Deux nouveaux mammifères du Jurassique
Nouvelles découvertes fascinantes faites en Chine:
Researchers say these new specimens show that early mammals, though small, were surprisingly diverse.
It lived in a temperate climate zone on the supercontinent Laurasia, surrounded by lush plants and lots of insects on the hilly shores of a lake, where it probably met an unlucky end. One day, it perhaps fell from a treetop and into the lake, where it drowned and, over time, became entombed in the sediment settling on the bottom.
Fast-forward 165 million years, and that lake bed has long dried up. It now lies within the borders of Inner Mongolia, where a fossil-hunting farmer found the creature's remarkably well-preserved skeleton in 2011.
This fossil-rich lake deposit, known as the Daohugou Formation, has already yielded dozens of extinct creatures from the Jurassic Period: a beaverlike swimming mammal, feathered dinosaurs known as Anchiornis, pterosaurs and prehistoric salamanders. When Zhe-Xi Luo, a professor of organismal biology and anatomy at the University of Chicago, saw Agilodocodon, he was amazed by its state of preservation.
"When we got into the study of Agilodocodon, we realized that the outline for the horny sheath of the claws is preserved," Luo told Live Science. "Those soft tissues are not preserved in the vast majority of mammals. It has a very long, curved narrow claw — one feature to show that it is a good climber."
The second creature, Docofossor brachydactylus, was also found by a fossil hunter — this time, in the 160-million-year-old lake deposits of the Ganggou Fossil Site in China's Hebei province, in 2012. Docofossor was even smaller than Agilodocodon — it probably stood just 3.5 inches (9 centimeters) tall and weighed up to 17 grams (0.6 ounces), the researchers said. The earliest underground-dwelling mammal ever found, Docofossor might have been similar to an African golden mole, with short, wide digits good for digging.
(...) The two fossils belong to an order of extinct mammals known as Docodonta, which share a common ancestor with modern mammals. Before the latest discovery, scientists mostly knew about docodonts from fossils of teeth, jaws and other bits of skulls. Previously, Luo said, scientists only had skeletons from the bodies of two docodonts: Castorocauda, the beaverlike swimming mammal found at Daohugou, and Haldanodon, another extinct small mammal found in a coal mine in Portugal in the 1970s. (...)
"What's new with this discovery of two additional docodonts is that one of them turns out to be a subterranean mammal with highly specialized digit patterns; the other is a bona fide excellent tree climber," Luo said. "From their locomotory functions, we can safely infer that docodonts' ecological diversity had a tremendous range — far more so than we previously anticipated."
(...) But one thing about the picture of mammals from the Mesozoic Era (which includes the Jurassic Period) still hasn't changed: These animals were small. Martin noted that the largest known mammal from this time is still the aforementioned Castorocauda, which had an estimated body mass of about 1.75 lbs. (0.79 kilograms). Mammals grew large only after the mass extinction event that wiped out the dinosaurs 65 million years ago.
David Krause, a professor of anatomical sciences at Stony Brook University in New York, said the discovery of such diversity among early mammals suggests one of two possibilities: that early mammals originated even earlier than scientists thought, or that they diversified very quickly. The only way to find out which option is right is to find more fossils — not only in China but in other parts of the world, Krause said in an email.
Pappochelys rosinae: l'ancêtre des tortues
It’s a primitive turtle, but it looks nothing like today’s dome-shelled reptiles. Resembling a broad-bodied, short-snouted lizard, the 240-million-year-old creature—dubbed Pappochelys rosinae—appears to be a missing link between prototurtles and their modern relatives, according to a new study. If so, the find could fill in a number of pieces about turtle evolution.
(...) About two dozen or so fossils of the creature have been recovered, all of them from 240-million-year-old rocks deposited as sediment on the floor of a shallow, 5-kilometer-long lake in what is now southern Germany.
(...) P. rosinae adults likely measured about 20 centimeters long, with half of that being a long, whiplike tail. (The species name is a combination of the Greek words for “grandfather turtle” and the person who helped clean rock from the fossils to prepare them for analysis.) Its peglike teeth suggest the animal fed on worms and other soft-bodied prey, Sues says.
(...) Unlike lizards, but much like the earliest known relative of turtles (Eunotosaurus, which lived in what is now South Africa about 20 million years earlier), Pappochelys’s ribs are broad, dense, and have a T-shaped cross section. In later, full-shelled species of turtles, those ribs are even wider and have fused with each other and certain bones in the shoulder girdle to form a carapace, or upper shell. But unlike the earlier Eunotosaurus, Pappochelys has gastralia, or belly ribs. These free-floating bones developed within the tissue of the underbelly, Sues says; in more evolved species of turtles, these gastralia broaden and fuse to form a plastron, or lower shell.
Because the fossils were originally entombed in lake floor sediments, the researchers suggest that Pappochelys spent a lot of its time in the water and around the lakeshore—a lifestyle similar to that of today’s marine iguanas, Sues says. So having broad, dense bones and gastralia would have acted like a diver’s weight belt, helping Pappochelys fight buoyancy and forage on the lake’s bottom. But these bones would also have had a beneficial side effect: They would have offered some degree of protection from predators, such as large amphibians or fish living in the lake, by deflecting or blunting their bites.
“In the water, predators can get you from all angles,” Sues notes. Over millions of years, evolution sculpted the bones to create the full set of body armor seen in modern-day turtles. The first full-shelled turtles show up in the fossil record about 205 million years ago.
Zhenyuanlong suni
Extrait de la nouvelle:
Researchers have discovered a new predatory dinosaur with a big body, short arms, and multiple layers of large feathers. And it roamed Early Cretaceous China 125 million years ago.
(...) Zhenyuanlong was between 126 and 165 centimeters (around 5 feet) long, and its proportionally short forelimbs supported large, feathered wings. Their findings, published in Scientific Reports this week, suggests that the diversity of feathered dinosaurs is even higher than we thought.
Being about a-meter-and-a-half long, Zhenyuanlong is one of the largest dinosaurs that’s been discovered with such a well-preserved set of bird-like wings and dense feathers on its tail. The feathers found on larger, previously described dinosaurs were simple filaments that resembled hair. The complex feathers of this new species are comprised of fine branches that stem from a central shaft.
Most Liaoning dromaeosaurids were small – between the size of a cat and a medium-sized dog – and they had long forelimbs with broad wings covered in feathers. The one exception is the two-meter-long (6.5-foot-long) Tianyuraptor, who also had comparatively shorter forelimbs but no preserved feathers.
Despite these bird-like wings, the researchers don’t think Zhenyuanlong could fly, at least not using the same muscle-driven flight we see in birds today. The purpose these wings served remains to be revealed.
(...) "This new dinosaur is one of the closest cousins of Velociraptor, but it looks just like a bird," Brusatte says in a statement. "It’s a dinosaur with huge wings made up of quill pen feathers, just like an eagle or a vulture. The movies have it wrong – this is what Velociraptor would have looked like too."
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.
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.
Représentation exacte d'un Psittacosaurus
Extrait de la nouvelle:
This Psittacosaurus also highlights how important research is for creating accurate paleoart. “The days when a paleoartist could simply fabricate any colour pattern they wanted for a dinosaur are gone,” Nicholls points out. “We have evidence of colour patterns in an increasing number of species, and where we don’t there are a great number of trends in nature that need to be considered, not to mention the variety of ways animals create colour.”
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