The classic family tree of dinosaurs consists of two subdivisions of early dinosaurs: ornithis, or dinosaurs containing birds, which include later triceratopes and stegosaurs; and saurish, or dinosaurs with lizard thighs, such as Brontosaurus and Tyrannosaurus.
However, in 2017, this classic view of dinosaur evolution was questioned, suggesting that perhaps the first to develop were dinosaurs with a lizard, a finding that abruptly rearranged the first major branches of the dinosaur family tree.
Now the MIT geochrologist, along with paleontologists from Argentina and Brazil, has found evidence to support the classic view of dinosaur evolution. The team’s results were published today in the magazine Scientific reports.
The team realized the fossil of Pisanosaurus, a small bipedal dinosaur that is believed to be the oldest surviving ornithicist in the excavation record. Researchers have found that the herbivore with birds dates back 229 million years ago, as well as around the time when the oldest lizard-eyed lizards are thought to have appeared.
Modern times indicate that the Ornithians and Sauris first appeared and departed from a common ancestor at about the same time, supporting the classical view of the evolution of the dinosaur.
Researchers also dated the rocks to the Ischigualasto Formation, a layered sedimentary rock in Argentina that is known to have preserved a large number of fossils from the earliest dinosaurs. Based on these fossils and others throughout South America, scientists believe that dinosaurs first appeared on the southern continent, which at that time merged with the supercontinent of Pangea. Then, it is believed, the early dinosaurs ran away and swelled around the world.
However, in a new study, researchers determined that the period during which the Ischigualasto Formation persisted coincided with another important geological deposit in North America, known as the Chinlov Formation.
The middle layers of the Chinlov Formation in the southwestern United States contain fossils of various fauna, including dinosaurs, which appear to be more evolving than the oldest dinosaurs. In the lower layers of this formation is missing any species of fossil animals, not to mention the early dinosaurs. This suggests that the conditions in this geological window prevented the preservation of any form of life, including the early dinosaurs, if they walked in this region of the world.
“If the Chinla and Iskigualasto formations overlap over time, the early dinosaurs may not have evolved in South America at first, but they may have roamed North America around the same time,” said Jahandar Ramezani, a researcher at MIT. Atmospheric and planetary sciences, which are co-authors of the study. “Those northern cousins just haven’t survived.”
Other researchers include first author Julia Desojo of La Plata National University and a team of paleontologists from institutions across Argentina and Brazil.
The oldest dinosaur fossils found in the Isigualasto Formation are concentrated in what is now a protected provincial park known as the “Valley of the Moon” in San Juan Province. The geological formation also extends beyond the park, albeit with fewer fossilized early dinosaurs. Instead, Ramezani and his colleagues tried to study one of the available outcrops of the same rocks outside the park.
They focused on Hoiada del Cerro Las Lajas, a less studied outflow of the Ischigualasto formation in the province of La Rioja, which another team of paleontologists studied in the 1960s.
“Our group picked up some field notes and excavated fossils from those early paleontologists, and thought we should follow in their footsteps to see why we could find out,” says Desojo.
During four expeditions between 2013 and 2019, the team collected fossils and rocks from various layers of the Las Lajas outcrop, including more than 100 new specimens of excavation, although none of these minerals were dinosaurs. However, they analyzed the fossils and found that they were comparable, both in species and relative age, to the fossil dinosaurs found in the park area of the same Ischigualast formation. They also found that the Iskgiualasto Formation in Las Lajas was much thicker and much fuller than the outcrop in the park. This gave them confidence that the geological strata in both places were deposited during one critical time interval.
Ramezani then analyzed samples of volcanic ash collected from several layers of Las Lajas outcrops. Volcanic ash contains zircon, a mineral that it separated from the rest of the sediment, and was measured for uranium and lead isotopes, the ratios of which give the age of the mineral.
Using this high-precision technique, Ramezani dated the samples to the top and bottom of the outcrop, and found that the sediments and any minerals preserved in them were deposited between 230 million and 221 million years ago. Because the team determined that the layered rocks in Las Lahas and the park were the same in type and relative timing, they could now also determine the exact age of the parks, which are rich in fossil outcrops of the park.
Moreover, this window resonates significantly with the time interval during which precipitation was deposited, thousands of kilometers to the north, in the Chinle Formation.
“For many years, people thought that the Chinla and Ischigualasto formations did not intersect, and based on this assumption, they developed a model of diachronic evolution, that is, the first early dinosaurs first appeared in South America and then spread to other parts of the world, including North America, says Ramezani, “We have now studied both formations extensively and shown that diachronic evolution is not really based on sound geology.”
Family tree preserved
Decades before Ramezani and his colleagues traveled to Las Lajas, other paleontologists explored the region and found numerous fossils, including the remains of Pisanosaurus mertii, a small grassy light framed by light. The fossils are now preserved in an Argentine museum, and scientists have returned here and there to determine whether it is a true dinosaur belonging to the Ornithic group, or a “basal dinosaur morph” – a species of dinosaur, with features that are almost, but not quite, a dinosaur.
“The dinosaurs we see in the Jurassic and Cretaceous periods are very evolving, and we can identify them perfectly, but in the late Triassic they were all very similar, so it’s very difficult to distinguish them from each other and from basal dinosaur morphs,” explains Ramezani.
His colleague Max Langer of the University of São Paulo in Brazil carefully analyzed the Pisanosaurus fossils preserved in the museum and concluded that he was indeed a dinosaur based on certain key anatomical features. sample. Based on the dating of Ramezan from the exposure and interpretation of Pisanosaurus, researchers have concluded that the oldest dinosaurs that took birds appeared about 229 million years ago – about the same time as their counterparts with lizard thighs.
“Now we can say that the first ornithists first appeared in the fossil record at about the same time as the Savrish, so we should not throw away the traditional family tree,” says Ramezani. “There’s all this discussion about where dinosaurs came from, how they diversified, what the family tree looked like. A lot of these questions have to do with geochronology, so we need really good, reliable age limits to help answer those questions.”
New data raise questions about when dinosaurs evolved in North America
Julia B. Desoyo et al., Late Triassic Ishigualasto formation in Cerro Las Lajas (La Rioja, Argentina): fossil tetrapods, high-resolution chronostratigraphy and faunal correlations, Scientific reports (2020). DOI: 10.1038 / s41598-020-67854-1
Provided by the Massachusetts Institute of Technology
Citation: The study sheds light on the evolution of the oldest dinosaurs (2020, July 29), obtained on July 29, 2020 from https://phys.org/news/2020-07-evolution-earliest-dinosaurs.html
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