The period before the first appearance of dinosaurs is called the Permian period and ended with a mass extinction event approximately 252 million years ago, probably because of heavy volcanic eruptions that caused the release of huge amounts of carbon dioxide and methane, which in turn caused the oxidation levels to rise in the atmosphere.
This extinction event may have killed as much as 90% of the species on Earth, including most marine animals and insects, but made room for other animals to evolve, like the dinosaurs, which evolved from earlier reptiles (Archosaurs) in the Triassic period.
When did the first dinosaurs appear?
Dinosaurs first appeared on Earth some 240,000,000 years ago (during the Triassic period), only 20 million years after the Permian mass extinction event. They probably evolved from reptiles like Lagosuchus or Lagerpeton. The first dinosaur was a small, two-footed omnivorous (eats both meat and plant matter) lizard with a long tail, called Eoraptor. The first dinosaurs were only about 1 meter (3 feet) long with small legs. They grew larger over millions of years, reaching lengths of 30 meters (100 feet), and some of them were even bigger! One of the largest dinosaurs ever discovered was Argentinosaurus, at 95 tons! That’s heavier than 15 African elephants! And almost as much as 95.000.000 ferrets (just saying).
It’s hard to imagine something that big walking on Earth.
The first dinosaurs were prey to other reptiles
Even though they are the most recognized prehistoric creatures, dinosaurs were not dominant when they first appeared. Other crocodile-like reptiles were just as big or even bigger than them, and heavy meat-eaters hunted them for food.
In fact, paleontologists believe that the ecological niches that dinosaurs occupied were originally held by another ancestor of reptiles, called Archosaurs. These creatures disappeared from Earth before the first dinosaurs appeared, and they may have even been their ancestors! Dinosaurs quickly filled these niches and started to thrive during the Jurassic period (200 million years ago), so much that they became dominant for 165 million years.
They evolved into many new species, and many of them were predators.
Why did they become so huge?
Dinosaurs became bigger and stronger because the food was abundant, and sauropods (known for their long neck, e.g., brontosaurus) became bigger as a defense against predators.
If you think about a dinosaur, you probably think about an animal with large jaw muscles and a powerful bite. However, Sauropods didn’t have big heads because there is no need for it when eating plants all day long – you would only get those muscles if you were a carnivore and needed them to crush bones.
Mammals accompanied dinosaurs in their evolution through different periods, but they were small and quickly fell prey to dinosaurs. So the time of mammals started when the era of dinosaurs was over. The first mammal might have looked like this —>
Where the dinosaurs come from - In a nutshell.
The dinosaurs probably appeared because of an extinction event 252.000.000 years ago that wiped out most animals and made room for new ones to evolve and occupy empty ecological niches. But, then, another extinction event wiped out the dinosaurs 65.000.000 years ago.