Andrew Flynn has been trying to understand the events surrounding the mass extinction that wiped out the dinosaurs since 2013 while working on a project he began during his Ph.D. at Baylor University.

The results of the New Mexico State University geology professor's research in collaboration with scientists at Baylor University, New Mexico Tech, the University of Edinburgh, the Smithsonian Institution and others around the world will be published in the Oct. 23 issue of the journal Science.

Flynn is the first author on the paper titled "Late-surviving New Mexican dinosaurs illuminate high end-Cretaceous diversity and provinciality" that takes a new look at the last-surviving dinosaur-dominated ecosystems in the San Juan Basin of northwestern New Mexico.

These last non-avian dinosaurs come from the Naashoibito Member in the De-Na-Zin Wilderness Area near Farmington, New Mexico. All dinosaurs, except for birds (which are their modern descendants), became extinct about 66 million years ago when an asteroid slammed into the Yucatan peninsula in Mexico, extinguishing about 75% of all life on Earth.

"The Naashoibito dinosaurs lived at the same time as the famous Hell Creek species in Montana and the Dakotas. They were not in decline. These were vibrant, diverse communities," said co-author Dan Peppe, Baylor professor of geosciences and Flynn's Ph.D. advisor.

Unlike well-studied dinosaur communities from the Hell Creek region, dominated by Triceratops and Edmontosaurus, the New Mexico dinosaur fauna is dominated by a very different kind of dinosaur – the large, long-necked sauropod Alamosaurus. Researchers estimate the Alamosaurus' body at about the length of two semi-tractor trailer rigs and weighing in at 30 to 80 tons, with a long neck and a height of about 30 to 50 feet. Imagine the Alamosaurus as the size of a blue whale but half as heavy.

"Our new data shows that the dinosaurs in New Mexico, which are made up of very different species than those found in Wyoming and North Dakota, are the same age," said Flynn. "What our new research shows is that dinosaurs are not on their way out going into the mass extinction. 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. Another key finding is that after the extinction event, the surviving mammals still retain the same north and south bio provinces. Mammals in the north and the south are very different from each other, which is different than other mass extinctions where it seems to be much more uniform."

Through Earth's history, the direction of the magnetic field flips between normal (where magnetic north is the same direction as today) and reversed (magnetic north is south). Scientists know when these magnetic field reversals happen and are able to use them to estimate when rocks were deposited.

Flynn and his fellow scientists measured the magnetic pole direction of the rocks and, combined with geochemical ages of crystals in sandstones of the same rocks, discovered the dinosaur fossils found in northern New Mexico were the same age as fossils of dinosaurs such as Tyrannosaurus and Triceratops found in the Hell Creek Formation located in the badlands of North Dakota and Montana.

"The extinction of the dinosaurs is the most famous instance of mass death in the history of Earth," said Steve Brusatte, professor of paleontology and evolution at the University of Edinburgh School of GeoSciences and co-author of the study. "An inconvenient truth is that until now paleontologists have had few fossils of dinosaurs unequivocally dated to the last few hundred thousand years of the Cretaceous, before the asteroid hit, so much of our understanding of the extinction was extrapolated from older fossils and statistical analyses.

"Now, in New Mexico, we have fossils of dinosaurs that were there right at the end. These were the dinosaurs that were greeted by the asteroid," Brusatte explained. "And when we compare them with the only other fossils accurately dated from this time, from further north, we can see they are much different. There clearly were many types of dinosaurs thriving up until that moment the asteroid ended it all."

"We're able to conclusively show that these dinosaurs are from the very end of the Cretaceous," Flynn said. "What we found is that these rocks were deposited in the last 380,000 years of the Cretaceous period. These were the very last dinosaurs alive in New Mexico before the asteroid impact."

Flynn's next research focus is to find fossil plants in the Naashoibito Member.

"I'm trained as a paleobotanist," Flynn said. "I study fossil plants, so finding the fossil plants there would allow us to show how different the flora was before and after the extinction in New Mexico. Our previous work in this area gives us a good idea of what the flora looks like just afterward, but we have no idea what it looks like just before. It might be very different, so that's the last missing step.

"We've looked for fossil plants, but so far, we've never found them. It doesn't mean they're not there, it just means we haven't found them yet."

The full article can be seen along with a Spanish version at https://newsroom.nmsu.edu/news/nmsu-professor-s-research-uncovers-last-surviving-dinosaurs-in-new-mexico/s/0edf78ea-8957-493e-b6e1-fdf7dabc9215