Forget the sale of the century. The auction house Sotheby’s has geared up for the sale of the epoch. On July 14 it opened live bidding on assorted fossils, but the pièce de résistance is lot 20, a rare 67-million-year-old Tyrannosaurus rex skeleton.

The specimen—dubbed Gus—is billed as one of the largest, most complete T. rexes ever found. Gus is expected to fetch up to $30 million and will go to the highest bidder, whether public museum or private collector. The latter have played an increasingly prominent role in buying fossils, with auction houses, according to paleontologists, contributing to the trend by building hype. But when private collectors swoop in and buy fossils at auction as luxury assets, those pieces of history are effectively lost to science.

By nearly all accounts, Gus is a big deal. In its description, Sotheby’s boasts that the specimen, which was discovered on a ranch in South Dakota, comprises “an incredible 183 fossil bone elements” making it “approximately 61% complete by bone count.” The fossil remains have been mounted in a custom steel armature along with replicas of the missing bones. The result is a reconstructed skeleton posed as if in hot pursuit, its mouth full of dagger teeth ready to tear into prey.

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Forget the sale of the century. The auction house Sotheby’s has geared up for the sale of the epoch. On July 14 it opened live bidding on assorted fossils, but the pièce de résistance is lot 20, a rare 67-million-year-old Tyrannosaurus rex skeleton.

The specimen—dubbed Gus—is billed as one of the largest, most complete T. rexes ever found. Gus is expected to fetch up to $30 million and will go to the highest bidder, whether public museum or private collector. The latter have played an increasingly prominent role in buying fossils, with auction houses, according to paleontologists, contributing to the trend by building hype. But when private collectors swoop in and buy fossils at auction as luxury assets, those pieces of history are effectively lost to science.

By nearly all accounts, Gus is a big deal. In its description, Sotheby’s boasts that the specimen, which was discovered on a ranch in South Dakota, comprises “an incredible 183 fossil bone elements” making it “approximately 61% complete by bone count.” The fossil remains have been mounted in a custom steel armature along with replicas of the missing bones. The result is a reconstructed skeleton posed as if in hot pursuit, its mouth full of dagger teeth ready to tear into prey.

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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.

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