Approximately 66 million years ago, a major asteroid impact led to the extinction of 75% of Earth’s species, including all non-avian dinosaurs. This catastrophic event radically changed the planet’s biodiversity, sparking discussions among paleontologists regarding the dinosaurs’ fate prior to this event. Many experts propose that around 5 to 10 million years before this extinction, dinosaur diversity was already in decline across North America, positioning them for vulnerability during the catastrophe.
To explore this hypothesis, researchers have reexamined fossils from the terminal dinosaur era, focusing on rock layers that were previously misdated. While many fossils have come from the Hell Creek Formation in Montana, recent studies have shifted attention to the Naashoibit Formation in New Mexico, a rock formation formed from ancient river deposits that is approximately 30 meters (about 100 feet) thick.
Scientists utilized a method called radiometric dating to establish the formation’s timeline by measuring the decay of radioactive elements in mineral grains. These grains must have crystallized prior to being incorporated into the Naashoibit Formation, allowing researchers to determine the maximum age of deposition.
The team analyzed 1,046 feldspar crystals from the Naashoibit Formation, identifying that the associated sandstone is younger than 66.9 million years at its base and older than 66.4 million years about 3.5 meters (11 feet) higher. This indicates that the dinosaur fossils discovered within the sandstone were formed roughly 400,000 years before the mass extinction event.
Following this, researchers also applied a technique known as magnetostratigraphy to assess the orientation of magnetic minerals in the rock. These minerals’ alignment reveals the Earth’s magnetic field direction at the time of formation. Given that the magnetic field has reversed many times in Earth’s history, these signals can help create a chronological framework for when the deposits formed.
To understand the original alignment of the magnetic minerals, the team examined conditions of normal polarity versus reverse polarity. They found a layer with positive polarity at the base and one with reverse polarity at the top of the Naashoibit Formation. The alignment of these magnetic fields corresponds with known polarity intervals, reinforcing that the Naashoibit member formed contemporaneously with the Hell Creek Formation.
To further analyze dinosaur diversity, researchers compared fossil records from various regions of western North America using statistical methods. Their findings revealed that temperature, rather than mere geographic location, was the most significant factor influencing diversity during extinction events.
The researchers concluded that a vibrant population of Naashoibit dinosaurs lived alongside those from the Hell Creek Formation. Earlier assumptions about a decline in dinosaur diversity stemmed from studies focused on colder northern climates, which exhibited lower diversity. These results suggest that warmer southern regions sustained rich dinosaur populations, indicating that dinosaurs were thriving up to the asteroid impact 66 million years ago.
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Source: sciworthy.com