Science, Tech, Math › Animals & Nature The Large, Meat-Eating Dinosaurs Share Flipboard Email Print Animals & Nature Dinosaurs Carnivores Basics Paleontologists Dinosaurs & Birds Herbivores Marine Reptiles Prehistoric Mammals Amphibians Birds Habitat Profiles Mammals Reptiles Wildlife Conservation Insects Marine Life Forestry Evolution View More By Bob Strauss Science Writer B.S., Cornell University Bob Strauss is a science writer and the author of several books, including "The Big Book of What, How and Why" and "A Field Guide to the Dinosaurs of North America." our editorial process Bob Strauss Updated January 29, 2020 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. 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. Large, Meat-Eating Dinosaurs 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 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. The Behavior of Large Theropods As with all carnivores, the main consideration driving the behavior of large theropods like allosaurs and abelisaurs was the availability of prey. As a rule, carnivorous dinosaurs were much less common than herbivorous dinosaurs (since it requires a large population of herbivores to feed a smaller population of carnivores). Since some of the hadrosaurs and sauropods of the Jurassic and Cretaceous periods grew to extreme sizes, it's reasonable to conclude that even the bigger theropods learned to hunt in packs of at least two or three members. One major topic of debate is whether large theropods actively hunted their prey, or feasted on already dead carcasses. Although this debate has crystallized around Tyrannosaurus Rex, it has ramifications for smaller predators like Allosaurus and Carcharodontosaurus as well. Today, the weight of the evidence appears to be that theropod dinosaurs (like most carnivores) were opportunistic: they chased down juvenile sauropods when they had the chance, but wouldn't turn up their noses at a huge Diplodocus that died of old age. Hunting in packs was one form of theropod socialization, at least for some genera; another may have been raising young. The evidence is sparse at best, but it's possible that larger theropods protected their newborns for the first couple of years, until they were big enough not to attract the attention of other hungry carnivores. Finally, one aspect of theropod behavior that has received a lot of attention in the popular media is cannibalism. Based on the discovery of the bones of some carnivores (such as Majungasaurus) bearing the tooth marks of adults of the same genus, it's believed that some theropods may have cannibalized their own kind. Despite what you've seen on TV, though, it's much more likely than the average allosaur ate its already-dead family members rather than actively hunting them down for an easy meal!