Rare fossils reveal a stunning scene from the final days of the dinosaurs

The heyday of the dinosaurs ended in fire. About 66 million years ago, an immense asteroid struck ancient Central America and sparked Earth’s fifth mass extinction, wiping out about 75 percent of known species.

Most of what we know about this fateful moment in life’s story comes from North America, among the haunts of dinosaurs like T. rex and Triceratops in the American West. Paleontologists know far less about how the catastrophe played out farther south—but a dinosaur bonebed recently uncovered in Argentina could help change that.

Called the Cañadón Tomás Quarry, the fossil site has “exceptional potential” to reveal what happened in South America as the mass extinction unfolded, says paleontologist Matthew Lamanna of the Carnegie Museum of Natural History. Discoveries at the site include bones from multiple duckbilled dinosaurs called hadrosaurs that may have lived together in a herd, as well as the tooth of a carnivorous dinosaur, a snake vertebra, and a small mammal jaw. These finds show that the rocks preserved both large and small animals from this prehistoric ecosystem.

Today the area is shrub-covered desert, but around 66 million years ago this part of South America was warm, wet, and covered with plants such as ferns and palms. Previous research has shown Cañadón Tomás had a meandering stream that wound toward the sea surrounded by broad floodplains, says paleontologist Noelia Cardozo of the National University of Patagonia San Juan Bosco. The freshwater habitat allowed the creatures found there to be buried and preserved, providing a rare look at life in this part of the world at the end of the Cretaceous period.

“There are far fewer sites that preserve fossils of land-living vertebrates that date to the very end of the Cretaceous in the Southern Hemisphere,” says Lamanna, who recently described the site at the annual Geological Society of America conference.

While fossil-bearing rocks of the right age are harder to find in the Southern Hemisphere, Lamanna says, experts have spent much more time and effort looking for end-Cretaceous fossil sites in the Northern Hemisphere. “There are fewer researchers, and they are often not as well-funded in the Southern Hemisphere,” says University of Chile paleontologist Alexander Vargas, who was not involved with the new research. The result has been a lopsided view of what transpired before and after the asteroid struck.

The Cañadón Tomás site helps change the story. With large herbivores, signs of carnivores, and material from smaller animals, the quarry provides a window to an entire ecosystem that thrived at the end of the age of dinosaurs.

New bones

In 2020 researchers from the National University of Patagonia San Juan Bosco were searching for new fossil sites in Patagonia when paleontologist Burno Alvarez found the end of a foot bone. It wasn’t just an isolated fragment. Working through torrential rain, the team kept searching until they found an entire bonebed.

Experts from the university soon found evidence of more bones at Cañadón Tomás, revealing there wasn’t just one dinosaur at the site, but several individuals of different ages—a possible indication of a herd.

The hadrosaurs piqued the interest of Lamanna because duckbills are relatively rare in South America’s rocks. Such finds on the continent often turn out to be new species, such as the hadrosaur Gonkoken that Vargas and colleagues described earlier this year from Chile.

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Even so, the site didn’t immediately jump out as extraordinary. “To be honest, I wasn’t super excited about the site right away,” Lamanna says. The field team was studying other fossil sites in the area on around the Río Chico headwaters. But all it took was a little sifting for small fossils to find something that would change Lamanna’s feelings about Cañadón Tomás.

“As soon as the little mammal jaw turned up, in two hours of looking, no less, my tune changed completely,” Lamanna says.

The upper jaw fragment, containing five teeth, came from a Cretaceous mammal called a regitheriid. “They were small herbivorous mammals, instantly recognizable by their highly specialized teeth that are riddled with ridges and grooves,” Lamanna says. The animal was probably rodent-like, about the size of a chipmunk, with teeth for grinding up plants. No one had found a fossil mammal jaw in the entire geologic basin. Suddenly scientists realized the site had the potential to reveal new details of multiple animals just before the extinction event.

The mammal wasn’t the only small, significant find. As experts dug in further, they found the tooth of a carnivorous dinosaur called an abelisaurid, like Carnotaurus, as well as a claw from a smaller theropod dinosaur called a noasaurid. The rocks of Cañadón Tomás also yielded the vertebra of a small snake, underscoring that the site preserves an array of life.

Filling in prehistory

The remains of small animals are important fossil site indicators. They often provide more information about the makeup of the local ecosystem than the large, wide-ranging dinosaurs. And because smaller skeletons decompose and break apart more readily than big, sturdy dinosaur bones, small fossils are rare finds. Mammal jaws with teeth in them, especially, help paleontologists better gauge how human’s ancient relatives fared over time.

The collection of fossils from Cañadón Tomás will act as a test on paleontologists’ hypotheses about this critical time. Some studies have suggested that the number of dinosaur species declined in the Northern Hemisphere during the end of the Cretaceous, perhaps making the animals more vulnerable to extinction. “It’s often assumed that, in the southern continents, these patterns mirrored those in the northern, but is that actually true?” Lamanna wonders.

While all non-avian dinosaurs went extinct after the impact, experts are still unclear about the fates of surviving species in the Southern Hemisphere. “It is possible that distance from the impact site may have favored the survival of some groups in the southern landmasses, such as monotreme mammals and the ancestors of modern marsupials,” Vargas says. This would help explain why these groups of mammals are present in southern landmasses today but almost entirely absent from those to the north.

Excavations and analysis of the fossils found at Cañadón Tomás are still ongoing. The field team plans to return to the site later this year and early in 2024, Cardozo says. Each new find has the potential to fill out a global picture of what the end of the Cretaceous was like, not long before the infamous impact. “More and better fossils are always great news,” Vargas says. And Lamanna is hopeful about what the field team may uncover during expeditions to come.

“If our team has already found fossils of a snake, two or three different kinds of dinosaurs, and a mammal, despite only having literally scratched the surface,” Lamanna says, “who knows what else might be there?”