Identifying the biggest, often deadly, dinosaurs that ever lived isn't as easy a task as you might think: sure, these giant beasts left giant fossils, but it's very rare to unearth a complete skeleton (tiny, bite-sized dinosaurs tend to fossilize all at once, but lumbering giants like Argentinosaurus can often only be identified by a single, massive neckbone). On the following slides, you'll find the biggest dinosaurs, according to the current state of research—as well as the biggest pterosaurs, crocodiles, snakes, and turtles.

01 of 20 Biggest Herbivorous Dinosaur - Argentinosaurus (100 Tons) Argentinosaurus. MathKnight and Zachi Evenor / Wikimedia Commons (CC BY-SA 3.0) Although paleontologists claim to have identified bigger dinosaurs, Argentinosaurus is the largest whose size has been backed up by convincing evidence. This gigantic titanosaur (named after Argentina, where its remains were discovered in 1986) measured about 120 feet from head to tail and may have weighed nearly 100 tons. Just one of the vertebra of Argentinosaurus is over four feet thick. Other, less-well-attested contenders for the "biggest dinosaur" title include Futalognkosaurus, Bruhathkayosaurus and Amphicoelias; a new contender, still unnamed and about 130 feet long, was recently discovered in Argentina.

02 of 20 Biggest Carnivorous Dinosaur - Spinosaurus (10 Tons) Spinosaurus. Mike Bowler / Wikimedia Commons You probably thought the winner in this category would be Tyrannosaurus Rex, but it's now believed that Spinosaurus (which had a huge, crocodile-like snout and a sail of skin sprouting from its back) was slightly heavier, weighing as much as 10 tons. And not only was Spinosaurus big, but it was agile as well: recent evidence points to it being the world's first identified swimming dinosaur. (By the way, some experts insist that the biggest meat-eater was the South American Giganotosaurus, which may have matched, and occasionally even outclassed, its northern African cousin.)

03 of 20 Biggest Raptor - Utahraptor (1,500 Pounds) Utahraptor (Early Cretaceous) at Museum of Ancient Life (Lehi, Utah). Wilson44691 / Wikimedia Commons Ever since its starring role in Jurassic Park, Velociraptor gets all the press, but this chicken-sized carnivore was positively anemic next to Utahraptor, which weighed in at a whopping 1,500 pounds (and was a full 20 feet long). Oddly, Utahraptor lived tens of millions of years before its more famous (and smaller) cousin, a reversal of the general evolutionary rule that tiny progenitors evolve into plus-sized descendants. Terrifyingly, the gigantic, curving hind claws of Utahraptor--with which it slashed and gutted prey, possibly including Iguanodon--measured nearly a full foot long.

04 of 20 Biggest Tyrannosaur - Tyrannosaurus Rex (8 Tons) J.M. Luijt / Wikimedia Commons / CC BY-SA 2.5 Poor Tyrannosaurus Rex: once considered (and often assumed) to be the world's biggest carnivorous dinosaur, it has since been surpassed in the rankings by Spinosaurus (from Africa) and Giganotosaurus (from South America). Thankfully, though, North America can still lay claim to the world's biggest tyrannosaur, a category that also includes not-quite-T.-Rex sized predators like Tarbosaurus and Albertosaurus. (By the way, there's evidence that T. Rex females outweighed males by a half ton or so -- a classic example of sexual selection in the theropod kingdom.)

05 of 20 Biggest Horned, Frilled Dinosaur - Titanoceratops (5 Tons) Pentaceratops - Guinness world record holder for largest skull. Kurt McKee / Wikimedia Commons If you haven't heard of Titanoceratops, the "titanic horned face," you're not alone: this ceratopsian dinosaur was only recently diagnosed from an existing species of Centrosaurus on display at the Oklahoma Museum of Natural History. If its genus designation holds up. Titanoceratops will slightly outclass the largest species of Triceratops, full-grown individuals measuring 25 feet from head to tail and weighing north of five tons. Why did Titanoceratops have such a massive, ornate head? The most likely explanation: sexual selection, males with more prominent noggins being more attractive to females.

06 of 20 Biggest Duck-Billed Dinosaur - Magnapaulia (25 Tons) Magnapaulia (Lambeosaurus laticaudus). Dmitry Bogdanov / Wikimedia Commons As a general rule, the biggest dinosaurs of the Mesozoic Era were the aptly named titanosaurs, represented on this list by Argentinosaurus (slide #2). But there were also some hadrosaurs, or duck-billed dinosaurs, that grew to titanosaur-like sizes, chief among them the 50-foot-long, 25-ton Magnapaulia of North America. Despite its enormous bulk, "Big Paul" (so named after Paul G. Hagaa, Jr., the president of the board of trustees of the Los Angeles Museum of Natural History) may have been capable of running on its two hind legs when pursued by predators, which must have made for an impressive sight!

07 of 20 Biggest Dino-Bird - Gigantoraptor (2 Tons) Gigantoraptor skeletal mount. doronko/Flickr.com Given its name, you might think Gigantoraptor should feature on this list as the biggest raptor, the honor currently bestowed on Utahraptor (slide #4). But even though this central Asian "dino-bird" was over twice the size of its North American cousin, it wasn't technically a raptor, but a gentler breed of theropod known as an oviraptorosaur (after the poster genus of the breed, Oviraptor). One thing we don't yet know about Gigantoraptor is whether it preferred to eat meat or vegetables; for the sake of its late Cretaceous contemporaries, let's hope it was the latter.

08 of 20 Biggest Bird Mimic Dinosaur - Deinocheirus (6 Tons) Deinocheirus mirificus restoration. Based on the skeletal diagram and description in Lee et al. (2014). FunkMonk/Wikimedia Commons It took a long time for Deinocheirus, the "terrible hand," to be correctly identified by paleontologists. The huge forelimbs of this feathered theropod were discovered in Mongolia in 1970, and it wasn't until 2014 (after the unearthing of additional fossil specimens) that Deinocheirus was conclusively pegged as an ornithomimid, or "bird mimic," dinosaur. At least three or four times the size of North American ornithomimids like Gallimimus and Ornithomimus, the six-ton Deinocheirus was a confirmed vegetarian, wielding its massive, clawed front hands like a pair of Cretaceous scythes.

09 of 20 Biggest Prosauropod - Riojasaurus (10 Tons) Riojasaurus skull cast, Copenhagen. FunkMonk (Micheak B.H.)/Wikimedia Commons Tens of millions of years before giant sauropods like Diplodocus and Apatosaurus ruled the earth, there were the prosauropods, the smaller, occasionally bipedal herbivores distantly ancestral to those late Jurassic behemoths. The South American Riojasaurus is the largest prosauropod yet identified, a 30-foot-long, 10-ton plant-eater of the late Triassic period, over 200 million years ago. You can detect the proto-sauropod bona fides of Riojasaurus in its relatively long neck and tail, though its legs were much more slender than those of its massive descendants.

10 of 20 Biggest Pterosaur - Quetzalcoatlus (35-Foot Wingspan) Life restoration of Quetzalcoatlus. Johnson Mortimer/Wikimedia Commons When measuring the size of pterosaurs, it's not weight that counts, but wingspan. The late Cretaceous Quetzalcoatlus couldn't have weighed more than 500 pounds soaking wet, but it was the size of a small airplane, and presumably capable of gliding long distances on its massive wings. (We say "presumably" because some paleontologists speculate that Quetzalcoatlus wasn't capable of flight, and instead stalked its prey on two legs, like a terrestrial theropod). Fittingly enough, this winged reptile was named after Quetzalcoatl, the feathered serpent god of the long-extinct Aztecs.

11 of 20 Biggest Crocodile - Sarcosuchus (15 Tons) Dinosaurios Park, Sarcosuchus. HombreDHojalata / Wikimedia Commons Better known as the "SuperCroc," the 40-foot-long Sarcosuchus weighed as much as 15 tons--at least twice as long, and ten times as heavy, as the biggest crocodiles alive today. Despite its enormous size, though, Sarcosuchus appears to have led a typical crocodilian lifestyle, lurking in the African rivers of the middle Cretaceous period and launching itself at any dinosaurs unlucky enough to draw too near. It's possible that Sarcosuchus tangled occasionally with another river-dwelling member of this list, Spinosaurus.

12 of 20 Biggest Snake - Titanoboa (2,000 Pounds) Titanoboa -- Smithsonian Natural History Museum. Ryan Somma/Flickr.com What Sarcosuchus was to contemporary crocodiles, Titanoboa was to contemporary snakes: an impossibly humongous forebear that terrorized the smaller reptiles, mammals, and birds of its lush habitat 60 or 70 million years ago. The 50-foot-long, one-ton Titanoboa prowled the humid swamps of early Paleocene South America, which--like King Kong's Skull Island--hosted an impressive array of giant reptiles (including the one-ton prehistoric turtle Carbonemys) a mere five million years or so after the dinosaurs had gone extinct.

13 of 20 Biggest Turtle - Archelon (2 Tons) 75 million year old "Archelon ischyros" from South Dakota. Mike Beauregard/Flickr.com Let's put the marine turtle Archelon into perspective: the largest testudine alive today is the Leatherback Turtle, which measures five feet from head to tail and weighs about 1,000 pounds. By comparison, the late Cretaceous Archelon was about 12 feet long and weighed in the neighborhood of two tons--not only four times as heavy as a Leatherback, and eight times as heavy as a Galapagos Tortoise, but twice as heavy as a Volkswagen Beetle! Weirdly enough, the fossil remains of Archelon hail from Wyoming and South Dakota, which 75 million years ago were submerged beneath the Western Interior Sea.

14 of 20 Biggest Ichthyosaur - Shastasaurus (75 Tons) A reconstruction of Shastasaurus sikanniensis. PaleoEquii/Wikimedia Commons Ichthyosaurs, the "fish lizards," were large, dolphin-like marine reptiles that dominated the seas of the Triassic and Jurassic periods. For decades, the biggest ichthyosaur was believed to be ​Shonisaurus, until the discovery of a super-sized (75 ton) Shonisaurus specimen prompted the erection of a new genus, Shastasaurus (after California's Mount Shasta). As huge as it was, Shastasaurus subsisted not on comparably sized fish and marine reptiles, but on soft-bodied cephalopods and other wee marine creatures (making it broadly similar to the plankton-filtering Blue Whales populating the world's oceans today).

15 of 20 Biggest Pliosaur - Kronosaurus (7 Tons) Kronosaurus queenslandicus. ДиБгд/Russian Wikipedia/Wikipedia Commons Not for nothing was Kronosaurus named after the mythical Greek god Cronos, who ate his own children. This fearsome pliosaur--a family of marine reptiles characterized by their squat torsos, thick heads perched on short necks, and long, ungainly flippers--ruled the seas of the middle Cretaceous period, eating pretty much anything (fish, sharks, other marine reptiles) that happened across its path. It was once believed that another famous pliosaur, Liopleurodon, outclassed Kronosaurus, but it now appears that this marine reptile was roughly the same size, and perhaps a bit smaller.

16 of 20 Biggest Plesiosaur - Elasmosaurus (3 Tons) Elasmosaurus Skeleton - Burke Museum of Natural History and Culture, Seattle, WA. The Lamb Family/Wikimedia Commons Kronosaurus was the largest identified pliosaur of the Cretaceous period; but when it comes to plesiosaurs--a closely related family of marine reptiles with long necks, slender trunks, and streamlined flippers--Elasmosaurus takes pride of place. This svelte undersea predator measured about 45 feet from head to tail and weighed a relatively petite two or three tons, and it preyed not on comparably sized marine reptiles, but smaller fish and squids. Elasmosaurus also figured prominently in the Bone Wars, the 19th-century feud between the famous paleontologists Edward Drinker Cope and Othniel C. Marsh.

17 of 20 Biggest Mosasaur - Mosasaurus (15 Tons) Fossil of Mosasaurus, an extinct mosasaur -- Natural History Museum of Maastricht. Ghedoghedo/Wikimedia Commons By the end of the Cretaceous period, 65 million years ago, ichthyosaurs, pliosaurs, and plesiosaurs (see previous slides) were either extinct or on the wane. Now the world's oceans were dominated by mosasaurs, fierce, streamlined marine reptiles that ate anything and everything--and at 50 feet long and 15 tons, Mosasaurus was the biggest, fiercest mosasaur of them all. In fact, the only creatures capable of competing with Mosasaurus and its ilk were slightly less enormous sharks--and after marine reptiles succumbed to the K/T Extinction, these cartilaginous killers ascended to the apex of the undersea food chain.

18 of 20 Biggest Archosaur - Smok (2,000 Pounds) Smok. Panek / Wikimedia Commons During the early to middle Triassic period, the dominant terrestrial reptiles were archosaurs--which were fated to evolve not only into dinosaurs but into pterosaurs and crocodiles as well. Most archosaurs weighed only 10, 20, or perhaps 50 pounds, but the euphoniously named Smok was the exception that proved the rule: a dinosaur-like predator that tipped the scales at a full ton. In fact, Smok was so big, and so demonstrably not a true dinosaur, that paleontologists are at a loss to explain its existence in late Triassic Europe-- a situation that may be remedied by the discovery of additional fossil evidence.

19 of 20 Biggest Therapsid - Moschops (2,000 Pounds) Moschops capensis - Middle Permian of South Africa. Dmitry Bogdanov/Wikimedia Commons For all intents and purposes, Moschops was the moo-cow of the late Permian period: this slow, ungainly, none-too-bright creature shuffled across the plains of southern Africa 255 million years ago, possibly in sizable herds. Technically, Moschops was a therapsid, an obscure family of reptiles that evolved (tens of millions of years later) into the very first mammals. And here's a bit of trivia to share with your friends: Way back in 1983, Moschops was the star of its very own kid's show, in which the title character shared its cave (somewhat inaccurately) with a Diplodocus and an Allosaurus.