To decide whether dinosaurs were the most successful animals ever to walk the Earth, we first have to set a few parameters. What is success, exactly? Does it relate to size, diversity or population relative to other animals living at the same time? Does the amount of time a life form has existed on Earth play a role? Or is it a mix of all of these?
In terms of size, the largest dinosaurs were definitely bigger than any known land animals. But if you extend your search to sea life, they're outranked by blue whales. Some diplodocid dinosaurs, members of the family that includes Diplodocus, Argentinosaurus and Seismosaurus, were 120 to 140 feet (36.5 to 43 meters) long. Blue whales are shorter -- they top out at about 100 feet (30 meters) [source: American Cetacean Society]. But since their bulk is spread out from nose to tail, blue whales are bigger overall.
Diversity is harder to measure. All dinosaurs fall into the scientific superorder known as dinosauria. This is a step between a class, such as insecta, and an order, such as hymenoptera -- the insects with membrane wings. Scientists don't know exactly how many dinosaurians have existed. While researchers have discovered about 530 dinosaur genera -- the classification just one step above species -- they estimate that there are 1,850 total genera [source: Wang]. It's hard to make a comparison between dinosaurs and other mammals because so many species are still undiscovered. One thing is clear, though. Dinosaurs and other reptiles dominated the landscape during the Mesozoic era.
Before their extinction 65 million years ago, dinosaurs had lived on Earth for about 180 million years. This seems like a long time, especially considering that humans have only been around for half a million years or so. But other animals that still exist today have been around since long before the first dinosaur took a breath. The first cockroaches, for example, appeared in the Carboniferous period, 360 million years ago [source: Kendall]. Dinosaurs, on the other hand, appeared only 245 million years ago, in the Triassic period. And as anyone who has flipped on the kitchen light and provoked a sudden scattering of dark shapes knows, cockroaches thrive today.
This brings us to what may be dinosaurs' biggest competitor for the title of most successful animal. Dinosaurs would probably come out on top if you narrowed the field to vertebrates that live on land. But invertebrate insects live all over the world, just like dinosaurs did. They're also significantly more diverse, and, as we mentioned, they existed before dinosaurs did. The one trait they lack is size -- the biggest insect is smaller than the tiniest dinosaur.