Dinosaur

A Tyrannosaurus rex!

Dinosaurs are a category of creatures of the clade Dinosauria, which first emerged around 250 million years ago, in the Triassic period. Generally these creatures have been classified as reptiles, with their name itself meaning "terrible" (deino) "lizard" (sauros), since their discovery and cataloging by modern science. However due to recent discoveries, it created a large divide in the Paleontology community on whether they should still be classified as inherently reptile or something completely new.

Dinosaurs came to be the dominant form of large, scaly (or feathery), land dwelling life forms across all of earth's major continents through the Jurassic and Cretaceous period. Dinosaurs were, and are, the biggest animals ever to live on the planet, however, what counts as THE biggest is often up in debate by the scientific community, as this usually this ends in flamewars between Sauropod-Fanboys and Cetacean-Fanboys (yes, they do exist, kind of like the relationship between Star Wars fanboys and Star Trek fanboys), but the more neutral agreement is Sauropods=Land and Whales=Ocean.

They are regarded as awesome for several reasons....

• They appeal to the sensational because they're "monsters", but unlike most, they actually DID exist.
• It's also been postulated that dinosaurs were the inspiration for dragons with people discovering dinosaur bones and making up stories about them, though that is debatable.
• Quite a few of the more massive dinosaurs were larger (as in much larger) then the biggest living land mammal, the African Elephant. In fact, the Paraceratherium orgosensis (a relative to the direct ancestor to the modern Rhinoceros) which was the largest land mammal to have ever existed, would be classified as "small-sized" by sauropod standards.
• The largest sauropods are so colossal that they could be classified as "Kaiju-Size" and could furthermore cause mini-earthquakes and create sonic booms with their tails.
• A popular idea is if the asteroid did not hit Earth, then it would be the Troodon (arguably the most intelligent non-avian dinosaur, which resembled Velociraptor) that would be building civilizations and spaceships rather than humans.

Tl;dr, Dinosaurs are NOT to be fucked with.

A modern day dinosaur of the theropod lineage

Most lines of dinosaur were extinguished during the Cretaceous–Paleogene Extinction Event some 65 million years ago. The only surviving branch of this clade are birds, which almost all scientists actually do consider dinosaurs (that's right, its fair to say that all birds everywhere are living dinosaurs). The other close surviving relatives of dinosaurs are the crocodiles, a different branch of the archosaur family that carries on its traditions of eating whatever they want and giving no fucks about mammals Contrary to popular belief, the Komodo dragon, a species of monitor lizard, was not related to the dinosaurs. It is, however, somewhat closely related to creatures commonly mistaken for dinosaurs - Mosasaurs. The dragon is notable for carrying Jurassic period bacteria so virulent that a single bite can fatally infect other animals.

Things people think are dinosaurs, but are not:

• Pterosaurs: These flying archosaurs had among their ranks the largest flying creatures that ever lived and were the closest clade to dinosaurs, but are not actually dinosaurs.
• Marine Reptiles: During the Mesozoic era there were a variety of large marine reptiles, including Ichthyosaurs, Plesiosaurs, Pliosaurs, Mosasaurs and various other sea beasties. As these creatures are big, reptillian, and look like dinosaurs, the general public often lumps them in as dinosaurs. In truth these are a number of separate families that took to the sea.
• Synapsids: Before the dinosaurs really took off as the dominant form of life during the Triassic period the Synapsids, sometimes known as "mammal-like reptiles" were king. One member of this clade is Dimetrodon or "that fin-back lizard thing' in the Land Before Time and those sets of dinosaur toys your mom bought you when you were a kid". You can tell they're not dinosaurs (and this is a very loose definition) thanks to an extra opening in the skull behind the eye where jaw muscles attached and later became the Sphenoid bone which sits behind the eyes, specialized teeth including canine teeth, among many other distinguishing traits. As the name suggests these guys became the early mammals which turned into us. It's unknown at what point they began to give birth to live young and lactate, although since fur was an extremely late adaptation its generally thought there was a point where something that looked like a dinosaur had lizard tits teats.

Major groups of dinosaurs

Prepare for SCIENCE.

Saurischians

The "lizard-hipped" dinosaurs. Ironically, birds are part of Saurischia.

Theropods

Easy name: Meat-eaters

Well known members: Tyrannosaurus, Velociraptor, Gallus

Awesome obscure members: Deinocheirus, Yi, Gypaetus

You know them well. The hunters of the prehistoric world. Theropods were likely the first group of dinosaurs to arise, and diversified quite a lot. Also the only group to survive the asteroid impact (and subsequent climate change).

Sauropods

Easy name: Long-necks

Well known members: Brontosaurus, Brachiosaurus, Diplodocus

Awesome obscure members: Amargasaurus, Shunosaurus, Dreadnoughtus

The other most well-known group. Titans with nothing on their minds other than eating. Notable for having the largest animals in the history of the planet (longer than whales, but not nearly as heavy).

Ornithischians

The "bird-hipped" dinosaurs. Might be weird that birds aren't in this group, but the terms were coined in the 1800s, so what can ya do.

Thyreophorans

Easy name: Armored Dudes

Well known members: Stegosaurus, Ankylosaurus, Euoplocephalus

Awesome obscure members: Kentrosaurus, Polacanthus, Scutosaurus

Armor, man. These guys where walking tanks with rows of bony armor, or spikes or plates, with many species having weaponized tails which (among the Stegosauridae family) are known informally as Thagomizers after a Gary Larson Far Side Cartoon. Yes really. Also notable because some scientists believe that they had colossal, sideways dongs. Seriously, how the hell else could they bone? Can't do normal fucking with those spikes in the way...

Ornithopods

Easy name: The guys with weird heads

Well known members: Iguanodon, Parasaurolophus, Edmontosaurus

Awesome obscure members: Kulindadromeus, Shangtungosaurus, Tsintaosaurus

Your standard "dino plant-eaters", though some theorize they could have actually been quite aggressive, which considering how many people are killed by plant eating hippos and elephants is quite probable.We also actually know what one type sounded like!

Marginocephalians

Easy name: The other guys with weird heads

Well known members: Triceratops, Protoceratops, Pachycephalosaurus

Awesome obscure members: Diabloceratops, Einosaurus, Nasutoceratops

Probably the last major group to arise. All originally bipedal, but some grew too massive for that. Also had a weird convergent evolution with birds, they had straight-up beaks.

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