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The study sheds light on the evolution of the earliest dinosaurs



The study sheds light on the evolution of the earliest dinosaurs

Microscopic crystals of uranium containing mineral zirconium were separated from rock samples and analyzed by the U-Pb isotope technique in the MIT Isotope Lab. These zircons gave an exact age of 221.82 ± 0.10 Ma for the upper Ischigualasto formation. Credit: Courtesy of the researchers

The classical dinosaur pedigree has at its base two subregions of early dinosaurs: orniteans or dinosaurs with the birds’ hips, which later include Triceratops and Stegosaurus; and Saurischians or hippos lizards such as Brontosaurus and Tyrannosaurus.

In 2017, however, this classic view of dinosaur evolution was challenged by evidence that perhaps the first-evolved lizard-dinosaurs evolved – a finding that dramatically changed the first major branches of the dinosaur family tree.

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

The team re-analyzed the fossils of Pisanosaurus, a small bipedal dinosaur that is considered to be the first surviving ornithology in the fossil record. The researchers found that the common herbivore dates back to 229 million years ago, which is also about the time when the oldest lizard-hipat Saurischians are believed to have appeared.

The new timing suggests that the Ornithiscians and Saurischians appeared and deviated from the common ancestor at about the same time, which supports the classical view of the evolution of dinosaurs.

Scientists have also dated rocks from the Ischigualasto formation, a layered sedimentary rock unit in Argentina that is known to have preserved many fossils of the earliest dinosaurs. Based on these fossils and others in South America, scientists believe that dinosaurs first appeared on the southern continent, which was associated with the Pangea supercontinent at the time. The early dinosaurs are then thought to have spread around the world.

However, in a new study, the researchers found that the period during which the Ischigualasto formation was saved overlaps with the timing of another important geological deposit in North America, known as the chinchilla formation.

The middle layers of the chinchilla formation in the southwestern United States contain fossils of various animals, including dinosaurs, which appear to be more evolved than the earliest dinosaurs. However, the lower layers of this formation have no animal fossil evidence of any kind, let alone early dinosaurs. This suggests that the conditions in this geological window prevented the preservation of any form of life, including early dinosaurs, as they walked through these particular regions of the world.

“If the Chinle and Ischigualasto formations overlap in time, the first dinosaurs may not have evolved in South America at first, but they may have roamed North America at about the same time,” said Jahandar Ramezani, a scientist at MIT’s Department of Atmospheric and Planetary Sciences. who co-authored the study. “Those northern cousins ​​may not have been preserved.”

Other researchers in this study are the first author Julia Desojo from the National University at the La Plata Museum and a group of paleontologists from institutions in Argentina and Brazil.

“In the footsteps”

The earliest dinosaur fossils found in the Ischigualasto formation are concentrated in the currently protected provincial park known as the “Moon of the Moon” in San Juan Province. The geological formation extends beyond the park, although with a smaller number of fossil early dinosaurs. Ramezani and his colleagues instead watched study one of the accessible exits of the same rocks, outside the park.

The study sheds light on the evolution of the earliest dinosaurs

Paleontologists pack fragments of fossil bones in plaster before transporting them to the laboratory for preparation and detailed studies. Credit: Courtesy of the researchers

They focused on the Hoyada del Cerro Las Lajas, a less studied starting unit of the Ischigualasto formation, in the province of La Rioja, which was examined by another team of paleontologists in the 1960s.

“Our group gave us some of the field notes and excavated fossils of those early paleontologists. We thought we should follow their steps to see what we could learn, ”says Desojo.

During four expeditions from 2013 to 2019, the team collected fossils and rocks from different layers of the Las Lajas starting point, including more than 100 new fossil specimens, although none of these fossils were dinosaurs. However, they analyzed fossils and found that they were comparable in species and relative age to fossils not found in the park in the area of ​​the same Ischigualasto formation. They also found that the Ischigualasto formation in Las Lajas was significantly denser and much more complete than the starting points in the park. This gave them the certainty that the geological strata in both localities were deposited in the same critical time interval.

Ramezani then analyzed samples of volcanic ash collected from several layers of Las Lajas bases. Volcanic ash contains zirconium, a mineral that has separated from the rest of the sediment and measured the isotopes of uranium and lead, the proportions of which give the age of the mineral.

With this high-precision technique, Ramezani dated samples from the top and bottom of the source and found that the sedimentary layers and all the fossils preserved in them were deposited 230 million to 221 million years ago. As the team determined that the layered rocks in Las Lajas and the park coincided in both species and relative timing, they could now also determine the exact age of the fossil fuel-rich bases of the park.

In addition, this window overlaps significantly with the time interval during which sediments were deposited thousands of miles north in the chinchilla formation.

“For many years, people believed that the Chinle and Ischigualasto formations did not overlap, and based on this assumption, they developed a model of diachronic evolution, meaning that the earliest dinosaurs first appeared in South America, then spread to other parts of the world, North America,” he says. Ramezani. “We have now studied both formations thoroughly and shown that diachronic evolution is not really based on sound geology.”

Preserved pedigree

Ten years before Ramezani and his colleagues set out for Las Lajas, other paleontologists explored the region and discovered numerous fossils, including the remains of Pisanosaurus mertii, a small, light-framed, herbivorous bull. The fossils are now kept in an Argentine museum, and scientists have turned to a true dinosaur belonging to the Ornithiscian group or a “basal dinosauromorph” – a species before a dinosaur whose characteristics are almost, but not entirely, a dinosaur.

“The dinosaurs we see in Jurassic and Chalk are very developed, and the ones we can identify nicely, but at the end of the Triassic they all looked very similar, so it’s very difficult to distinguish them from others and from basal dinosauromorphs,” Ramezani explains.

His collaborator Max Langer from the University of São Paulo in Brazil carefully re-analyzed the Pisanosaurus fossil preserved in the museum and, based on certain key anatomical features, concluded that it was indeed a dinosaur – and even more so that it was the earliest ornithology saved. specimen. Based on Ramezan’s dating on the source and interpretation of Pisanosaurus, the researchers concluded that the earliest dinosaurs with bird flanks appeared about 229 million years ago – about the same time as their lizard counterparts.

“We can now say that the earliest ornititines first appeared in fossil records at about the same time as the Saurischians, so we should not throw away the conventional pedigree,” says Ramezani. “It’s all these debates about where dinosaurs appeared, how they diversified, what the pedigree looked like. Many of these questions are related to geochronology, so we need really good and robust age restrictions to be able to answer these questions. ”


New evidence raises questions about when dinosaurs evolved in North America


More information:
Julia B. Desojo et al., Late Triassic formation of ischigualasts in Cerro Las Lajas (La Rioja, Argentina): fossil tetrapods, high-resolution chronostratigraphy and fauna correlations, Scientific reports (2020). DOI: 10,1038 / s41598-020-67854-1

Provides a technology institute in Massachusetts



Citations: Study sheds light on evolution of earliest dinosaurs (2020, July 29), obtained on July 29, 2020 from https://phys.org/news/2020-07-evolution-earliest-dinosaurs.html

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