The Study

Fabien Condamine, a research scientist at the French National Center for Scientific Research and the Institute of Evolutionary Science of Montpellier, along with his team, compiled data from over 1,600 dinosaur fossils, representing 247 species from six prominent dinosaur families during the Late Cretaceous.

These families included herbivorous ankylosaurs (armored dinosaurs), ceratopsians (horned dinosaurs), hadrosaurs (duck-billed dinosaurs), as well as carnivorous tyrannosaurs, dromaeosaurids, and troodontids (bird-like dinosaurs).

The findings indicated that both herbivorous and carnivorous dinosaurs had been declining for around 10 million years prior to the K-Pg extinction event. “Our analysis of the six most prevalent dinosaur families throughout the Cretaceous shows that while they thrived initially, about 76 million years ago, they began to experience a downturn, with rising extinction rates and a decline in new species emerging,” Condamine explained.

The researchers meticulously recorded fossil occurrences to establish the approximate ages of species' appearances and disappearances. They acknowledged that fossil records represent only a small portion of the Late Cretaceous dinosaur population, as many specimens remain undiscovered. Condamine noted that their models compensated for these gaps in the fossil record.

“The models we employed help estimate the true ages of species’ appearances and extinctions, allowing us to construct diversity curves spanning from their origins to their extinctions,” he said.

The models provided insights into the number of dinosaur species that existed at various points over the last 40 million years of their era, revealing a decline in diversity among all six families, albeit to varying degrees. For example, herbivore diversity sharply fell during the last ten million years of the Cretaceous, particularly among ankylosaurs and ceratopsians, while troodontids showed only a minimal decline in the final five million years.

This decline appears to correlate with heightened extinction rates among older species, with fewer new, resilient species emerging.

Factors of Decline

Climate change likely played a significant role in the dinosaurs' decline. At the end of the Cretaceous, the North Atlantic experienced a drastic temperature drop of 7 °C (12.6 °F). As temperatures fell, herbivorous dinosaurs dwindled in numbers, which may have subsequently impacted carnivorous species that relied on them for food, according to Condamine.

Herbivores are essential to their ecosystems, and their decline could destabilize these environments, leading to an extinction cascade. The research also indicated that longer-lived dinosaur species were more susceptible to extinction, possibly due to their inability to adapt to the changing conditions.

Cooling temperatures may have also affected dinosaur reproduction, akin to modern crocodiles and turtles. If this were true, a shift in embryo sex ratios could have contributed to the loss of diversity during this period of global cooling.

“This cooling trend directly correlates with the increased extinction rates observed in dinosaurs 10 million years before the asteroid impact,” Condamine stated. “Dinosaurs were mesothermic, reliant on their environment’s temperature for their activity.”

Duck-billed dinosaurs, specifically hadrosaurs, might have played a role in this decline among herbivores. The study revealed that the emergence of new hadrosaur species was associated with increased extinction rates of ankylosaurs and ceratopsians, suggesting competition among herbivorous dinosaurs.

Caution in Interpretation

Despite the study's advancements over previous research, its findings should be approached with caution. For instance, it remains unclear whether the observed decline in diversity was primarily due to increased extinction rates, decreased speciation, or a combination of both, as noted by David ?erný, a doctoral candidate at the University of Chicago.

“We're navigating a complex web of inferences; if initial assumptions are flawed, further conclusions may also falter,” ?erný remarked. “Without confidence in whether non-avian dinosaurs faced a decline, exploring the reasons behind that decline becomes less relevant.”

Additionally, the study did not encompass every dinosaur species or family. The examined groups predominantly resided in the Northern Hemisphere, while southern continents hosted diverse dinosaur families such as carnivorous abelisaurids and herbivorous titanosaurs, which may have persisted until the end of the Cretaceous.

Moreover, a decline in species diversity does not necessarily equate to fewer individual animals overall. An ecosystem can be robust with fewer species but may contain a large population of a single family, indicating that dinosaurs may not have been in immediate peril or unable to continue dominating the planet had the asteroid not struck.

“This reduction in diversity may have made dinosaurs more vulnerable to the catastrophic impact of the asteroid, but I doubt it signified a critical crisis or that they were doomed to extinction without the asteroid,” commented Steve Brusatte, a paleontologist at the University of Edinburgh not involved in the study.

At that time, the Atlantic Ocean supported more large predator species than any known ecosystem. Since these predators went extinct alongside the terrestrial dinosaurs, it’s evident that the asteroid’s impact affected more than just the already declining species.

Epilogue

This recent paper adds to the growing body of research examining whether dinosaurs were in decline prior to the asteroid impact that ultimately led to their extinction. While this study employs advanced statistical modeling techniques that address some issues related to fossil record gaps, it does not definitively resolve the critical question.

Thus, further evidence is needed to draw more conclusive insights.

Relevant Reads:

• Tyrannosaurs: The ‘Kings’ of the Cretaceous
  • Meet the famous T.rex and its kin…
• Hadrosaurs: The ‘Duckbills’ of the Cretaceous
  • Meet one of the most successful dinosaur groups of the Mesozoic…
• Horns, Beaks & Impressive Frills: The Mighty Ceratopsians
  • Introducing the remarkable Triceratops and its kin…
• Dromaeosauridae: Meet the “Raptors” of the Mesozoic
  • The feathered predators with the sickle-shaped claws…

References

Condamine, F.L., Guinot, G., Benton, M.J. et al. Dinosaur biodiversity declined well before the asteroid impact, influenced by ecological and environmental pressures. Nat Commun **12,* 3833 (2021). https://doi.org/10.1038/s41467-021-23754-0*