Utah paleontologists probing for signs of dinosaurs' rise
Paleontology • Chinle Formation proves to be rich in clues about the environment where dinosaurs first evolved.
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For paleontologists Randall Irmis and Andrew Milner, the tiny stuff matters, especially when you're exploring the dawn of big reptiles. Microscopic fossilized pollen, two-inch fishes, even the color of the rock that bones are embedded in say a lot about the landscapes dinosaurs roamed, the climate, what they ate and what their prey ate.

And, more important, such details can help explain how they evolved during the late Triassic more than 200 million years ago to dominate the Earth's land forms for millions of years.

Among the best places to look for clues is Utah's Chinle Formation, the dark red and orange conglomerates deposited when the Colorado Plateau was a swampy tropical place.

While exploring federal lands in southeast Utah, Irmis and his team of students and volunteers discovered the remains of small fish and a phytosaur. These finds could lead to troves of ancient bones when they return next year with an excavation permit from the Bureau of Land Management.

"It's not just a few scales and bones, but the whole skeleton of many different fish," said Irmis, a curator at the Natural History Museum of Utah. He blogs about his excavations in The New York Times.

The BLM has asked the news media to not disclose the exact location to deter vandalism or theft of the publicly owned resource.

The phytosaur was a crocodilelike reptile that prowled freshwater environments for prey that included early dinosaurs. The dominant predator, phytosaurs died off like so many other species at the end of the Triassic.

Phytosaur species have a long snout like modern crocodiles, but the nostrils are close to the eyes rather than protruding from the end of the snout. This is evidence that the crocodile did not descend from the phytosaur and the two creatures developed long snouts independently in a process known as "convergent evolution," according to Milner, a paleontologist with St. George's Dinosaur Discovery Site at Johnson Farm.

He and his colleagues have found numerous phytosaurs, as well as early crocodiles that look nothing like their modern descendants.

"When phytosaurs went extinct, it opened up a niche for crocs to evolve," Milner said. The disappearance of the phytosaur also made room for other reptiles.

But dinosaurs were slow to rise to prominence in North America.

They became large and numerous in Europe and in Asia by 215 million years ago, but it was not until the mass extinction that closed the Triassic 200 million years ago that they took off here.

Irmis and Milner hope those little fish and other clues hidden in Chinle sites will help explain why.

"We knew they had good potential and hadn't been searched before. There is so much good rock in Utah that hasn't been searched," said Irmis, who joined the University of Utah's geology faculty not long after earning his doctorate at University of California, Berkeley, in 2008.

Irmis' research aims to reconstruct the ecosystems that characterized the Colorado Plateau when reptiles began evolving upward in size and in diversity.

Utah is already famous for yielding dinosaur bones from the Jurassic and Cretaceous periods.

But the state is also rich in fossil evidence from the two periods that bookend these dinosaur ages that began and ended with massive die-offs — the Triassic, 250 to 200 million years ago, and the Paleocene, 66 to 23 million years ago when mammals rose to fill the vacuum left by dinosaurs' extinction.

The sediments that created the Chinle were deposited at the end of the Triassic and its slope-forming rock, which appears purplish from a distance, is exposed in many places on the Colorado Plateau.

During those times North America was close to the equator. As the Pangea supercontinent broke apart and North America crept northward, the climate became drier. Irmis is interested in how ancient life adapted to these changes. Right now the Triassic fossil record is full of gaps that he hopes to fill.

Irmis' team has yet to identify the species of fish fossils he collected last summer, but it is certain these fish inhabited streams as opposed to lakes.

"It was a large, broad river basin, heavily forested with huge conifers. There were slow-moving rivers packed with all sorts of critters," Milner said. "But as you move up the Chinle, we see changes, with drying out, flash floods. It started getting salt domes and uplift."

The team has yet to discover any dinosaur specimens in the area, which is not surprising, given how rare they were in the early stages of their evolution.

But Milner has found tracks and recovered a limb and other parts of an aetosaur, an armored herbivore. The hope is dinosaur remains will soon emerge from the cobbly outcrops of Chinle rock in southeastern Utah.

"If we find any dinosaurs," Irmis said, "they would be small meat-eaters, not much larger than a dog."

bmaffly@sltrib.com