10 Prehistoric Creatures that Grew to Dinosaur-Like Sizes

A List of Dinosaur-Sized Prehistoric Animals

The Greek prefix "dino" (meaning "great" or "terrible") is extremely versatile--it can be attached to just about any kind of giant animal besides dinosaurs, as demonstrated by the examples below.

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Dino-Cow - The Auroch

Heck's Cattle, the modern equivalent of the Auroch (Wikimedia Commons).

Not all megafauna mammals went extinct toward the end of the last Ice Age, about 10,000 years ago. For example, the Auroch, a slightly larger predecessor of the modern dairy cow, managed to survive in eastern Europe until the start of the 17th century AD and roamed the Netherlands as late as 600 AD. Why did aurochs become extinct? Well, the obvious answer is that the burgeoning human populations of first-millennium Europe hunted them down for food. But as so often happens, encroaching human settlement also whittled down the aurochs' natural habitat, to the point where they simply did not have room enough to breed.

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Dino-Amoeba - Gromium

A relative of the dino-amoeba (Mic-UK).

Amoebas are tiny, transparent, primitive creatures, mostly inoffensive except when they're colonizing your intestinal tract. But recently scientists discovered a mega-amoeba called Gromia, an inch-in-diameter spherical blob that inhabits the sea bottoms of the Bahamanian coast. Gromia makes its living by rolling slowly along deep-sea sediments (top speed: about an inch a day), sucking up any microorganisms it happens across. What makes Gromia important, from a paleontological perspective, is that the tracks it creates on the sea bottom are very similar to the fossilized tracks of as-yet-unidentified organisms from the Cambrian period, about 500 million years ago.

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Dino-Rat - Josephoartigasia

Dino-Beaver: Castorocauda. Wikimedia Commons

Pretty much any type of animal--not just reptiles--will evolve to as large a size as necessary to fill an available ecological niche. Consider Josephoartigasia mones, a gigantic rodent that lived in South America about four million years ago. Judging by its almost two-foot-long head, paleontologists think this mega-rat weighed over 2,000 pounds or as much as a full-grown bull--and it may have successfully fought off saber-toothed cats and swooping birds of prey. Despite its size, though, Josephoartigasia appears to have been a relatively gentle plant-eater, and it may or may not be the last word in gigantic prehistoric rodents, pending further discoveries.

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Dino-Turtle - Eileanchelys

Odontochelys, a relative of Eileanchelys.

You might think the discovery of a new species of marine turtle ranks right up there with, say, finding oil in Saudi Arabia. The difference is, this turtle lived about 165 million years ago, during the late Jurassic period, and represents an intermediate form that succeeded the landbound turtles of the preceding Triassic. Near-complete fossils of this medium-sized, domed reptile, Eileanchelys waldmani, were discovered by researchers in Scotland's Isle of Skye, which had a much more temperate climate 165 million years ago than it does today. This find demonstrates that turtles were more ecologically diverse, at earlier times, than anyone had previously suspected.

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Dino-Crab - Megaxantho

Dino-Crab: Megaxantho. Cornell University

Giant crabs with oversized right claws are the poster crustaceans for sexual selection: male crabs use these huge appendages to attract females. Recently, paleontologists discovered the fossil of an especially giant-clawed crab of the aptly named Megaxantho family, which lived during the late Cretaceous period alongside the last of the dinosaurs. What's interesting about this crab--besides its enormous size--is the prominent tooth-shaped structure on its giant claw, which it used to pry prehistoric snails out of their shells. Also, this species of Megaxantho lived 20 million years earlier than paleontologists had previously thought, which may prompt some rewriting of the "crustaceans" section of biology textbooks.

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Dino-Goose - Dasornis

Dasornis (Senckenberg Research Institute).

Sometimes it seems as if every animal living today had at least one oversized ancestor. Consider Dasornis, a gigantic, goose-like prehistoric bird that lived in southern England about 50 million years ago. This bird's wingspan measured about 15 feet, making it bigger than any eagle alive today, but its weirdest feature was its primitive teeth, which it used to hold onto fish after it scooped them out of the sea. Could Dasornis have been an offshoot of the pterosaurs, the flying reptiles that dominated the skies of the Cretaceous period? Well, no: pterosaurs went extinct 15 million years before Dasornis glided onto the scene, and anyway, we all know that birds evolved from landbound dinosaurs.

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Dino-Frog - Beelzebufo

Dino-Frog: Beelzebufo. Nobu Tamura

Tens of millions of years ago, frogs (and other prehistoric amphibians) were usually on the wrong end of the food chain, tasty mid-afternoon hors d'oeuvres for carnivorous dinosaurs snacking between meals. So it's poetic justice that researchers in Madagascar recently unearthed a bowling-ball-sized frog that may have fed on baby dinosaurs. Beelzebufo (whose name translates as "devil frog") weighed 10 pounds, with an exceptionally wide mouth well-suited to scarfing down tiny reptiles. This frog lived during the late Cretaceous period, about 65 million years ago--and one can only speculate about the size it might have attained if it hadn't been pulverized in the K/T Extinction.

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Dino-Newt - Kryostega

Eryops, a close relative of Kryostega (Wikimedia Commons).

One of the rules of evolution is that organisms tend to evolve (or "radiate") to fill open ecological niches. During the early Triassic period, the role of "big, dangerous land animal that eats anything that moves" hadn't yet been taken by carnivorous dinosaurs, so you shouldn't be shocked by the discovery of Kryostega, a giant amphibian that roamed Antarctica 240 million years ago. Kryostega looked more like a crocodile than a salamander: it was 15 feet long, with a long, narrow head studded with huge upper and lower teeth. If you're wondering how any creature--much less an amphibian--could survive in prehistoric Antarctica, bear in mind that the southern continent used to be much more temperate than it is today.

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Dino-Beaver - Castoroides

Dino-Beaver: Castoroides. Field Museum of Natural History

Long story short: beavers the size of black bears prowled North America three million years ago. To judge by recent fossil discoveries, the giant beaver Castoroides survived right up until the last Ice Age, when it disappeared along with other plus-sized megafauna mammals, like Woolly Mammoths and Giant Sloths--both because the vegetation these creatures fed on wound up buried beneath gigantic glaciers, and because they were hunted to extinction by early humans. By the way, you'd think beavers the size of grizzly bears would've built dams the size of the Grand Cooley, but (if they ever existed) none of these structures have survived down to the present day.

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Dino-Parrot - Mopsitta

Mopsitta (Wikimedia Commons).

There's something about discovering a 55-million-year old parrot that brings out the wacky side of paleontologists--especially if that parrot is dug up in Scandinavia, thousands of miles from the tropics. The bird's scientific name is Mopsitta tanta, but researchers have taken to calling it "Danish Blue," after the deceased ex-parrot in a famous Monty Python sketch. (It doesn't help that the sketch parrot was described as "pining for the fjords.") All joking aside, what does Danish Blue tell us about parrot evolution? Well, for one thing, the world was clearly a hotter place 55 million years ago--it's even possible that parrots originated in the northern hemisphere, before finding a permanent home further south.