Few issues in paleontology are as confusing as the classification of theropods--the bipedal, mostly carnivorous dinosaurs that evolved from archosaurs during the late Triassic period and persisted until the end of the Cretaceous (when the dinosaurs went extinct). The problem is, theropods were extremely numerous, and at a distance of 100 million years it can be hard to distinguish one genus from another based on fossil evidence, much less to determine their evolutionary relationships. (See a gallery of large theropod pictures.)
For this reason, the way paleontologists classify theropods is in a state of constant flux. So, I'm going to add fuel to the Jurassic fire by creating my own informal sorting system. I've already addressed tyrannosaurs, raptors, therizinosaurs, ornithomimids and "dino-birds"--the more evolved theropods of the Cretaceous period--in separate articles on this site. This piece will mostly discuss the "big" theropods (excluding tyrannosaurs and raptors) that I've dubbed the 'saurs: allosaurs, ceratosaurs, carnosaurs, and abelisaurs, to name just four sub-classifications.
Here are brief descriptions of the classifications of large theropods currently in (or out of) vogue:
Abelisaurs. Sometimes included under the ceratosaur umbrella (see below), abelisaurs were characterized by their large sizes, short arms, and (in a few genera) horned and crested heads. What makes the abelisaurs a useful group is that they all lived on the southern supercontinent of Gondwana, hence the numerous fossil remains found in South America and Africa. The most notable abelisaurs were Abelisaurus (of course), Majungatholus and Carnotaurus.
Allosaurs. It probably won't seem very helpful, but paleontologists define an allosaur as any theropod more closely related to Allosaurus than to any other dinosaur (a system that applies equally well to all the theropod groups listed below; just substitute Ceratosaurus, Megalosaurus, etc.) In general, allosaurs had large, ornate heads, three-fingered hands, and relatively large forearms (compared to the tiny arms of tyrannosaurs). Examples of allosaurs include Carcharodontosaurus, Giganotosaurus and the huge Spinosaurus.
Carnosaurs. Confusingly, the carnosaurs (Greek for "flesh-eating lizards") includes the allosaurs, above, and is sometimes taken to embrace the megalosaurs (below) as well. The definition of an allosaur pretty much applies to a carnosaur, though this broader group includes such relatively small (and sometimes feathered) predators as Sinraptor, Fukuiraptor and Monolophosaurus. (Oddly enough, as yet there's no genus of dinosaur named Carnosaurus!)
Ceratosaurs. This designation of theropods is in even greater flux than the others on this list. Today, the ceratosaurs are defined as early, horned theropods related to (but not ancestral to) later, more evolved theropods like tyrannosaurs. The two most famous ceratosaurs are Dilophosaurus and, you guessed it, Ceratosaurus.
Megalosaurs. Of all the groups on this list, megalosaurs are the oldest and least respected. This is because, early in the 19th century, pretty much every new carnivorous dinosaur was assumed to be a megalosaur, Megalosaurus being the first theropod ever officially named (before the word "theropod" was even coined). Today, megalosaurs are rarely invoked, and when they are, it's usually as a subgroup of carnosaurs alongside the allosaurs.
Tetanurans. This is one of those groups that's so all-inclusive as to be practically meaningless; taken literally, it includes everything from carnosaurs to tyrannosaurs to modern birds. Some paleontologists consider the first tetanuran (the word means "stiff tail") to have been Cryolophosaurus, one of the few dinosaurs to be discovered in modern Antarctica.
Next page: the Behavior of Large Theropods, and a List of Notable Genera