4 août 2015

Les dinosaures de l'Utah

Le NYT a un intéressant article à propos d'un des sites paléontologiques sur monde, le Monument National Grand Staircase-Escalante en Utah.

En voici quelques extraits:

(...) In the past 15 years, Dr. Titus and his colleagues at the bureau (...) have excavated tens of thousands of fossils from an extraordinary part of the Grand Staircase monument called the Kaiparowits Plateau, a 50-mile-long, high-elevation ridge.

One of the richest troves of fossils from the Late Cretaceous Period, the Kaiparowits is providing a window into the hothouse world that was home to the dinosaurs in their twilight, about 10 million years before their sudden extinction.

There are a number of well-preserved Late Cretaceous sites in the western United States, including New Mexico’s San Juan Basin; the Judith River, the Two Medicine region and the Hell Creek formation in Montana; and Dinosaur Provincial Park in Alberta, Canada. But the Kaiparowits stands out for the sheer number of well-preserved, unique fossils. Finds from this ancient ecosystem are challenging long-held assumptions about dinosaur physiology, evolution and environment.

Most fossils have been excavated from a part of the plateau called the Kaiparowits formation, a multilayered band of sandstone and mudstone dating from 76.6 million to 74.5 million years ago. Quickly deposited layers of sand and mud buried the fossils in a pristine state.

Their preservation is spectacular: articulated skeletons, fossilized skin, plants so shockingly fresh that their delicate leaves can be peeled right off the rocks. When they are illuminated beneath an epifluorescence microscope, their cuticles, or waxy leaf coverings, fluoresce bright green, revealing their cellular structures.

(...) As many as four species of horned dinosaurs lived here 77 million years ago — twice as many as have been discovered at contemporaneous sites in North America, said Scott Sampson, a paleontologist at the Denver museum, whose team has excavated in the Kaiparowits since 2004.

Hadrosaurs, or duck-billed dinosaurs, are also common in the Kaiparowits, and two new species of tyrannosaurs have been found on the plateau: the 12-foot-tall Teratophoneus currei (“monstrous murderer”), which died 75 million years ago; and Lythronax argestes (“king of gore”), at 81 million years old the oldest true tyrannosaurid known to science.

(...) Today the Kaiparowits is a craggy expanse of shrub-covered rock and sheer cliffs with little moisture; plants hungrily shoot their roots 30 feet down to suck calcium phosphate from still-buried fossils, sometimes destroying them in the process.

But 75 million years ago, the region was a steamy, swampy, coastal forest in southern Laramidia, a narrow continent stretching from Alaska to Mexico that formed about 96 million years ago when the Western Interior Seaway bisected North America, separating the land mass into two continents — Laramidia in the west and Appalachia in the east — for more than 20 million years.

Located about 60 miles from the sea in a flat basin crisscrossed by rivers, lakes and ponds, the Kaiparowits was a water-saturated, multistory forest. Giant pine trees draped with moonseed vines towered over an Everglades-wet forest floor blanketed with gingers, ferns, duckweed, water lettuce and floating, flowering plants.

“It was an extremely diverse, high biomass forest,” said Ian Miller, curator of paleobotany at the Denver museum, which has collected more than 12,000 plant specimens from 75 sites in the area.

(...) All this material supported the site’s showiest residents: the dinosaurs. The Kaiparowits is “the Shangri-La of dinosaurs,” Dr. Sertich said. “It gives you the opportunity to answer pretty much any question you can come up with about how dinosaurs lived and how they evolved.”

One of the most intriguing questions: How did the ecosystem support so many large-bodied dinosaurs?

In the Late Cretaceous, southern Utah was home to nine species of animals that weighed well over 2,000 pounds as adults. Compare that with present-day Africa, which supports only a handful of animals that big: elephants, giraffes, hippopotamuses, buffaloes and rhinoceroses.

“It’s a tiny land mass,” Dr. Sampson said of the plateau. “How did you get so many giants in such a small piece of real estate?”

The answer may lie in the leaves, specifically those from the moonseed family. Found in abundance in the Kaiparowits, the heart-shaped moonseed leaves indicate the presence of a dense vine system in the Late Cretaceous.

Today, forests with the highest biomass have “tons and tons” of vines, Dr. Miller said. The presence of so much moonseed suggests that this ecosystem was fantastically dense, a salad bar for giants.

“To get that many bigger-than-hippo-sized herbivores living in a small space, you need an incredible plant ecosystem to support them,” he said. “This is unprecedented in the Cretaceous. We’ve never seen a forest that was so heavily dominated by vines in the fossil record really anywhere on the planet.”

(...) Specialized adaptation might have eventually posed a problem for these locavores, who might not have easily weathered changing environmental conditions, fluctuating temperatures and changes in sea level common in the last 10 million years of the Cretaceous.

This could explain why fewer big dinosaurs are found in the fossil record as it approaches the mass extinction around 66 million years ago, after which mammals diversified and thrived.

But not everyone is convinced these dinosaurs were so deeply rooted in particular habitats. “That’s a good story,” said Spencer Lucas, a paleontologist at the New Mexico Museum of Natural History & Science (...) “But do I believe it? The jury is still out.”

(...) The area, Dr. Evans said, is adding to our knowledge of ancient western North America, which “is really our only high resolution window into the time period leading up to and through the extinction of the dinosaurs and into the age of mammals.”

He added, “This is really the only place we can study the causes of dinosaur extinction in any detail.”

National Geographic a également un article intéressant sur les dinosaures de l'Utah ici.

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