Dinosaurs have ruled the Earth for about 174 million years. Here's what we know about their history. The history of dinosaurs covers a long period of time with different creatures.

Dinosaurs were a successful group of animals that appeared between 240 and 230 million years ago and began to rule the world until a giant asteroid hit the Earth about 66 million years ago. During that time, dinosaurs evolved from a group of creatures mostly about the size of dogs and horses to the most massive beasts that ever existed on land.

Some carnivorous dinosaurs shrank over time and evolved into birds. So, in this sense, only non-avian dinosaurs became extinct.

During the approximately 174 million years that dinosaurs existed, the world changed dramatically. When dinosaurs first appeared in the Triassic period (251.9 to 201.3 million years ago), they roamed the Pangaea. But when asteroids hit at the end of the Cretaceous period (145 million to 66 million years ago), the continents were in roughly the same position as they are today.



1. What is a dinosaur?


The oldest definitive dinosaur fossils dating back to about 231 million years ago come from the Izquigualasto Provincial Park in northwestern Argentina and include the genera Herrera, Erechosaurus, and Erechosaurus. Scientists are still debating whether Niassaurus, a genus found in Tanzania about 240 million years ago, was also an early dinosaur or dinosaur taxon (including dinosaurs and their close relatives) Edinburgh in Scotland, said Steve Brusat, a paleontologist at the University of Tanzania. 

Whenever they first appeared, the unique anatomy of dinosaurs distinguished them from other groups of animals. Dinosaurs are primordial dinosaurs, an evolutionary branch (a group of different animals that share a common ancestor) that includes crocodiles, pterosaurs, dinosaurs, and birds. After the end of the Permian extinction about 252 million years ago, sauropods emerged. Over time, some sauropod dinosaurs, including dinosaurids, adapted to an upright posture, which means that their legs were underneath their bodies rather than out to the sides. 

The dinosaur evolutionary branch was coined in 1842 by British paleontologist Richard Owen, who listed the carnivorous theropod suborder Brontosaurus, the long-necked lizard-footed cetacean, and the ornithischian avian dinosaur as the first known species evolutionary branches of dinosaurs.

It turns out that each of these dinosaurs represents one of the three major dinosaur groups.


2. Types of dinosaurs


According to the Paleontology Database, there are 1,545 scientifically described dinosaur species as of 2021. brusatte says that about 50 previously unknown species are described each year, which means that about one newly discovered species is described each week. 

All of these dinosaurs belong to one of three groups: Ornithischia, Lizard-footed and Theropoda.

Ornithischian dinosaurs had a backward-pointing pubic bone on their hips, hence the name Ornithischian dinosaurs. Ornithischians include beaked herbivores such as stegosaurs and duck-billed dinosaurs, as well as horned dinosaurs such as Triceratops and armored dinosaurs such as Aetosaurus. Some ornithischians walked on four legs, while others walked on two. 

Lizard-footed dinosaurs are long-necked, large-bellied dinosaurs with small heads and columnar limbs. The group includes lizard-footed dinosaurs (e.g., Leontosaurus), their smaller predecessors (including Chromatosaurus), and the super-large lizard-footed dinosaurs known as titanosaurs (e.g., Dauntless and Argentinosaurus), which were among the largest terrestrial animals that ever existed.

The suborder Theropoda was a group of carnivorous dinosaurs, although some changed their diet to herbivorous or omnivorous. Theropods include Tyrannosaurus rex and Velociraptor, as well as birds, all of which evolved from small theropods.

According to the book "Rediscovering Dinosaurs," theropods and lizard-footed animals had lizard hips or "reptile hips," which can also be seen in modern crocodiles and lizards.

Historically, it was thought that reptilian rump theropods and lizard-footed animals were more closely related to each other than to ornithischians.


3. When did dinosaurs live?


Dinosaurs lived during most of the Mesozoic Era, a geological era that lasted from 252 million years to 66 million years ago. The Mesozoic includes the Triassic, Jurassic, and Cretaceous periods. Dinosaurs originated from small dinosaur-like ancestors during the Triassic period, when the climate was harsh and dry.

During the Jurassic period (201.3 to 145 million years ago), dinosaurs dominated, and some became enormous in size. For example, the earliest titanosaur, Vouivria damparisensis, dates to 160 million years ago. It weighed about 33,000 pounds and was more than 50 feet long. Iconic dinosaurs of this period include Brontosaurus, Brachiosaurus, Diplodocus, and Stegosaurus. During the Jurassic period, flowering plants evolved and birds, including primitive birds, first appeared, Brusatte said, adding that "there was a small extinction at the end of the Jurassic that we know very little about."

The dominance of dinosaurs continued during the Cretaceous period as continents grew farther apart. Notable dinosaurs of this period include Tyrannosaurus rex, Triceratops, Spinosaurus and Velociraptor. The largest recorded dinosaurs, including Argentinosaurus, date back to the Cretaceous period. The Cretaceous period ended with the Cretaceous-Tertiary (K-Pg) extinction event, when a 6-mile (10 km) wide asteroid collided with Earth, leaving an impact crater over 110 miles (180 km) in diameter on the Yucatan Peninsula now known as Mexico.


4. Why did the dinosaurs become extinct?


How the dinosaurs behaved before the asteroid hit the Earth is open to debate. Some studies suggest that dinosaur extinctions were increasing and diversity was decreasing during the Late Cretaceous, especially among herbivorous dinosaurs. But these studies rely on incomplete fossil data and models that may not tell the whole story.

In the aftermath of asteroid collisions, long-term suffering is accompanied by chaos. Collisions cause enormous damage, including shock waves, heat pulses, wildfires, tsunamis, volcanic eruptions, deadly acid rain, and earthquakes. Dust and dirt raised by the asteroids hovered in the air. "The shower of hot dust raised global temperatures within hours of the impact and cooked living animals too large to seek shelter," says Kruk. "Small animals that could hide underground, underwater or in caves or large tree trunks may have been able to survive the initial heat wave."

Scientists have often wondered in the past whether the eruption of the Deccan isthmus, now in India, played a role in the mass extinction. But recent studies suggest the Deccan may have had little impact. In all likelihood, the asteroid was the cause of the extinction.


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