Ichthyostega – The First Vertebrate with Limbs? ★★

Stensiö's Fish Roof
(Art by SeismicShrimp◊, CC BY 4.0)

Okay, technically this animal isn't an amphibian — if anything, it's closer to fish than to any living amphibian. Nonetheless, it has been historically lumped with true amphibians, so we're putting it here.

Ichthyostega ("fish roof") has long been one of the icons of evolution. Like Archaeopteryx, it has been often cited as a "missing link" between two main vertebrate groups — that being fish and tetrapods (which is to say all mammals, birds, reptiles, and amphibians). Thus, it is often thought of as one of the first land vertebrates, and when it was named in 1932 by Swedish paleontologist Gunnar Säve-Söderbergh, it was indeed the oldest known. Since then, even older animals have been found, such Acanthostega and Tiktaalik. Four species are attributed to this genus — I. stensioei (the original), I. eigili, I. watsoni, and I.kochi.

Ichthyostega is the oldest animal we've covered so far, inhabiting the Late Devonian roughly 360 mya in what is today Greenland. During its time, Greenland really was a green land, covered with forests, swamps, lakes, and rivers and populated by arthropods, such as early insects, arachnids, and millipedes (though none could fly yet and all were still tiny). As a tetrapod-ancestor, Ichthyostega was fairly large for its day, measuring about 5ft/1.5m long and weighing 50 lbs/22.7 kg. It looked like a cross between a finned salamander and a fish with legs, and was indeed one of the first animals to have true legs. These were already similar to those of modern animals, except they had seven digits on each foot — the vast majority of later vertebrates have no more than five. It was capable of hauling itself onto land like a seal and moving about in a similar manner to a mudskipper, as its legs weren't quite capable of moving the way fully terrestrial animals could. Thus, it would have stayed near water hunted there too, likely using its legs to escape aquatic predators or slowly drag itself to new bodies of water. The respiratory systems of its low, flat, fish-like skull (the inspiration for its name) indicates it possessed both lungs and gills, and additional remains tell us it had several other fishy traits, including scales, a strong tail with an eel-like fin, and possibly even a lateral line to sense underwater! It most certainly laid its eggs in water, although its hard to say what its larvae would have been like. If modern amphibians and lungfish are any indication, it would have had gills as a baby and only developed lungs as an adult.

Ichthyostega and its tetrapod-ancestors are collectively known as tetrapodomorphs, with Ichthyostega being among the more derived forms, which are called stegocephalians. More primitive forms, such as Eusthenopteron (see next folder), strongly resembled lungfish and coelacanths, which are their closest living relatives. But despite being more "fish" than "amphibian", Ichthyostega and other stegocephalians are often given the traits of true amphibians, such as naked skin, croaking voices, tadpole-like larvae, and eggs resembling frogspawn. A good example of this is Walking with Monsters, which featured a close relative of Ichthyostega named Hynerpeton. True amphibians actually first appeared in the Carboniferous, with many tetrapodomorphs actually coexisting alongside them in the ancient swamps and rivers of the period.

Diplocaulus – The Hammerhead Salamander ★

Salamander-Formed Double Stem
(Art by Entelognathus◊, CC BY-SA 4.0)

Unlike Ichthyostega, Diplocaulus was a true amphibian. Specifically, it was one of the lepospondyls, a group of amphibians that thrived during the Carboniferous and Permian before going extinct at the end of the latter period. Their exact relationships are uncertain and they may not in fact represent a valid grouping, rather a miscellany of unrelated stegocephalians, true amphibians, and reptile ancestors. Regardless, Diplocaulus was the largest lepospondyl at 3.3ft/1m and 10 lbs/4.5 kg. Named in 1877 by Edward Drinker Cope during the Bone Wars, Diplocaulus is known from five valid species that primarily lived across the Late Carboniferous and Early Permian of North America. The two best known species are the original D. salamandroides of the Late Carboniferous circa 300 mya and the extremely common D. magnicornis of the Early Permian approx. 280 mya (making it a contemporary of the famous predatory protomammal Dimetrodon).

Diplocaulus means "double stem", and this refers to its unique boomerang-like head, which has made it a very common sight in paleo-books. The purpose of its bizarre-looking head protrusions has been a headache to paleontologists, and theories have included an attachment point for gills, a defense against being swallowed by predators, a hydrofoil-like swimming device, a tool to excavate the bottom of lakes, and a means of showing off to the opposite sex. Some have even suggested it to have supported flaps of skin that would have made the animal look like a stingray when alive. Regardless, Diplocaulus was fairly "normal" in appearance once you looked past the enigmatic "horns", with a long, flat, newt-like body that would have undulated like an eel when swimming. Its eyes and nostrils were placed at the top of its flattened head and were very close to each other — the exact opposite of a modern hammerhead shark, whose eyes and nostrils are at the extremities of the "hammer".

Like modern amphibians, Diplocaulus inhabited wetlands; it too was mainly aquatic and fed on small water critters, while in turn being hunted by larger amphibians and Dimetrodon. One particularly remarkable discovery was of eight juvenile Diplocaulus in a fossilized burrow, three of which had their heads bitten in half. The bite marks perfectly matched those of Dimetrodon, and it appears that the unfortunate amphibians were dug by the predator as they were hibernating during a drought, as many modern amphibians do. If it was anything like most modern amphibians, it was likely clumsy on land and agile in the water. But because of the many uncertainties surrounding the lepospondyls, we can't say for certain if it had tadpoles like other amphibians (though it most certainly laid its eggs in water) or a lateral line like fish and possibly tetrapodomorphs.

Eryops – The Alligator Frog ★

Frogs, toads, newts, and salamanders, as well as the little-known caecilians, are collectively known as lissamphibians. They are the only living group of amphibians today and highly successful in their present niches, but they only arose at the start of the Triassic. In the preceding Permian and Carboniferous periods, other amphibians ruled the waterways, and the most successful of them all were the temnospondyls, whose diversity at their peak outshined even modern amphibians. Many of them were quite large compared to lissamphibians and resembled salamanders mixed with crocodiles, with the most iconic of such traditionally being Eryops megacephalus ("big-headed drawn-out face").

Like Diplocaulus, Eryops was named in 1877 by Edward Drinker Cope as part of the Bone Wars. At 10ft/3m long and 489 lbs/222 kg, it was looked like a small crocodile, and has indeed classically been compared to them in documentary media. However, its tail was much shorter and its body was much more compact compared to any modern croc, and as an amphibian it would have had naked skin. Nonetheless, like a crocodile, it had short, strong legs and a massive, very alligator-like head (the inspiration for its name) with eyes placed at the top of the skull and a large snout. Its teeth were different from those of any gator, though, being thinner and more numerous, more like a gharial's. Like many extinct amphibians, particularly temnospondyls, Eryops is sometimes wrongly portrayed with reptilian scales and claws, making it look more like an actual crocodile.

Eryops lived in North America during the Early Permian some 280 mya, sharing its swampland habitat with early protomammals like Dimetrodon and Edaphosaurus, as well as Diplocaulus and many other amphibians, including several fellow temnospondyls. Like Diplocaulus, it is believed to have been mainly aquatic, feeding primarily on smaller amphibians, fish, and invertebrates while in turn being eaten by Dimetrodon. As an amphibian, it would have laid its eggs in water, and based on the fossils of other temnospondyls' larvae, the babies would have looked simply like miniaturized adults.

Mastodonsaurus – The Biggest Amphibian Ever? ★

The temnospondyls managed to weather through the massive extinction event at the end of the Permian that gave way to the Triassic and continued to be a major player in freshwater ecosystems. It is in this period do we see the emergence of another extremely well-known temnospondyl, the massive Mastodonsaurus (sometimes misspelled "Mastodontosaurus"). This 20ft/6m long, 600 lbs/272 kg behemoth and was long considered the largest amphibian of all time. This title has since been disputed by the discovery of a fragmentary skull belonging to a previously obscure Early Permian temnospondyl from Brazil named Prionosuchus that appears to represent an individual some 30ft/9m long (most other specimens of Prionosuchus are about 6.5ft/2m long). However, Mastodonsaurus is still the largest known on a non-fragmentary basis and is overall better understood by science.

In fact, Mastodonsaurus was the very first temnospondyl recognized by science, but like a lot of early paleontological finds, its history is quite convoluted. It began in 1828 when German physician Georg Friedrich Jaeger found a broken tooth he named Mastodonsaurus, meaning "breast-toothed lizard", as he mistakenly believed the tooth's damaged appearance (which made it look a bit like a nipple) was its natural state. At the same site, Jaeger also found a partial skull he named "Salamandroides", recognizing it as an amphibian while assuming his tooth to be from a reptile (he rectified his mistake in 1833 when he found a complete skull). In 1841 however, Sir Richard Owen, the man who gave us the word "dinosaur", objected to Mastodonsaurus as an inaccurate name, citing the mistaken nature of the tooth, the fact it was an amphibian not a reptile, and how the name could be misconstrued as meaning "mastodon lizard" in reference to the animal's immense size (indeed, it has been misstated as such by many). Thus he proposed the new name of "Labyrinthodon", referencing the strange "labyrinthic" inner structure of the tooth when seen in a cross-section. Owen also erected the order Labyrinthodontia for his renamed animal and many other giant amphibians being discovered at the time, all of which had the same tooth patterns (although why is uncertain). These days however, "labyrinthodonts" are no longer considered a valid term, instead being composed of various temnospondyls, stegocephalians, and reptiliomorphs (see next entry). Ichthyostega is an example of one animal that got lumped with the "labyrinthodont" name, hence why it is commonly called an amphibian despite not being one. Regardless, Owen's "Labyrinthodon" name didn't stick around either, as the rules of scientific nomenclature mean a name cannot be changed no matter how ill-fitting it is. Many species of Mastodonsaurus have been named, but most are now invalid or considered their own genera (such as Cyclotosaurus or Capitosaurus), and only four are accepted today, including the original M. jaegeri.

Mastodonsaurus lived in Middle Triassic Germany about 235 mya, just before the arrival of the first dinosaurs. It was overall similar to Eryops in form, with a short tail and limbs, as well as a compact body. Its huge 4ft/1.2m-long head was similar to that of Eryops too, only there were two teeth in the lower jaw larger than all the rest. They were so big in fact that they seem to have penetrated the upper jaw, as supported by two small holes on the snout that they fit neatly through. Some illustrations choose to have the teeth exposed, but others instead cover them up with skin, sometimes making these impressive features entirely invisible in their take on the living animal. Given its huge size and vaguely gator-like form, Mastodonsaurus likely lived similarly to a crocodile, lunging at prey that came too close to the water while also snacking on fish. It was also likely incapable of supporting itself on land because of its sheer bulk, and mass graves containing hundreds of individuals that died when their lakebed homes dried up in a drought are known. These in turn have given us knowledge of the full body of Mastodonsaurus, as it was almost solely known from its skull prior. It is for this reason that Sir Richard Owen originally restored Mastodonsaurus with a tailless body like that of a giant toad, his reconstructions being still visible at Crystal Palace Park in London today.

The temnospondyls' success cannot be understated. They produced a variety of forms, from the traditional crocodile-like freshwater predators to more terrestrial animals (that still needed to lay their eggs in water though) and even oceangoing species (all living amphibians are all freshwater-bound or terrestrial and cannot survive in saltwater). But as the Triassic progressed, they began to decline Mastodonsaurus was one of their last hurrahs, as semiaquatic archosaurs, like the ancestors of crocodiles, started outcompeting them. The vast majority were wiped out by the end-Triassic mass extinction event, but a few managed to hang on in Asia and Australia. The last known temnospondyl Koolasuchus of Walking with Dinosaurs fame, lived in the Early Cretaceous of the latter continent, surviving only because Australia was isolated in the Antarctic Circle at the time and thus too cold for crocs. However, current research suggests the temnospondyls may not be as extinct as once thought. Rather, it seems some smaller Permian forms evolved into the modern lissamphibians, meaning the familiar frogs, toads, newts, and salamanders of today are living temnospondyls in the same way birds are living dinosaurs!

Seymouria – Nearly Reptile ★

Reptiliomorphs (sometimes called "anthracosaurs") represent the transitional forms between traditional amphibians, like lissamphibians and temnospondyls, and the first amniotes (reptiles, birds, and mammals). Regardless, they were quite a diverse group, ranging from land-dwelling critters that still needed to lay their eggs in water to fully aquatic beasts similar to the temnospondyls. We don't know if they already had scaly skin like reptiles, or were naked like amphibians though. We have already met one reptiliomorph in Westlothiana, but traditionally, the best-known among them has been Seymouria baylorensis, named in 1904 by Ferdinand Broili after the town of Seymour, Texas, near where it was discovered and Baylor County, where Seymour is in.

Like Westlothiana, Seymouria was so similar to reptiles that it was originally identified as a true reptile. It lived in the Early Permian at the same time and place as the likes of Diplocaulus and Eryops, being likely hunted by Dimetrodon as well. Compared to the previously mentioned amphibians, it lived a terrestrial lifestyle, with short but strong limbs, a stocky head and body, and a short tail that altogether made it look a bit like a lizard or a salamander. Measuring about 2ft/60cm and weighing about 3 lbs/1 kg, it likely lived as a small, opportunistic predator of invertebrates. Fossil remains of tadpoles from relatives indicate Seymouria still had to lay its eggs in water, despite its land-living lifestyle, and these babies would have resembled those of newts, complete with gills. Indeed, it was this remarkable discovery that confirmed to scientists that Seymouria was a true amphibian and not merely a an extremely basal reptile as once believed. Nonetheless Seymouria was a historically common inclusion in textbooks as the "missing link" between reptiles and amphibians and is also known for appearing in Walking with Monsters (albeit unnamed) as a potential thief of Dimetrodon eggs. Two other species of Seymouria are known in S. sanjuanensis and S. grandis, with the former having also been reported from Europe.

Between the stegocephalians, lepospondyls, temnospondyls, and reptiliomorphs, it might come as a surprise to learn that prehistoric lissamphibians are rare in Fictionland. Most that do appear are never of any specific forms, despite the fact we know quite a few. A good example of this is the "tailed frog" that jumps near Littlefoot in The Land Before Time , which was entirely an invention of the move. This might be due to the fact most extinct lissamphibians are identical in size and shape to modern ones, so they lack the Rule of Cool factor that makes the other groups more interesting to us.

Other prehistoric amphibians

Sorry, these aren't here. If you're looking for Triadobatrachus, Karaurus, Eocaecilia, Beelzebufo, Phlegetontia, Platyhystrix, Cacops, Branchiosaurus, Gerrothorax, Metoposaurus, Pholiderpeton, Diadectes, and others, see here.

Megalodon – Jaws on Steroids ★★★

Big-Tooth Ear-Tooth
(Art◊ by EvolutionIncarnate, CC BY-SA 4.0)

It's usually accepted that the biggest/most spectacular prehistoric animals lived in the Age of Reptiles, the mythical Mesozoic. Well, sharks give us a notable exception in famous Megalodon. This gigantic shark lived, not alongside the dinosaurs, but after their reign, during the Cenozoic, the time when mammals were dominating terrestrial ecosystems worldwide.

But wait. Megalodon (literally "big tooth") is not the name of its genus; it's that of its species. Its teeth had long been collected by people, but they were only recognized as shark teeth in the 17th century, presumed by naturalists of the day as having belonged to sharks that died during Noah's Flood. Before that, they were commonly considered to be petrified serpent or dragon tongues. Megalodon teeth would be recognized as belonging to a previously unknown extinct shark in 1835 by Swiss-American naturalist Louis Agassiz, who initially gave it the name "Carcharias megalodon" (the same genus as the sand tiger shark), later amended in 1843 to "Carcharodon megalodon" (the genus of the great white shark). However, scientists have since pointed out that Megalodon teeth have a band at the bottom unseen in a great white's teeth, and for this reason, it was reclassified as "Carcharocles megalodon" in the 2000s, then amended the following decade to Otodus megalodon after "Carcharocles" was deemed the latter's synonym. Otodus ("ear tooth") was coined by Agassiz in 1843 and includes several other shark species that lived from the Paleocene to the Pliocene, with Megalodon being the last and largest. As the naming history suggests, Megalodon was once considered the sister species of the great white (part of the lamnid family of sharks), but today, it is part of a different family, the otodontids. Nonetheless, lamnids and otodontids are closely related, both part of an order of sharks known as Lamniformes, which also includes the mako shark, thresher shark, basking shark, megamouth shark, goblin shark, and sand tiger shark.

As typical of prehistoric sharks, most common fossil remains of Megalodon are jaws and teeth, not the softer cartilaginous remainder of its skeleton, which rarely fossilizes. Said jaws are common in museums and educational media, often depicted with some people inside to show how immense they are. That said, we don't actually know how big Megalodon was. Some sensational sources describe it as 100ft/30.5m long and 100 tons, as big as a blue whale, but no serious paleontologist believes this. Most estimates ranged from 34ft/10.5m to 67ft/20.3m, with weight estimates ranging between 13 and 59 tons, more akin in size to a whale shark and still bigger than the biggest great white. However, a study of fossilized vertebral centrae (the equivalents of backbones in sharks, which have skeletons made out of cartilage) in 2024 suggests that the average size was around 66ft/20m and exceptionally huge individuals could reach 80 ft/24.4m. Megalodon is typically reconstructed as resembling an oversized great white, due to the historical belief they were closely related and their similar lifestyles as big predatory sharks, but this might not have quite been the case in life (though its hard to say for certain).

Megalodon was one of the most successful apex predators ever. Its fossils are known from virtually every corner of the globe, and its tenure as top carnivore of the Cenozoic oceans lasted some 20 million years, from the Early Miocene roughly 25 mya to the Early Pliocene, about 5 mya (for comparison, T. rex only had a reign of about 2 million years, from 68-66 mya). This actually makes the last living Megalodons contemporaries of the first humans! Megalodon was a specialist hunter of cetaceans, and its bite marks have been found on whale skeletons. It is also known to have eaten seals, manatees, turtles, and large fish, including other sharks. It likely hunted its prey by ambush, hiding in deeper water and then lunging out to deliver a killing blow from below with its massive body and huge 7.1in/180mm long teeth (the largest of any shark ever). Its bite force is estimated to be the most powerful for any animal ever, ranging anywhere between 20-40 thousand pounds or 9-18 thousand kilograms (the strongest bite of any animal alive today goes to the saltwater crocodile at a "mere" 3000 lbs/1300 kg, while T. rex is about 8000 lbs/3600 kg). However, Megalodon would not have been the sole top carnivore of its day; stiff competition existed in the form of the highly intelligent and similarly predatory macroraptorial sperm whales, particularly the similar-sized Livyatan. Fossils of small Megalodon teeth found in what were once warm, shallow, coastal waters suggest that like modern great whites, juvenile Megalodon lived in such "nurseries" and fed on fish until they were big enough to hunt whales in the open ocean. As a lamniforme shark, Megalodon would have given live birth as all living members do.

Megalodon's sudden extinction during the Pliocene is something of a mystery, but the most popular theory is that climatic changes did it in. In particular, the formation of the Isthmus of Panama during this time (the same event that ushered the Great American Interchange) would have closed off an important hunting, breeding, and migration area for Megalodon and caused major shifts in the world's currents and weather systems. This would have led to a cooling climate and lowered sea levels, depriving Megalodon of coastal nurseries and instigating whales to move towards polar waters too cold for the giant shark. Unable to adapt, Megalodon became extinct, allowing the more adaptable great white shark and newly-evolved orcas to enter its niche. Without Megalodon to hunt them and a surge in the abundance of plankton and krill as a result of the cooling waters, baleen whales underwent a new explosion in diversity, going from the smaller forms hunted by Megalodon and Livyatan to the gargantuan species that roam our oceans today. Claims that Megalodon went extinct during the Pleistocene have turned out to be misdated specimens, and it is most certainly not secretly still alive in some unexplored part of the deepest ocean as popularized by the infamous fake documentary Megalodon: The Monster Shark Lives (it simply isn't able to survive such a cold, food-scarce environment).

As the previously mentioned exaggerated size estimates and fraudulent claims of modern survival indicate, Megalodon is probably the one prehistoric creature that gets almost as much sensationalism as T. rex. But despite being know to science for over a century and the sheer Rule of Cool it invokes, Megalodon was largely relegated to documentary media until the end of the 20th century when it became an object of fascination for shark-themed media following on Jaws, most notably the book Meg and its movie adaptation The Meg. Unsurprisingly, it is a favorite of schlocky low-budget Syfy shark movies, in which crude CGI Megalodons chomp down on dinosaurs and other Mesozoic reptiles (despite it living long after the Mesozoic ended), destroy ships, and even jump out of the water to take down planes. Megalodon is frequently cast as the ultimate predator, but this obviously cannot be verified —Sea Monsters had it as "only" the third-most-dangerous marine superpredator of prehistory, after the sea reptiles Tylosaurus and Liopleurodon.

Dunkleosteus – Living-Tank Fish ★★

Terrell's Dunkle's Bone
(Art◊ by PlacodermReconstructions, CC BY 4.0)

The placoderms were a group of armored fish that lived well before the dinosaurs, during the Silurian and Devonian periods, being particularly common in the latter before going extinct at its end. They were the first vertebrates to possess jaws, and are believed by some to be the ancestors of all other jawed vertebrates. Most were small, obscure animals, but the big exception is the most famous of them all — Dunkleosteus terrelli.

Dunkleosteus means "David Dunkle's bone", named after a paleontologist. It received its name in 1956 courtesy of Jean-Pierre Lehman, but it has actually been known to science since 1873, when John Strong Newberry classified it as a species of the small, obscure genus Dinichthys ("terrible fish"), with the "terrelli" part of the name honoring its discoverer, Ohio-based hotel owner and amateur fossil collector Jay Terrell. You'll often seen Dunkleosteus go by the Dinichthys name in old books and art because of this. Regardless, Dunkleosteus is only known from its skull, as placoderms, like sharks, have cartilage skeletons that don't usually fossilize. For this reason, we also aren't certain how big it is. Size estimates have varied across the decades between 13-33ft/4-10m in length and 1466 lbs/665kg to nearly 2 tons in weight. Either way, we're quite certain that it was one of the biggest placoderms.

Dunkleosteus lived during the Late Devonian some 360 mya, and its fossils have been found across North America, Europe, and Africa. It has traditionally been restored with an eel-like body based on comparisons with its smaller but better-preserved relative Coccosteus, but scientists today believe it would have had a more shark-like body, complete with a dorsal fin — a build much better suited for its lifestyle as an apex predator that lived on a steady diet of sharks, ammonites, and other placoderms out in the open ocean. This also suggests that Dunkleosteus was likely quite agile, contrary to historical restorations of it as a sluggish animal encumbered by its size and armor.

The most distinctive and notorious feature of Dunkleosteus however is its head, covered in bony armor and equipped with strange scissor-like "teeth". These were in fact extensions of its armor, plates of sharpened bone that were only further sharpened by simply being gnashed together. Studies of its jaw reveal that it probably sucked food up like a vacuum as many modern fish do, using its tooth-plates to then slice through prey like a guillotine. Since it couldn't chew, it would have had to regularly regurgitate the armor and bones of its prey in the same way owls regurgitate pellets of indigestible bones and fur, and its fossilized vomit is a common find. Additionally, several Dunkleosteus fossils preserve evidence of being attacked by other Dunkleosteus, which has led some to suggest that they were active cannibals like many fish today are. Based on some exquisite specimens of smaller placoderms, it's likely Dunkleosteus gave live birth; in fact, placoderms were probably the first vertebrates to do this. And while Dunkleosteus is traditionally depicted with its "teeth" and armor exposed, they may have actually been covered up in life, as is the case with most fish today.

Despite its status as "the living tank fish", Dunkleosteus has not gained much attention outside of paleo-books. Its most prominent appearance to date is Sea Monsters, where it's ranked as the fifth-most-dangerous ocean superpredator of all time, after two marine reptiles, Megalodon, and the early whale Basilosaurus. This version of Dunkleosteus is given cat-like eyes and blood-red coloration to emphasize the "sea monster" image, but most portrayals show it with round pupils like a typical fish, and more generic colors.

Xiphactinus – Overgrown Herring ★

The marine reptiles were not the only large ocean predators of the Mesozoic. Asides from sharks, one of the most well known is Xiphactinus audax, a huge bony fish that roamed oceans worldwide across the entire Late Cretaceous (with most fossils being found in North America's former Western Interior Seaway from about 80 mya). At 20ft/5m long and 2 tons, it was larger than most great white sharks and rivalled a small orca in size. Among bony fish alive today, the only ones of comparable size are the most massive — large sturgeons and the ocean sunfish (or mola mola). However, Xipactinus wasn't the biggest bony fish ever — that honor goes to the Late Jurassic filter-feeder Leedsichthys, estimated to be about 30-50ft/9-16m long.

Xiphactinus audax means "courageous sword ray", coined in 1870 by pioneering American paleontologist Joseph Leidy, whom we've previously met as the namer of the dire wolf, the short-faced-bear, the American lion, Uintatherium, Megacerops, and Hadrosaurus. It is sometimes nicknamed "the X-fish", but older works often call it "Portheus molossus", a synonym coined in 1871 by Edward Drinker Cope during the Bone Wars. "Portheus" references one of the 50 sons of the evil King Lycaon in Classical Mythology, while "molossus" is the name of a large and fierce breed of mastiff from Ancient Greece. Indeed, Xiphactinus is also called "the bulldog fish" by some, as its protruding lower jaw slightly resembles that of a bulldog. This, together with its long, pointed teeth, slender, powerful body, and lengthy fins, clearly indicate it was a fast-moving predator, something akin to a huge, fanged tarpon. It would have behaved similarly to a tuna or barracuda, pursuing prey and swallowing victims whole (although we don't know if it lived in schools or not). Several fossils exist of individuals with prey were nearly half their size in their guts, likely having been killed as a result of their victims thrashing inside and rupturing vital organs.

Xiphactinus is the best-known of an extinct order of bony fish called Ichthyodectiformes, which lived during the Jurassic and Cretaceous, and were all fast-moving predators similar to Xiphactinus itself. Its reputation as a voracious hunter has earned it appearances in quite a few documentary media, most notably Sea Monsters and Prehistoric Planet. Both series depict it hunting the man-sized diving bird Hesperornis, with the former show also having it fall victim in turn to the dominant marine reptiles of the Cretaceous (especially large mosasaurs like Tylosaurus).

Eusthenopteron – Fish Conquer the Land ★

Unlike mammals, birds, reptiles, and amphibians, fish are not a single class of vertebrates. Rather, they are composed of several different classes, some of which are now extinct. Placoderms are an example of an extinct class of fish, but sharks (together with rays and the obscure chimeras) form a still-living class in the cartilaginous fish, or chondrichthyes. As for bony fish, they actually comprise two closely related classes — ray-finned fish, or actinopterygians (Xiphactinus is one of them), and lobe-finned fish, or sarcopterygians. The sole bony fish in the latter category alive today are lungfish and the coelacanth, but their prehistoric members include the tetrapodomorphs. We've already met an advanced tetrapodomorph in Ichthyostega, but here we'll cover a more primitive form.

Named in 1881 by Joseph Frederick Whiteaves, Eusthenopteron is known from about half a dozen species (the original being E. foordi) and over 2 thousand specimens, all found in Quebec, Canada near the municipality of Nouvelle (which has this prehistoric fish on its town seal). About the same size as Ichthyostega, it lived in rivers and lakes about 385 mya at the end of the Middle Devonian, and its name means "good strong fin". This name references the classic feature of all lobe-finned fish, two pairs of muscular, fleshy fins supported by limb bones (ray-finned fish only have thin spines covered by skin for fins) — the predecessors to tetrapod legs. Otherwise, it was a fairly typical fish in appearance, with an elongated, coelacanth-like body and a distinct three-lobed tail fin resembling a trident. Its head was long and flat like that of Ichthyostega, with lots of sharp, tiny teeth, likely adapted for eating smaller fish. Also like Ichthyostega, it had the strange labyrinthic inner tooth patterns characteristic of many tetrapodomorphs and early amphibians, further revealing to us just how close it was to the ancestry of tetrapods.

Eusthenopteron is frequently coupled with Ichthyostega in paleo-books to show how vertebrates came onto land for the first time. As a result, it's frequently depicted in paleoart as being able to breathe air and crawl out of the water like a lungfish. However, Science Marches On, and nowadays, it is believed Eusthenopteron was a strictly aquatic animal like the coelacanth. Nonetheless, a common sight in science textbooks is a linear representation of how an animal like Eusthenopteron evolved into Icthyostega, with the animals between usually being Panderichthys, Tiktaalik, and Acanthostega in that order, each one slightly younger and more derived than the last, and unlike Eusthenopteron, all able to clamber onto land.

Cephalaspis & Pteraspis – Fish or Shellfish? ★

Jawless fish represent the most primitive of all fish lineages, but again, they are not a singular class. Their living representatives, the hagfish and lampreys, both form separate classes, but many extinct classes are known. Many of these possessed armor, and were thus once put together in a single class called the ostracoderms, which is now only used as an informal term for the multiple classes it has since been split into. They were ubiquitous from the Early Ordovician to the end of the Devonian, and the two perhaps most oft-represented members in media are Cephalaspis lyelli ("Charles Lyell's head shield", after a highly influential Scottish geologist) and Pteraspis rostrata ("beaked wing shield").

Cephalaspis and Pteraspis were first both identified in 1835 by Louis Agassiz, who also named Megalodon as part of his extensive work on fossil fishes. The former was originally considered a second species of the latter (it got its own name in 1847, thanks to Rudolf Kner), but Cephalaspis has had a general problem of being used as a wastebasket taxon, with dozens of species assigned to the genus and most likely representing new names. Regardless, both lived in European seas during the Early Devonian about 400 mya, with their fossils being extremely numerous wherever they are found. Both were tiny, with Cephalaspis being 11.8in/30cm and Pteraspis being 7.8in/20cm. Cephalaspis' body had a simple eel-like tail fin and larger pectoral fins, while Pteraspis had no pectoral fins at all but a forked tail fin. Their head armor was also quite distinct; Cephalaspis had a flat, shovel-shaped head without any ornamentation and Pteraspis had a narrow, pointy head with a small spike further to the back. These in turn reveal their particular lifestyles. Whereas lampreys and hagfish are parasites and scavengers, Cephalaspis and Pteraspis were inoffensive filter-feeders, the former slowly sifting through muck on the seabed and Pteraspis swiftly swimming in the water column with ease.

Pteraspis and Cephalaspis are the namesake of their respective groups of ostracoderms, those being the cephalaspidomorphs and the pteraspidomorphs, the former being suspected by some to represent the ancestors of placoderms and thus all jawed vertebrates. More specifically in the case of Cephalaspis, it was among the most advanced cephalaspidomorphs, which are known as osteostracans, who are the closest of the bunch to jawed vertebrates. It was for this reason that Cephalaspis was featured in Walking with Monsters as the protagonist of the Silurian segment, escaping the claws of giant scorpions and migrating into freshwater to spawn like salmon. That last behavior has absolutely no basis in fossil evidence and is extremely unlikely, given its lifestyle as a slow-moving bottom-feeder.

Other prehistoric fish

Sorry, these aren't here. If you're looking for Coelacanthus, Mawsonia, Dipterus, Palaeoniscum, Cretoxyrhina, Hybodus, Stethacanthus, Helicoprion, Climatius, Bothriolepis, Coccosteus, Arandaspis, Drepanaspis, Birkenia, Haikouichthys, the Conodonts, and others, see here.

Ammonites – The Symbol of the Mesozoic ★★★

Is there anyone who hasn't seen those spiral stony shells emerging from the surrounding rocks? Ammonites, more technically ammonoids, have always been among the most iconic fossil invertebrates, together with the trilobites. Over 10 thousand species split amongst six orders are lived in oceans worldwide over a span of nearly 350 million years — even longer than the trilobites. They first appeared during the Early Devonian about 410 mya, but reached their prime during the Dinosaur Age before going extinct at the end of the Cretaceous alongside the nonbird dinosaurs, marine reptiles, and pterosaurs, when the asteroid struck some 66 mya. They are so common in the Mesozoic in fact that like trilobites for the Paleozoic, they are used as "index fossils" for to determine the age of Mesozoic rocks, with individual species indicating specific ages.

Ammonites are cephalopods, thus cousins to octopuses and squids. This fact was first recognized in 1789 by French zoologist and mollusk expert Jean-Guillaume Bruguière, who pointed out their similarities to the shells of nautiluses (which despite appearances, are not living ammonites; we'll explain later). Before that, ammonites were long ascribed with all kinds of supernatural traits, claimed by some to be coiled snakes turned to stone (a common name for their fossils are "snakestones"; some people even carved snake heads onto them!) and used by others as magical charms. The word "ammonite" comes from Roman naturalist Pliny the Elder, who called their fossils "ammonis cornua", or "horns of Amun", after an Egyptian god often portrayed with ram's horns. This would become the basis of their official scientific name, Ammonoidea, coined in 1884 by Karl von Zittel (prior to that, they had been classified among living cephalopods).

Despite the abundance of their shells, the soft bodies of ammonites are only rarely preserved. The earliest reconstructions had them as strange sailing animals that floated on the surface of the water, based on the little-known paper nautilus, or argonaut (actually a type of octopus). Most though preferred them as nautilus-like creatures that freely swam under the ocean's surface in the water column, but other than that, all restorations were educated guesses. In the 21st century however, the first fossils preserving the ammonites' soft parts were discovered, validating numerous long-held suspicions about their life appearance. Not only did these verify that ammonites were more closely related to octopuses and squids than to nautiluses as suspected by some, but they confirmed that ammonites had ten squid-like tentacles with suckers, a beak, a small siphon for jet propulsion, and even an ink sac. They also showed complex eyes similar to those of octopus and squid, and special muscles that allowed them retreat inside their shells.

Ammonites lived a variety of lifestyles. Many were excellent swimmers, but others were slow-moving bottom-dwellers. Their diets varied from species to species, with some being active hunters and others being filter feeders. Their hard shells were excellent protection against predators, as shown by some ammonites with teeth marks left by ichthyosaurs or mosasaurids that tried to break the shell in vain. Like nautiluses, their shells also let them function like miniature submarines, with small chambers they could fill with water or empty to control buoyancy and depth. Fossil evidence suggests ammonites produced planktonic larvae that lived in shallow surface waters, in contrast to nautiluses, which lay their eggs on the deeper seafloor and have babies that look just like tiny adults. This likely played a major role in why ammonites were wiped out by the asteroid that ended the Cretaceous, as their babies would have been much more vulnerable to its devastating effects.

Ammonites also varied extraordinarily in size; the smallest known were 0.4in/10mm in diameter and the biggest could reach 6.6ft/2m. In media, ammonites are almost always shown with the classic curly, laterally-flattened shell; these are known as homomorph ammonites. During the Cretaceous, some ammonites developed different shell shapes, becoming known as heteromorph ammonites. Some had loosely uncoiled shells, some had u-shaped shells, some had spiny tower-like shells, some just had straight shells, and others had shapes so bizarre they must be seen to be believed. No one is certain how these ammonites lived; none were likely to have been fast animals, and they may have been either octopus-like bottom-dwellers, jellyfish-like free-floaters, or stationary animals like anemones and sponges. Like trilobites, ammonites are common as ambient prehistoric animals in media, rarely ever being identified as a particular species. Prehistoric Planet is one of the few aversions of this, with a few segments dedicated wholly to ammonites and several being namedropped, on top of also being one of the rare instances of heteromorph ammonites appearing in media.

Trilobites – The First Successful Invertebrates ★★★

Hydrocephalus sp.
(Model by Vladislav Egorov & Jaagup Metsalu◊, CC BY-SA 4.0)

Few groups of animals can claim the level of success achieved by the trilobites. These ancient arthropods populated oceans worldwide for about 270 million years, more than a hundred million years longer than the reign of the non-avian dinosaurs and many, many times longer than humans have existed. With over 22 thousand species divided into 10 orders, their fossils are among the most abundant of all extinct animals, so much so that they are used as "index fossils". That is to say Paleozoic rocks can be easily recognized just because they almost certainly contain at least one trilobite — and said trilobite's species can be used to determine the age of the rocks.

Indeed, trilobites might be taken as the Paleozoic's unofficial symbol. They lived worldwide through almost the entire Paleozoic era, first appearing the Cambrian about 520 mya and being wiped out by the devastating mass extinction at the end of the Permian some 251 mya. Their heyday however was the Ordovician and Silurian, and they became increasingly rare from the Devonian onwards, likely as a result of the evolution of jawed fish. They were also the first prehistoric animals recognized, before Pterodactylus, Mosasaurus, or any Ice Age mammals. The earliest record of their study is from 1698 when Rev. Edward Lhwyd dubbed a trilobite fossil he found as a flatfish skeleton. Proper research however began in 1749 when Charles Lyttleton found a second specimen near Dudley, England. He identified his find as an insect he nicknamed "The Dudley Locust", but in 1754, Manuel Mendez da Costa declared it a crustacean he nicknamed "Pediculus marinus major trilobos" ("large three-lobed marine louse"). This became the basis for the "trilobite" name, officially coined in 1771 by Johann Walch. But long before any of this, trilobite fossils had been collected as amulets, pendants, and other ornaments by many cultures worldwide, with one such decorative being 15 thousand years old — the time of the Pleistocene Ice Age.

"Trilobite" means "three lobes", referring to the way their bodies were divided. They had a head, a segmented body called the thorax, and a rear bit called the pygidium, but their thoraxes also had three parts in the longitudinal sense, with a middle segment called the axial lobe and the two lateral ones called pleural lobes. They looked a bit like flattened woodlice or horseshoe crabs, but formed their own class that was not closely related to any modern arthropod group. Nonetheless, as arthropods, they had many pairs of legs (up to 100) and one pair of antennae; they also had many pairs of gills on account of being aquatic (although all of these features are rarely preserved, unlike their hard shells). Most species also had two large compound eyes like those of insects, and trilobites were in fact among the first creatures capable of seeing images. Beyond that however, they were amazingly diverse. Most lived on the bottom, but others were active swimmers and a few could even crawl onto land; some were scavengers, some predators, and others filter feeders; and some had unusual features, including horns, spikes, eyestalks, disproportionately huge eyes, and even eyelessness; a few could even curl up for protection like pillbug woodlice do. They also ranged in size from the 0.06in/1.5mm to the 28in/72cm. Fossilized trilobite larvae are also known, and they tell us that trilobites added segments as they grew, shedding their exoskeletons just like crustaceans.

In fiction, trilobites can often be seen in underwater visuals, usually as ambient animals that skitter about on the ocean floor. They might even show up in the Mesozoic alongside dinosaurs, despite having actually gone extinct millions of years before the first dinosaurs. They also tend to be used as an indicator of something being "prehistoric". For example, in the original Godzilla (1954), a living trilobite is found in one of Godzilla's footprints, revealing his primeval origins to the characters. However, it is extremely rare for any media, fiction or documentary, to make an effort to identify any trilobite species. Said trilobites are pretty much always one of the bog-standard stereotypical bottom-dwelling forms; the more unusual species pretty much never appear.

Meganeura – The First Giant Flyer ★★

Mony's Big Nerve
(Art◊ by Qohelet12, CC BY-SA 4.0)

Popularly referred to as a "giant dragonfly", Meganeura ("big nerve", a reference to its heavily-veined wings) was in fact a part of a related group of insects called Meganisoptera (formerly called Protodonata), informally known as "griffinflies". Actual dragonflies and damselflies belong to the order Odonata, and while Meganeura was extremely similar to them, its lineage has been extinct since the end of the Permian.

Named in 1885 by Charles Brongniart, Meganeura lived in the densely forested tropical swamps of Late Carboniferous Europe about 300 mya and is known from three species — the original M. monyi, M. brongniarti, and M. vischerae. Its most iconic quality was its size. With a wingspan of 2.5ft/75cm and a body length of 1.6ft/50cm, it was about the size of a crow (not an eagle as is sometimes said), and thus represents not just one of the largest insects of all time (rivalled only by some of its fellow meganisopterans), but also the oldest Giant Flyer in the fossil record.

As stated above, Meganeura had the same shape as modern dragonflies, with a slender body, huge eyes with excellent vision, short antennae, and two pairs of independently moving wings. Meganeura also had large, powerful mandibles, contrary to the small, less visible mouthparts of true dragonflies. It would have been a very powerful flyer, perhaps as fast as many modern birds. And like true dragonflies, it was a carnivore that fed on smaller insects, catching them with spiny legs, just like dragonflies do. Its agility would have kept it safe from the giant amphibians and large fish that were the top predators of its world, and some speculate it might have even hunted the diminutive early reptiles and protomammals around during its day. Fossils of Meganeura nymphs are known too, and like modern dragonfly nymphs, they were fierce aquatic predators.

In fiction, Meganeura tends to be an ambient animal in fiction, that exists mostly to emphasize a prehistoric setting, just like trilobites and ammonites. Older works tend to portray it living alongside dinosaurs, despite the fact that it had already died out long before the dinosaurs arrived on the scene. A good example is in the first Jurassic Park novel, where a cloned Meganeura is present in the park without any particular explanation.

Arthropleura – The Super-Millipede of the Coal Forests ★★

Armored Jointed Sides
(Art◊ by Prehistorica CM, CC BY 4.0)

Known from Late Carboniferous Europe and North America, appx. 300 mya, Arthropleura armata ("armored jointed sides") was an 8.2ft/2.5m long millipede that's claim to fame is being the biggest known terrestrial invertebrate of all time. Named in 1854 by Archaeopteryx namer Hermann von Meyer, it was well-armored creature, safely protected from predators by a combination of its sheer size and strong exoskeleton. As it is mainly known from shed pieces of its exoskeleton and fossilized trackways, much of its nature was a mystery, including its exact appearance and diet (indeed, it wasn't even identified as a millipede until 1911; Von Meyer thought it was a crustacean). However, comparisons with modern millipedes, as well as fossilized gut contents, indicated it was likely an inoffensive, slow-moving herbivore that fed on the rotting vegetation abundant in the swamp-forests it shared with Meganeura, while also possessing a wide, flattened body like that of a trilobite (there are modern millipedes with similar body shapes). New fossils (including the first properly preserved head) reported in 2024 confirmed much of these beliefs, but also revealed some surprising new details — that it had fairly large centipede-like antennae, surprisingly large mandibles for a vegetarian (albeit without the prey-grasping, venom-injecting adaptations of centipedes and other carnivorous arthropods), and most shockingly, short eyestalks, a trait unseen in any modern millipedes or centipedes.

Arthropleura and Meganeura were far from the only Big Creepy-Crawlies alive during the Carboniferous (another well-known form is the Early Carboniferous Pulmonoscorpius, a 28in/70cm long scorpion), but they do raise the question of how they got so big. The traditional explanation goes like this. Terrestrial arthropods breathe using a system of tubes running through their body that absorb oxygen, and since this is dependent on the amount of oxygen in the air, they suffocate over a certain size if oxygen quantities aren't sufficient enough for absorption. As studies indicate the Carboniferous had more oxygen than other periods (caused by the vast swaths of forest that developed in the Devonian without any plant-eaters around), arthropods were able to get bigger. The increased oxygen explanation has sometimes been given to justify the vast sizes achieved by sauropod dinosaurs, but oxygen levels during the Mesozoic were actually the same as today.

However, the classic "more oxygen = bigger bugs" hypothesis has been called into question by some, as large terrestrial arthropods continued to exist in the Permian, after oxygen levels nosedived to less ridiculous levels. Thus, it has been alternatively suggested that the giant bugs were simply able to get as massive as they did because there were no large terrestrial vertebrates to compete with or predate on them. Tellingly, large flying insects persisted into even the Triassic, albeit at relatively smaller sizes, only vanishing around the same time the first flying vertebrates (pterosaurs) appeared. It's also worth noting at this point that not every creepy-crawly alive during the Carboniferous was as huge as Arthropleura and Meganeura. Plenty of regular-sized bugs lived alongside them, but none of those are seen as particularly interesting by non-experts.

Arthropleura, alongside Meganeura, has long been a staple of literature discussing prehistoric life, especially of the Carboniferous, but unlike the "dragonfly", the giant millipede doesn't make many appearances in fiction. It has shown up in a few documentaries though, such as Walking with Monsters and Prehistoric Park, both of which show it as being able to rear up like a cobra. This is a behavior we have no evidence for, and it's debatable if this was even possible, given its weight. The most noteworthy appearance of an Arthropleura in a non-educational work is probably Primeval, which instead decided to portray it as a giant venomous centipede.

Pterygotus – Scorpions in the Sea? ★

Eurypterids are popularly known as "sea scorpions", but they were not scorpions, or even arachnids. Rather, they are cousins of both true arachnids and horseshoe crabs (which, while we're at it, aren't crabs or any other kind of crustacean). They did look a bit like scorpions though — if scorpions evolved lived to underwater, that is. They had large compound eyes with excellent vision, lobster-like bodies, and six pairs of limbs. They also had two respiratory systems — one for breathing underwater and another able to breathe air. The second set, combined with fossil trackways, tells us they were likely able to come onto land for brief periods, possibly to breed. And despite their colloquial name, they were not exclusive to the ocean; some species lived in freshwater.

Eurypterids first appeared in the Middle Ordovician some 465 mya and vanished at the end of the Permian about 252 mya, becoming one of many casualties in the colossal end-Permian mass extinction. These aquatic arthropods were major predators in underwater ecosystems of the Paleozoic, although they experienced a major decline from the Devonian with the evolution of jawed fish (which ousted them from the niche of apex predator). Nonetheless, they were extremely successful, producing forms that ranged in size from 0.8in/2cm to 8.5ft/2.6m — the latter being the largest arthropods of all time. They are divided into two major subgroups — the free-swimming eurypterines (identifiable by their paddle-shaped last pair of legs) and the bottom-crawling stylonurines (which possessed spidery legs).

The most famous of all eurypterids was Pterygotus ("the winged one"), which was among the bigger ones at a body length of 5.7ft/1.7m and 66 lbs/30 kg. It is the prototype of the pterygotids, a family of eurypterines whose members include the largest eurypterids. Pterygotus had the flattened, streamlined body and paddle-shaped hind limbs common to all eurypterines, but as a pterygotid, it also had a paddle-shaped tail and, most significantly, lobster-like pincers on the first pair of limbs. All these traits tell us that Pterygotus and its fellow pterygotids were powerful, fast-moving predators that chased after prey in the ocean's water column, catching them in their pincers and tearing them apart. As pterygotids coexisted with the first jawed fish, it's widely believed their distinct traits, including their large size, were evolved in an effort to compete with them.

About two dozen species (about a quarter of which are deemed dubious, including the original "P. problematicus") have been assigned to the Pterygotus name, which was originally coined in 1839 by Louis Agassiz (the same man who named Megalodon) for fossils discovered in Britain by local quarrymen (who nicknamed their finds "seraphims"). Agassiz however, believed his animal to be a fish and only recognized it as an arthropod in 1844; however, he recategorized it as a crustacean, which eurypterids were considered to be until the late 1850s. Pterygotus was a long-lived genus, existing from the Middle Silurian some 430 mya to the Late Devonian about 370 mya, with its fossils being found worldwide. It also has a history as a wastebasket taxon, with many other eurypterid genera once being put under its name, such as Acutiramus and Jaekelopterus.

Being among the biggest and fiercest of its kind, Pterygotus is a popular inclusion in docu-media about the Paleozoic: it memorably appeared in Walking with Monsters as the top predator of the Silurian, ambushing the giant, aquatic true scorpion Brontoscorpio to feed its babies.

Cameroceras/Endoceras & the "Giant Orthocone" – A Kraken in a Cone ★

The nautiluses that inhabit the deeper waters of the Indo-Pacific today are the last of an ancient legacy. Despite appearances, they are not ammonites, but rather, members of an even more ancient lineage of cephalopods — the nautiloids. As cephalopods, nautiloids are believed to be the ancestors of the ammonites, as well as living octopus and squid (together known as coeloids). They had the tentacles, beaks, and siphons of all cephalopods, but compared to their more derived kin, they have simple eyes with poor vision, no suckers on their tentacles, and no ink sac. Their chambered shells strongly resemble those of ammonites and had the same purpose, but they can be told apart by how nautiloids have smooth, straight walls separating each chambers while ammonites have wrinkled, curved walls. Additionally, whereas a nautiloid's siphon nests in the middle of its tentacles, an ammonite's is under its tentacles.

Although the modern nautilus possesses a helix-shaped shell like those of most ammonites, many extinct nautiloids possessed long, straight shells resembling ice cream cones. These nautiloids are dubbed "orthocones", and the most oft-featured goes by the name Cameroceras ("chambered horn"). Named in 1842 by Timothy Conrad, Cameroceras is known from 15+ species that lived through the entire Ordovician (485 - 443 mya) in oceans across North America, Europe, and Asia. Most had shells ranging between 2.3ft/70cm to 6.6ft/2m long and, but you may be more familiar with a species claimed as having a shell length of 30ft/9.1m, dubbed "Cameroceras giganteum". This Real Life Kraken, popularly dubbed the "giant orthocone", is claimed to be the largest of all nautiloids and was most famously featured in Sea Monsters.

Only one problem. "Cameroceras giganteum" doesn't actually exist. First off, the giant orthocone's real name is Endoceras giganteum. The genus Endoceras ("inner horn", named in 1847 by James Hall) is known from quite a few species living across the Middle to Late Ordovician (470-443 mya) of Europe, North America, and Australia, most of which are the same size as most Cameroceras species. However, it has historically been lumped with Cameroceras by some scientists due to how similar they are, and evidently, Sea Monsters followed this interpretation. Secondly, all known shells of Endoceras giganteum are a somewhat smaller 19ft/5.7m long. The alleged supergiant shell, originally reported in 1955 by Rousseau H. Flower from a New York quarry, is now widely believed to have been an extreme miscalculation, with the supposed specimen having been unfortunately lost. But even if "Cameroceras giganteum" wasn't really that giant or really a Cameroceras at all, it was still the largest animal of its day. The giant orthocone likely spent most of its life slowly hovering just above the seafloor of Late Ordovician North America some 450 mya, feeding on smaller bottom-dwelling animals like trilobites (specimens with pieces bitten out of them by a nautiloid's beak are known) and eurypterids.

As the great size of Endoceras giganteum might indicate, nautiloids had their heyday in the Ordovician (and the succeeding Silurian), after debuting in the Late Cambrian, approx. 495 mya. The evolution of ammonites and coeloids during the Devonian and Carboniferous respectively caused the group to enter a long, drawn-out decline. The last orthocone-type nautiloids vanished sometime during the Late Triassic, leaving behind the modern helix-shelled forms, which experienced a brief resurgence after the extinction of the ammonites at the end of the Cretaceous. However, their new prosperity during the Mammal Age suddenly crashed during the Oligocene and Miocene, leaving behind only the few modern species found in the Indo-Pacific today. For a long time, the nautiloids' near-extinction in the Cenozoic was attributed to the oceans getting colder, but in 2022, new research revealed an unexpected additional culprit — seals. Seals consume shelled prey by biting on and sucking/shaking the soft-bodied animal out of its armor— something sharks and whales are unable to do— and the decline of nautiloids during the Cenozoic coincides with the evolution of seals. Tellingly, all of the places the modern nautiluses inhabits today are entirely seal-free.

Belemnites – Stone Lightning and Devil Fingers ★

The fossils of many marine invertebrates are so common and widespread that before they were understood to be the petrified remains of long-gone animals, people made up all sorts of stories about them. We've already mentioned an example with ammonites being claimed as petrified snakes, but another, lesser-known group of extinct cephalopods have had wild legends attached to them as well. These were the belemnites (technically belemnitids), easily recognized in the fossil record by the straight, pointed arrow-like shape of their shells. In ancient times, these were believed to be the petrified remains of lightning bolts, and other people nicknamed them "Devil's fingers"

In Real Life, belemnites were close relatives of octopus, squid, and cuttlefish, being part of the coeloid grouping just like them. The so-called "Devil's fingers" were invisible in life, hidden inside the animal and making them resemble a regular squid or cuttlefish. Note that squids and cuttlefish have internal shells too (octopuses have lost them entirely), but this is a much simpler structure than in belemnites — a thin, transparent structure called the "pen" or "gladius" in squids and the flat, oval-shaped cuttlebone of cuttlefish (a popular dietary supplement for certain pets, like parrots and turtles). It was the similarity between "Devil's fingers" and cuttlebone that allowed English naturalist John Samuel Miller to be the first person to identify them as cephalopods and give them their name (itself derived from "bélemnon", the Greek word for "dart") back in 1823. His hypothesis was vindicated shortly afterwards in 1826 when Mary Anning, the woman who found [[Plesiosaurus and Ichthyosaurus, discovered belemnite fossils with the soft parts still preserved, complete with ink sacs. We also know through these discoveries that the belemnite body extended beyond its fins, in contrast to squid and cuttlefish, whose fins form at the very end of their body.

Known from over 2 thousand species found in oceans worldwide, belemnites first appeared at the end of the Middle Triassic about 235 mya. They quickly took to a life as highly active, fast-moving swimmers, with shells ranging in length from 1.2in/3cm to 2.3ft/70cm. They did many of the same things modern squid do today: spraying ink, swimming using their lateral fins, catching prey with ten arms full of their suckers, seeing complex images with their large eyes, etc. Their heyday proved to to be the Early and Middle Jurassic, and with the evolution of squids and octopus in the Late Jurassic and Early Cretaceous, they began to decline. By the end of the Late Cretaceous, they were relegated solely to Europe and North America before being wiped out alongside the ammonites, non-avian dinosaurs, pterosaurs, and marine reptiles by the asteroid. In popular media, belemnites tend to be shown as ambient animals for prehistoric placements, but it's not uncommon to hear them referred to as proper squids, which they weren't. And despite appearances, they were not the ancestors of squids either, but rather the most primitive branch of the coeloid family tree.

Anomalocaris – The First Predator of the Seas ★

Canadian Anomalous Shrimp
(Art◊ by Junnn11, CC BY-SA 4.0, cropped from original)

About 538 million years ago, one of the most important events in the history of life occurred — the so-called "Cambrian Explosion". This event marks the sudden appearance of a staggering variety of complex lifeforms — some of the first-ever animals. And among the "wonderful life" (to reference Stephen Jay Gould) that emerged in this period, the most famous of all is Anomalocaris canadensis ("anomalous shrimp from Canada").

Anomalocaris lived about 500 mya in the shallow seas that covered what is today the Rocky Mountains of British Columbia in western Canada. The rocks from which is is known from, the world-famous Burgess Shale, represent one of the best sites in the world to find examples of the kinds of animals that arose from the Cambrian Explosion, many of which looked like something from another planet. Anomalocaris was no exception. It had a soft body with numerous lateral fin-like structures to swim above the seafloor, compound eyes on stalks, a pair of mustache-like "arms" at the front of its head, and a circular mouth on the underside of its head with "teeth" placed in a circular fashion like a lamprey. This bizarre appearance, along with its name, reveal a singular Real Life case of Mix-and-Match Critter. You see, the name Anomalocaris was initially invented in 1892 by Joseph Whiteaves for a shrimp known solely from its tail. Then in 1911, Charles Doolittle Walcott described a strange disc-like jellyfish named Peytoia nathorsti and a sea cucumber named "Laggania cambria", the latter of which was then argued in 1978 to actually be a sponge. Regardless, in 1985, new fossils retrieved from the Burgess Shale revealed the truth — the "shrimp", "jellyfish", and "sea cucumber" were all actually the respective "arms", mouth, and body fragments of one bigger animal, one that science kept the "shrimp"'s name for. However, in 2012, Peytoia was resurrected after new fossils revealed it and "Laggania" to represent a close relative of Anomalocaris, with Anomalocaris itself having been discovered to have had a mouth structure made of three parts rather than the four of Peytoia.

Like many Burgess Shale lifeforms, Anomalocaris was an ancestral member of a modern phylum — in this case, the arthropods. Specifically, Anomalocaris was a member of the dinocardids, or "terrible shrimps", so-called because their ranks included the first apex predators to evolve. This status, along with the fact it was among the largest animals of the Cambrian, means that Anomalocaris gets a lot of hype in media as "the first Sea Monster". But at only 1.2ft/38cm long, it would have been very harmless by today's standards. Nonetheless, Anomalocaris was the biggest and fiercest animal of the Burgess Shale, over eight times the size of its prey. 2in/5cm long oddities like Hallucigenia, Pikaia, Wiwaxia, and Opabinia (the last one also a dinocardid) would have been snatched up in the "arms" of Anomalocaris and sucked into its maw. In fact, it's quite possible that the evolution of the dinocardids set up the important role of predators in ecology and evolution, driving animals to evolve a variety of defenses or to directly compete with their predators and each other. It's also worth noting that additional species of Anomalocaris have also been recorded from China, Australia, and the United States, but have since now reclassified as their own genera, like Houcaris, Echidnacaris, and Lenisicaris.

While the strange animals of the Burgess Shale are a staple of paleo-books, Anomalocaris is the only one that has made regular appearances in popular media. This, of course, is because it was one of the largest animals of the Cambrian and among the earliest examples of a large apex predator. A good example of Walking with Monsters, where its size is exaggerated to that of a human! Interestingly, despite being virtually unknown across most non-paleontology media internationally, Burgess Shale critters are extremely popular in Japan, and Anomalocaris is again the most popular. For example, the Pokémon Anorith and its evolved form Armaldo are both based on it, while franchise rival Digimon features Anomalocarimon. Also, in Seton Academy: Join the Pack!, the titular school's principal, Amano Karorisu, is also an Anomalocaris.

Other prehistoric invertebrates