Rare look at the last days of the dinosaurs shows they were thriving, scientists say

By Ashley Strickland, CNN

(CNN) — A site in the San Juan Basin of northwestern New Mexico is providing a rare glimpse into the last days of the dinosaurs.

Rocks and fossils at the Naashoibito Member site show an ecosystem that was filled with a diverse population of dinosaurs just before they disappeared from Earth.

Paleontologists have long debated if the dinosaurs suddenly went extinct when a 6.2-mile-wide (10-kilometer-wide) asteroid crashed into Mexico’s Yucatán Peninsula 66 million years ago at the end of the Cretaceous Period, or if they were in a gradual decline and living in weakened ecosystems ahead of the catastrophic event.

Answering that question requires finding fossils and dating the surrounding rock to come up with an accurate timeline of the site. But identifying fossils in an area accurately dated to just before the extinction event is rare.

A key site of interest for paleontologists has been the well-studied Hell Creek and Fort Union Formations, located in what is now Montana, Wyoming and the Dakotas.

Hell Creek has preserved evidence of multiple species of Triceratops and Edmontosaurus, as well as rocks, dated to the end of the Cretaceous Period, or just before the dinosaurs went extinct. But the dinosaur community there completely lacked any long-necked species, causing scientists to wonder if those had already disappeared.

Now, new research has dated rocks in the Naashoibito Member to the same time period as the Hell Creek Formation, revealing what kinds of dinosaurs lived in different parts of North America just a few hundred thousand years before going extinct.

Among the dinosaurs that lived in the Naashoibito Member was Alamosaurus, one of the largest long-necked dinosaurs that ever lived, according to a new study published Thursday in the journal Science.

“What our new research shows is that dinosaurs are not on their way out going into the mass extinction,” lead study author Andrew Flynn, assistant professor in the department of geological sciences at New Mexico State University, said in a statement. “They’re doing great, they’re thriving, and that the asteroid impact seems to knock them out. This counters a long-held idea that there was this long-term decline in dinosaur diversity leading up to the mass extinction making them more prone to extinction.”

Cracking the rock time code

Dinosaur fossils were first found in the San Juan Basin beginning in the mid- to late 1800s.

But dating the rock layers containing the fossils is more complicated, Flynn said. First, the layers need to be exposed so they can be studied, and then the rocks themselves must be dated.

“These factors make dinosaur-bearing rocks from the last 400,000 years of the Cretaceous quite rare,” Flynn said. “Additionally, it takes years of work to establish the ages of rocks, with multiple lines of evidence needing to be collected, which sometimes discourages people from going to new areas to work.”

The fossil record is not neatly organized, and some chapters are missing.

“Species might have failed to enter the fossil record, they might have been preserved as fossils but not yet discovered, or they might have been discovered but lack essential context, such as precise estimates of geological age,” wrote Dr. Lindsay Zanno, head of paleontology at the North Carolina Museum of Natural Sciences, in a related article published with the research. Zanno was not involved in the study.

The Naashoibito Member site is problematic because erosion has stripped it of the perfect layers of rock found at other sites in North America, making it difficult to determine when the layers were created, Flynn said.

Work at the site to determine a more detailed timeline of dinosaur habitation began in 2011. The team members measured the thickness and location of different rock layers, collected samples for testing and identified all of the spots where fossils were found over the last century and a half. Sample collection lasted about three to four years just for Cretaceous rocks, while another four to five were spent on the overlying rock that shows when mammals lived at the site after the dinosaurs went extinct.

One way the team was able to date the rocks was by comparing the samples with Earth’s magnetic field, which periodically changes direction.
For instance, right now, magnetic north aligns with the direction of north. But sometimes, magnetic north can align with the direction of south. Scientists have a clear understanding of when these reversals have occurred over time. By measuring the direction of the magnetic field when the rock layers were created, researchers can narrow down the time frame.

“The end-Cretaceous mass extinction, fortunately for us, took place during a relatively short period of reversed polarity, which makes it much easier to say what our data correlates to,” Flynn said.

The team also used radiometric dating — or measuring the decay of certain elements in rocks to determine their ages — on sand grains from sandstone in the Naashoibito Member.

Their results showed that the dinosaur fossils were from a 380,000-year window leading up to the mass extinction event. The team also determined that mammals appeared about 350,000 years after the mass extinction event.

Regions of diverse dinosaurs

The new study paints a portrait of two very different dinosaur communities divided between the north and south regions of the continent.

Both areas had dinosaurs in common, such as Tyrannosaurus rex and Torosaurus. Fossils of duck-billed dinosaurs were also found at each location, but they belonged to different groups. There were no giant, long-necked sauropods to be found in the north, which for a long time led some paleontologists to think that after millions of years of pushing the boundaries of evolution with their humongous sizes, they had gone extinct, said study coauthor Steve Brusatte, a professor of paleontology and evolution at the University of Edinburgh in Scotland.

But they flourished in the south, including Alamosaurus, which reached 100 feet (30 meters) long, a height of 30 to 50 feet (9 to 15 meters) and weighed over 30 tons, Brusatte said. The fact that one of the biggest dinosaurs, and one of the largest animals ever to live on Earth, witnessed an asteroid striking the planet truly illustrates how dinosaurs were thriving to the very end, he said.

“I can imagine the scene, one minute a jet plane-sized dinosaur was shaking the ground as it walked, the next minute the whole Earth was shaking with the energy unleashed by the asteroid,” Brusatte said.

Part of the reason for the differing groups divided between north and south was likely due to climate conditions, Flynn said.

The Naashoibito Member site was like a warm and humid tropical forest, similar to conditions in modern Panama, while the Hell Creek area was at a much lower elevation and included the cooler conditions of an inland sea coastal plain.

“They were doing what dinosaurs had been doing for over 150 million years, adapting to their local conditions, dividing up niches in the food chain, varying in size and shape and diet, exhibiting rich diversity across the landscape,” Brusatte said. “There is no sign that these dinosaurs were in any trouble.”

The new evidence about diverse, late-surviving dinosaurs in New Mexico is exciting, said Michael Benton, professor of vertebrate paleontology at the School of Earth Sciences at the UK’s University of Bristol. Benton was not involved in the new study, and his previous research has suggested a decline in dinosaur populations before the asteroid strike.

“However, this is just one location, not a representation of the complexity of dinosaur faunas at the time all over North America or all over the world,” Benton wrote in an email.

“As the authors also show in the paper, in general dinosaurs of the last 6 million years of the Cretaceous were less diverse, falling from 43 species beforehand to 30 species in western North America. We would suggest that there is evidence for overall declines in dinosaurs towards the end of the Cretaceous, with individual rich faunas where climates were favourable.”

But Darla Zelenitsky, associate professor within the department of Earth, energy, and environment at the University of Calgary, said she believes the new findings could change how researchers think about dinosaurs in North America before the mass extinction event.

Zelenitsky was not involved in the new research, but her team’s ongoing work in Alberta uncovering fossil eggshells linked to diverse dinosaur species aligns with the findings about stable dinosaur populations.

“The research team uncovered compelling new evidence (…) that dinosaurs were still going strong ‘til the very end,” Zelenitsky wrote in an email.

The new study may focus on dinosaurs, but the fact that 75% of species on Earth disappeared at the same time contains a lesson that remains applicable today, Brusatte said.

“Sudden climate and environmental change can catch animals and ecosystems unaware,” he said, “and can defeat even the strongest and most iconic of species.”

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