The Deeper You Go, The More Prehistoric Fish Look Like Monsters

Added: 2026-06-10

[00:00:02]A creature with a self-sharpening blade for a face, a monster with eyes mounted on stalks, a predator with a venomous spine growing from its skull. These are prehistoric fish that belong in a horror movie. Except they weren't fiction.

[00:00:15]Evolution built every single one and then destroyed most of them. One of them even went on to become your great great ancestor ever measured, living or extinct. That's enough to punch through shark cartilage, ammonite shells, and the armor plating of other fish. Nothing in the Devonian ocean could take a direct hit from this and survive. And if you're thinking, "Well, at least it probably stuck to eating other species."

[00:00:39]No. We found juvenile Dunkleosteus bones in the regurgitated stomach contents of adults. This thing ate its own children.

[00:00:47]380 million years ago, the ocean had a self-sharpening guillotine swimming around in it, and it was so hungry it would eat anything it could catch, including little Jimmy, its baby.

[00:00:59]Dunkleosteus was a blade, but evolution wasn't done experimenting with jaws, cuz something else was out there growing teeth that would never stop and would never fall out. The teeth never fell out. That is the first thing you need to understand about Edestus. In normal sharks run a conveyor belt system. Old teeth drop away, new ones push forward from behind. Mouth stays fresh. Edestus didn't work like that. Its teeth grew from two strips in the center of its mouth, one on the roof and one on the floor, and as new teeth pushed up from behind, the old ones didn't go anywhere.

[00:01:32]They just curved outward and forward, growing past the lips, getting longer every year of the animal's life. Picture a fish with two arcs of saw blade teeth in opposite direction in opposite directions, like serrated tusks having a slow motion collision that never quite arrives. That was Edestus, a living sawmill the size of a great white shark.

[00:01:53]The killing mechanism was even worse than the way it looked. When Edestus bit down, the lower jaw didn't just close, it pulled backward towards the throat at the same time. So, the upper teeth would hold prey in place with the lower teeth dragged across the flesh in a sawing motion with every bite cycle. It wasn't just eating, it was butchering and swallowing in real time. One motion for the kill, one motion for the feed happening together. This thing filled the apex predator slot that opened up after Dunkleosteus went extinct. The ocean needed something new at the top of the food chain, and evolution delivered a creature with scissors for a face that grew more dangerous every year it stayed alive. Edestus was horrifying, >> [music] >> but at least scientists understood what they were looking at when they found it.

[00:02:38]The next fish we're going to talk about, well, they spent over a hundred years trying to figure out where the teeth were even supposed to go. For a hundred years, scientists kept finding the same fossil, a perfect spiral of teeth coiled like a nautilus shell. Some of them were over two feet across, and there was never anything attached. No skull, no body, just teeth spiraling inward in a continuous chain where each one was bigger than last. The theories got weird fast. Maybe the spiral stuck out of its nose like a weapon. Maybe it mounted on the back like a fin. Maybe it grew from the chin and curved upward. Maybe it grew from the chin and curved forward.

[00:03:15]For a hundred years, the best minds in paleontology were basically just guessing because all they had was this bizarre coil of teeth and nothing else to go on. But then in 2013, someone ran a CT scan on a specimen from Idaho that still had part of a skull attached. And the answer was worse than any of the guesses. The tooth whirl was inside the lower jaw curling backward into the throat. When Helicoprion closed its mouth, the spiral rolled forward like a pizza cutter, slicing through whatever it had caught. The teeth weren't falling out and growing back. They were staying attached in a continuous spiral pushed down towards the throat as new ones grew into hide. And here is the twist, it technically wasn't even a shark. The skull puts it closer to ratfish, which are those weird bulbous-eyed things [music] from deep deep water.

[00:04:03]Helicoprion would have been rounder, stranger, with big glassy eyes built for hunting the dark. The ocean had a circular saw with a fish attached. But if you thought freshwater was any safer than the ocean, it wasn't because something bigger was waiting in the rivers.

[00:04:20]Rhizodus was the largest freshwater fish that has ever existed. Not the largest prehistoric freshwater fish, but the largest one period. Nothing that has ever lived in a river or a lake, past or present, has been bigger than this thing. It stretched around 18 ft long and weighed over a ton. And sitting at the front of its mouth were two fangs.

[00:04:43]Not regular teeth, but actual fangs.

[00:04:45]Each one about 9 in long, and behind them the rest of the jaw was packed with smaller but equally sharp teeth ready to hold whatever the fangs caught. The way it hunted was basically identical to a crocodile. Lurk in the murk, wait for something to come close, and then explode upward and grab. The fangs would punch in first, then just like a croc, it would shake violently, thrashing side to side until the prey tore apart into pieces small enough to swallow. But here's the detail that should make you nervous about prehistoric river banks.

[00:05:16]There's evidence that Rhizodus could lunge partially out of the water to grab things on land. Fossil trackway impressions suggest its thick muscular fins were strong enough to shove its front half onto shore. So, if you were an early amphibian 330 million years ago walking down to the river for a drink, this thing could launch itself at you from below the surface. If that doesn't belong in a horror movie, I don't know what does. The swamp wasn't safe, the river wasn't safe, and apparently the shoreline wasn't safe either. Nowhere near the water was off limits. Rhizodus hunted by waiting for prey to come to it, but in the Cretaceous ocean, something else was hunting so recklessly, so aggressively, that it literally ate itself to death.

[00:06:02]Zyphactis looked like a tarpon designed by someone who has seen far too many sleep paralysis demons. It was 20 ft long, covered in thick bony scales, with a face full of 3-in fangs pointing in random directions, like the mouth couldn't decide where to aim. Everything about the skull says grab and do not let go. This thing was such an aggressive feeder that we have a fossil showing exactly how far it was willing to push itself. It's called the fish within a fish, and it's exactly what it sounds like. A 14-ft Zyphactis that died with a 6-ft fish still whole inside its stomach. Not digested, not partially eaten, but whole, swallowed in one piece. The prey apparently thrashed around inside the gut and ruptured something vital, killing the predator that had just eaten it. This animal was so greedy, it literally ate itself to death. Scientists now believe Zyphactis may have been warm-blooded, or at least partially warm-blooded like modern tuna and great whites. And if that's true, it explains a lot. A warm-blooded fish has metabolism that never stops demanding food. It has to eat constantly just to keep running. So, picture a 20-ft predator that couldn't slow down, cruising the Cretaceous seas and forcing anything it could catch down its throat.

[00:07:15]Based on the fossil record, it could catch plenty. The Western Interior Seaway, the shallow ocean that split North America in half during the Cretaceous, was absolutely full of these things. Fast, aggressive, warm-blooded, and apparently willing to swallow prey so large that it killed them. Zyphactis was a pure killing machine, but not everything evolution built was obviously deadly at first. Some of it was just strange. And now we need to talk about the shark with an ironing board growing out of its back. Most of Stethacanthus looked exactly like you'd expect a shark to look. Streamlined body, standard fins, nothing unusual. But growing out of its back, right where the dorsal fin should be, was a flat anvil-shaped platform covered in tiny tooth-like spikes. It's like someone had welded an ironing board made of sandpaper to a shark's spine. And it gets weirder. The same kind of spike patch showed up on the top of its skull. Two rough tooth-covered surfaces, one on the back, one on the head, that scientists have spent decades trying to explain. The guesses are all over the place. Mating display, maybe males with bigger, spikier platforms got more attention.

[00:08:26]Mimicry, so it looked like a larger predator's open mouth from above. Some kind of sensory organ that we don't understand yet. Nobody actually knows.

[00:08:35]Evolution built this thing and kept building it for 80 million years, and we haven't really found out a reason why it exists. That's the part that makes Stethacanthus unsettling in a different way than the blade jaws or the buzz saws. Those make obvious sense to us now. They're weapons, and weapons have clear purposes.

[00:08:54]This thing is evolution doing something for reasons it decided to keep to itself. We can only guess, and all of our guesses sound kind of insane.

[00:09:02]Whatever the ironing board was for, it worked, because Stethacanthus survived from the late Devonian all the way through the Carboniferous. Over 80 million years of success with an anatomy that still doesn't make sense to anyone alive today. Stethacanthus was strange, but clearly survivable. Another prehistoric shark, though, pushed the weird even further, with a venomous spine growing straight out of its skull, and a body shaped like an eel, and a survival run that makes 80 million years look short. Xenacanthus didn't swim like a shark. Its body was long and ribbony, almost like an eel, with a single fin running the entire length of its back and flowing straight into the tail.

[00:09:42]Where modern sharks power through the water with that signature tail stroke, Xenacanthus moved in slow, snaking waves, more like a sea snake anything that you would call a shark. But the spine is what makes it nightmare fuel.

[00:09:55]Growing out of the back of its skull, pointing backward and slightly up, was a long, sharp, bony spike. Think of a stingray's barb, except mounted on a shark's head instead of its tail. And based on the structure, scientists believe [music] it may have been venomous. It would have been a defense mechanism that could stab and poison anything that tried to grab it from behind. Oh, and it lived in fresh water, rivers, swamps, lakes. This was a venomous eel shark that had given up on the ocean entirely and moved into the places where animals go to drink. But the most unsettling thing about Xenacanthus isn't the spine or the venom. It's how long it lasted. This animal first showed up around 360 million years ago and didn't go extinct until the end of the Triassic, roughly 202 million years ago.

[00:10:45]That is That's 160 million years of continuous survival. It lived through the worst mass extinction in Earth's history, the one that killed over 90% of marine species, and just kept going like it didn't notice. Some nightmares don't need to be the biggest or the scariest.

[00:11:03]They just need to refuse to die.

[00:11:06]But something else took the weapon attached to face concept and scaled it up to something absurd. A 2-m barbed harpoon that could sense heartbeats through the mud. Onchopristis could feel your heartbeat from across the riverbed.

[00:11:20]The underside of its snout, all 2 m of it, was covered in electroreceptors.

[00:11:26]These are the same sensors modern sharks used to pick up tiny electrical fields, the kind muscles and hearts give off except oncopristis had them spread across a face mounted weapon the length of a person lying down. The snout itself was lined on both sides with teeth but not like the smooth teeth on a modern sawfish. These had backward pointing barbs like fish hooks designed to snag and hold whatever they slashed through.

[00:11:51]Oncopristis would sweep that thing through the water or drag it across the riverbed and anything it caught would get hooked, tangled and pulled towards the mouth. There was no escape once you were snagged. It lived in the river systems of what's now North Africa around 100 million years ago. The same waterways where spinosaurus hunted and we know exactly what happened between them because there's a vertebrae from an oncopristis that was found lodged in a spinosaurus jaw. The fish with a 2 meter electro sensory harpoon for a face was getting eaten by dinosaurs. That's the Cretaceous for you. Even your apex fish are somebody else's lunch. The whole ecosystem was predators stacked on top of predators all the way up. Oncopristis was terrifying but we always knew what it looked like. Sawfish relatives exist today so the body plan was never a mystery. What came after was a different kind of mystery. Scientists had nothing but teeth to work for with 200 years for this creature and when they finally found a body in 2024 every single thing they'd assumed about it turned out to be wrong. The teeth were flat. That was the problem. For nearly 200 years all scientists had were these broad rigid grinding plates. Some the size of your palm obviously designed for crushing hard shell prey.

[00:13:09]The logical assumption was obvious. Slow moving bottom feeder something like a nurse shark puttering along the seafloor looking for clams and crabs. That assumption shaped everything about how we pictured Triacodus. Reconstruction showed the sluggish animal cruising the ocean bottom gently crunching shellfish.

[00:13:28]Nothing to worry about if you could swim. But then in 2024, complete skeletons turned up in limestone quarries in Mexico, and the body wasn't anything like what anyone expected.

[00:13:39]Tycodus was shaped like a Mako, streamlined to fast, powerful, built for open water and pursuit. The tail was crescent-shaped, the kind you see on high-speed hunters. The pectoral fins were broad and triangular. This wasn't a bottom dweller at all. This was a pursuit predator that happened to have crushing plates instead of cutting teeth. Scientists now think it chased armored prey through the water column, sea turtles, ammonites, anything with a shell that thought it was safe because it could hide inside something hard. But Tycodus didn't need to slice through armor. It just bit down and crushed.

[00:14:14]Picture a 30-ft shark built for speed with over a thousand grinding teeth packed into a jaw nearly a meter wide, actively running down armored prey in open water. That's what was out there in the Cretaceous, and we didn't even know it up until a few years ago. Tycodus was ancient history, over 75 million years gone. But what about something more recent, something close enough to now that it might actually feel familiar, but not in a good way?

[00:14:43]You know what piranhas are, small, angry fish with triangular teeth that can strip flesh from bone where they swarm.

[00:14:49]Scary enough in a movie, manageable in real life if you're not bleeding in the wrong river, because there are many misconceptions about piranhas. Most of the different piranhas out there are not that aggressive, uh unless you are bleeding and like a wounded animal. You will be fine most likely if you swam and you're not like actively bleeding. But now let's scale that up to three. Mega piranha swam the rivers of South America around 10 million years ago. Recent enough that early, and I mean early human ancestors were already walking upright in Africa while this thing was still hunting. It grew to nearly 4 ft long, two to three times the size of the largest piranhas alive today. With teeth arranged in a strange zigzag pattern that let it crush bone and shell along with flesh. The bite force is where it gets absurd. Pound for pound, mega piranha bit harder than Megalodon, harder than Dunkleosteus. This is pound for pound, not total bite force. It's harder than any fish should be possible at that body size. Something that you could technically fit in a large bathtub was putting out more relative force than the largest ocean predators in history.

[00:15:55]The zigzag teeth suggest that it wasn't picky about diet. That pattern sits halfway between the flesh slicing teeth of meat-eating piranhas and the crushing plates of their plant-eating relatives, which if you didn't know, there are vegetarian piranhas out there. I used to keep them in a fish tank when I was a child. Mega piranha could probably do both, slice through muscle and crush through armor in the same meal. There's a strong case that it waited at riverbanks for mammals coming to drink.

[00:16:21]10 million years ago, you wander down to the wrong South American river and something the size of a bulldog with more bite force than a great white is sitting just below the surface waiting.

[00:16:32]Mega piranha was genuinely terrifying, but it was still under 4 ft long. The next fish we're going to talk about went the opposite direction. They were so massive that their size became its own kind of problem. This next fish, Victorian scientists actually named it problematicus at first because they gave up trying to understand what they were looking at. Yeah, prehistoric naming is fun. Bones kept arriving in fragments, they were too big, too weirdly shaped, impossible to assemble into anything recognizable. So, they threw up their hands and called it the problem fish.

[00:17:04]Honestly, fair. That's what I do with problems in my life, too. Modern estimates put Leedsichthys at around 50 ft long and 45 metric tons, which makes it the largest bony fish that has ever existed. Bigger than most whale species, bigger than any fish with a skeleton swimming in the oceans today. But, it was a filter feeder, cruising through Jurassic waters with its enormous mouth open, draining plankton through a honeycomb-like mesh built into its gills. It sounds like a gentle giant, nothing to worry about unless you're microscopic. Except, it shared those oceans with pliosaurs, which were massive short-neck marine reptiles with jaws designed for exactly one purpose.

[00:17:43]And we found Leedsichthys bones covered in their bite marks, chunks torn from the fins, deep gouges raked across the ribs. This whale-sized fish was getting attacked by things that saw 50 ft of bulk as a target, not a warning. That's the cost of being enormous. You're visible from a distance, and you're worth the effort. And in the Jurassic, there was always something willing to make that effort. 50 ft long, and it still wasn't safe. It was just a bigger meal.

[00:18:13]Leedsichthys was massive, but completely unarmored. No plating, no weapons, just size and hope. Another giant, though, from an earlier era tried a different strategy by covering itself in thick bone from head to chest. It looked like one of the most terrifying predators in the ocean, but it also ate plankton.

[00:18:33]Titanichthys was Dunkleosteus' cousin, the same armored face plating, the same Late Devonian seas, the same general body plan. A massive fish wrapped in interlocking bone from snout to mid-torso. If you saw one of these things swimming towards you, every instinct would scream that you were about to die the same way Dunkleosteus killed things, crushed, sheared, and swallowed in pieces. But, if you look at the jaw, you'll notice something's off.

[00:18:59]The bone plates are small. There's no cutting edge anywhere. The whole structure is mechanically weak. It was built for opening wide, not biting down.

[00:19:08]A 2020 study confirmed what the anatomy already hinted at. Titanichthys was a filter feeder. That armored nightmare face was attached to an animal that plankton. It swam around with his mouth hanging open, letting water pour through and straining out whatever tiny organisms drifted in. No hunting, no violence, just cruising and filtering hour after hour. The image is almost funny. 25 ft of bone-plated menace, and it was basically the Devonian version of a whale shark. All threat display and zero follow-through.

[00:19:41]But, here's where it stops being funny.

[00:19:44]Titanichthys lived in the exact same waters as Dunkleosteus, the self-sharpening blade fish that we started the video with. They were neighbors in the same ocean, the same era, probably within swimming distance of each other. And Dunkleosteus almost certainly ate it. Because Titanichthys looked terrifying, but it wasn't. The next creature we're going to talk about looks so completely wrong that scientists spent decades arguing whether it was even an animal. It is So, what kind? Remember the eyes mounted on stalks from the beginning of this video that we showed you? Well, here it is, the Tully monster. And it's exactly as wrong-looking as that description made it sound. The body was soft and roughly tube-shaped, like a fat worm or a stubby eel. The tail had horizontal fins that made it swim almost like a cuttlefish.

[00:20:31]So far, it's weird, but manageable. But then you get to the front and everything breaks down. Instead of a face, there's a rigid horizontal bar crossing the body with a swollen bulge at each end of it.

[00:20:43]Those bulges are its eyes. Not eyes on a head, but eyes mounted on a crossbar pointing in opposite directions, watching everything at once. The mouth isn't where you'd expect it, either.

[00:20:55]It's at the end of a long, flexible trunk sticking out from the front of the body, tipped with a tiny claw holding eight small teeth. Imagine an elephant's trunk shrunk down and fitted with a grabbing tool, and attached to a worm with its eyes on a stick. That's the Tully Monster. For decades, scientists classified this thing as basically everything. Worm, mollusk, arthropod, some kind of bizarre slug, Demi Lovato.

[00:21:20]Nobody agreed because nothing about it made sense. Then, in 2016, a research team finally cracked it. Tullimonstrum was a vertebrate, a relative of lampreys. It had a notochord and gill structures, [music] the works. And now it's the state fossil of Illinois, because of course it is.

[00:21:38]Tullimonstrum lived about 300 million years ago, but to understand where all these creatures actually come from, the blade jaws, the buzz saws, the eyes on stalks, we need to go back even further to when vertebrates were just starting their first experiments in existing at all. 450 million years ago, nothing walked on land. No insects, no plants with roots, no vertebrates of any kind breathing air. The continents were bare rock baking under a sun that looked slightly different than it does now. And in the shallow seas along their edges, some of the earliest fish were running their first experiments in not getting killed. Sacabambaspis was one of them.

[00:22:19]About 10 inches long, covered in bony armor from head to mid body, with no jaws and no paired fins. It couldn't bite anything. It couldn't out-swim anything. All it could do was suck organic material off the seafloor through a small mouth on its underside and try not to attract attention. The eyes are what stick with you. They were huge. Mount- I mean, look at this thing.

[00:22:40]It looks terrifying. They were mounted at the very front of its armored head like the headlamps of an old truck, staring forward with the fixed expression of something that knows it's being watched, because it was. The Ordovician seas were ruled by giant nautiloids and sea scorpions, invertebrate predators that saw Sacabambaspis as an easy snack wrapped in a crunchy shell. No weapons, no way to fight back.

[00:23:04]Its entire survival strategy was armor and awareness. Be hard to eat, see the threat coming. That was the whole strategy. This is what evolution looked like before invented jaws, before it built guillotine mechanisms and buzzsaws and harpoon snouts. The earliest vertebrates weren't predators. They were prey, soft, scared things hiding inside shells, watching in all directions for the things trying to kill them.

[00:23:29]Sacabambaspis was ancient, but there's something even older.

[00:23:33]From the Cambrian, over 500 million years ago, something so small and soft it had no defense at all. It had four eyes, and it is the reason that you are watching this video. Haikouichthys was about 1 in long. It lived 518 million years ago during the Cambrian explosion, that brief window when complex animal life first started branching into all the body plans that we would eventually recognize. It's one of the earliest vertebrates ever found, maybe the earliest. It had no jaws, no armor, no weapons of any kind. Just a soft, flexible body with muscle segments running along its length. A notochord where a spine would eventually evolve, and gills for breathing, and four functioning eyes, two pointed sideways, two pointed straight up. 360° of awareness at all times, because everything in the Cambrian ocean wanted to eat it, and everything did.

[00:24:29]Haikouichthys was the perfect prey item, soft, nutritious, completely unable to fight back. Giant arthropod predators hunted it, anomalocarids with grabbing arms, early armored things with crushing mouths. Anything that could catch it took a shot. The Cambrian was not kind to soft, tiny things, but enough of them survived, enough reproduced, and their descendants became everything. Fish, amphibians, reptiles, the dinosaurs, birds, mammals, whales even trace back to this. You trace back to this. Every nightmare that we've covered, the self-sharpening guillotine, the buzzsaw mouth, the venomous skull spine, the 2-m harpoon face, the eyes on stalks, it all of it descends from something like Haikouichthys, and so do you. The most disturbing creature in this video isn't the scariest one. It's the one that became everything else. Every horror movie monster that evolution has built, that lineage started here in a 1-in fish with four eyes and no way to defend itself getting eaten over and over in a Cambrian ocean that showed it no mercy.

[00:25:38]It survived anyway, and 500 million years later, it became all of us. You enjoyed watching this video, make sure to give it a like and comment down below what other creatures you'd like to see me talk about in future videos. Make sure to also subscribe to the channel and hit the bell notification so you don't miss out on any of the times that we upload. And if you want to watch another one of my videos, you can click the one that's on your screen right now, and I'll see you