Around 66 million years ago, a catastrophic asteroid impact led to the extinction of approximately 75% of Earth’s species, including all dinosaurs except for their avian descendants, the non-avian dinosaurs. An ongoing debate in paleontology revolves around whether dinosaurs were already on the decline prior to this devastating event. Many researchers suggest that about 5 to 10 million years before the extinction, there was a notable decrease in dinosaur biodiversity across the United States, making them more vulnerable to extinction.
To explore this theory, scientists recently re-evaluated fossils from rock layers dating to the end of the dinosaur era that had previously been misdated. Previous studies primarily analyzed fossils from the well-known Hell Creek Formation in Montana. In contrast, the recent research focused on the Nashoibito Formation, a rock formation exposed in New Mexico, composed of ancient river deposits and approximately 30 meters (about 100 feet) thick.
To accurately determine the formation age of the Nashoibito member, scientists utilized a process known as radiometric dating, measuring the decay of radioactive elements within its mineral grains. This technique allowed researchers to establish that these mineral grains must have crystallized prior to their incorporation into the Naashoibito Formation, thus indicating the maximum age of deposition.
The study involved the analysis of 1,046 feldspar crystals from the Naashoibito Formation, which is home to two dinosaur species. Findings revealed that the sandstone located within 5 meters (approximately 16 feet) of the Naashoibito Formation’s lower boundary is younger than 66.9 million years, while layers about 3.5 meters (11 feet) up indicate ages younger than 66.4 million years. This data implies that the sandstone contained dinosaur fossils that formed roughly 400,000 years before the extinction event.
Following this, researchers measured the orientation of magnetic minerals within the rock using a methodology called magnetostratigraphy. The magnetic orientation of these minerals reflects the state of Earth’s magnetic field at that period. Given the Earth’s magnetic field has reversed numerous times throughout history, these signals can help create a chronological framework of when the deposits were formed.
Researchers aimed to ascertain whether the magnetic minerals were aligned as they are currently (normal polarity) or in an opposite orientation (reverse polarity). They identified one rock layer exhibiting positive polarity at the bottom of the Naashoibito Formation and another layer with reverse polarity at the top. The alignment of these magnetic fields corresponds with established polarity intervals consistent with the feldspar ages. Collectively, this data indicates that the Nashoibito member was formed at the conclusion of the dinosaur era, roughly contemporaneously with the Hell Creek Formation.
Next, the research team sought to determine whether non-avian dinosaurs retained their diversity across different areas of western North America during this era. By employing statistical grouping techniques, they analyzed fossil distributions in various locations throughout the continent to assess environmental similarities and differences. Their findings indicated that temperature, rather than geographical location alone, served as the primary predictor of diversity during extinction events.
The study concluded that a diverse assemblage of Nashoibito dinosaurs coexisted with dinosaurs from the Hell Creek Formation. Previous assumptions that dinosaurs had already begun to decline were based on studies focused on the colder northern regions exhibiting lower diversity. The new results imply that warmer southern climates still supported vibrant dinosaur populations. Altogether, this evidence suggests that dinosaurs remained diverse and thriving as recently as 66 million years ago, reinforcing the notion that their extinction was primarily due to the asteroid impact.
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Source: sciworthy.com
