Most people think of dinosaurs as big, ferocious and extinct reptiles. That's largely true, but there are some misconceptions. Dinosaurs came in all shapes and sizes. Dinosaurs were the largest land animals of all time, but a great number of dinosaurs were smaller than a turkey.
Dinosaurs first appeared between 247 and 240 million years ago. They ruled the Earth for about 175 million years until an extinction event 65.5 million years ago wiped out all of them, except for the avian dinosaurs. Scientists don't agree entirely on what happened, but the extinction likely was a double or triple whammy involving an asteroid impact, choking chemicals from erupting volcanoes, climate change and possibly other factors.
Only the big, classic dinosaurs are extinct. Birds are living dinosaurs, most experts believe. Think of that next time a pigeon strafes you.
Fossils show that some of the more advanced dinosaurs had feathers or feather-like body covering, but many of them didn't fly and probably didn't even glide. Archaeopteryx, which was for a long time considered to be the first bird (although this status is not certain), could likely launch itself from the ground, but probably couldn't fly far, according to unpublished research presented at the 2016 Society of Vertebrate Paleontology meeting in Salt Lake City, Utah. Instead, feathers, likely helped these bird-like dinosaurs stay warm as juveniles or send signals to other individuals.
Many people think extinct flying reptiles called pterosaurs were dinosaurs. They were dinosaurs' closest relatives, but technically not dinosaurs. Pterosaurs had hollow bones, relatively large brains and eyes, and, of course, the flaps of skin extending along their arms, which were attached to the digits on their front hands. The family includes Pterodactyls, with elaborate, bony head crests and lack of teeth. Pterosaurs survived up until the mass die-off 65 million years ago, when they were going the way of the dodo along with marine reptiles and other nonavian dinosaurs.
Dinosaur fossils were first recognized in the 19th century. In 1842, paleontologist Richard Owen coined the term dinosaur, derived from the Greek deinos, meaning "terrible" or "fearfully great," and sauros, meaning "lizard" or reptile." Scientists classify dinosaurs into two orders — Saurischians and Ornithischians— based on the structure of the bones in their hips. (This saurischian and ornithischian grouping is now disputed. See the "Family tree update" section below to learn more.)
Most of the well-known dinosaurs — including Tyrannosaurus rex, Deinonychus and Velociraptor — fall into the order known as Saurischian dinosaurs (pronounced sor-ISK-ee-en). These "reptile-hipped" dinosaurs have a pelvis that points forward, similar to more primitive animals. They are often long-necked, have large and sharp teeth, long second fingers, and a first finger that points strongly away from the rest of the fingers.
Saurischians are divided into two groups – four legged herbivores called sauropods and two-legged carnivores called theropods (living birds are in the theropod lineage).
Theropods walked on two legs and were carnivorous. "Theropod" means "beast-footed" and they are some of the fearsome and most recognizable dinosaurs — including Allosaurus and T. rex.
Scientists have wondered whether large theropods — such as Giganotosaurus and Spinosaurus — actively hunted their prey, or simply scavenged carcasses. The evidence points to the animals working together as opportunistic hunters: they would bring down prey, but also eat animals that were lying around. When fossil-hunters found bones with bite marks on them, they wondered if theropods engaged in cannibalism. It appears now that the animals may have scavenged their own kind, but they didn't hunt down their own.
Sauropods were herbivores with long heads, long necks, and long tails. They were among the largest land animals ever, but they likely had small brains. The gentle giants like leaf-eating Apatosaurus, Brachiosaurus and Diplodocus are part of this family.
Ornithischian (pronounced or-neh-THISK-ee-en) dinosaurs, a group that includes horned and frilled Triceratops, spiked Stegosaurus and armored Ankylosaurus, are more mild-mannered, plant eaters.
These dinosaurs were beaked herbivores. Smaller than the sauropods, the Ornithischia (meaning "bird-hipped") often lived in herds and were prey to the larger species of dinosaurs. Interestingly, the Ornithischia shifted from a two-legged to a four-legged posture at least three times in their evolutionary history and scientists think they could adopt both postures early in their evolutionary history.