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.

Flying on

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this status is not certain), could likely launch itself from the ground, but probably couldn't fly far, according to unpublished research presented at the 2016 Society of Vertebrate Paleontology meeting in Salt Lake City, Utah.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 went the way of the dodo along with marine reptiles and other nonavian dinosaurs.

Hip check

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.

Ornithischia

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.

In 2017, a metaphorical bombshell hit the paleontology world regarding the dinosaur family tree. A study published in the journal Nature suggested that this hip-oriented classification was incorrect. Rather, theropods are likely close cousins with the ornithischian dinosaurs, and the two groups — the theropods and Ornithischia — form a newly identified group known as Ornithoscelida, the researchers said.

The old (left) and the new (right) dinosaur family tree. (Image credit: University of Cambridge)

The finding came about after researchers realized that theropods and Ornithischia had many anatomical features in common. If the updated tree is correct, it may explain why both theropods and Ornithischia have feathers, while other dinosaurs don't.

However, this hypothesis will need to be tested and retested over the next several years before the paleontology community can fully accept it.

Marine reptiles

During the age of the dinosaurs, a lot was happening below the surface of the world's oceans. The "fish flippers," or ichthyopterygia, includes Ichthyosaurus — the streamlined, tuna- and dolphin-shaped ocean-going predators. This abundant family of marine reptiles largely went extinct at the end of the Jurassic Period.

Dinosaur cloning

Despite the popularity of the "Jurassic Park" franchise, it would be incredibly difficult, if not impossible, to clone a dinosaur. In order to clone a dinosaur, researchers would need dinosaur DNA. But there is no known surviving dinosaur DNA on record (the oldest recovered and authenticated DNA sample belongs to a 700,000-year-old horse that lived in ancient Canada.)

However, some dinosaur organic matter has withstood the test of time. Researchers have uncovered a number of soft tissues from the Mesozoic era, including 80-million-year-old blood vessel belonging to a duck-billed dinosaur and 130-million-year-old proteins in an early bird fossil. But blood vessels and proteins, unlike DNA, cannot be used to clone animals.

There is a concerted effort to reverse engineer a chicken into a dinosaur (remember, birds are the descendants of theropod dinosaurs), but the team still has a ways to go.

Additional reporting by staff writer Laura Geggel.

More on dinosaurs

Time periods

Precambrian: Facts About the Beginning of Time

Paleozoic Era: Facts & Information

Mesozoic Era: Age of the Dinosaurs

Cenozoic Era: Facts About Climate, Animals & Plants