A Brief History of Dinosaurs
By Robin Lloyd
Most people think of dinosaurs as big, ferocious and extinct reptiles. That’s largely true, but there are some misconceptions.
The word dinosaur, which means "terrible lizard," was coined in 1842, but now we know that dinosaurs aren’t lizards, although they share a common ancestor. Lizards as a group are primitive compared to dinosaurs.
Dinosaurs were the largest land animals of all time, but a great number of dinosaurs were smaller than a turkey.
Dinosaurs as a group got bigger over time until an extinction event 65 million years ago wiped out all but bird-like 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.
Yet only the big, classic dinosaurs are extinct. Birds are living dinosaurs, most experts believe. Think of that next time a pigeon strafes you. The carnage continues.
Many people think extinct flying reptiles called pterosaurs were dinosaurs. They were dinosaurs’ closest relatives but technically not dinosaurs.
Fossils from the past ten years 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. Instead, feathers, rather than being an adaptation for flight, helped these bird-like non-birds stay warm as juveniles.
Scientists classify dinosaurs into two major groups based on the structure of the bones in their hips.
Most of the well-known dinosaurs, including Tyrannosaurus rex, Deinonychus, maniraptors, and birds, fall into a category known as Saurischian dinosaurs (pronounced sor-ISK-ee-en). These "reptile-hipped" dinosaurs have a pelvis that points forward, similar to more primitive animals.
Saurischian dinosaurs, 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.
Ornithischian (pronounced or-neh-THISK-ee-en) dinosaurs, a group that includes horned and frilled ceratopsians, Iguanodontids, armored stegosaurs, and duck-billed hadrosaurs, are more mild-mannered, plant eaters. All have "bird hips," or a backward-pointing pelvis (although ironically they are not the ancestors of birds), which is more stable and gives them that lumbering look. They also have a beak-like bump on the front of their lower jaws that Saurischians lack, and their teeth tend to be smaller and blunter.
Saurischians are divided into two groups – four legged herbivores called sauropods and two-legged carnivores called theropods (living birds are theropods).
Big feet, small brains
Sauropods, such as Diplodocus, Brachiosaurus and Apatosaurus, were among the largest land animals ever, had small brains, and large guts for digesting leaves and grasses.
Theropods, such as velociraptor, allosaurus and albertosaurus, are more agile and have large eyes for spotting prey, sharp teeth for slicing prey, and "grasping claws."
Theropods are the ancestors of extinct birds, including Archaeopteryx, the "first bird," and living birds, which actually evolved from a different branch of the dinosaur family tree than Archaeopteryx.
Scientists are starting to learn about dinosaur behavior. For instance, theropod dinosaurs exhibited some of the same behaviors as living birds, such as sleeping with their heads tucked under their forelimbs and nesting to protect their eggs.
Quoted from LiveScience, posted on December 1, 2005