The study sheds light on the evolution of the earliest dinosaurs


Uranium-bearing minerals were isolated from microcrystalline rock samples of zircon and analyzed by the U-Pb isotopic technique at the MIT Isotope Lab. These zircones attained an exact age of 221.82 Ma 0.10 Ma for the upper Ischigulasto Formation. Sincerely: Courtesy of Researchers

There are two subdivisions of early dinosaurs based on the classic dinosaur family tree: ornithischian, or bird-hipped dinosaurs, including later triceratops and stegosaurus; And saurischian, or lizard-shaped dinosaurs, such as Brontosaurus and Tyrannosaurus.


In 2017, however, this classical view of the evolution of dinosaurs was put into question with evidence that perhaps lizard-shaped dinosaurs evolved first – a discovery that dramatically reduced the first major branches of the dinosaur family tree Re-arranged.

Now an MIT geochronologist with paleontologists from Argentina and Brazil has found evidence to support the classical view of the evolution of dinosaurs. The team’s findings are published today in the magazine Scientific report.

The team cut fossils of Pisanosaurus, a small bipolar dinosaur considered to be the earliest preserved ornithischian in the fossil record. Researchers determined that the bird — Habibor Habibor — dates back to 229 million years ago, which is also around the time when lizard-hunted saurischians appear.

New times suggest that ornithischians and saurischians first appeared at the same time from a common ancestor and came at the same time, supporting the classical view of the evolution of dinosaurs.

Researchers also ate rocks from the Iskigulasto Formation, a layered sedimentary rock unit of Argentina known to preserve an abundance of fossils of very early dinosaurs. Based on these fossils and others in South America, scientists believe that dinosaurs first appeared in the southern continent, which at the time was linked together with the supercontinent of Pangea. Early dinosaurs are believed to be mutilated and fired around the world.

However, in the new study, researchers determined that the period in which the Ischigulasto Formation overlapped with the timing of another important geological deposit in North America is known as the Chinle Formation.

The middle layers of the Chinle Formation in southwestern America contain fossils of various fauna, including dinosaurs, which appear to be more developed than early dinosaurs. The lower layers of this formation, however, lack any kind of animal fossil evidence, let alone early dinosaurs. This suggests that conditions within this geological window prevented the preservation of any form of life, including early dinosaurs, if they moved to this particular region of the world.

Research scientist at MIT’s Earth Department, Jahandar Ramezhani, says, “If the Chinle and Ischigulasto formations run out in time, the early dinosaurs may not have evolved in South America, but they may have evolved in North America at the same time Roaming around. ” Atmospheric, and Planetary Sciences, who co-wrote the study. “Those northern cousins ​​are not yet protected.”

The other authors of the study are the first authors, the Julia Desojo National University of La Plata Museum and a team of paleontologists from institutions in Argentina and Brazil.

“After the walk”

The earliest dinosaur fossils found in the Iskigulasto Formation is a protected provincial park in San Juan Province known as the “Valley of the Moon”. Geological formations also extend beyond the park, with fewer fossils of early dinosaurs. Ramizani and his colleagues looked outside the park to study accessible outbreaks of the same rocks.

The study sheds light on the evolution of the earliest dinosaurs

Paleontologists wrapping pieces of fossil bone in a plaster jacket prior to transport to the laboratory for preparation and detailed studies. Sincerely: Courtesy of Researchers

He focused on the less studied Hoyada del Cerro Las Lajas of the Ichigualasto Formation in the province of Lo Rioja, which was discovered by another team of paleontologists in the 1960s.

“Our group put their hands on some field notes and excavated fossils from those early paleontologists and thought we should follow in their footsteps,” says Desajo.

In four expeditions between 2013 and 2019, the team collected fossils and rocks from various layers of Las Lajas Outcorp, including more than 100 new fossil specimens, although none of these fossils were of dinosaurs. Nevertheless, he analyzed the fossils and found that they were comparable in both species and relative age to the Nondinosaurus fossils found in the park area of ​​the same Iskigulasto Formation. They also found out that the outcrop at the Ischigulasto Formation Park in Las Lajas was significantly thicker and more complete than the outcrop. This led them to believe that the geological layers in both locations accumulated during the same critical time interval.

Ramejani then analyzed volcanic ash samples collected from several layers of the Las Lajas outcrop. Volcanic ash contains zircon, a mineral that separates it from the rest of the sediment, and is measured for isotopes of uranium and lead, the ratios of which attain the age of the mineral.

With this high-precision technique, Rezmani sampled from the top and bottom of the outcrop, and found that sedimentary layers, and any fossils preserved within them, were deposited 230 million to 221 million years ago. Since the team determined that the layered rocks and parks in Las Lajas matched in both species and relative time, they could now also determine the exact age of the park’s more fossil-containing outbreaks.

Furthermore, this window overlaps substantially with the time interval at which sediments were deposited thousands of kilometers north of the Chinle Format.

“For many years, people thought that the Chinle and Iskigulasto Formations did not overlap, and based on that assumption, they developed a model of diachronous evolution, meaning that the earliest dinosaurs first appeared in South America, Then spread to other parts of the world. North America, “Ramizani says. “We have now studied both structures extensively, and have shown that in reality development is not based on sound development.”

A family tree, preserved

Before Ramazani and his colleagues set out for Las Lazas, other paleontologists explored the area and discovered several fossils, including the remains of Pisanosaurus Merti, a small, light-furred, ground-dwelling herbivore . The fossils are now preserved in an Argentinean museum, and scientists have gone back and forth as to whether this is a true dinosaur belonging to the Ornithisian group, or a “basal dinosaurosomorph” —a sort of pre-dinosaur, which has almost features. But not completely, dinosaurs.

“The dinosaurs we see in the Jurassic and the Cretaceous are very developed, and which we can recognize well, but in the late Triassic, they all look the same, so it is very difficult to separate them from each other. , And from the basal dinosaurosomorph, “Ramadani explains.

His colleague Max Langer of the University of São Paulo, Brazil painstakingly cut the museum-preserved fossil of Pisanosaurus, and concluded, based on some key anatomical features, that it is indeed a dinosaur and even more, that it The earliest preserved ornithischian specimen. Based on Ramazzani’s extraterrestrial dating and interpretation of Pisanosaurus, the researchers concluded that the oldest bird-hyped dinosaurs appeared around 229 million years ago – around the same time as their lizard-hide counterparts.

“Now we can say that the first ornithischians first appeared in the fossil record at about the same time as solarians, so we should not throw away the traditional family tree,” says Ramizani. “These are all debates about where the dinosaurs appeared, how they diversified, what the family tree looked like. A lot of those questions are tied to geomorphology, so to help us answer these questions really Good, strong age is needed. ”


New evidence raises questions about when dinosaurs evolved in North America


more information:
Julia b. DiSojo et al, The Late Triassic Iscigualsto Formation in Cerro Las Lajas (La Rioja, Argentina): fossil tetrapods, high-resolution chronostratigraphy, and faunal correlation, Scientific report (2020). DOI: 10.1038 / s41598-020-67854-1

Provided by Massachusetts Institute of Technology

Quotes: The study sheds light on the evolution of early dinosaurs (2020, July 29). Retrieved July 29, 2020, from https://phys.org/news/2020-07-evolution-earliest-dinosaurs.html

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