Dinosaurs Thrived Until Their Last Moments — Evidence from 340,000 Years Before the End in New Mexico: A Challenge to the Dinosaur "Decline Theory"

What's "New": The Timestamp of "Just Before"

The research team dated the lithofacies of the Naashoibito Member in northwestern New Mexico using records of magnetic polarity reversals and the decay of radioactive isotopes contained in microcrystals within rocks (such as volcanic glass). As a result, it was found that the youngest strata of the same layer correspond to about 340,000 years before the K–Pg boundary, suggesting that the dinosaur communities persisted until just before extinction. This directly counters the view that "dinosaurs were already in a long-term decline phase."Reuters

The Last Dinosaur World Differed Between "North" and "South"

In the same era (latest Cretaceous), in the northern regions (such as the Hell Creek Formation in Montana), ceratopsians like Triceratops and hadrosaurids were dominant, whereas in the southern regions (New Mexico), the giant sauropod Alamosaurus, Torosaurus, and crested hadrosaurids had a significantly different lineup. This indicates "provinciality," where environmental factors such as temperature differences divided communities by region, rejecting the simplification that "North America had a single, homogeneous dinosaur fauna."Reuters

That's Why It's Strong: Methodological Duplication

The strength of this study lies in the point that it converges results using different chronological methods (magnetostratigraphy × radioactive isotopes). Since the fossils themselves do not contain carbon and **^14C dating is ineffective**, it is standard to indirectly read time from the surrounding strata. The significance of multiple independent methods pointing to "just before the end" is substantial.AP News

On the Other Hand, Cautionary Views: The Limitations of a Single Site and "North American Bias"

Michael Benton and others from the University of Bristol point out that "while the richness of a single locality can be demonstrated, it cannot immediately determine trends across all of North America or globally." In fact, there are ongoing suggestions that the long-term trend of dinosaur diversity at the end of the Late Cretaceous is heavily influenced by exposure area (preservation bias), leaving room for the simple dichotomy of "decline theory vs. thriving theory" to be unresolved. Nonetheless, it is reasonable to read that the consensus that **"the asteroid was the main cause"** has been rather reinforced.The Guardian

SNS Reactions Summary

• Sharing & Dissemination: Articles from The Guardian and major media were widely shared on Bluesky, with the headline **"thriving"** catching on. It was perceived as an "update to the conventional wisdom that 'dinosaurs were in decline.'"Bluesky Social

• Community Discussion: On Reddit, threads were created in r/Dinosaurs and science-related subs, focusing on discussions like "Generalizing from a single site is dangerous" and "the dual approach of magnetostratigraphy + isotopes is solid," highlighting the methodological and extrapolation limits.Reddit

• The Image of the "Last Giant": An illustration depicting Alamosaurus as the "giant of the end" went viral, visualizing the surprise and romance of **"giant sauropods remaining until the end."**Reuters

Rewriting the Narrative: "What If There Was No Impact?"

This study supports a scenario of **"a sudden catastrophe toppling the rulers"** rather than the narrative that "dinosaurs went extinct because they were 'weakened'." The implication that mature ecosystems differing between north and south were functioning suggests a high possibility that the dinosaur dynasty would have continued if there had been no impact.The Guardian

Remaining Challenges: The "Non-North American" Gap

While North America, especially the western part, has favorable stratigraphic conditions and can be richly discussed, the terminal dinosaur fauna of the southern hemisphere and high latitudes remains fragmentary. If multi-regional comparisons with equivalent chronological precision advance, the relationship between "local prosperity" and "global trends" should become clearer.National Geographic

Research Notes (Bullet Points)

• Subject: Naashoibito Member, New Mexico (San Juan Basin)

• Method: Magnetostratigraphy + Radioactive Isotopes (Microcrystals in Rocks)

• Age: Approximately 340,000 to 400,000 years before the K–Pg boundary

• Included Dinosaurs: Alamosaurus, Tyrannosaurus, (Torosaurus, etc.)

• Significance: Diverse and Healthy Until Just Before the End, Marked Regional Differences Even Within North America

• Caution: Single Site / North American Bias, Cannot Conclusively Determine Global TrendsReuters