Around 66 million years ago, an asteroid wiped out 75% of the species on Earth. This event caused the extinction of all dinosaurs except birds, referred to as non-avian dinosaurs. However, an open question in paleontology is whether the dinosaurs were already declining before the asteroid hit. Many scientists think that around 5 to 10 million years before the extinction event, dinosaurs became less biologically diverse across America, making them more prone to extinction.
To investigate this hypothesis, scientists recently revisited fossils in a rock layer from the end of the dinosaur era that previous researchers had incorrectly dated. The bulk of the fossils from this period had previously been collected from a different section of rocks called the Hell Creek Formation in Montana. These researchers studied a rock layer in New Mexico that is exposed to the surface, known as the Naashoibito Member. It’s approximately 30 meters (about 100 feet) thick and formed from ancient river sediments.
To determine when the Naashoibito Member formed, the scientists measured the decay of radioactive elements within its mineral grains over time, a technique called radiometric dating. The scientist reasoned that these mineral grains must have crystallized before being transported and incorporated into the Naashoibito Member, so their youngest ages should provide a maximum depositional age.
They determined the ages of 1,046 crystals of the mineral feldspar from 2 dinosaur-bearing rock layers in the Naashoibito Member. Based on these ages, the researchers found that a sandstone within 5 meters (about 16 feet) of the lowest boundary of the Naashoibito Member must be younger than 66.9 million years old. They found that the other layer, which is around 3.5 meters (11 feet) from the base of the lowest boundary, must be younger than 66.4 million years old. These age ranges indicated that the sandstone contains dinosaur fossils formed 400,000 years before their extinction.
Next, the researchers measured the direction of magnetic minerals inside the rocks, using a practice called magnetostratigraphy. The direction of these minerals indicates where Earth’s magnetic field was pointing at that time. Scientists have shown that the Earth’s magnetic field has flipped multiple times over Earth’s history, so they can use these magnetic signals to create a timescale for when the sediments formed.
The researchers wanted to determine whether the magnetic minerals were lined up in a direction like today, called normal polarity, or the opposite, called reverse polarity. The researchers found one rock layer with normal polarity at the base of the Naashoibito Member, and one with reverse polarity in the upper part of the Naashoibito Member. These magnetic field directions lined up with 2 known polarity intervals over a timeline that matched the feldspar dates. Together, these data demonstrated that the Naashoibito Member formed at the end of the dinosaur era, around the same time as the Hell Creek Formation.
Next, the researchers wanted to test whether non-avian dinosaurs maintained their diversity across different regions of Western North America during this time period. They compared fossil patterns from locations across North America using statistical grouping methods that determine how similar or different their environments were. They found that temperature, rather than geographical location alone, was the best predictor of diversity at the extinction event.
The team concluded that a diverse population of Naashoibito dinosaurs lived at around the same time as the Hell Creek dinosaurs. Earlier scientists surmised that dinosaurs were already on the decline because they studied the colder northern regions, where diversity was lower. However, these results suggest that warmer conditions in the south still supported diverse dinosaur populations. Together, this evidence suggests that dinosaurs were still diverse and thriving 66 million years ago, and that their extinction was indeed a result of the asteroid impact.
