NC scientists solve a dinosaur mystery

N.C. scientists use rise and fall of prehistoric islands to explain duck-bill variations

CorrespondentSeptember 30, 2012 

When Terry Gates was a grad student in paleontology at the University of Utah, two dinosaur fossils were being excavated about 1,000 miles apart – one in the Grand Staircase Escalante National Monument in southern Utah, the other in western Montana.

Gates took part in the dig in southern Utah, where a dinosaur skull had been found poking out of a large sandstone block that rested atop a 500-foot hill.

In 2011, more than a decade after the two discoveries, scientists determined that the fossils belonged to the same species, one of the oldest duck-billed dinosaurs found in North America. They named the species Acristavus gagslarsoni, or “non-crested grandfather,” because the skulls of both lacked ornamentation most of their successors sported a few million years later.

Both sets of fossils were about 79 million years old, and their age harbored a revelation about the Campanian, a geologic age that lasted from 83 million years to 72 million years ago and was part of the late Cretaceous epoch.

“Duck-billed dinosaurs were free to walk up and down (the mountainous belt, which ran north to south),” Gates said. “No one knew that. There was no data.”

Gates, now a postdoctoral researcher at Ohio University and a research associate at the N.C. Museum of Natural Sciences, then took this revelation a step further. In a paper published recently in the scientific journal PLoS One he and his co-authors used Acristavus’ ability to roam to solve a paleontological mystery.

Working with Gates were Lindsay Zanno, director of the paleontology and geology research laboratory at the Raleigh museum’s new Nature Research Center and a research assistant professor at N.C. State, and Albert Prieto-Marquez of the Bavarian Geological and Paleontological Collection in Munich, Germany.

Scientists had long wondered why the variety of North American duck-billed and horned dinosaur species increased rapidly in the late Campanian age and then decreased suddenly before non-avian dinosaurs went extinct worldwide about 65 million years ago.

Gates, Zanno and Prieto-Marquez suggested that in the late Campanian approximately two species of duck-billed and horned dinosaur species evolved every million years, because Achristavus’ successors were no longer able to roam as freely. Mountains of the Laramide range had started to rise and about 75 million years ago were tall enough to create barriers between dinosaur habitats and cut populations off from each other. As a result, they evolved into different species.

But by the late Cretaceous, geologic and climate changes in North America again extended the dinosaurs’ habitats – and the rate at which new duck-billed and horned dinosaur species slowed to about one every 2 million years.

Time and difference

The theory squares with what scientists know about consequences of habitat fragmentation, which can occur through, for example, volcanic eruptions, fire or land clearing to farm and construct roads and human settlements.

As competition for food and other resources increases among organisms surviving in smaller and disconnected portions of their former habitat, species become threatened or endangered.

That’s what has happened with tigers, the largest big cats in Asia. In the past century, Asian tigers have been competing for space with expanding human populations. More than 90 percent of the tigers’ historic habitat has been destroyed, degraded or fragmented, according to the World Wildlife Fund.

Today, the number of wild tigers has dwindled to about 3,200 in Asia, a 97 percent reduction, according to the WWF. Tiger species on the islands of Java and Bali have become extinct.

On Sumatra, their last stronghold in Indonesia, fewer than 400 remain in an area measuring one-third of the island, down from about 1,000 tigers in the late 1970s.

What tigers don’t have going for them that duck-billed and horned dinosaurs did is time. Lots of time.

Building mountains takes millions of years – enough time for new species of duck-billed and horned dinosaurs to come about while some relatives go extinct.

“When this happens on a natural cycle, it doesn’t outpace replacement,” said Zanno.

Life on Laramidia

During the Campanian most continents were in the same place as they are today, but satellite photos of North America would have shown a lot less land. The air was warmer, the poles were free of ice and snow and sea levels were much higher than today, Gates said.

West Coast and East Coast states would have been under water, and a seaway hundreds of miles wide extended from the Gulf of Mexico to the Arctic Ocean and into Hudson Bay, dividing the continent into three large islands.

Laramidia is what scientists named the island that stretched more than 1,200 miles from modern Canada to Mexico. The narrow strip of land between the mountains and the seaway is where scientists believe non-avian dinosaurs roamed – at least that’s what the fossil record suggests.

Much of the strip was very green and lush, probably covered with low-lying plants, including ferns and flowering trees, Gates said. He compared the scenery to a dreamscape of a modern-day forest, similar but different. Where Utah is today, the strip of land was swampy, he said. In Texas it was fairly dry. Along with the dinosaurs, birds, fish, frogs, salamanders, lizards and mammals no bigger than a raccoon were likely to inhabit the strip.

Duck-billed and horned dinosaurs were plant eaters that probably lived in herds. Both were common, had few defenses and were the primary prey for large meat-eaters such as the Tyrannosaurus rex, just like water buffaloes and wildebeest are prey for lions in the African savannah today, Gates said.

After 79 million years ago, mountains of the Laramide range started to erupt east to west across the narrow strip, essentially creating fragmented dinosaur habitats.

The fossil record suggests duck-billed and horned dinosaurs developed crests and horns that differed from habitat to habitat. Each species used its ornamentation as a signal to recognize a mate, Zanno said, which would have prevented different species from mating with each other.

As the Laramide range rose and the climate turned cooler, the seaway along eastern Laramidia started to narrow. By 69 million years ago, it had receded so much that land bridges connected Laramidia to the two islands to the east, forming what resembled the North American landmass of today.

The variety of duck-billed and horned dinosaurs decreased once the land bridges allowed the animals to spread out over a much larger area, Gates, Prieto-Marquez and Zanno suggested: The non-restricted habitat slowed genetic exchange and species variation.

The changes the duck-billed and horned dinosaurs went through as the Laramide mountains rose and the seaway receded serves as an example for what happens to organisms when the climate and habitats change, Gates said.

“We know species evolve,” he said, “but it is difficult for us to envision the process over millions of years given our 75-year life spans.”

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