It's often mistaken for a genuine dinosaur, but Dimetrodon lived long before the first dinosaurs evolved--280 million years ago, during the Permian period. An example of a pelycosaur--among the first reptiles to live entirely on land--it went kaput about 250 million years ago, along with countless other species, in the Permian/Triassic Extinction event.

Dimetrodon's most spectacular feature was its large sail, which ran along the entire length of its back. Paleontologists speculate that the main purpose of this sail was to regulate this reptile's temperature: it soaked up sunshine during the day (much like a solar panel) and also helped dissipate energy if the creature became overheated. Secondarily, this sail may have served as a mating display, and helped Dimetrodon look a lot bigger in the eyes of any hungry carnivores lurking nearby.

Read more about prehistoric reptiles like Dimetrodon: Before the Dinosaurs - Archosaurs, Pelycosaurs and "Mammal-Like Reptiles"

Illustration: Wikimedia Commons

It's not true that all the cool dinosaur names are already taken. Witness Helioceratops (Greek for "sun horned face"), an early ceratopsian recently named by Chinese paleontologists. After its scattered fossils were discovered in 2003, it took six years for experts to conclusively "diagnose" this middle-Cretaceous herbivore, which was considerably smaller than later, more famous ceratopsians like Triceratops and Pentaceratops.

Just how small was Helioceratops? Here's the quote from Xinhua, the Chinese news agency: "A Helioceratops looks bigger than a pig but smaller than a cow." My question is, did it go "oink" or "moo?"

Among the earliest predatory dinosaurs of the Jurassic period, Dilophosaurus was also one the least threatening (unless, of course, you happened to be a small, quivering herbivore). This carnivore was slightly built, much different than the thick, relatively plodding body plans of later theropods like Allosaurus. Presumably, it made up in speed what it lacked in sheer bulk.

As for the distinctive crests over Dilophosaurus' eyes, paleontologists believe these were sexually selected--that is, males with these structures had a better chance of mating with females. Although sexual selection may explain similar features on other dinosaurs, when it comes to Dilophosaurus, this theory is on much better ground--since only some fossils (presumably the males) have been found to have these crests. (By the way, despite what you saw in Jurassic Park, there's absolutely no evidence that Dilophosaurus spit poison at its enemies before zooming in for the kill.)

Illustration: Wikimedia Commons

It looks like dental paleontology may be a promising career. First, researchers in Japan found the fossilized teeth of an as-yet-unidentified tyrannosaur that measured about 17 feet long (this was big news because the fossil dates from the early Cretaceous, about 140 million years ago, before tyrannosaurs had attained plus sizes). Now, a team in Spain has found the nearly 10-centimeter-long tooth of an unidentified allosaur, which may have measured as long as 15 meters. The original title of the paper makes the discovery sound even more impressive: "Diente de Un Gran Dinosaurio Teropodo."

It's not hard to guess why teeth persist so well in the fossil record: the same characteristics that make them suited for crunching through bone (or tough, woody plants) also make them resistant to erosion. That's why so many dinosaurs have been "diagnosed" simply on the basis of their teeth, though it helps to have other bone fragments scattered nearby.