Study sheds light on the evolution of the first dinosaurs

The classic dinosaur family tree has two subdivisions of the earliest dinosaurs at its base: ornithischians, or bird-shaped dinosaurs, which include the later Triceratops and Stegosaurus; and the saurisquios, or dinosaurs with lizard hips, such as the brontosaurus and the tyrannosaurus.

However, in 2017, this classic view of dinosaur evolution was challenged with evidence that perhaps lizard-hipped dinosaurs evolved first, a finding that dramatically rearranged the first major branches of the dinosaur family tree.

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

The team re-analyzed fossils of Pisanosaurus, a small bipedal dinosaur believed to be the earliest ornithisian conserved in the fossil record. The researchers determined that the bird-hipped herbivore dates back 229 million years, which is also roughly when the first lizard-hipped saurischians are believed to have appeared.

The new moment suggests Ornithiscians and Saurischians first appeared and broke away from a common ancestor at around the same time, supporting the classic view of dinosaur evolution.

The researchers also dated rocks from the Ischigualasto Formation, a layered sedimentary rock unit in Argentina that is known to have preserved a large number of fossils from the earliest dinosaurs. Based on these fossils and others in South America, scientists believe that dinosaurs first appeared on the southern continent, which at that time merged with the Pangea supercontinent. The earliest dinosaurs are believed to have diverged and spread across the world.

However, in the new study, the researchers determined that the period during which the Ischigualasto Formation was deposited overlaps with the timing of another major geological deposit in North America, known as the Chinle Formation.

The middle layers of the Chinle Formation in the southwestern US contain fossils of various fauna species, including dinosaurs that appear to be more evolved than the earliest dinosaurs. However, the lower layers of this formation lack any animal fossil evidence of any kind, let alone the earliest dinosaurs. This suggests that the conditions within this geological window prevented the preservation of any life form, including the first dinosaurs, if they walked through this particular region of the world.

“If the Chinle and Ischigualasto formations overlap in time, then the first dinosaurs may not have evolved for the first time in South America, but they could also have been wandering around North America at the same time,” says Jahandar Ramezani, a scientist. Researcher from the MIT Earth Department, Atmospheric and Planetary Sciences, who were co-authors of the study. “Those northern cousins ​​may not have been preserved”

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

“Following the steps”

The first dinosaur fossils found in the Ischigualasto Formation are concentrated in what is now a protected provincial park known as “Valle de la Luna” in the province of San Juan. The geological formation also extends beyond the park, albeit with fewer early dinosaur fossils. Instead, Ramezani and his colleagues sought to study one of the accessible outcrops of the same rocks, outside the park.

They focused on Hoyada del Cerro Las Lajas, a less-studied outcrop of the Ischigualasto Formation, in the province of La Rioja, that another team of paleontologists explored in the 1960s.

“Our group put in our hands some of the field notes and fossils excavated from those early paleontologists, and they thought we should follow in their footsteps to see what we could learn,” says Desojo.

During four expeditions between 2013 and 2019, the team collected fossils and multi-layered rocks from the Las Lajas outcrop, including more than 100 new fossil specimens, though none of these fossils were from dinosaurs. However, they analyzed the fossils and found that they were comparable, both in species and in relative age, to the non-dinosaur fossils found in the park region of the Ischigualasto Formation itself. They also discovered that the Ischigualasto Formation at Las Lajas was significantly thicker and much more complete than the outcrops in the park. This gave them confidence that the geological layers at both locations were deposited during the same critical time interval.

Ramezani then analyzed samples of volcanic ash collected from various layers of the Las Lajas outcrops. Volcanic ash contains zircon, a mineral that separated it from the rest of the sediment and measured the isotopes of uranium and lead, the proportions of which produce the age of the mineral.

With this high-precision technique, Ramezani dated samples from the top and bottom of the outcrop, and discovered that the sedimentary layers and fossils preserved within it were deposited between 230 and 221 million years ago. Since the team determined that the layered rocks at Las Lajas and the park coincide in both species and relative time, they could now also determine the exact age of the park’s most fossil-rich outcrops.

Furthermore, this window overlaps significantly with the time interval over which sediments were deposited, thousands of kilometers to the north, in the Chinle Formation.

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

A family tree, preserved

Decades before Ramezani and his colleagues headed to Las Lajas, other paleontologists had explored the region and unearthed numerous fossils, including remains of Pisanosaurus mertii, a small, light-framed, ground-dwelling herbivore. The fossils are now preserved in an Argentine museum, and scientists have debated whether it is a true dinosaur belonging to the Ornithian group, or a “basal dinosauromorph”, a kind of pre-dinosaur, with characteristics that are almost, but not of the everything, dinosaur.

“The dinosaurs we see in the Jurassic and Cretaceous are highly evolved, and we can identify them very well, but in the late Triassic they all looked very much alike, making it very difficult to distinguish them from each other and from the basal dinosauromorphs,” Ramezani explains .

His collaborator Max Langer, from the University of São Paulo in Brazil, carefully analyzed the Pisanosaurus fossil preserved in the museum and concluded, based on certain key anatomical characteristics, that it is in fact a dinosaur and, what is more, that It is the oldest preserved Ornithian. shows. Based on the Ramezani outcrop dating and Pisanosaurus interpretation, the researchers concluded that the first bird-hipped dinosaurs appeared about 229 million years ago, around the same time as their lizard-hipped counterparts.

“We can now say that the first Ornithiscians first appeared in the fossil record at about the same time as the Saurischians, so we shouldn’t throw out the conventional family tree,” says Ramezani. “There are all these debates about where the dinosaurs appeared, how they diversified, what the family tree looked like. Many of those questions are tied to geochronology, so we need really good and robust age restrictions to help answer these questions.” .