Most people don't have to travel too far to find some kind of dinosaur exhibit. In addition to touring shows like "Walking with the Dinosaurs," there are museums with dinosaur displays all over the world. But if you want to get a really good look at a dinosaur, you may not have to travel at all. Simply look at any bird you can see outside your home.
The prevailing scientific view is that whether you're seeing a hummingbird, a robin, a flamingo or an ostrich, you're seeing a descendant of dinosaurs. In fact, some scientists go so far as to call birdsavian dinosaurs and to call all other dinosaurs non-avian dinosaurs.
The thought that a giant carnivore like Tyrannosaurus rex has something in common with an ordinary wren might seem foreign or even far-fetched. This is especially true since people often describe dinosaurs as reptiles. But the idea that dinosaurs became birds has been around for more than 100 years. In 1868, Thomas Henry Huxley described evidence that birds evolved from dinosaurs. This is currently the most widely-held scientific theory about the origin of birds, and it's helped shape today's view of dinosaurs as swift and agile instead of plodding and clumsy.
All of these ideas come from the same source as everything we know -- and don't know -- about dinosaurs. To learn about dinosaurs, researchers have to study physical clues and put these clues into the context of current scientific knowledge. This can be a tricky process. There were no humans on Earth when dinosaurs lived, so there are no written records or illustrations of exactly how they behaved or what they looked like. All we have are bone and egg fossils, collections of footprints called trackways, and our knowledge of living animals.
This evidence has given scientists lots of material to study, but there are still plenty of unanswered questions about dinosaurs. One of the biggest is whether dinosaurs were more like reptiles or birds. In this article, we'll use this question as our framework for understanding dinosaurs. We'll examine common beliefs about how dinosaurs lived, moved and reproduced. We'll start with the basics -- what makes a dinosaur a dinosaur and why many of the most well-known prehistoric reptiles aren't dinosaurs at all.