Megafauna aren’t just big; they have to be surprisingly big. A giraffe is not surprisingly big, nor is an elephant; because we expect them to be big. But we expect mice to be small. So imagine the sort of OMGWTF?! moment you would experience if you saw a mouse the size of puma hissing it’s way towards you at eye height. Exactly. That’s what makes it an example of Megafauna; being able to lord it over its closely related cousins with its horrendously unnatural looking majestic giganticness, and extra fangs.
To help represent the sphincter disintegrating terror of these big chaps, my artistically endowed chum Ewan has done some illustrations of suitable in situ examples. After you've read this you should definitely go to his blog, because it rocks. Like a boulder (and boulders are 100% rock).
5. Meganeuroptera aka really big dragonflies
These dudes were a metre long. Grab a tape measure and check how big that is, and continue reading when you’ve wiped the spilled tears of shock and wonder from your monitor.
If a regular dragonfly alighted on your wrist, you might chuckle with delight and marvel for a brief instant at the pretty shimmer of effervescent iridescence before, with a flap of its delicate crystal wings, it was spiralling up into the sun dappled canopy, or some such. Compare with Meganeuroptera; if one of these landed on your arm you wouldn’t be able to see your arm underneath it. Nor could you see anything at all, what with its horrible scaly wings scratching at your face and eyes as it burst open your stomach with its steel teeth  and treated itself to a faceful of gizzards.
Fortunately, humanity and Meganeuroptera never coexisted, indeed they relied on an air-oxygen content a hefty 163% of modern level to fuel their malign existence, and they have been mercifully smothered by a more modern clime.
4. Colossal squid
Colossal squid exhibit abyssal gigantism, which is to say, the deeper into the blackest depths you sink, the more likely it is that your puny submarine will be torn contemptuously in two by a massive spiked tentacle, before you are drawn into massed rings of teeth quite like those that monster had in the third star wars film except also with tentacles. And more the fool you. Also, the tentacles have hooks and extra teeth. Unlike most Megafauna, Colossal squid are extant rather than extinct, and very occasionally a dead one floats up on a beach somewhere. But nowhere near as often as humans are lost at sea. Eh? Think about it…
3. Haast’s Eagle
These were a bird of prey with a wingspan of 3 metres. Which sounds big, but isn’t all that big compared to some modern birds. And they were 15kg or so, which is not so heavy, you might think. They had rather heftier claws than anything we’re used to, however, and neck muscles like a squad of angry sailors in a sack, which stood them in good stead for some serious dive bombing. (Or perhaps ‘depth charging’, if you want to keep the maritime metaphor afloat.)
Haast’s Eagle mainly fed on moas, a flightless bird, which weighed about 200 kg. The eagles smashed into the moas at high speed from above, using one leg to disembowel and the other to tear open the neck, in a rather deftly dextrous manner, before retreating back to their eyries for a round of margaritas and high fives .
“Hold on a tick!” You might say. “Moas were 200kg? But humans don’t weigh that much!” And since humans and Haast’s eagles lived on the New Zealand islands at the same time, you might reasonably wonder if Haast’s eagles preyed on humans; when a palaeontologist was asked this question he shrugged and said “Yah, Probably. Look at the claws on them.”
I imagine some sort of epic battle ‘twixt man and bird raged.
Who won? Ever seen a Haast’s eagle hovering over a car park?
2. Jaekelopterus aka big sea scorpion
If you subscribe to the notion of a benevolent god creating all of the creatures of the Earth, I would invite you to reconsider this opinion by way of the example of the giant sea scorpion. God must surely have been in an extremely foul mood that day. Possibly it was just after humans turned out to be such a shower of ungrateful bastards, and He wanted something to set on them?
Anyway, this animal is the evolutionary equivalent of a dark alley full of spines, claws and a cloying sense of aquatic foreboding. And although called a sea scorpion, it is speculated to have lived in fresh water rivers and lakes, not in saltwater seas.
Just think; there could totally be a Jaekelopterus skulking in a puddle between you and the shops, or lurking in a children’s paddling pool. There probably isn’t, but that’s just what makes it so terrifying; never knowing when you might hear the brittle clatter of chitin on tarmac. Click clack snap.
1. Megalodon aka giant great white shark
If you were told that megalodons were a really big type of shark you might chuckle genially and say “I think you’ll find that sharks are big enough as it is, old chap,” and then point out that I promised only to discuss animals that were bigger than expected, and that sharks were already as big as one might reasonably expect. “I’ve seen several documentaries on sharks; I know their scale,” you might haughtily impart with a knowledgeable nod and a flicker of condescension dappling your lip.
Yeah, well, megalodons are significantly bigger than that, so nyer.
While in conversation about the awesome majesticallity of yon shark, illustrious Ewan had a biblical revelation. Noting that the Megalodon had jaws which could open up to ten feet wide, Ewan postulated that this may go some way to explain the story of Jonah and the whale. It's possible, he said, that the whale was in fact a Megalodon, meaning Jonah was probably a dolphin. I found this an impeccable line of logic, and concurred. Did I mention that you should definitely be heading over to his blog?