the fossil record’s fidelity, or its quality. Since the 1970s, there’s been an understanding that the fossil record isn’t entirely precise; it offers a skewed representation of the past,” explained Chris Dean, the study’s lead author and a paleontology research fellow at University College London.
“Only recently, with the advent of extensive databases of fossil discoveries, have we begun to fully grasp the extent of this bias issue,” he added.
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Occupancy Modeling: A Novel Approach
To gain a clearer understanding of the circumstances surrounding the dinosaurs’ demise, Dean and his colleagues employed a statistical technique known as occupancy modeling. This method, currently utilized in ecology and conservation, estimates the probability of a species’ presence in a location, accounting for potential oversights or failures to detect a species even when present. Dean emphasized that this research is the first application of occupancy modeling to dinosaur studies on such a broad scale.
A paleontologist prospecting for dinosaur fossils in North American Cretaceous rocks. Research suggests limited late Cretaceous rock exposure on Earth’s surface might obscure dinosaur diversity understanding from that period.
“Implementing a new methodology is always challenging,” Dean remarked. “This study isn’t definitive, and further discussion is anticipated.”
Focus on Major Dinosaur Groups
For this research, scientists concentrated on four major dinosaur families:
- Ankylosauridae: Armored herbivores like Ankylosaurus.
- Ceratopsidae: Large, horned herbivores including Triceratops.
- Hadrosauridae: Duck-billed dinosaurs.
- Tyrannosauridae: Carnivores such as Tyrannosaurus rex.
“We examined these broader categories to increase our data pool,” Dean stated. “North America was divided into a large grid to pinpoint fossil sites, document fossil discoveries, and record the frequency of fossil searches in these areas.”
This data was input into a computer model. Dean and his team then compared the actual fossil record against the model’s projections, revealing discrepancies.
Addressing Gaps in the Fossil Record
The model indicated that, throughout the 18-million-year timeframe, the proportion of land inhabited by these four dinosaur groups remained relatively constant. This suggests their potential habitat areas were stable, and extinction risk was consistently low.
One factor potentially distorting dinosaur diversity patterns is the scarcity of exposed rock from that period, limiting fossil discoveries.
“Our study demonstrates that the apparent decline is more likely due to a smaller sampling window. This is caused by geological shifts in late Mesozoic fossil-bearing layers—resulting from tectonics, mountain formation, and sea-level changes—rather than actual biodiversity fluctuations,” explained study coauthor Alfio Alessandro Chiarenza, a Royal Society Newton International Fellow at University College London.
“Dinosaurs were likely not inevitably headed for extinction at the Mesozoic’s end,” Chiarenza asserted. “Absent the asteroid, they might still coexist with mammals, lizards, and their avian descendants.”
Insights into Dinosaur Extinction Biases
Paleontologist Darla Zelenitsky from the University of Calgary, who was not involved in the research, commented that the study effectively highlights biases impacting scientists’ comprehension of dinosaur diversity trends leading up to the extinction event.
“Due to the rock record’s nature, detecting dinosaurs and understanding their diversity patterns in the pre-mass extinction window has proven more challenging for paleontologists,” she noted.
“It’s logical, given known biases from the rock record that can obscure true biological patterns. Increased rock exposure enhances our chances of finding dinosaur fossils, improving our grasp of their diversity patterns.”
Divergent Views on Dinosaur Decline
Mike Benton, a vertebrate paleontology professor at the University of Bristol, described the paper as “thorough and detailed” but contested its conclusion that dinosaur diversity wasn’t declining before the extinction. Benton’s prior research suggests dinosaurs were already in decline before the asteroid impact. He was not part of this study.
“This paper proposes the ‘reduction’ could be a statistical artifact,” Benton stated via email. “It indicates that, in my view, the reduction could be either real or explained by decreased sampling.”