{"id":18826,"date":"2020-08-01T17:58:21","date_gmt":"2020-08-01T17:58:21","guid":{"rendered":"https:\/\/www.revoscience.com\/en\/?p=18826"},"modified":"2020-08-01T18:04:52","modified_gmt":"2020-08-01T18:04:52","slug":"study-sheds-light-on-the-evolution-of-the-earliest-dinosaurs","status":"publish","type":"post","link":"https:\/\/www.revoscience.com\/en\/study-sheds-light-on-the-evolution-of-the-earliest-dinosaurs\/","title":{"rendered":"Study sheds light on the evolution of the earliest dinosaurs"},"content":{"rendered":"\n

Geological evidence suggests the known dinosaur groups diverged early on, supporting the traditional dinosaur family tree<\/strong><\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Jennifer Chu, MIT News Office<\/strong><\/p>\n\n\n\n

CAMBRIDGE, Mass. — The classic dinosaur family tree has two subdivisions of early dinosaurs at its base: the Ornithischians, or bird-hipped dinosaurs, which include the later Triceratops<\/em> and Stegosaurus<\/em>; and the Saurischians, or lizard-hipped dinosaurs, such as Brontosaurus<\/em> and Tyrannosaurus<\/em>.<\/p>\n\n\n\n

In 2017, however, this classical view of dinosaur evolution was thrown into question with evidence that perhaps the lizard-hipped dinosaurs evolved first \u2014 a finding that dramatically rearranged the first major branches of the dinosaur family tree.<\/p>\n\n\n\n

Now an MIT geochronologist, along with paleontologists from Argentina and Brazil, has found evidence to support the classical view of dinosaur evolution. The team\u2019s findings are published today in the journal Scientific Reports.<\/em><\/a><\/p>\n\n\n\n

The team reanalyzed fossils of Pisanosaurus<\/em>, a small bipedal dinosaur that is thought to be the earliest preserved Ornithiscian in the fossil record. The researchers determined that the bird-hipped herbivore dates back to 229 million years ago, which is also around the time that the earliest lizard-hipped Saurischians are thought to have appeared.<\/p>\n\n\n\n

The new timing suggests that Ornithiscians and Saurischians first appeared and diverged from a common ancestor at roughly the same time, giving support to the classical view of dinosaur evolution.<\/p>\n\n\n\n

The researchers also dated rocks from the Ischigualasto Formation, a layered sedimentary rock unit in Argentina that is known for having preserved an abundance of fossils of the very earliest dinosaurs. Based on these fossils and others across South America, scientists believe that dinosaurs first appeared in the southern continent, which at the time was fused together with the supercontinent of Pangaea. The early dinosaurs are then thought to have diverged and fanned out across the world.<\/p>\n\n\n\n

However, in the new study, the researchers determined that the period over which the Ischigualasto Formation was deposited overlaps with the timing of another important geological deposit in North America, known as the Chinle Formation.<\/p>\n\n\n\n

The middle layers of the Chinle Formation in the southwestern U.S. contain fossils of various fauna, including dinosaurs that appear to be more evolved than the earliest dinosaurs. The bottom layers of this formation, however, lack animal fossil evidence of any kind, 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 walked this particular region of the world. <\/p>\n\n\n\n

\u201cIf the Chinle and Ischigualasto formations overlap in time, then early dinosaurs may not have first evolved in South America, but may have also been roaming North America around the same time,\u201d says Jahandar Ramezani, a research scientist in MIT\u2019s Department of Earth, Atmospheric, and Planetary Sciences, who co-authored the study. \u201cThose northern cousins just may not have been preserved.\u201d<\/p>\n\n\n\n

The other researchers on the study are first author Julia Desojo from the National University of La Plata Museum, and a team of paleontologists from institutions across Argentina and Brazil.<\/p>\n\n\n\n

\u201cFollowing footsteps\u201d<\/strong><\/p>\n\n\n\n

The earliest dinosaur fossils found in the Ischigualasto Formation are concentrated within what is now a protected provincial park known as \u201cValley of the Moon\u201d in the San Juan Province. The geological formation also extends beyond the park, albeit with fewer fossils of early dinosaurs. Ramezani and his colleagues instead looked to study one of the accessible outcrops of the same rocks, outside of the park.<\/p>\n\n\n\n

They focused on Hoyada del Cerro Las Lajas, a less-studied outcrop of the Ischigualasto Formation, in La Rioja Province, which another team of paleontologists explored in the 1960s.<\/p>\n\n\n\n

\u201cOur group got our hands on some of the field notes and excavated fossils from those early paleontologists, and thought we should follow their footsteps to see what we could learn,\u201d Desojo says.<\/p>\n\n\n\n

Over four expeditions between 2013 to 2019, the team collected fossils and rocks from various layers of the Las Lajas outcrop, including more than 100 new fossil specimens, though none of these fossils were of dinosaurs. Nevertheless, they analyzed the fossils and found they were comparable, in both species and relative age, to nondinosaur fossils found in the park region of the same Ischigualasto Formation. They also found out that the Ischigualasto Formation in Las Lajas was significantly thicker and much more complete than the outcrops in the park. This gave them confidence that the geological layers in both locations were deposited during the same critical time interval.<\/p>\n\n\n\n

Ramezani then analyzed samples of volcanic ash collected from several layers of the Las Lajas outcrops. Volcanic ash contains zircon, a mineral that he separated from the rest of the sediment, and measured for isotopes of uranium and lead, the ratios of which yield the mineral\u2019s age.<\/p>\n\n\n\n

With this high-precision technique, Ramezani dated samples from the top and bottom of the outcrop, and found that the sedimentary layers, and any fossils preserved within them, were deposited between 230 million and 221 million years ago. Since the team determined that the layered rocks in Las Lajas and the park match in both species and relative timing, they could also now determine the exact age of the park\u2019s more fossil-rich outcrops.<\/p>\n\n\n\n

Moreover, this window overlaps significantly with the time interval over which sediments were deposited, thousands of kilometers northward, in the Chinle Formation.<\/p>\n\n\n\n

\u201cFor many years, people thought Chinle and Ischigualasto formations didn\u2019t overlap, and based on that assumption, they developed a model of diachronous evolution, meaning the earliest dinosaurs appeared in South America first, then spread out to other parts of the world including North America,\u201d Ramezani says. \u201cWe\u2019ve now studied both formations extensively, and shown that diachronous evolution isn\u2019t really based on sound geology.\u201d<\/p>\n\n\n\n

A family tree, preserved<\/strong><\/p>\n\n\n\n

Decades before Ramezani and his colleagues set out for Las Lajas, other paleontologists had explored the region and unearthed numerous fossils, including remains of Pisanosaurus mertii<\/em>, a small, light-framed, ground-dwelling herbivore. The fossils are now preserved in an Argentinian museum, and scientists have gone back and forth on whether it is a true dinosaur belonging to the Ornithiscian group, or a \u201c basal dinosauromorph\u201d \u2014 a kind of pre-dinosaur, with features that are almost, but not quite fully, dinosaurian.<\/p>\n\n\n\n

\u201cThe dinosaurs we see in the Jurassic and Cretaceous are highly evolved, and ones we can nicely identify, but in the late Triassic, they all looked very much alike, so it\u2019s very hard to distinguish them from each other, and from basal dinosauromorphs,\u201d Ramezani explains.<\/p>\n\n\n\n

His collaborator Max Langer from the University of S\u00e3o Paulo in Brazil painstakingly reanalyzed the museum-preserved fossil of Pisanosaurus<\/em>, and concluded, based on certain key anatomical features, that it is indeed a dinosaur \u2014 and what\u2019s more, that it is the earliest preserved Ornithiscian specimen. Based on Ramezani\u2019s dating of the outcrop and the interpretation of Pisanosaurus<\/em>, the researchers concluded that the earliest bird-hipped dinosaurs appeared around 229 million years ago \u2014 around the same time as their lizard-hipped counterparts.<\/p>\n\n\n\n

\u201cWe can now say the earliest Ornithiscians first showed up in the fossil record roughly around the same time as the Saurischians, so we shouldn\u2019t throw away the conventional family tree,\u201d Ramezani says. \u201cThere are all these debates about where dinosaurs appeared, how they diversified, what the family tree looked like. A lot of those questions are tied to geochronology, so we need really good, robust age constraints to help answer these questions.\u201d<\/p>\n\n\n\n

This research was mainly funded by the National Council for Scientific and Technical Research (Argentina) and the S\u00e3o Paulo State Research Support Foundation (Brazil). Geochronologic research at the MIT Isotope Lab has been supported in part by the U.S. National Science Foundation.<\/p>\n","protected":false},"excerpt":{"rendered":"

Geological evidence suggests the known dinosaur groups diverged early on, supporting the traditional dinosaur family tree<\/p>\n","protected":false},"author":2,"featured_media":18827,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[17],"tags":[],"class_list":["post-18826","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-research"],"featured_image_urls":{"full":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2020\/08\/dinosaur-family.jpg",639,426,false],"thumbnail":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2020\/08\/dinosaur-family-200x200.jpg",200,200,true],"medium":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2020\/08\/dinosaur-family-300x200.jpg",300,200,true],"medium_large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2020\/08\/dinosaur-family.jpg",639,426,false],"large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2020\/08\/dinosaur-family.jpg",639,426,false],"1536x1536":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2020\/08\/dinosaur-family.jpg",639,426,false],"2048x2048":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2020\/08\/dinosaur-family.jpg",639,426,false],"ultp_layout_landscape_large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2020\/08\/dinosaur-family.jpg",639,426,false],"ultp_layout_landscape":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2020\/08\/dinosaur-family.jpg",639,426,false],"ultp_layout_portrait":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2020\/08\/dinosaur-family.jpg",600,400,false],"ultp_layout_square":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2020\/08\/dinosaur-family.jpg",600,400,false],"newspaper-x-single-post":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2020\/08\/dinosaur-family.jpg",639,426,false],"newspaper-x-recent-post-big":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2020\/08\/dinosaur-family-550x360.jpg",550,360,true],"newspaper-x-recent-post-list-image":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2020\/08\/dinosaur-family-95x65.jpg",95,65,true],"web-stories-poster-portrait":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2020\/08\/dinosaur-family.jpg",639,426,false],"web-stories-publisher-logo":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2020\/08\/dinosaur-family.jpg",96,64,false],"web-stories-thumbnail":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2020\/08\/dinosaur-family.jpg",150,100,false]},"author_info":{"info":["RevoScience"]},"category_info":"Research<\/a>","tag_info":"Research","comment_count":"0","_links":{"self":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/18826","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/comments?post=18826"}],"version-history":[{"count":0,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/18826\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/media\/18827"}],"wp:attachment":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/media?parent=18826"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/categories?post=18826"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/tags?post=18826"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}