River Monsters Full Episode - Prehistoric Terror

Added: 2026-05-05

[00:00:00]I'm Jeremy Wade. For more than 30 years, I've been investigating underwater mysteries.

[00:00:07]>> Is a fish and uncovering nightmarish beasts.

[00:00:14]>> But if I thought I'd seen the worst, I'd be wrong.

[00:00:21]Half a billion years of evolution has seen millions of fish come and go. among the monsters that were stranger, deadlier, and bigger than anything alive today.

[00:00:36]But with the evidence locked in stone, it's been impossible for me to pursue these prehistoric predators.

[00:00:44]Now, however, breakthroughs in technology mean I've found a way to unlock their secrets.

[00:00:50]>> It's amazingly clear. You've got upper jaw, lower jaw. And now that the door to the prehistoric past is open, I have to go through.

[00:00:59]>> This is the first time I've been so close to such a large predator. I'm going on an epic journey around the globe and back in time to hunt down the greatest river monster that ever lived.

[00:01:22]In 30 years of tracking down monsters, I've always relied on a fishing line to get results.

[00:01:31]>> They do exist. They do exist.

[00:01:34]>> But this mission is different. I need to find a way to connect with another time because the monsters I'm hunting now lived millions, even hundreds of millions of years ago.

[00:01:50]I'm going to hunt the special living river monsters that still carry echoes of the distant past. They're going to help me find the biggest, baddest river monster of all time.

[00:02:05]Where better to begin than with the ultimate instrument of death and destruction?

[00:02:13]This is a cast of a tooth that's around 260 million years old. It belonged to the very first river monster.

[00:02:25]One way to help me picture my prehistoric contenders is to draw them out in sand. Life size.

[00:02:34]This 10-ft long serpentine swamp monster was a lethal ambush predator known as the xenac or eel shark.

[00:02:44]These were the first prehistoric predators to take over fresh water.

[00:02:51]In Texas, you still find their fossils and similar habitat today.

[00:02:58]So, that's where I'm starting my investigation.

[00:03:04]these freshwater sharks were hunting, I wouldn't recognize the vegetation. It' be quite different to what we've got here. But the general feel, the general habitat would be quite similar. You'd have these channels, but then off the channels, you'd have lots of trees standing very close together.

[00:03:19]Eel sharks were the top predator of warm swamps and rivers like these in the prehistoric past, and ambush hunters are defined by their habitat.

[00:03:31]If I can find the top predator here today, then maybe it can shed light on the rise of the very first river monster.

[00:03:39]Normally, I tend to fish with one rod, and I tend to hold that rod or have it very close to me.

[00:03:45]With more than one line, I can cover a lot more water.

[00:03:55]This has taken 20 yards or so.

[00:03:59]It's still going. It's moving right to left.

[00:04:03]>> There. Still going. It's going across the river now.

[00:04:08]Awesome.

[00:04:12]Got to tighten up and go. Okay.

[00:04:17]Yeah. This is a good size fish.

[00:04:22]There it is. There it is. There it is.

[00:04:33]Got her, guys.

[00:04:35]>> Fantastic.

[00:04:37]>> It's an alligator gar, a living fossil that's been unchanged for more than 60 million years and is close to the size of the prehistoric eel shark.

[00:04:49]Take it to the beach here where we can actually just gently slide it up on the sand.

[00:04:56]But I'm wary of any predator that weighs in at over 120 lb.

[00:05:06]>> This fish's body is packed with muscle to launch surprise attacks, just as I imagined the eel sharks was.

[00:05:15]>> Right, this creature is very much the top predator in the water these days.

[00:05:19]And that's a double row of teeth it's got on the upper jaw.

[00:05:23]But the eel shark took its hardware to an even more devastating level.

[00:05:29]>> If you can imagine something where every tooth has two prongs, >> then any attempt to wrigle free would shred the prey alive. This 10-ft long prehistoric predator was the largest freshwater fish of its time and grew bigger than any fish in these waters today.

[00:05:47]>> The xenacanth, an eelike shark, would have been 3 and 1/2 ft longer than this.

[00:05:54]Eel sharks dominated the world's swamps and rivers for more than a 100 million years, hunting the same waters as the ancestors of the gar.

[00:06:28]Eel sharks were the kings of stealth, freshwater swamp monsters that ambushed their victims at close range.

[00:06:36]But many factors make for a successful predator, like strength, size, and speed.

[00:06:47]Evidence is emerging of a fast swimming open water predator which definitely ticks all these boxes and it's being excavated from landlocked Kansas.

[00:07:01]I'm on an excavation site where this lightningast predator has been found but its exact location is secret so that the site's not looted. But I can tell you it lies in the smoky hills of Kansas in the heart of Tornado Alley.

[00:07:22]Normally we can't see what's beneath the surface of the earth without digging a hole. But what's happened here is that nature in the form of wind and water has done the excavation for us. And as I walk down into these chalk deposits layer by layer, it's literally like traveling back in time.

[00:07:42]And just about everywhere you look, emerging into the light for the first time in 85 million years are the remains of extinct creatures. And they're mostly things like these clams that you can see here.

[00:07:56]But there were also predators.

[00:08:00]This tooth belonged to a lightning fast river monster called Zactinus.

[00:08:07]Some of the bodies exumed from this ancient seabed have reached more than 17 ft long.

[00:08:14]To get my first glimpse of this deadly speed merchant, I'm going to the Rocky Mountain Dinosaur Resource Center, where its fossils are cleaned, cast, and then reconstructed into three-dimensional skeletons.

[00:08:34]Now that I can see the whole thing in three dimensions, there are parts of it that really do remind me of fish that I've seen in the flesh. The way the jaws operate is uncannily like the Goliath tiger fish.

[00:08:49]And the shape of the body is somewhat like a giant tarpon. The tarpon uses this streamlined body design to cut through the water with minimal drag.

[00:09:00]It's explosive on the line and reaches weights of 350 lb.

[00:09:05]>> Yeah. Yeah. Yeah.

[00:09:06]>> But Zactamus could top a ton.

[00:09:11]This is the biggest complete specimen of Zacttonus that's ever been found. Here's the tail, dorsal fin, and all the way down here giving a total length of 18 1/2 ft, the head. And there are bones from other individuals that suggest they could have grown even bigger.

[00:09:38]The shape of its skeleton, especially the design of its petrol fins and tail, is very close to another open water predator living today.

[00:09:49]I'm heading south of the equator to the Indian Ocean to try to catch a sailfish, the fastest fish on the planet, which in terms of sheer speed could be a very close match for the torpedo-like Zactinus.

[00:10:06]The large eye sockets on the Zifactton skull suggest to me that it was a sight predator, just like the sailfish.

[00:10:14]We have some teasers off the back in addition to the wake of the boat. And there are baits swimming fast across the surface. I'm waiting for something to hit one of those.

[00:10:44]Oh yeah.

[00:10:47]Oh, I think it's off. No, that'll happen very, very quickly. It's just jumping there in the distance.

[00:11:01]I think I'm on I'm going to come that side. Here we go.

[00:11:05]This fish can strip line in explosive bursts approaching 70 mph.

[00:11:11]>> All right, let's get some line back in.

[00:11:12]Oh, jump. Jump. Jump. Jump. Jump. Jump.

[00:11:15]And again. I'm going to come round to the front. Ran to the front. Ran to the front.

[00:11:20]>> If I can keep it on the hook, it should tire enough for me to bring it in.

[00:11:25]Slowly going in line.

[00:11:34]>> Sailfish have been known to impale fishermen during the struggle to bring them on board.

[00:11:40]This is a real first for me.

[00:11:43]>> Sailfish.

[00:11:45]That's the reason it's called that. This fish is said to be the fastest fish on the planet today. Uh clocked at nearly 70 mph.

[00:11:55]The sail is a bit of a mystery. Some say it's used to corral smaller fish, others that it improves maneuverability. But when it's down, I can see how closely this fish shares a body plan with seactness.

[00:12:09]>> It's just this streamlined tubular shape built for speed. Very characteristic shaped tail, like a bit of aeronautical engineering.

[00:12:18]>> It's almost identical to the giant fossil tail.

[00:12:23]So too are the pectoral fins.

[00:12:26]But that's where the similarities end.

[00:12:29]Zactinus had jaws that could bite fish like this in half. The zifactus was the biggest fish of this general body design that's ever lived. It grew up to about 20 ft long.

[00:12:46]85 million years ago. Zacttonus hunted open waters for fast swimming fish. as big as I am.

[00:12:54]And those prey fish had to be fast because in the open ocean, there's nowhere to hide.

[00:13:10]In terms of speed, size, and hardware, Zactinus has to be up there as one of the greatest river monsters that's ever lived.

[00:13:23]But one prehistoric Leviathan grew to three times its length. And with monsters, size counts for a lot.

[00:13:38]There is a living fish that can give me a sense of scale for this prehistoric leviathan, but it roams for thousands of miles through open ocean.

[00:13:54]I'm looking for the biggest fish that's alive on our planet today. But the ocean is a very big place. So up here is the only realistic way I'll even have a chance of locating one.

[00:14:11]Close up. The surface of the water is too choppy to see through. But from up here, it flattens out. If I'm lucky, I'll see dark shapes lurking in the deeper water outside this Indian Ocean reef.

[00:14:32]I think we got something here. Something down here.

[00:14:40]Right, time to try and get a closer look.

[00:14:44]With this position captured on GPS, I just hope it will stay around long enough for me to track it down in the water.

[00:14:53]I've never tried to go after a fish this big, and I'm starting to wonder if I should. If I get up close to this one, it's going to be in a whole different league.

[00:15:11]The sound of spray is the first indication that there are giants nearby.

[00:15:19]>> Well, there's certainly some big animals on the moon.

[00:15:23]But those are humpback whales, which are mammals. What I'm looking for is a very big fish.

[00:15:36]A dark shadow betrays a giant beneath the surface.

[00:15:47]Emerging from the gloom is not a whale, but a whale shark. The largest fish on the planet today.

[00:15:57]That thing is just massive. It's a good 20 foot long.

[00:16:04]But that's a small one. They grow to about twice that length. That is the size and weight of the school bus.

[00:16:14]But compared with what came before, the whale shark is small fry.

[00:16:19]>> Incredible. Huge as these things are, even a fully grown one is smaller than the biggest fish that ever lived. That was about the size of a train car.

[00:16:37]This prehistoric shadowy beast has to be a contender for the greatest river monster ever. It patrolled our planet's waters long before giants like whale sharks or whales came on the scene. The sheer scale of the biggest ever river monster is something that can only be appreciated outdoors.

[00:17:00]Normally when I go to the beach, I'm there to bring in a monster. I never get to play in the sand.

[00:17:08]>> Today, I'm going to create my own life-size image of this contender for the ultimate river monster.

[00:17:15]From the tip of the snout to the end of its tail, we're talking more than 50 ft.

[00:17:24]So, you could line me up eight times along its length.

[00:17:30]And at up to 12 ft long, you could line up two of me along each fin.

[00:17:36]This fish was like a giant ocean glider.

[00:17:40]Then there's its enormous mouth.

[00:17:44]It had a huge gape. I could pretty much walk into its mouth, but I wouldn't find any savage fangs in there.

[00:17:53]This giant ate plankton like the largest whales do today.

[00:18:01]Long before Wales came on the scene, Leedzikis was the first planktone eating Leviathan.

[00:18:14]No bony skeleton fish has ever grown near this size again. It may have topped 20 tons, yet it flew through prehistoric oceans like a great glider.

[00:18:29]And once Leedzikthis vanished, it would be many millions of years before any other plankton feeder approached its phenomenal size.

[00:18:42]There's no denying that the lead is an unsurpassed giant, but despite its monstrous size, I think the ultimate river monster has to be a predator, a deadly hunter.

[00:18:58]That still leaves Zactinus as the strongest contender so far. But I've just found out that one of the most feared freshwater fish alive today, the piranha, had a giant, even more horrific prehistoric relative. Enter mega piranha.

[00:19:24]These photographs show a unique and very disturbing fossil from the banks of the Rio Parana in Argentina. It's part of a fish's jaw with three teeth. Based on the size and curvature of this jaw fragment, its owner was a giant piranha 3 ft long.

[00:19:44]If it was swimming alongside me, it would extend from my head to my waist.

[00:19:50]And if the mega piranha shared the aggressive pack hunting behavior of these guys, then the mega piranha is a truly terrifying candidate for the baddest river monster of all time.

[00:20:05]But with nothing else to go on, how can I bring this beast to life?

[00:20:12]I'm going to scale up the piranha's body to match scientists estimates of mega piranha.

[00:20:22]I've also had the piranha skull laser scanned, then scaled up and 3D printed.

[00:20:35]The biggest piranha that's ever lived with the deadliest smile I've ever seen.

[00:20:43]This could take a hand off. And that's just for starters.

[00:20:47]I can scarcely begin to imagine what a pack of hungry mega piranhas might be capable of.

[00:21:00]Even a school of redbellied piranhas, the most feared fish in the Amazon today, wouldn't stand a chance against mega piranha.

[00:21:27]But we only have a single solitary fossil for this entire species. We don't really know anything about its behavior.

[00:21:37]If it was a pack hunter, then I pity anything else in the water.

[00:21:42]But not all piranhas hunt in packs. So despite its horrific teeth, I'm going to park this one until more evidence turns up, which is exactly what has happened with another fiercely armed prehistoric predator.

[00:21:59]I've heard from a scientist who's just solved a fish century old puzzle about a much bigger, stranger, and possibly more deadly contender.

[00:22:10]This is an announcement from more than a hundred years ago of a great discovery by a Russian scientist, Alexander Carpinsky.

[00:22:20]And this is the strange fossil at the heart of the puzzle. At first sight, this fossil looks like the familiar ammonite, but in Karpinsky's fossil.

[00:22:31]These are teeth.

[00:22:36]Karpinsky named the mystery fish helico pryion, which means spiral saw. To put that into more modern language, let's call it the buzzsaw killer.

[00:22:51]This predator belonged to an obscure group of fish that's all but disappeared.

[00:22:57]But I'm told that there's a relative hanging on in the Pacific Northwest off the coast of Canada.

[00:23:05]The Buzzaw killer's hardware is an evolutionary one-off.

[00:23:11]But if I can bring up its distant cousin, I might get clues to the rest of the mystery predator's body.

[00:23:20]This fish goes by various names, including spookish and ghost shark. And I'm hoping if I can get my hands on one that it will help me start to construct some kind of more complete picture of the extinct buzzaw killer.

[00:23:36]I'm told the fish I'm after doesn't go for baits on a line. So I've joined forces with the Bamfield Marine Sciences Center to find another way to bring this fish up from the dark depths.

[00:23:50]We're going to set a troll.

[00:23:56]>> This is our course. We started the troll here.

[00:24:00]>> Yeah. We're in about 300t of water. So, what lives down there is not your familiar kinds of fish. I mean, it's it's very dark and cold down there.

[00:24:07]>> Very dark. It's very cold. And because we're fishing on the bottom, we're getting fish that tend to be the bottom dwelling fish.

[00:24:16]I don't usually condone trollling, but this operation is run by scientists who are studying the seabed 300 ft below.

[00:24:30]>> I think we've been towing for a little over half an hour now, so that should be enough time to get some fish into the net. So, let's go haul it back and see what we've got.

[00:24:41]I can barely imagine the cast of creatures that hide and hunt in that deep domain.

[00:25:01]There seems to be quite a lot in there.

[00:25:03]Can't see what it is.

[00:25:26]What's this?

[00:25:34]This is the fish that I was after. And I'm handling it with these gloves because it's got a sharp venomous spine on the dorsal fin there. This is the spookish, the ghost shark, also known as rat fish.

[00:25:50]>> I've never seen anything like this anywhere in the world. belongs to a group of fish called chimera which in Greek mythology the chimera was part lion part goat part snake and looking at this it does rather look like an assemblage of different fish all stitched together like Frankenstein's monster strange as it is it is actually related to sharks but its lineage diverged from the rest of the sharks that we see today a long long time ago Go.

[00:26:25]>> The reason I've got my hands on this creature is to find clues to the buzzaw killer's body. It has certain similarities. It's got cartilage for the skeleton. Uh the skin is very similar.

[00:26:37]Small little toothlike dentacles. Did the buzzaw killer have the same spooky eyes to see in the dark ocean depths.

[00:26:48]The ratfish doesn't have a tooth whirl, but it does have fused blades in its mouth. So, weird dental hardware runs in the family. It does look very strange, but it fits perfectly with where these fish live and what they feed on. They're down there on the bottom, and they're feeding on things like shellfish, crabs, and worms.

[00:27:09]Pulling up the ratish from the depths has shown me how alien and bizarre the buzzsaw killer's family is.

[00:27:19]Now I need to find the man who made the connection between these two creatures to build a better picture of my killer contender.

[00:27:28]I'm meeting him in the place where these mysterious blade discs keep turning up, Idaho.

[00:27:37]The sight of this phosphate mine is now more than 600 miles from the coast. But once it was at the bottom of the sea of Phosphoria, a shallow, virtually landlocked sea, which was home to a population of helica pryan.

[00:27:54]And last year, Dr. Leaf Tapanila from Idaho State University solved the century old mystery of how these lethal blade spirals were used by analyzing a unique and now priceless specimen.

[00:28:11]So that is it.

[00:28:12]>> That is the fossil.

[00:28:15]Looking at this, the the first thing I think you think of is is it looks like a saw blade.

[00:28:22]>> But this fossil isn't unique for what's on the surface, but for what lies beneath.

[00:28:28]Dr. Tapanila used CT X-rays to scan the fossil and revealed for the very first time the cartilage jaws that held the bizarre tooth whirl.

[00:28:40]The next step was to bring these jaws to life with 3D printing. And this is the result.

[00:28:48]>> This is an exact one to one replication of what's in the rock to scale.

[00:28:53]>> Life size.

[00:28:54]>> Life size. You're holding a helicopan that no one ever has touched.

[00:28:59]>> The only thing that's missing from this now is the tooth world.

[00:29:06]This animal is meant to eat meat, >> but not bone. Very little wear has been found on any teeth, suggesting these tooth whs were designed to eat boneless prey like squid or sharks. This whirl belonged to a buzzsaw killer the size of a great white, but they got a lot bigger than that. We have spirals that go up over 2 and 1/2 ft in diameter, which means the skull is now pushing us up to 4 feet and gives us an animal that reaches well past 20, 25, maybe even up to 30 ft for the largest helicoprines.

[00:29:46]>> With this knowledge, I'm able to imagine such a beast in action. Perhaps on a moonlit night when ancient squid called bellum knights gather near the seabed to mate.

[00:30:12]Any fish armed with a weapon like this has to rank very highly on the list of the deadliest river monsters ever.

[00:30:35]For me, sheer size and incredible weaponry push the buzzsaw killer way ahead of us.

[00:30:48]But now my challenge is to find out if there's anything bigger, bad, or even more terrifying that can possibly rival it.

[00:31:02]Let me take you on a journey back through time.

[00:31:05]If this line here represents today and this line here represents 400 million years ago, then all the monsters that I'm hunting are somewhere in this space. Now, the vast majority of fish alive today are the so-called rayfinned fish. They've got very obvious bony rays in their fins.

[00:31:33]There are tens of thousands of these species, including piranhas, tiger fish, and catfish.

[00:31:42]This group is also home to zactinus, benef, and mega piranha.

[00:31:49]But when you dig back into the fossil record, the number of rayfinned fish steadily diminishes until there are none around. The other significant group of fish around today are the sharks and their relatives.

[00:32:06]Sharks originated significantly further back than the rayfin fish. This was the era of the eel shark and the buzzsaw killer.

[00:32:17]>> But there was another group of fish.

[00:32:20]They were on the scene for more than 50 million years. But then they all became extinct. There are no descendants alive today.

[00:32:31]These were the placardms or plate skins, ancient armored fish that started a monumental arms race. Not only were their heads encased in bony armor, but these were also the prehistoric fish that pioneered and perfected biting jaws. And one armored fish grew into a gargantuan bone crunching giant, the Duncalostius.

[00:32:58]And this is how big its head armor was.

[00:33:03]Somehow I have to figure out whether this long extinct beast is the greatest river monster that ever lived.

[00:33:17]There's only one living fish I know that even approaches the ancient ancestry, giant size, and armor plating of the Duncalostius.

[00:33:27]It lurks in the dramatic landscape of Canada's Fraser Canyon in the wilds of British Columbia. But fishing here is extremely challenging. This is tremendously deep, powerful, turbulent water. And even though I've been fishing a long time, I have to remind myself that it's possible for creatures to live and hunt in water like this. Who knows what could be down the bottom of this water?

[00:33:56]Yeah. So, we're all set. Baits are down.

[00:34:00]Just waiting there.

[00:34:04]Below me, these churning waters plunge into more than 150 ft of darkness.

[00:34:12]Because of the current, there's a certain amount of line movement and rod movement. But when a fish takes one of those baits, it should be fairly unmistakable.

[00:34:27]>> Yeah, that's that's a good size, I think.

[00:34:49]Here we go. Chance to get a little bit of line back.

[00:34:55]Yank.

[00:34:59]Um, I think it might be approaching the surface.

[00:35:04]Oh, here we here we go. Here we go. Here we go. Here we go. Yep. Here we go.

[00:35:09]Ah, >> there it is. There it is.

[00:35:13]>> It's a 140 lb white sturgeon. The largest fish in North American rivers.

[00:35:19]Sturgeon belong to the most ancient group of rayfinned fish around today.

[00:35:24]And they're incredibly longived.

[00:35:26]>> 184 12. That's just over 6 foot, isn't it?

[00:35:30]>> Heavy fish. Two of us struggling to hold this up briefly, but by sturgeon standards, this isn't big. They grow uh up to three times the length of this.

[00:35:39]Hard to believe.

[00:35:41]>> That's around the same size range as the bone crusher.

[00:35:46]The sturgeon's bony plates are unusual in living fish. And its body helps me imagine the strength and power of the bone crusher's body.

[00:35:55]>> It really is a strange looking beast.

[00:35:57]But back when these started to emerge, this look was actually very common, armor plating. And this is one of the few remnants of that age that survives to this day.

[00:36:09]>> This sturgeon certainly seems like a last link to a lost armored predator.

[00:36:18]>> It's going.

[00:36:21]>> I wonder what horrors these strange fish have witnessed in their 300 million years on the earth.

[00:36:29]Coming to this canyon has rather been like stepping back in time.

[00:36:36]It really is quite a spooky feeling to catch such an ancient and oddlooking fish from such an ancient and dramatic landscape.

[00:36:48]To find more concrete clues to the size and appearance of the bone crusher, I'm heading back to the US to the Rocky River in Ohio.

[00:37:04]This river might not look a very promising place to look for fish, but in fact, this is a very special place because this river over time has dug into the bedrock here. And this cliff is actually the bed of an ancient sea laid down layer after layer over the period of millions of years. And in those layers are the remains of fish. Some of them in an exceptional state of preservation and some of them some absolute monsters.

[00:37:36]The Cleveland shales revealed to the world for the very first time the true scale and power of the bone crusher.

[00:37:44]And David Chapman of the Cleveland Museum of Natural History is still fishing for its heavyweight fossils.

[00:37:52]These hard rocks are unforgiving, crushing precious clues into hundreds of flattened fragments, which have to be painstakingly reassembled. Back at the lab, we have to clean the fragments very carefully. In an afternoon, we can clean off usually a surface area equivalent to a large postage stamp. Once we get all those pieces cleaned, then we reassemble them into more recognizable elements of the skeleton.

[00:38:25]>> But some bones from the top of the bone crusher's immense skull armor are so huge they've withstood millions of years of crushing pressure.

[00:38:36]And that is incredibly solid and thick.

[00:38:40]This must have been about the thickest bone ever in any fish.

[00:38:44]>> As far as we know, it was. These were very massively built animals.

[00:38:51]>> Now I finally have a chance to come face to face with one of them. The very first oceangoing apex predator.

[00:39:06]And this is it. This is Duncalostius.

[00:39:09]And I have to say I'm having a bit of trouble believing that this was actually a fish. It's unlike anything I've caught or seen or anything that's alive on the planet today.

[00:39:21]Based on the remains of close relatives, experts here think the bone crusher reached lengths of around 20 ft.

[00:39:31]And you just can't fail to notice these huge fangs. They're not actually teeth, but sharpened bare jawbones.

[00:39:41]It's estimated that the bone crusher could slam its jaws shut with a force of around 8,000 lb per square in inside 500s of a second. Possibly the strongest fish bite ever and horrifically more powerful than any creature alive today.

[00:39:59]And what all of this tells us is that this wasn't a fish that merely grabbed prey, which had to be small enough to swallow whole. This could slice through flesh and bone, which meant it could go after much larger prey and bite clean mouthfuls out of them. And this took predation to a whole new level.

[00:40:55]As a contender for the ultimate prehistoric river monster, the Bone Crusher is hitting hard.

[00:41:03]But there's one more group of fish with even more formidable roots.

[00:41:08]The armored fish weren't the only other group of ancient fish thriving back here in prehistory.

[00:41:16]There were also the lobe fins.

[00:41:21]They are still hanging on by a thread.

[00:41:26]There's only a handful left and the most famous is the celacanth. It was thought to be extinct until one was found in the Indian Ocean in the 1930s.

[00:41:39]When it was first found, the celacanth was nicknamed old fourlegs because of its strange fleshy paired fins which look like simple limbs.

[00:41:50]It's a nickname that's very apt because way back in prehistory, some lobe fins branched off to eventually become all modern-day four-limmed vertebrates, including us.

[00:42:06]And one prehistoric lobe fin cousin was a gargantuan heavyweight, a 23t 4tonon river monster. The risodont, a predatory freshwater fish, the size and weight of a killer whale.

[00:42:28]330 million years ago, they hunted lakes and rivers in Scotland.

[00:42:37]The first time I came to the Hunterian Museum in Glasgow, it was to see the plesiosaur, the reptile some believe is behind the biggest aquatic monster myth of all time, Loch Ness.

[00:42:51]But I've heard that massive risodont teeth found near here leave the plesiosaur's deadly hardware in the dust.

[00:43:02]Risodont means rooted tooth. And you can clearly see from where the enamel ends that fully half of this weapon was buried in the jawbone. This tusk was clearly designed to drive into flesh and bone and rip prey apart. Now, the size of the teeth suggests large prey.

[00:43:26]And at 23 ft long, this fish had more than enough muscle to take on anything else in the water.

[00:43:35]But apart from its huge tusks and fangs, most of its remains are the bony scales that made up a suit of full body armor.

[00:43:45]There's no fish alive today that comes anywhere near the risodont size, appearance, and aggression.

[00:43:53]But in Australia, that is a river monster that does.

[00:43:57]The esturine crocodile.

[00:44:02]To help bring the prehistoric risodon to life, I'm submerging with one of the largest crocs alive. Getting as close as possible without being torn apart.

[00:44:22]His missing limbs are the price a predator pays to stay at the top.

[00:44:28]At 18 ft long, Chopper is three times my length, but the giant risodon could have been another 5 ft longer.

[00:44:42]And I'm just trying to sort of calm myself down a little bit and get into proper observation mode.

[00:44:51]This giant aquatic predator deploys the same ambush strategy as the risodont.

[00:44:57]And the power of that strike is rooted in the tail.

[00:45:03]>> It's half the body length. So on an 18 ft animal like this, you got a 9 ft tail. And that's just solid muscle. It works in the same way as a fish's tail.

[00:45:12]And it's all about propulsion.

[00:45:15]Judging by size alone, the risodon's massive tail must have packed more than twice the croc's muscle power.

[00:45:24]It's uncomfortably close view. I'm getting of the teeth and jaw of this animal.

[00:45:30]>> These crocs have the strongest recorded bite of any animal alive today. Close to £4,000 per square in for a beast this size.

[00:45:42]A 23 ft 4tonon risodont might pack more than double that.

[00:45:50]Chopper has given me a visceral insight into the awesome killer potential of this prehistoric fish.

[00:45:58]I'm starting to think the risodont could even outbite the bone crusher.

[00:46:04]It's not every day you get that close to a crocodile in the water and live to tell the tale.

[00:46:21]It's time to put this gargantuan fish in context. In Scotland, where it terrorized lakes and rivers 330 million years ago.

[00:46:33]My journey halfway around the world has given me the final information I needed to picture this river monster's general body plan and size.

[00:46:43]And now I'm trying to imagine the world that that beast lived in.

[00:46:50]Millions of years before solid ice carved out the locks, risodon stalked prey as big as a man in murky, slow flowing waters.

[00:47:32]At the end of my search for the greatest river monster of all time, It's a very close contest between the Bone Crusher and the Risodont, but I now know where I'd place my bet.

[00:47:47]So, for me, there's no question the Risodont is the ultimate prehistoric river monster.

[00:47:54]>> As my run of bad luck extended, I found myself thinking more and more about these beliefs. By now, I'm tired and starting to lose my focus.