01 of 16 How Prehistoric Animals Size Up Next to Human Beings Note the teeny-tiny human being in the lower left corner. Sameer Prehistorica The size of prehistoric animals can be difficult to comprehend: 50 tons here, 50 feet there, and pretty soon you're talking about a creature that's as much bigger than an elephant as an elephant is bigger than a house cat. In this picture gallery, you can see how some of the most famous extinct animals that ever lived would have sized up against an average human being--which will give you a good idea what "big" really means!

02 of 16 Argentinosaurus Argentinosaurus, compared to a full-grown human being. Sameer Prehistorica The largest dinosaur for which we have compelling fossil evidence, Argentinosaurus measured over 100 feet from head to tail and may have weighed in excess of 100 tons. Even still, it's possible that this South American titanosaur was preyed upon by packs of the contemporary theropod Giganotosaurus, a scenario you can read about in detail in Argentinosaurus vs. Giganotosaurus - Who Wins?

03 of 16 Hatzegopteryx Hatzegopteryx, compared to a full-grown human being. Sameer Prehistorica Less well known than the equally giant Quetzalcoatlus, Hatzegopteryx made its home on Hatzeg Island, which was isolated from the rest of central Europe during the late Cretaceous period. Not only was Hatzegopteryx's skull ten feet long, but this pterosaur may have had a wingspan of a whopping 40 feet (though it probably only weighed a few hundred pounds, since a heavier build would have made it less aerodynamic).

04 of 16 Deinosuchus Deinosuchus, compared to a full-grown human being (Sameer Prehistorica). Dinosaurs weren't the only reptiles that grew to enormous sizes during the Mesozoic Era. There were also gigantic crocodiles, notably the North American Deinosuchus, which measured over 30 feet from head to tail and weighed as much as ten tons. As intimidating as it was, though, Deinosuchus would have been no match for the slightly earlier Sarcosuchus, aka the SuperCroc; this African crocodile tipped the scales at a whopping 15 tons!

05 of 16 Indricotherium Indricotherium, compared to an African elephant and a full-sized human being. Sameer Prehistorica The biggest terrestrial mammal that ever lived, Indricotherium (also known as Paraceratherium) measured about 40 feet from head to tail and weighed in the vicinity of 15 to 20 tons--which put this Oligocene ungulate in the same weight class as the titanosaur dinosaurs that vanished off the face of the earth 50 million years before. This giant plant-eater probably had a prehensile lower lip, with which it ripped the leaves off the high branches of trees.

06 of 16 Brachiosaurus Brachiosaurus, compared to a full-grown human being. Sameer Prehistorica Granted, you probably already have a sense of how big Brachiosaurus was from repeated viewings of Jurassic Park. But what you may not have realized is how tall this sauropod was: because its front legs were significantly longer than its back legs, Brachiosaurus could attain the height of a five-story office building when it reared its neck up to its full height (a speculative posture which is still a subject of debate among paleontologists).

07 of 16 Megalodon Megalodon, compared to a full-grown human being. Sameer Prehistorica There's not much to say about Megalodon that hasn't all been said before: this was fins-down the biggest prehistoric shark that ever lived, measuring anywhere from 50 to 70 feet long and weighing as much as 100 tons. The only ocean dweller that matched Megalodon's heft was the prehistoric whale Leviathan, which briefly shared this shark's habitat during the Miocene epoch. (Who would prevail in a battle between these two giants? See Megalodon vs. Leviathan - Who Wins?)

08 of 16 The Woolly Mammoth The Woolly Mammoth, compared to a full-grown human being. Sameer Prehistorica Compared to some of the other animals on this list, the Woolly Mammoth was nothing to write home about--this megafauna mammal measured about 13 feet long and weighed five tons soaking wet, making it only slightly bigger than the biggest modern elephants. However, you have to put Mammuthus primigenius in the proper Pleistocene context, where this prehistoric pachyderm was both hunted and worshiped as a demigod by the earliest humans.

09 of 16 Spinosaurus Spinosaurus, compared to a full-grown human being. Sameer Prehistorica Tyrannosaurus Rex gets all the press, but the fact is that Spinosaurus was the more impressive dinosaur--not only in terms of its size (50 feet long and eight or nine tons, compared to 40 feet and six or seven tons for T. Rex) but also its appearance (that sail was a pretty cool accessory). It's possible that Spinosaurus occasionally grappled with the huge prehistoric crocodile Sarcosuchus; for an analysis of this battle, see Spinosaurus vs. Sarcosuchus - Who Wins?

10 of 16 Titanoboa Titanoboa, compared to a full-grown human being (Sameer Prehistorica). The prehistoric snake Titanoboa made up for its relative lack of heft (it only weighed about a ton) with its impressive length--fully grown adults stretched 50 feet from head to tail. This Paleocene snake shared its South American habitat with equally huge crocodiles and turtles, including the one-ton Carbonemys, with which it may occasionally have grappled. (How would this battle have turned out? See Carbonemys vs. Titanoboa - Who Wins?)

11 of 16 Megatherium Megatherium, compared to a full-grown human being. Sameer Prehistorica It sounds like the punchline to a prehistoric joke--a 20-foot-long, three-ton sloth in the same weight class as the Woolly Mammoth. But the fact is that herds of Megatherium were thick on the ground in Pliocene and Pleistocene South America, rearing up on their stocky hind legs to rip the leaves off trees (and fortunately leaving the other mammalian megafauna to themselves, since sloths are confirmed vegetarians).

12 of 16 Aepyornis Aepyornis, posed next to a full-grown human being (Sameer Prehistorica). Also known as the Elephant Bird--so called because it was legendarily huge enough to carry off a baby elephant--Aepyornis was a 10-foot-tall, 900-pound, flightless resident of Pleistocene Madagascar. Unfortunately, even the Elephant Bird was no match for the human settlers of this Indian Ocean island, who hunted Aepyornis into extinction by the end of the 17th century (and also stole its eggs, which were over 100 times larger than those of chickens).

13 of 16 Giraffatitan Giraffatitan, posed next to a full-grown human being (Sameer Prehistorica). If this picture of Giraffatitan reminds you of Brachiosaurus (slide #6), that's no coincidence: many paleontologists are convinced that this 80-foot-long, 30-ton sauropod was actually a Brachiosaurus species. The truly remarkable thing about the "giant giraffe" was its almost comically long neck, which allowed this plant-eater to lift its head to a height of almost 40 feet (presumably so it could nibble on the tasty upper leaves of trees).

14 of 16 Sarcosuchus Sarcosuchus, compared to a full-grown human being (Sameer Prehistorica). The largest crocodile that ever walked the earth, Sarcosuchus, aka the SuperCroc, measured about 40 feet from head to tail and weighed in the neighborhood of 15 tons (making it slightly more menacing than the already pretty menacing Deinosuchus, pictured in slide #4). Intriguingly, Sarcosuchus shared its late Cretaceous African habitat with Spinosaurus (slide #9); there's no telling which reptile would have had the upper hand in a snout-to-snout standoff.

15 of 16 Shantungosaurus Shantungosaurus, compared to a full-grown human being (Sameer Prehistorica). It's a common myth that sauropods were the only dinosaurs to reach double-digit tonnage, but the fact is that some hadrosaurs, or duck-billed dinosaurs, were almost as massive. Witness the truly gigantic Shantungosaurus of Asia, which measured 50 feet from head to tail and weighed about 15 tons. Amazingly, as huge as it was, Shantungosaurus may have been capable of running for short bursts on its two hind feet, when it was being chased by predators.