Dinosaurs evolved from archosaur ancestors in the Middle Triassic, diversified into various body types, and increased in size after the Triassic-Jurassic extinction event. During the Jurassic period, they diversified even more impressively, with herbivores acquiring extreme sizes and unique armor, and predators getting bigger and smarter.
More broadly, there were two orders of dinosaurs: bird-flanked and lizard-flanked. Bird-flanked dinosaurs (Ornithischians) consisted of several herbivores including Stegosaurus, Triceratops, Ankylosaurus hadrosaurs (duck-billed dinosaurs), and thick-headed dinosaurs such as Dracorex. The lizard-flanked dinosaurs (saurischians) included the theropods, which included all carnivorous dinosaurs, and the sauropods, which were the largest animals that ever walked the Earth.
The dinosaur evolved from archosaur ancestors in the Middle Triassic, about 230 million years ago, not long after the largest mass extinction in Earth’s history. Archosaurs are the group that includes modern birds and crocodilians, but they were universally reptilian in appearance at the time. Common features of archosaurs include teeth set into sockets, special holes in the skull to reduce weight, and extra notches on the femur for muscle attachments. Dinosaurs are essentially just archosaur reptiles with limbs held erect under the body.
Quickly, dinosaurs diversified into a range of body types. The first dinosaur is thought to have been a small, bipedal predator. About 215 million years ago, an extinction event wiped out many of the dinosaurs’ biggest competitors, including the basal archosaurs and mammalian ancestors, the therapsids. Before this extinction, dinosaurs made up only 1-2% of land animals, and after it they accounted for 50-90%. Dinosaurs diversified to fill the empty niches, including medium sized predators, large predators, medium sized herbivores, large herbivores, etc.
Although dinosaurs diversified early on, they didn’t increase in size until after the Triassic-Jurassic extinction event 200 million years ago, which wiped out the remaining competing dinosaurs even more aggressively. The earliest dinosaurs were “small” sauropods (up to 20 feet), which had long necks and walked on four legs, eating plants, and theropod carnivorous dinosaurs (with a superficially T. rex-like body plan that were only the size of humans .
During the Jurassic period, dinosaurs diversified even more impressively. The interaction between predators and prey gave rise to many adaptations on both sides: Herbivorous dinosaurs acquired extreme sizes or unique arrangements of armor or weapons, such as tail clubs or spikes. Predators have gotten bigger and smarter, with bigger teeth, stronger jaws, and a longer head to hold them. This has culminated in giants like the T. rex.
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