The Large, Meat-Eating Dinosaurs

The Large, Meat-Eating Dinosaurs Few issues in paleontology are as confusing as the classification of theropodsthe 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, Im going to add fuel to the Jurassic fire by creating my own informal sorting system. Ive already addressed tyrannosaurs, raptors, therizinosaurs, ornithomimids and dino-birdsthe more evolved theropods of the Cretaceous periodin separate articles on this site. This piece will mostly discuss the big theropods (excluding tyrannosaurs and raptors) that Ive 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 wont 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, threes 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 theres 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, its usually as a subgroup of carnosaurs alongside the allosaurs. Tetanurans. This is one of those groups thats 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, its 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 wouldnt 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 its 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. (However, its also possible that some theropod kids were left to fend for themselves from birth!). 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, its believed that some theropods may have cannibalized their own kind. Despite what youve seen on TV, though, its much more likely that the average allosaur ate its already-dead family members rather than actively hunting them down for an easy meal!