A study indicates that dinosaurs were not in decline before the meteorite impact, as some theories suggest

This paper takes a number of lines of evidence (mostly geologic dating and a census of land vertebrates from the end of the Age of Dinosaurs in North America), and combines them to show that dinosaurs were not declining in diversity toward the end of their reign.

The fossil record is always spotty. Not all organisms or species will wind up being fossilized, and not every location on the map will have fossils forming. So there are always questions about what the real diversity of a group was in the past, and how that diversity changed with time.

This new study helps clarify an important issue: whether the Alamosaurus beds (the Naashoibito Member) of the American Southwest were the same age as the classic Hell Creek deposits of Montana, Wyoming, and the Dakotas (our classic dinosaur community from the end of the Cretaceous), or if they might be older in time. The new study combines different types of geologic data to show that the Naashoibito Member is in fact a latest Cretaceous deposit, equivalent in time to the Hell Creek. To put this another way, there were dinosaurs who were part of the Naashoibito community who would have been around to see the effects of the terminal impact.

So since we now know that the Hell Creek and the Naashoibito were equivalent in age, we see that those southwestern dinosaurs were part of the overall North American diversity at the very end. And this shows that the diversity of dinosaurs was not declining (as some researchers had thought, as far back as the late 1800s), but was still going strong.

[Regarding possible limitations] This study tells us about North America, but there is still much of the rest of the world to consider. There are some end-Cretaceous dinosaur sites in Central Europe, in India, in Siberia, and in China. There probably are some in Argentina: that is a region where this kind of detailed fine-scale geologic dating will help to resolve how South American dinosaur diversity was changing (if at all).

I read the article on the dinosaur extinction in the journal of Science that proposed the dinosaurs "were not in long-term decline but were instead ended abruptly by the end-Cretaceous asteroid impact". This ‘theory’ was first proposed by Luis Alvarez, Nobel Prize winner and his Geologist son Walter in 1980 with great fanfare. Ever since then the theory has been repeated with little or no change and most people believe it. The article is not of good quality because much information is omitted.

In contrast, the alternate theory of climate and environmental change leading to the dinosaurs long-term decline over 300 to 400 hundred thousand years is well documented by dinosaur experts. But the asteroid impact theory remains unquestionably accepted by many dinosaur experts to this day.

What is the true age of the asteroid impact? This question has been consistently ignored and/or denied by impactors, including discoveries of pristine impact glass the spherules in 1999 and 2000 and new drilling in 2003. The true age the Chicxulub impact was discovered to have crashed in Yucatan 200,000 years BEFORE the mass extinction of the dinosaurs 66 million years ago. The impact caused no significant environmental change or extinctions. It was a mere blip in our universe as life continued as usual.

Then 66  million years ago volcanic eruptions exploded caused unprecedented inferno and rapid extinction of the dinosaurs along with the smallest species survivor, the tiny single-celled Guembelitria cretacea that still survives today. This is the mass extinction that caused the end of the dinosaurs and other species 66 million years ago. And so it’s not an asteroid that caused the mass extinction, but massive volcanic eruptions in India.

[About the article] Rather poor quality with the same errors and incomplete data repeated. Existing evidence has been outdated for decades, but still today the same “theories are repeated”. There is no new evidence, just repeat of old believes. I see no new data, just the same old believes that never changed.

This paper is of good quality and certainly very interesting, providing new data, especially in terms of dating, and showing in all cases the presence of a fairly diverse fauna of dinosaurs around 340,000 years before the meteorite impact. 

he study challenges the idea of a uniform dinosaur fauna during the Maastrichtian period (latest Cretaceous), suggesting that dinosaurs were thriving and diverse on a regional level until their abrupt extinction following the asteroid impact. Another very interesting point is that the authors demonstrates that faunal provinciality persisted across the extinction event, countering the idea that the extinction homogenized ecosystems.

In my opinion, several important weaknesses can be identified:

• Short term patterns of extinction: 

It is well established that two hypotheses prevail regarding the extinction of the dinosaurs: the Deccan volcanism that began around 300,000 years before the Cretaceous-Paleogene (K-Pg) boundary, reaching its peak during the last 100,000 years of the Cretaceous period, and the Chicxulub impact. Despite the improved dating of the Naashoibito Member, the temporal resolution (~340,000 years from the K–Pg boundary) may still be too coarse to capture finer-scale ecological or evolutionary dynamics (e.g., huge volcanism leading to their decline and final extinction during the impact). Especially since the dinosaurs have been found around 20m below the KPg boundary and the age of the level is 66.38 Ma.  

Although the study sheds light on the Naashoibito Member in southern Laramidia, its findings are limited to North America. This limits the applicability of the conclusions to global dinosaur extinction patterns. 

Like other regions, the fossil record in the San Juan Basin is subject to sampling biases. While the study acknowledges this, it does not fully address how these biases might influence interpretations of diversity and provinciality. 

The paper briefly mentions evidence from Europe and South America, but these regions are not included in the quantitative analyses (idem for India). This restricts the scope for generalizing findings about dinosaur diversity and extinction on a global scale. 

• Long term patterns of extinction: 

To date, the best paper on the long-term decline of dinosaurs prior to the K-Pg boundary impact is undoubtedly the 2021 paper by Condamine et al. published in Nature Communications. The authors cite it in their paper, but do not really address it. The Condemine et al paper discusses the decline in dinosaur biodiversity prior to the asteroid impact 66 million years ago.  The study analyzed six key dinosaur families and found evidence of a decline in diversity beginning around 76 million years ago, long before the Cretaceous–Paleogene (K–Pg) extinction event.  This decline was driven by environmental and ecological factors, including global climate cooling and a drop in the diversity of herbivorous dinosaurs, likely due to competition from hadrosaurs.

Condamine et al also suggests that older dinosaur species were more susceptible to extinction during this period, potentially because they were unable to adapt to changing environments.  It concludes that long-term environmental changes, such as global cooling and shifts in herbivore diversity, made dinosaurs more vulnerable to the catastrophic asteroid impact that ultimately caused their extinction.

The geographic range of a species is often considered a key factor in its extinction rate, with those that are widely distributed typically being buffered against extinction.  However, the pattern is reversed in the case of dinosaurs.  During the Campanian (pre-declining phase), dinosaurs exhibited higher endemism, meaning they were more locally species-specific. In contrast, in the Late Campanian – Maastrichtian (the declining phase that started 76 million years ago, before the K-Pg), dinosaurs had a more widespread distribution.  Despite this broader geographic range, extinction rates increased during the Maastrichtian period, suggesting that geographic range alone did not protect dinosaurs from extinction.

In conclusion, I believe that this paper does not contradict the idea that dinosaurs began to decline approximately 76 million years ago, during the Late Campanian period.  This decline was characterised by a substantial rise in extinction rates, surpassing speciation rates and resulting in negative net diversification.  This decline was driven by factors such as global cooling and a decrease in the diversity of herbivorous dinosaurs, making them more vulnerable to extinction even before the asteroid impact at the Cretaceous–Paleogene (K–Pg) boundary 66 million years ago.