These are animals which are the largest ever known of their kind. All of them have gone extinct and some of them are predecessors of animals we know existing today. Some went extinct long before we ever came to be, and others went extinct largely because we came to be.

Megalodon

Imagine a great white shark but more than twice as large. That’s what megalodon was—an enormous great-white-like shark (emphasis on like) that died out 1.5-million-years-ago. Megalodon’s approximate size capacity remains somewhat disputed, but according to some reputable estimators, megalodon’s growth potential had exceeded sixty-feet in length. For the sake of perspective, consider that the maximum size potential of a modern-day great white maxes out at about twenty-four-feet in length and that that length is a generous figure.

Megatherium

Megatherium, which translates, as “great beast” was an elephant-sized ground sloth endemic to South America from the Late Pliocene epoch to the end of Pleistocene epoch. During its reign, Megatherium was one of the largest land animals on Earth, and remains today as one of the largest land mammals ever known, weighing up to four metric tonnes and measuring over six-meters from head to tail.

The exact cause of Megatherium’s extinction is disputed, but as one of the more widely accepted theories of at least one strong contributing factor of Megatherium’s extinction points to the rise of modern man—giant ground sloth hunters.

Gigantopithecus blacki

Gigantopithecus, named as from the Ancient Greek gigas, which translates as “giant,” is an extinct genus of ape, which is believed to have existed from about nine-million-years-ago to about one-hundred-thousand-years-ago, in regions of central-south-east Asia. At current, there are three known species of extinct Gigantopithecus, the largest of which by far is Gigantopithecus blacki, the largest known primate of all time.

Known only through fossil records of its teeth and mandibles, the exact size of Gigantopithecus blacki is to some extent indeterminable but can still be reasonably estimated at twelve-hundred-pounds in mass and 9.8-feet in standing height—however, as no pelvic or leg bones of Gigantopithecus has yet been discovered, the dominant view is that Gigantopithecus walked on all fours, as do all modern apes, although according to one minority opinion of experts, Gigantopithecus blacki characterized true bipedal locomotion as do we.

But all in all, as its fossil record is severely limited, much about Gigantopithecus is unfortunately highly disputable, but if there is one thing which may be known with good certainty it’s that Gigantopithecus blacki was one gigantic primate, notably much larger than even the largest silverback gorillas around today.

Dating methods indicate Gigantopithecus blacki died out about 100,000 years ago, making G. blacki a former contemporary of modern man, we ourselves, Homo sapiens.

Sarcosuchus imperator

Not technically a crocodile; taxonomically distinct yet seemingly very much the same, the genus of Sarcosuchus is genus to what is most widely considered the largest species of crocodile-like reptile ever to have lived: Sarcosuchus imperator.

With fossils of its skull measuring at record lengths of as much as 5.2-feet long, S. imperator’s total length is reasonably estimated at thirty-nine-feet, making it nearly twice as large as a full-grown modern day saltwater crocodile, which the largest living reptile on Earth.

One can only imagine how frightening it would be to ever actually see an S. imperator in person; literally I’d probably shit my pants. I’d only hope that I’d be too small to be on its menu, but for all that I know I’d probably make for a great snack.

Because of the slender design of its snout, one might initially theorize that S. imperator’s diet had probably compared to that of a modern-day gharial, which although is quite large itself, poses quite small risk of harm to humans as gharials are strict fish-eaters. However, unlike gharials, S. imperator’s teeth are not needle-like—which is ideal for grasping fish—but instead are smooth and sturdy and they interlock when S. imperator’s jaws close, which is why most experts believe that S. imperator’s general diet would most probably compare most accurately to that of modern-day Nile Crocodiles, which are known for hunting large terrestrial animals. But while Nile crocodiles might ambush drinking wildebeests, S. imperator most likely had ambushed actual literal dinosaurs. In any case, I’m not losing any sleep over S. imperator being extinct.

Titanoboa cerrejonensis

Titanoboa, which means “titanic boa,” is an extinct genus of snake that lived and went extinct during the Paleocene epoch, a ten-million-year period immediately subsequent of the extinction of the dinosaurs. The only known species of Titanoboa is T. cerrejonensis, which, measuring at more than forty-feet-long and weighing as much as twenty-five-hundred-pounds, is the longest and largest known snake to have ever lived.

The size of modern-day snakes is often terribly exaggerated. Although there have been several claims of anacondas and reticulated pythons being as large thirty-feet in length, no such claims have ever been proved. In fact, in 1910, Theodore Roosevelt offered a prize reward to anyone who could find a snake thirty-feet in length; the prize had remained unclaimed for decades, eventually growing in worth to fifty-thousand-dollars, but no one could find such a snake. In 2002, the prize finally died, coinciding with the death of its closest-qualifying snake, a female reticulated python named Samantha, homed by the Brooklyn Zoo she was twenty-six-feet long at time of death.

Raphus cucullatus

Commonly known as the dodo, the extinct giant flightless bird that is the dodo, which we often tend to associate with idiocy, is also the largest pigeon that’s ever existed; it’s closest living relative is the Nicobar pigeon.

Subfossil remains show that dodos stood at about one-meter tall and weighed probably about forty-pounds, which made them a worthy food source for hungry sailors. Unfortunately for the dodo, as they’d evolved in isolation from true predation they’d rendered entirely fearless of humans, and that coupled with their inability to fly had made them easy pickings for us.

Steller’s sea cow

Steller’s sea cow was pretty much like any manatee around today, except that it grew to at least thirty-feet in total length. Yeah. That’s a gigantic manatee, the largest member ever of the order Sirenia, in fact. Discovered by humans in 1741, the Steller’s sea cow, which had long been an abundant species throughout the North Pacific, had gone completely extinct just twenty-seven-years after humans found them.

Wiped out by sailors and seal hunters, Steller’s sea cows were popular hunting game, and being as large, and harmless, and slow moving as they were, humans found them quite easy to capture.