Some mighty peculiar reptiles

An unearthly discovery
Dinosaurs began appearing in the Triassic era, between 245 and 208 million years ago. The family tree is divided into the two large groups, based on the shape of their hip bones:

	Saurischians, the "reptile-hipped" dinosaurs, included both carnivores and herbivores (defined). Sue and her fellow tyrannosaurs were saurischians, as were diplodocus and some of the other giant herbivores.
	Ornithischians, or "bird-hipped," dinosaurs, were all herbivores.

Dinosaurs were an extremely durable group of animals. They originated about 220 million years ago and evolved until the great Cretaceous extinction event of about 64 million years ago wiped them out and allowed mammals to take over the world, biologically speaking.

Chris Pladziewicz chipping stone from a Triceratops rib to make it light enough to carry, © Jeff Miller/UW-Madison Office of News and Public Affairs.

Although dinosaur fossils have been eroding out of rocks for millions of years, they did not become objects of scientific study until a giant jaw bone of an extinct marine reptile was found in a chalk quarry in Holland in 1770. One reason the previous dinosaur finds were not understood to be an extinct group of organisms was religious dogma that held that God had created all animals in their present form. If that was true, how could anyone imagine that fossils represented extinct organisms?

Already a crowd pleaser
But the jaw bone -- of a reptile eventually named Mosasaurus -- was unique enough to undermine that dogma. The sabotage was increased by several discoveries in England, where scientists began reconstructing the animals from whatever remains they had. (That the reconstructions were not always accurate did not seem to dampen public enthusiasm for the ancient monsters.)

Here's a page from the British Museum on -- could it be dinos?

In 1855, dino-teeth were found in the United States. Soon, two academic rivals -- Edward Drinker Cope and Othniel Charles Marsh -- made enormous contributions to the budding science of paleontology. (The rivalry arose in part because Marsh had had the temerity (defined) to point out that Cope had put the head on the wrong end of a dinosaur reconstruction. (Is it bad form to point out a rival's bad form?)

So he had it backwards?
Yes, but unlike some scientific rivalries, at least this one was productive: The 1870s and 1890s, expeditions led by the two men brought back 130 new dinosaur species from digs in Colorado, Montana, Wyoming and elsewhere.

In the 1920s, Roy Chapman Andrews led expeditions for the American Museum of Natural History in New York, which recovered a ton of cool dinos from Mongolia. (Andrews also wrote "All About Dinosaurs," which I loved as a kid.)

Over the years, 775 genera (defined) of dinosaurs have been named, but the phylogeny is in continual dispute. Here's a discussion of the problems in counting dino species.

Contrary to popular impression, not all of the giant, bizarre creatures from the Mesozoic Era (defined) were dinosaurs. Dinosaurs were all reptiles, so that excludes fish, amphibians, mammals and birds. That means the pterosaurs, the flying reptiles of the Mesozoic Era were not dinos. (That doesn't mean they weren't cool, however.)

Here's a species map of the dinosaurs, showing who's related to whom. Invaluable when you have to find the heir who inherited old Steggy's seaside swamp after her demise.

One distinction is that dinosaurs had legs under their bodies, while other large critters of the day had legs splayed to the side. This not only made dinos better runners, it gave a unique structure to their hips, knees and ankles. (If there's one thing paleontologists like better than a cold beer after a hot day in the field, it's unique structure -- since most remains are fragmentary, a giveaway structure is extremely desirable.)

True or false: fossils are just a bunch of dusty old bones...

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