Tag Archives: sharks

Episode 216: Gentle Giant Sharks



Let’s learn about some of the biggest sharks in the sea–but not sharks that want to eat you!

Further reading:

‘Winged’ eagle shark soared through oceans 93 million years ago

Manta-like planktivorous sharks in Late Cretaceous oceans

Before giant plankton-eating sharks, there were giant plankton-eating sharks

An artist’s impression of the eagle shark (Aquilolamna milarcae):

Manta rays:

A manta ray with its mouth closed and cephalic fins rolled up:

Pseudomegachasma’s tooth sitting on someone’s thumbnail (left, photo by E.V. Popov) and a Megachasma (megamouth) tooth on someone’s fingers (right):

The megamouth shark. I wonder where its name came from?

The basking shark, also with a mega mouth:

The whale shark:

Leedsichthys problematicus (not a shark):

Show transcript:

Welcome to Strange Animals Podcast. I’m your host, Kate Shaw.

This week we’re going to look at some huge, weird sharks, but they’re not what you may expect when you hear the word shark. Welcome to the strange world of giant filter feeders!

This episode is inspired by an article in the brand new issue of Science, which you may have heard about online. A new species of shark is described in that issue, called the eagle shark because of the shape of its pectoral fins. They’re long and slender like wings.

The fossil was discovered in 2012 in northeastern Mexico, but not by paleontologists. It came to light in a limestone quarry, where apparently a quarry worker found it. What happened to it at that point isn’t clear, but it was put up for sale. The problem is that Mexico naturally wants fossils found in Mexico to stay in Mexico, and the authors of the study are not Mexican. One of the authors has a history of shady dealings with fossil smugglers too. On the other hand, the fossil has made its way back to Mexico at last and will soon be on display at a new museum in Nuevo León.

Fossils from this quarry are often extremely well preserved, and the eagle shark is no exception. Sharks don’t fossilize well since a shark’s skeleton is made of cartilage except for its teeth, but not only is the eagle shark’s skeleton well preserved, we even have an impression of its soft tissue.

The eagle shark was just slightly shorter than 5 ½ feet long, or 1.65 meters. Its tail looks like an ordinary shark tail but that’s the only ordinary thing about it. The head is short and wide, without the long snout that most sharks have, it doesn’t appear to have dorsal or pelvic fins, and its pectoral fins, as I mentioned a minute ago, are really long. How long? From the tip of one pectoral fin to the other measures 6.2 feet, or 1.9 meters. That’s longer than the whole body.

Researchers think the eagle shark was a filter feeder. Its mouth would have been wide to engulf more water, which it then filtered through gill rakers or some other structure that separated tiny animals from the water. It expelled the water through its gills and swallowed the food.

The eagle shark would have been a relatively slow swimmer. It glided through the water, possibly flapping its long fins slowly in a method called suspension feeding, sometimes called underwater flight. If this makes you think of manta rays, you are exactly correct. The eagle shark occupied the same ecological niche that manta rays do today, and the similarities in body form are due to convergent evolution. Rays and sharks are closely related, but the eagle shark and the manta ray evolved suspension feeding separately. In fact, the eagle shark lived 93 million years ago, 30 million years before the first manta remains appear in the fossil record.

The eagle shark lived in the Western Interior Seaway, a shallow sea that stretched from what is now the Gulf of Mexico straight up through the middle of North America. Because it’s the only specimen found so far, we don’t know when it went extinct, but researchers suspect it died out 65 million years ago at the same time as the non-avian dinosaurs. We also don’t have any preserved teeth, which makes it hard to determine what sharks it was most closely related to. Hopefully more specimens will turn up soon.

Now that we’ve mentioned the manta ray, let’s talk about it briefly even though it’s not a shark. It is big, though, and it’s a filter feeder. If you’ve never seen one before, they’re hard to describe. If it had gone extinct before humans started looking at fossils scientifically, we’d be as astounded by it as we are about the eagle shark—maybe even moreso because it’s so much bigger. Its body is sort of diamond-shaped, with a blunt head and short tail, but elongated fins that are broad at the base but end in drawn-out points.

Manta rays are measured in width, sometimes called a wingspan since their long fins resemble wings that allow it to fly underwater. There are two species of manta ray, and even the smaller one has a wingspan of 18 feet, or 5.5 meters. The larger species can grow 23 feet across, or 7 meters. Some other rays are filter feeders too, all of them closely related to the manta.

The manta ray lives in warm oceans, where it eats zooplankton. Its mouth is wide and when it’s feeding it moves forward with its mouth open, letting water flow into the mouth and through the gills. Gill rakers collect tiny food, which the manta ray swallows. It has a pair of fins on either side of the mouth that are sometimes called horns, but which are properly called cephalic fins. Cephalic just means “on the head.” These fins help direct water into the mouth. When a manta ray isn’t feeding, it closes its mouth just like any other shark, folding its shallow jaw shut. For years I thought it closed its mouth by folding the cephalic fins over it, but that’s not the case, although it does roll the fins up into little points. The manta ray is mostly black with a white belly, but some individuals have white markings on the back and black speckles and splotches underneath. We talked about some mysteries associated with its coloring in episode 96.

The eagle shark isn’t the only filter feeding shark. The earliest known is Pseudomegachasma, the false megamouth, which lived around 100 million years ago. It was only described in 2015 after some tiny shark teeth were found in Russia. The teeth looked like those of the modern megamouth shark, although they’re probably not related. The teeth are only a few millimeters long but that’s the same size as teeth from the megamouth shark, and the megamouth grows 18 feet long, or 5.5 m.

Despite its size, the megamouth shark wasn’t discovered until 1976, and it was only found by complete chance. On November 15 of that year, a U.S. Navy research ship off the coast of Hawaii pulled up its sea anchors. Sea anchors aren’t like the anchors you may be thinking of, the big metal ones that drop to the ocean’s bottom to keep a ship stationary. A sea anchor is more like an underwater parachute for ships. It’s attached to the ship with a long rope on one end, and opens up just like a parachute underwater. The tip of the parachute has another rope attached with a float on top. When the navy ship brought up its sea anchors, an unlucky shark was tangled up in one of them. The shark was over 14 ½ feet long, or 4 ½ m, and didn’t look like any shark anyone had ever seen.

The shark was hauled on board and the navy consulted marine biologists around the country. No one knew what the shark was. It wasn’t just new to science, it was radically different from all other sharks known. Since then, only about 100 megamouth sharks have ever been sighted, so very little is known about it even now.

The megamouth is dark brown in color with a white belly, a wide head and body, and a large, wide mouth. The inside of its lower lip is a pale silvery color that reflects light, although researchers aren’t sure if it acts as a lure for the tiny plankton it eats, or if it’s a way for megamouths to identify each other. It’s sluggish and spends most of its time in deep water, although it comes closer to the surface at night.

The basking shark is even bigger than the megamouth. It can grow up to 36 feet long, or 11 meters. It’s so big it’s sometimes mistaken for the great white shark, but it has a humongous wide mouth and unusually long gill slits, and, of course, its teeth are teensy. It’s usually dark brown or black, white underneath, and while it spends a lot of its time feeding at the surface of the ocean, in cold weather it spends most of its time in deep water. In summer, basking sharks gather in small groups to breed, and sometimes will engage in slow, ponderous courtship dances that involve swimming in circles nose to tail.

But the biggest filter feeder shark alive today, and possibly alive ever, is the whale shark. It gets its name because it is literally as large as some whales. It can grow up to 62 feet long, or 18.8 meters, and potentially longer.

The whale shark is remarkably pretty. It’s dark gray with a white belly, and its body is covered with little white or pale gray spots that look like stars on a night sky. Its mouth is extremely large and wide, and its small eyes are low on the head and point downward. Not only can it retract its eyeballs into their sockets, the eyeballs actually have little armored denticles to protect them from damage. The body also has denticles, plus the whale shark’s skin is six inches thick, or 15 cm.

The whale shark lives in warm water and migrates long distances. It mostly feeds near the surface although it sometimes dives deeply to find plankton. It filters water differently from the megamouth and basking sharks, which use gill rakers. The whale shark has sieve-like filter pads instead. The whale shark doesn’t need to move to feed, either. It can gulp water into its mouth by opening and closing its jaws, unlike the other living filter feeders we’ve talked about so far.

We talked about the whale shark a lot in episode 87, if you want to know more about it.

All these sharks are completely harmless to humans, but unfortunately humans are dangerous to the sharks. Even though they’re all protected, they’re vulnerable to getting tangled in nets, killed by ships running over them, and killed by poachers.

One interesting thing about these three massive filter feeding sharks is their teeth. They all have tiny teeth, but the mystery is why they have teeth at all. Their teeth aren’t just tiny, they have a LOT of teeth, more than ordinary sharks do. It’s the same for the filter feeding rays. They have hundreds of teensy teeth that the animals don’t use for anything, as far as researchers can tell. One theory is that the babies may use their teeth before they’re born. All of the living filter feeders we’ve talked about, including manta rays, give birth to live pups instead of laying eggs. The eggs are retained in the mother’s body while they grow, and she can have numerous babies growing at different stages of development at the same time. The babies have to eat something while they’re developing, once the yolk in the egg is depleted, and unlike mammals, fish don’t nourish their babies through umbilical cords. Some researchers think the growing sharks eat the mother’s unfertilized eggs, and to do that they need teeth to grab hold of slippery eggs. That still doesn’t explain why adults retain the teeth and even replace them throughout their lives just like other sharks. Since all of the filter feeders have teeth although they’re not related, the teeth must confer some benefit.

So, why are these filter feeders so enormous? Many baleen whales are enormous too, and baleen whales are also filter feeders. Naturally, filter feeders need large mouths so they can take in more water and filter more food out of it. As a species evolves a larger mouth, it also evolves a larger body, and this has some useful side effects. A large animal retains heat even if it’s not actually warm-blooded. A giant fish can live comfortably in cold water as a result. Filter feeding also requires much less effort than chasing other animals, so a giant filter feeder has plenty of energy for a relatively low intake of food. And, of course, the larger an animal is, the fewer predators it has because there aren’t all that many giant predators. At a certain point, an adult giant animal literally has no predators. Nothing attacks an adult blue whale, not even the biggest shark living today. Even a really big great white shark isn’t going to bite a blue whale. The blue whale would just bump the shark out of the way and probably go, “HEY, STOP IT, THAT TICKLES.” The exception, of course, is humans, who used to kill blue whales, but you know what I mean.

Let’s finish with a filter feeder that isn’t a shark. It’s not even closely related to sharks. It’s a ray-finned fish that lived around 165 million years ago, Leedsichthys problematicus. Despite not being related to sharks and being a member of what are called bony fish, its skeleton is partially made of cartilage, so fossilized specimens are incomplete, which is why it was named problematicus. Because the fragmented fossils are a problem. I’m genuinely not making this up to crack a dad joke, that’s exactly why it got its name. One specimen is made up of 1,133 pieces, disarticulated. That means the pieces are all jumbled up. Worst puzzle ever. Remains of Leedsichthys have been found in Europe and South America.

As a result, we’re not completely sure how big Leedsichthys was. The most widely accepted length is 50 feet long, or 16 meters. If that’s anywhere near correct, it would make it the largest ray-finned fish that ever lived, as far as we know. It might have been much larger than that, though, possibly as long as 65 feet, or 20 meters.

Leedsichthys had a big head with a mouth that could open extremely wide, which shouldn’t surprise you. Its gills had gill rakers that it used to filter plankton from the water. And we’re coming back around to where we started, because like the eagle shark, Leedsichthys had long, narrow pectoral fins. Some palaeontologists think it had a pair of smaller pelvic fins right behind the pectoral fins instead of near the tail, but other palaeontologists think it had no pelvic fins at all. Because we don’t have a complete specimen, there’s still a lot we don’t know about Leedsichthys.

The first Leedsichthys specimen was found in 1886 in a loam pit in England, by a man whose last name was Leeds, if you’re wondering where that part of the name came from. A geologist examined the remains and concluded that they were part of (wait for it) a type of stegosaur called Omosaurus. Two years later the famous early palaeontologist Othniel Marsh examined the fossils, probably rolled his eyes, and identified them as parts of a really big fish skull.

In 1899, more fossils turned up in the same loam pits and were bought by the University of Cambridge. IA palaeontologist examined them and determined that they were (wait for it) the tail spikes of Omosaurus. Leeds pointed out that nope, they were dorsal fin rays of a giant fish, which by that time had been named Leedsichthys problematicus.

In 1982, some amateur palaeontologists excavated some fossils in Germany, but they were also initially identified as a type of stegosaur—not Omosaurus this time, though. Lexovisaurus. I guess this particular giant fish really has been a giant problem.

You can find Strange Animals Podcast at strangeanimalspodcast.blubrry.net. That’s blueberry without any E’s. If you have questions, comments, or suggestions for future episodes, email us at strangeanimalspodcast@gmail.com. If you like the podcast and want to help us out, leave us a rating and review on Apple Podcasts or just tell a friend. We also have a Patreon at patreon.com/strangeanimalspodcast if you’d like to support us that way.

Thanks for listening!


Episode 212: The River of Giants



Thanks to Pranav for his suggestion! Let’s find out what the river of giants was and what lived there!

Further reading:

King of the River of Giants

Spinosaurus was a swimming dinosaur and it swam in the River of Giants:

A modern bichir, distant relation to the extinct giants that lived in the River of Giants:

Not actually a pancake crocodile:

A model of Aegisuchus and some modern humans:

Show transcript:

Welcome to Strange Animals Podcast. I’m your host, Kate Shaw.

A while back, Pranav suggested we do an episode about the river of giants in the Sahara. I had no idea what that was, but it sounded interesting and I put it on the list. I noticed it recently and looked it up, and oh my gosh. It’s amazing! It’s also from a part of the world where it’s really hot, as a break for those of us in the northern hemisphere who are sick of all this cold weather. I hope everyone affected by the recent winter storms is warm and safe or can get that way soon.

The Sahara is a desert in northern Africa, famous for its harsh climate. Pictures of the Sahara show its huge sand dunes that stretch to the horizon. This wasn’t always the case, though. Only about 5,500 years ago, it was a savanna with at least one lake. Lots of animals lived there and some people too. Before that, around 11,000 years ago, it was full of forests, rivers, lakes, and grasslands. Before that, it was desert again. Before that, it was forests and grasslands again. Before that, desert.

The Sahara goes through periodic changes that last around 20,000 years where it’s sometimes wet, sometimes dry, caused by small differences in the Earth’s tilt which changes the direction of the yearly monsoon rains. When the rains reach the Sahara, it becomes green and welcoming. When it doesn’t, it’s a desert. Don’t worry, we only have 15,000 more years to wait until it’s nice to live in again.

This wet-dry-wet pattern has been repeated for somewhere between 7 and 11 million years, possibly longer. Some 100 million years ago, though, the continents were still in the process of breaking up from the supercontinent Gondwana. Africa and South America were still close together, having only separated around 150 million years ago. The northern part of Africa was only a little north of the equator and still mostly attached to what is now Eurasia.

Near the border of what is now Morocco and Algeria, a huge river flowed through lush countryside. The river was home to giant animals, including some dinosaurs. Their fossilized remains are preserved in a rock formation called the Kem Kem beds, which run for at least 155 miles, or 250 km. A team of paleontologists led by Nizar Ibrahim have been working for years to recover fossils there despite the intense heat. The temperature can reach 125 degrees Fahrenheit there, or 52 Celsius, and it’s remote and difficult to navigate.

For a long time researchers were confused that there were so many fossils of large carnivores associated with the river, more than would be present in an ordinary ecosystem. Now they’ve determined that while it looks like the fossils were deposited at roughly the same time from the same parts of the river, they’re actually from animals that lived sometimes millions of years apart and in much different habitats. Bones or even fossils from one area were sometimes exposed and washed into the river along with newly dead river animals. This gives the impression that the river was swarming with every kind of huge predator, but it was probably not quite so dramatic most of the time.

Then again, there were some really fearsome animals living in and around the river in the late Cretaceous. One of the biggest was spinosaurus, which we talked about in episode 170. Spinosaurus could grow more than 50 feet long, or 15 m, and possibly almost 60 feet long, or 18 m. It’s the only dinosaur known that was aquatic, and we only know it was aquatic because of the fossils found in the Kem Kem beds in the last few years.

Another dinosaur that lived around the river is Deltadromeus, with one incomplete specimen found so far. We don’t have its skull, but we know it had long, slender hind legs that suggests it could run fast. It grew an estimated 26 feet long, or 8 meters, including a really long tail. At the moment, scientists aren’t sure what kind of dinosaur Deltadromeus was and what it was related to. Some paleontologists think it was closely related to a theropod dinosaur called Gualicho, which lived in what is now northern Patagonia in South America. Remember that when these dinosaurs were still alive, the land masses we now call Africa and South America had been right in the middle of a supercontinent for hundreds of millions of years, and only started separating around 150 million years ago. Gualicho looked a lot like a pocket-sized Tyrannosaurus rex. It grew up to 23 feet long, or 7 meters, and had teeny arms. Deltadromeus’s arms are more in proportion to the rest of its body, though.

Some of the biggest dinosaurs found in the Kem Kem beds are the shark-toothed dinosaurs, Carcharodontosaurus, nearly as big as Spinosaurus and probably much heavier. It grew up to 40 or 45 feet long, or 12 to almost 14 meters, and probably stood about 12 feet tall, or 3 ½ meters. It had massive teeth that were flattened with serrations along the edges like steak knives. The teeth were some eight inches long, or 20 cm.

Researchers think that Carcharodontosaurus used it massive teeth to inflict huge wounds on its prey, possibly by ambushing it. The prey would run away but Carcharodontosaurus could take its time catching up, following the blood trail and waiting until its prey was too weak from blood loss to fight back. This is different from other big theropod carnivores like T. rex, which had conical teeth to crush bone.

Dinosaurs weren’t the only big animals that lived in and around the River of Giants, of course. Lots of pterosaur fossils have been found around the river, including one species with an estimated wingspan of as much as 23 feet, or 7 meters. There were turtles large and small, a few lizards, early snakes, frogs and salamanders, and of course fish. Oh my goodness, were there fish.

The river was a large one, possibly similar to the Amazon River. In the rainy season, the Amazon can be 30 miles wide, or 48 km, and even in the dry season it’s still two to six miles wide, or 3 to 9 km. The Amazon is home to enormous fish like the arapaima, which can grow up to 10 feet long, or 3 m. Spinosaurus lived in the River of Giants, and that 50-foot swimming dinosaur was eating something. You better bet there were big fish.

The problem is that most of the fish fossils are incomplete, so paleontologists have to estimate how big the fish was. There were lungfish that might have been six and a half feet long, or 2 meters, a type of freshwater coelacanth that could grow 13 feet long, or 4 meters, and a type of primitive polypterid fish that might have been as big as the modern arapaima. Polypterids are still around today, although they only grow a little over three feet long these days, or 100 cm. It’s a long, thin fish with a pair of lungs as well as gills, and like the lungfish it uses its lungs to breathe air when the water where it lives is low in oxygen. It also has a row of small dorsal fins that make its back look like it has little spikes all the way down. It’s a pretty neat-looking fish, in fact. They’re called bichirs and reedfish and still live in parts of Africa, including the Nile River.

There were even sharks in the river of giants, including a type of mackerel shark although we don’t know how big it grew since all we have of it are some teeth. Another was a type of hybodont shark with no modern descendants, although again, we don’t know how big it was.

The biggest fish that lived in the River of Giants, at least that we know of so far, is a type of ray that looked like a sawfish. It’s called Onchopristis numidus and it could probably grow over 26 feet long, or 8 meters. Its snout, or rostrum, was elongated and spiked on both sides with sharp denticles. It was probably also packed with electroreceptors that allowed it to detect prey even in murky water. When it sensed prey, it would whip its head back and forth, hacking the animal to death with the sharp denticles and possibly even cutting it into pieces. Modern sawfish hunt this way, and although Onchopristis isn’t very closely related to sawfish, it looked so similar due to convergent evolution that it probably had very similar habits.

The modern sawfish mostly swallows its prey whole after injuring or killing it with its rostrum, although it will sometimes eat surprisingly large fish for its size, up to a quarter of its own length. A 26-foot long Onchopristis could probably eat fish over five feet long, or 1.5 meters. It wouldn’t have attacked animals much larger than that, though. It wasn’t eating fully grown Spinosauruses, let’s put it that way, although it might have eaten a baby spinosaurus from time to time. Spinosaurus might have eaten Onchopristis, though, although it would have to be pretty fast to avoid getting injured.

But there was one other type of animal in the River of Giants that could have tangled with a fully grown spinosaurus and come out on top. The river was full of various types of crocodylomorphs, some small, some large, some lightly built, some robust. Kemkemia, for instance, might have grown up to 16 feet long, or 5 meters, but it was lightly built. Laganosuchus might have grown 20 feet long, or 6 meters, but while it was robust, it wasn’t very strong or fast. It’s sometimes called the pancake crocodile because its jaws were long, wide, and flattened like long pancakes. Unlike most pancakes, though, its jaws were lined with lots and lots of small teeth that fit together so closely that when it closed its mouth, the teeth formed a cage that not even the tiniest fish could escape. Researchers think it lay on the bottom of the river with its jaws open, and when a fish swam too close, it snapped it jaws closed and gulped down the fish. But obviously, the pancake crocodile did not worry spinosaurus in the least.

Aegisuchus, on the other hand, was simply enormous. We don’t know exactly how big it is and estimates vary widely, but it probably grew nearly 50 feet long, or 15 meters. It might have been much longer, possibly up to 72 feet long, or 22 meters. It’s sometimes called the shield crocodile because of the shape of its skull.

We don’t have a complete specimen of the shield crocodile, just part of one skull, but that skull is weird. It has a circular raised portion called a boss made of rough bone, and the bone around it shows channels for a number of blood vessels. This is unique among all the crocodilians known, living and extinct, and researchers aren’t sure what it means. One suggestion is that the boss was covered with a sheath that was brightly colored during the mating season, or maybe its shape alone attracted a mate. Modern crocodilians raise their heads up out of the water during mating displays.

The shield crocodile had a flattened head other than this boss, and its eyes may have pointed upward instead of forward. If so, it might have rested on the bottom of the river, looking upward to spot anything that passed overhead. Then again, it might have floated just under the surface of the water near shore, looking up to spot any dinosaurs or other land animals that came down to drink. Watch out, dinosaur! There’s a crocodilian!

Could the shield crocodile really have taken down a fully grown spinosaurus, though? If it was built like modern crocodiles, yes. Spinosaurus was a dinosaur, and dinosaurs had to breathe air. If the shield crocodile hunted like modern crocs, it was some form of ambush predator that could kill large animals by drowning them. You’ve probably seen nature shows where a croc bursts up out of the water, grabs a zebra or something by the nose, and drags it into the water, quick as a blink. The croc can hold its breath for up to an hour, while most land animals have to breathe within a few minutes or die. The shield crocodile and spinosaurus also lived at the same time so undoubtedly would have encountered each other.

Then again, there’s a possibility that the shield crocodile wasn’t actually very fearsome, no matter how big it was. It might have been more lightly built with lots of short teeth like the pancake crocodile’s to trap fish in its broad, flattened snout. Until we have more fossils of Aegisuchus, we can only guess.

Fortunately, palaeontologists are still exploring the Kem Kem beds for more fossils from the river of giants. Hopefully one day soon they’ll find more shield crocodile bones and can answer that all-important question of who would win in a fight, a giant crocodile or a giant swimming dinosaur?

You can find Strange Animals Podcast at strangeanimalspodcast.blubrry.net. That’s blueberry without any E’s. If you have questions, comments, or suggestions for future episodes, email us at strangeanimalspodcast@gmail.com. If you like the podcast and want to help us out, leave us a rating and review on Apple Podcasts or just tell a friend. We also have a Patreon at patreon.com/strangeanimalspodcast if you’d like to support us that way and get twice-monthly bonus episodes as well as stickers and things.

Thanks for listening!