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The following groups of prehistoric reptiles, to quote Lore Sjoberg, "are not technically speaking dinosaurs but end up in the dinosaur section of the plush toy aisle nonetheless." This can get quite confusing, as no matter what your dictionary tells you, scientists have been waffling on the subject of dinosaurs for as long as there has been a name for them.
Pterosaurs (usually called Pterodactyls in media and in pop-language) are often referred as the "flying dinosaurs"; they actually were closely related with them, but technically were not true dinosaurs.
Pterosaurs in media
In Real Life, pterosaurs were the main flying beings in the Age of Dinosaurs, coexisted with their land-living relatives for 160 million years and eventually went extinct together with the last dinosaurs. Like about dinosaurs, there are several issues about ptero-portrayals in media. They go far further than simple Anachronism Stew and Misplaced Wildlife, they regard every pterosaurian biological feature. Here were fall in the Critical Research Failure field,and it’s easy to imagine ptero-scientists cry more than every other colleague.
- In fiction, these "reptiles" usually act as air-born terrors to menace humans and/or other animals, usually grabbing them with improbable eagle-like feet and magically strong enough to lift and carry their victims in flight, often giving them to their nestlings just like eagles. In Real Life pterosaurs had very weak legs with non-opposable digits and caught their food with their mouth instead. And it has been recently discovered they didn't have any nestlings to feed: pterosaurs reproduced like lizards, buring and abandoning their soft eggs, until the hatchlings (termed "flaplings" by some paleontologists) emerged, strong-boned and ready to fly on their own.
- Expecially in older media, they are often portayed with Bat Wings, tying into the whole Dinosaurs Are Dragons thing. Actually their wing-membranes were substained by only one overly-long digit, the fourth one (not the fifth as sometimes heard), which was as robust as the rest of the forelimb. The 1°, 2°, and 3° finger were normal-sized and protruded from the anterior wing-edge like the 1° digit of modern bats. More persistant is the depiction of the wings as loose, bat-like flaps of skin--in Real Life, the wing membranes were thin but taut sheets of muscle, containing several distinct layers of complex muscle fibers which essentially made them into shape-shifting organs for on the fly adjustments.
- We don’t know at all which kind of sounds they emitted in Real Life, but expect to hear them screeching loudly and continuously (usually in a mixed crow-vulture-seagull manner), and also fanning their wings as noisily as possible.
- They tend to be represented always huge. In Real Life there were many types of pterosaurs (just like what happens to modern birds) and they were astonishingly diverse in size, being from the size of a crow up to a small airplane.
- They weren't either dinosaurs, nor were they birds. Further, they were not even the ancestors of any modern flier: instead, they were only relatives of dinosaurs (and thus birds, of course).
Its worth noting, however, that Science Marches On has been a crucial factor in pterosaur portraits. Like dinosaurs, pterosaurs have undergone a sort of scientific "renaissance" since the seventies: traditionally described as awkward, cold-blooded gliders, then it has become clear that they were instead active and efficient fliers, covered with down like several dinosaurs and with high-rate metabolism. Nevertheless, even today they remain one of the most enigmatic group of prehistoric beasts, as their fossil record has always been one of the scarcest of all. Sadly, these issues are usually glossed over in pop-media, even today. See Ptero-Soarer to get into this in depth.
Very few pterosaurs have been portrayed in non-docu media, despite their notable variety in Real Life. Among pterosaurs listed here, only three can be called “pterodactyls” without getting totally wrong: Pteranodon, Quetzalcoatlus, and of course Pterodactylus. The other two, Rhamphorhynchus and Dimorphodon, were more primitive than the former and are usually called “rhamphorhynchs”.
Surprisingly, stock pterosaurs are not (necessarily) the biggest/coolest-looking ones --three out of five are not bigger than an eagle or a stork. Instead, they were among the very first scientifically-described kinds, in the XIX century. Pterodactylus, Rhamphorhynchus, and Dimorphodon,  were discovered in Europe before the 1820s (the decade in which the “first dinosaurs” were named). The last two were initially classified as “Pterodactylus” and recognized distinct only after the 1820s. With its 7 m/24 ft wide wingspan, Pteranodon was found in USA in the last quarter of the XIX century, during the “Bone Wars”.  Its sheer size (“Whoa the biggest flier ever!) and its crest soon made it the new iconic pterosaur, and still preserves its status today.
Several interesting new pterosaurs were discovered in the second half of the XX century, but only one managed to achieve some consideration in media: Quetzalcoatlus, because was the only one clearly bigger than the pteranodont, and the new “biggest flying animal ever”. In the 2000s, Ornithocheirus gained some popularity as well thanks to a memorable apparition in Walking with Dinosaurs, but only because was (wrongly) described as the biggest flying animal ever existed. The others (Dsungaripterus, Pterodaustro, Tapejara, Eudimorphodon, Sordes, and so on) were largely ignored outside dino-books and documentaries. If you’re searching for these and other non-stock pteros, go here.
Toothed or toothless? Pteranodon ***
Pteranodon lived 86-84 million years ago on what were then the shorelines of Kansas and other midwestern US states. It had the typical traits of the most evolved pterosaurs, the Pterodactyloids aka “literal pterodactyls”. It had an elongated head, weak hindlimbs, only a hint of tail, and very long wings with a huge “wing-finger”, while the other digits were very small and maybe almost useless. It was one of the most specialized flying animals that ever lived, but very clumsy on land, where it arguably walked slowly on all fours.
It’s thought pteranodonts lived a bit like modern seashore birds, laying its eggs on cliffs and using ascendent winds to take off. Like modern albatross, they could have been vagrant or migratory. Contrary to what is sometimes said, Pteranodon probably didn’t survive enough to see the meteorite--its fossil record ends a dozen million years before the mass-extinction. Among pterosaurs, only Azdarchids have left fossils from 65 mya: one of them was Quetzalcoatlus (see below).
The backwards-pointing crest is the most striking feature of Pteranodon. It was laterally flattened in Real Life, but in media expect to see it with a conical shape, often resembling a horn. The real purpose of this crest is still unclear. It is traditionally described as a sort of balancing-pole to better balance the long head, or as a rudder to maintain it stabilized during flight. However, this doesn’t explain why only males had such a big crest, while the females’ one was extremely shortened. Once was thought long- and short-crested individuals belonged to different species.  This bony protrusion could simply be a display device, like is hypothized as well for Stegosaur’s plates, Triceratops’ horns and frill, Spinosaurus “sail”, and so on. This could explain why the males’ one was so big compared with the females.
In popular portrayal, an ever-present mistake is to show Pteranodonts as toothy creatures. When present, these teeth usually resemble those of the other well-known ptero, Rhamphorhynchus (see below). The fact that the genus’ name ends in “odon” (meaning tooth in Greek) could lead people in error, too. Actually, “odon” is preceded by the greek “privative A” (becoming “an” when followed by a vowel). Thus, “anodon” means with no teeth. (See also Ludodactylus.)
In Real Life, Pteranodon was substantially a giant pelican in behaviour. Its long toothless beak was useful to catch fish, and the shape of its lower jaws seem to show a sort of “pouch” to store fish in flight, even though this is not sure. If alive today, Pteranodon could not be that danger for us folks as shown everywhere in media. Weighing only about 20 kg , was too light to lift a 70 kg man up in the air. And if it really could have done so, certainly didn’t with its weak hindlimbs but with the mouth instead. Finally, since its beak was straight and smooth-edged, a child could easily have got out of it by wriggling.
Pteranodon’s nifty crest on its skull, along with the fact to have holded the size-record for almost a century, has made it THE pterosaur in popular imagination. Its iconic status among pterosaurs coud be partially justified. It’s not only one of the first discovered pterosaurs, but perhaps also the most common in fossil record. Hundreds of specimens are known, while most other pterosaur kinds are much, much rarer, often known from a single individual. The vast majority of Pteranodons belong to the species all people know, Pteranodon longiceps.
- Features in the second Volvic bottled water ad.
Dragon-tailed fisher: Rhamphorhynchus **
Rhamphorhynchus (“sharp beak”) was first found in Germany in the XIX century, and lived in the Late Jurassic in the same locations of several other pterosaurs and also Archaeopteryx and Compsognathus. It was the second named pterosaur after the prototypical Pterodactylus, and the first one known with a long tail. It’s the namesake of the primitive pterosaurs called Rhamphorhynchoids. Like Archaeopteryx, Rhamphorhynchus has left exquisite remains. Some specimens with prints of wing membranes are known since the XIX century. It was just these Rhamphorhynchuses that definitively showed pterosaurs were airborn critters, not water-living as believed by some at the time.
Rhamphorhynchus had a wingspan of about 2 m / 6 ft, relatively short wings, a narrow snout slightly pointing upwards, robust hindlimbs, and long tail. Two recognizable traits are the protruding teeth and the diamond-shaped “fin” put vertically on its tail-tip. As Rhamphorhynchus lived in coastal lagoons, the teeth were probably apt to catch fish in flight. The typical tail-fin was made of soft tissue (it’s know only thanks to prints in the rocks).
Rhamphorhynchus’ tail has often been compared with a dragon’s or a devil’s, and the large teeth give it a deceptively “menacing” look (actually it wouldn’t be more dangerous than a gull if alive today). We now know it was covered in hair-like structures like its close relative Sordes pilosus (which just means “hairy devil”)
Its striking look, the earliness of its discover and the completeness of many remains have contributed to make Rhamphorhynchus the second most commonly portrayed pterosaur in media—expecially the older ones. Rhamphorhynchus has become quite rare today--pterosaurs shown in the most recent movies generally are Pteranodon, Quetzalcoatlus, or (more often) imaginary Giant Fliers.
Like Pteranodon, Rhamphorhynchus tends to be portrayed incorrectly in fiction, often as big as a pteranodont or even bigger. Another classic mistake is to show rhamphorhynchs with flexible tails (a bit like what happens to “raptors”) and sometimes with triangular fins. Actually their tail was stiffened by bony tendons, and was a steering device during flight.  Finally, a very Undead-Horse subtrope is to apply the “devilish” rhamphorhynchoid tail to every other pterosaur, expecially Pteranodon. Actually pterodactyloid pterosaurs had stubby tails without distinction.
Divine beast: Quetzalcoatlus *
This is, for now, the Ultimate Giant Flyer among prehistoric animals. Lived in North-America at the very end of the Cretaceous 70-65 mya (but some think its distribution was worldwide). Quetzalcoatlus was discovered in the 1970s in Texas from incomplete remains. Its wingspan was extimated from 10 m / 35 ft up to 16 m / 50 ft, with the lower range being the most likely. Of course, pop-media have often followed the higher one. This “living airplane” took the Pteranodon’s reputation over as “the biggest flier ever” in those years. Its describer named it from an Atzec divinity: Quetzalcoatl, the “feathered snake” (the animal itself is often called “the quetzalcoatl”.)
Quetzalcoatlus was long described as similar to an upscaled Pteranodon. Indeed, both animals had a long toothless beak and an atrophied tail. Some depictions show Quetzalcoatlus with a small cranial crest, others show it crestless. Compared to a pteranodont, Quetzalcoatlus had much longer/stronger hindlimbs, a bigger body, a longer/stiffer neck, and shorter/wider wings with the “hands” closer to the wingtips than to the shoulders. Its diet has long been an enigma (Fish? Small reptiles? Carrions?). In the 2000s, scientists re-studied its anatomy, and today Quetzalcoatlus is thought a mainly terrestrial animal, walking on its four long limbs, and eating small land critters like a stork. 
The astonishing thing is, in spite of being as tall as a giraffe when on land, Quetzalcoatlus could still fly. Giving its size, it should have been an extremely powerful flier, capable to frequent several habitats, and maybe even to travel worldwide. An almost-identical relative, Hatzegopteryx, was recently described from Europe. It was extimated even bigger than Quetzalcoatlus, but they could be the same animal.
Despite their impressiveness, Quetzalcoatlus has received great attention only in dino-books and documentaries, while is still rare in films and dino-stories. No matter if, with its size and terrestrial habits, it could have been the only pterosaur potentially dangerous for humans if alive today. A downsized Quetzalcoatlus flying robot capable to flap its wing was built in the 1990s by an paleo-amateur: the first ptero-flight after 65 million years.
The first named Mesozoic reptile: Pterodactylus *
Rarely will you see any other pterosaur in fiction, but if you do, it will either be Pterodactylus or Dimorphodon. In the original novel The Lost World (1912), the two scientists argue if the pterosaurs they meet are one of the two. Whatever kind they were, they are enormously oversized—even bigger than a Pteranodont. In Real Life both were not bigger than a large seagull. In recent stories, Pterodactylus and Dimorphodon were rarely (if ever) portrayed, but are common in documentaristic media due to their historical relevance.
Pterodactylus was the first fossil recognized by science as belonging to a prehistoric animal completely different to the modern ones (well before the first dinosaurs); this happened in year 1809, when concepts like "extinct" and "antediluvian" (the word "prehistoric" came later) were still highly controversial. Obviously, Pterodactylus was the very first flying reptile ever found, and even early paleontologists tended to refer to the whole group as 'pterodactyls' long before the name Pterosaur was coined; this explains why pterodactyl has become the stock name of pterosaurs.
Several Pterodactylus species were recognized in the past. Today only one has remained valid, Pterodactylus antiquus, about the same size of a Rhamphorhynchus. Some alleged Pterodactylus species were described from individuals no bigger than a sparrow: this led the genus to be described as “one of the tinies pterosaurs ever”. Recent research suggests these specimens were actually juveniles, which deceptively resemble miniaturized adults. The fact that newborn pterosaurs were virtually identical to adults is a recent discovery which strongly astonished scientists and paleo-fans: none of the still-living fliers shows powered flight soon after its birth.
Pterodactylus was one of the first appeared pterodactyloid pterosaurs (Late Jurassic Europe), and shared its habitat with the more archaic Rhamphorhynchus. Its body-shape was more similar to a miniaturized pteranodont than to a rhamphorhynch, with long, thin jaws, elongated wings, weak hindlimbs, small “wing-hands”, and stubby tail. Usually depicted as a generic-as-it-gets pterosaur, the latest findings show it had a small crest made of skin on the back of its head. Moreover, its apparently generic teeth could have been specialized for something---traditionally described as a fish- or insect-hunter, Pterodactylus could have been a filter-feeder in coastal lagoons (though not so specialized as another relative, Pterodaustro).
A toothed toucan: Dimorphodon *
Dimorphodon was a rhamphorhynchoid that lived in Early Jurassic, 190 mya. Found in England, it shared with Rhamphorhynchus the elongated stiffened tail, but we don’t know if it had a “fin”. Its more striking trait is its oversized skull, even bigger than the body itself! Despite appearences, the head of Dimorphodon was lightened by wide openings in the skull, and the animal couldn’t have troubles to lift it, a bit like modern toucans and hornbills with their beaks.
Its name, “two-shaped teeth”, recalls that of the famous Dimetrodon (“two-measured teeth”). It had two kinds of teeth (while most pterosaurs have only one). Some teeth were bigger and sparse among the smaller ones. With this kind of dentition, the feeding habits of Dimorphodon have always been matter of speculation. It could have been a fisher, a hunter of small land animals, an insectivore, or all these things.
More archaic than Rhamphorhynchus, Dimorphodon shows several primitive traits which betray how dinosaurs and pterosaurs were closely related. The three free wing-fingers were strong and large-clawed, not unlike those of a feathered theropod. The hindlegs were long and powerful, and the animal could have walked bipedally used its long tail for balance. Its stocky skull was more similar to a theropod dinosaur than to a pterosaur, with nasal openings on the tip of the nose (most pterosaurs had nostrils just in front of the eyes).
A strange anatomical thing is one elongated digit in each hindfoot, which could have been attached to the wing membrane. If so, the dimorphodont could have used it to better-control the flight, like modern bats do with their feet. More evolved pterosaurs lost this super-toe altogether. Despite its primitive anatomy, Dimorphodon was a fully pterosaur with all the pterosaurian traits. (See also Eudimorphodon.)
Sorry, these ones aren't here. If you're looking for Nyctosaurus, Pteranodon sternbergi, Dsungaripterus, Ornithocheirus, Anurognathus, and others, see here.
Similarly, these animals are often collectively referred to as "swimming dinosaurs", but this time they were not close relatives of true dinosaurs; some of them were not even related each other. If you'd like to see a seagoing dinosaur, watch March of the Penguins.
Sea-reptiles in media
Four main groups of sea reptiles can be recognized in media: Plesiosaurs, Ichthyosaurs, Mosasaurs, and Pliosaurs. In Real Life there were other sea-going reptiles in the Mesozoic, but being less-impressive than the former, they don’t gain much attention. (except for the giant turtle Archelon, that shows up occasionally). As with most prehistoric animals, only the largest will be mentioned from each group: Elasmosaurus for Plesiosaurs, Liopleurodon for Pliosaurs, and Tylosaurus for Mosasaurs --with the exception of the Ichthyosaurs. Ichthyosaurs will be represented only by Ichthyosaurus which was actually small for the group, though that won't stop them writers from making it bigger. However, in old media Plesiosaurus is frequent as well, even though was a very small member of the eponymous group (but don’t worry: it is shown regularly oversized as well).
Interestingly, unlike dinosaurs, marine reptiles were already well-known to science at the beginning of the 19th century. Their fossil record is overall wealthier and better-preserved than that of the dinosaurs. Significantly, the very first “antediluvian” reptiles entered in narrative media were not dinosaurs, but the Ichthyosaur and the Plesiosaur which battle each other in the novel Journey to The Center of The Earth written by Jules Verne in 1864. Both animals were enormously oversized and depicted in a very fanciful way: if you read the novel, you'll find them more like Mix and Match Critter-related sea-monsters than their Real Life counterparts.The “ichthyosaur” is similar to a mixup of whales-crocodiles-dragons-snakes-whatnot, and has not the familiar fish-like shape of a Real Life Ichthyosaurus. The plesiosaur is a bit more realistic, but has a serpentine neck and the shell of a sea turtle. However, some of these errors are due to Science Marches On, as we’ll se in the single sections below.
Possibly thanks to Verne, the battle against prehistoric marine reptiles has become stock in paleo-art and pop-culture, just like its land-placed equivalent "Carnivorous vs Herbivorous Dinosaur". One of the opponents is always a long-necked Plesiosaur, while the other may alternate between a Mosasaur, an oversized Ichthyosaurus, or a Pliosaur. In these portraits, ichthyosaurs, mosasaurs, and pliosaurs tend to be used indifferently, often confused each other and portrayed as generic “giant swimmers”.
Another long-standing cliche makes sea-reptiles the pterosaurs’ archenemies. You probably have already seen the scene of a "prehistoric leviathan" emerging abruptly from the surface of the sea, grabbing a giant flying reptile with its jaws (the victim is usually Pteranodon), and dragging it underwater to eat it. In Real Life this could be possible only for the biggest mosasaurs and pliosaurs. Giant ichthyosaurs did roam the seas, but in their time pterosaurs were still very small. Even though giant plesiosaurs like Elasmosaurus could have interacted with giant pterosaurs, their small mouth was unable to swallow whole Giant Fliers like Pteranodon. .
Turtles inside a snake's body: Plesiosaurs **
Plesiosaurs (more correctly, plesiosauroids) are the most distinctive marine reptiles, and lived worldwide throughout the Mesozoic, 210-65 mya. With their long necks, massive body, short tails, small heads, and four paddle-like limbs, their look may recall that of a “flippered brontosaur”, but they were actually very different than a sauropod dinosaur. They were carnivorous like all known marine reptiles. With their small mouths, they arguably ate only small items, like fish, juvenile reptiles, or shellfish. Their hunting tecniques are still matter of discussion -- active hunting, ambush-predation, bottom-feeding or even partial filter-feeding are all possible. They had pointed teeth protruding from their jaws, but were perhaps covered by lips in the living animals. For obvious reasons, expect to see plesiosaurs with ever-visible teeth fitted outside their mouth, often oversized and more protruding than in Real Life, literally making a snake-like head.
The association with snakes and turtles seems a constant when talking about plesiosaurs. Even scientists once used to describe these animals as “turtles into a snake’s body” or “snakes into a turtle’s body”. Their body was actually turtle-like (except for the lack of shell of course), and their neck had a huge number of vertebrae (even 76 in Elasmosaurus!). Classic depictions show plesiosaurs with extremely flexible necks capable to coil and to dart like a snake. Science Marches On however, and it was discovered in the 2000s (thanks to simulations in CGI) their neck was much more rigid than previously thought, a bit like what is happened to sauropod dinosaurs.
These animals are traditionally described as slow turtle-like swimmers, using their four flippers as oars and propelling awkwardly their bulk through the water. In classic paleo-art plesiosaurs will usually be portrayed in a swan-like posture when emerged, and will use their neck as a periscope when swimming underwater. According to biomechanical studies, they’d kept their neck straight to better plough the water, and used their flippers to literally “fly” underwater, even though the exact movement of the flippers is still uncertain (see also “Pliosaurs” below). Maybe plesiosaurs were among the most skilled swimming animals of all time. Like whales compared with dolphins, larger species should have been less-agile than the smaller ones. Some portrayals show plesiosaurs with a sort of fin at the end of their tail, but it’s only speculative. If really present, it acted only as a rudder, as plesiosaur’s tail was too weak to propel the animal.
The commonly shown species is Elasmosaurus. It was one of the largest plesiosauroids, 40ft/13m long (like a grey whale), but since only a small portion of its length was of body, it weighed “only” 8-10 tons (like a large killer whale). The 20ft/7m long neck made more than half the entire length, and was actually longer than the body. Living in Late Cretaceous in the inland North-American Sea, the elasmosaur was discovered in the USA during the Bone-Wars. Its describer, Edward Cope, made an astounding mistake in its first attempt to rebuilt its skeleton (see Prehistoric Life). The prototypical Plesiosaurus was the first described plesiosaurian (1810s), even before Megalosaurus and Iguanodon. First found in England, it was much smaller (16 ft long),earlier (first Jurassic), and much shorter-necked than Elasmosaurus.
Possibly thanks to their dinosaurian-look and the association with snakes, Plesiosaurs have been the most iconic and depicted sea reptiles in media. Like dinosaurs and pterosaurs, expect to see them as scary monsters with an killing attitude towards humans. If alive today, even the biggest Elasmosauruses wouldn’t be more aggressive than most whales (although they could unintentionally capsize your tiny boat or raft). And every time a plesiosaur shows up, someone will bring up the Loch Ness Monster.
Kron and his (oversized) son: Pliosaurs *
The long-necked plesiosauroids belong to the Sauropterygian supergroup, which also includes the Pliosaurs (or Pliosauroids) and other less-known groups of sea-reptiles. Together, Plesiosauroids and Pliosauroids make the Plesiosauria, aka “plesiosaurs” in broader sense, originated from the same common ancestor in the Triassic seas. Their relationship of Sauropterygians with modern reptiles has long been unclear. Once, they were placed with Ichthyosaurs in their own subgroup (see further), and not related with any still-living reptilian group. Today, plesiosaurs sensu lato (pliosaurs included) are thought not closely related to ichthyosaurs. They seem only loosely related with modern lizards, rather that with crocodiles and dinosaurs.
Like Plesiosauroids, Pliosauroids too were widespread throughout the Mesozoic, 218-65 mya. Both subgroups shared the same body plan, with rigid bodies, short tails, and the two pairs of powerful flippers--perhaps alternately-moved making a typical “double-wings” swimming effect (as seen in Walking with Dinosaurs). The difference stays in front of their shoulders. Pliosaurs had very short stocky necks, and their head was far bigger than an Elasmosaurus. Their teeth were less-numerous, but much longer and stronger: like elasmosaurs, expect to see them visible when the mouth closed, even though they could have been hidden by lips in Real Life. Despite the resemblances, the head-anatomy of plesios and plios was the same. Both had eyes and nostrils placed above to see out of water when the remaining head was submerged. Both shared a singular trait: each nostril had two chambers like fish, possibly giving them a directional sense of smell. We don’t know if they passed more time near the surface or in the deeps, but some think pliosaurs were more deep-creatures than plesiosaurs.
Pliosaurs were variably-sized, some were not bigger than dolphins, but the biggest one are candidates for “the largest sea reptile” title – even though their size has often been exagerrated. Among the latter, Liopleurodon and Kronosaurus, were among the top-predators of the Jurassic and early Cretaceous oceans respectively. Once extimated 16 m long (10 m are more likely), Kronosaurus is named from Cronus, a Greek goddity who devoured its own offsprings (Zeus and Poseidon among them). About the same length was the less-coolly named Liopleurodon, virtually identical to the former but with less teeth. As is usual with marine superpredators, both are usually depicted as merciless ever-hungry killing machines.
Despite this, Pliosaurs have been the least-portrayed group of sea-reptiles, and still remain mainly documentary-animals. The kronosaur has long been the most commonly shown pliosaur in books and documentaries until 1999, when a memorable appearance of an extraordinarily oversized  Liopleurodon on Walking with Dinosaurs rapidly made it the new iconic member of the family (very similarly to Deinonychus à Velociraptor thanks to Jurassic Park). In the show, an old male liopleurodon was described weighing 150 tons (a bit less than the Blue Whale) and the biggest predator of all time. In particular, the scene in which he pulls the ichthyosaur in pieces disturbed many viewers (even though the sad final scene where he’s stranded and slowly dies gives it a bit of humanity). To give the idea about how the animal remained impressed in pop-consciousness: all successive depictions have shown Liopleurodons with the WWD blue-white color-patterns.  While WWD may have made Liopleurodon the Stock Pliosaur in place of Kronosaurus, Charlie the Unicorn managed to bring it to the status of minor Internet meme.
Fishes, dolphins, or lizards? Ichthyosaurs *
Ichthyosaurs have the very evocative name of “fish-lizards”: they really resembled large fish in shape and swimming style, but recall modern dolphins as well thanks to their flippers and their long “nose”. Like fishes and unlike dolphins, they had four flippers (foreflippers were usually bigger) and an upright caudal fin. As a group, Ichthyosaurs were the most ancient marine reptiles, and were widespread from the Middle Triassic until the Late Cretaceous, 245-90 mya, but got extinct 25 years before the mass-extinction. Once, ichthyosaurs, plesiosaurs and pliosaurs were put together in their own reptilian supergroup, the “Euryapsids”, unified by having a single pair of skull-openings on the top of the skull. Today, “euryapsids” are firmly put in the Diapsid supergroup (which also included dinosaurs, pterosaurs and all living reptiles except maybe turtles). Icththyosaurs were among the very first diapsids to have evolved: since they are not related with any modern animal group (thus preventing good comparisons), they still remains quite mysterious critters.
Descended from a still-unknown land-living ancestor (remember every lineage of marine reptiles did descend from four-limbed terrestrial reptiles), Ichthyosaurs were the most marine of all marine reptiles and never came onto land, not even for laying their eggs. In fact, their young were born alive just like modern dolphins, as we can see in some fossils of mothers dead with their offspring just getting out of their body. We now know that plesiosaurs also reproduced in the same way and probably never left the water either---however, the scene of a long-necked plesiosaur which crawls on the seashore like a sea-lion is a staple in artworks. Ichthyosaurs are extremely abundant in fossil record: several individuals are preserved with soft tissue and, sometimes, even the print of the whole body. Thanks to the latter, we know they had a dorsal fin and a crescent-shaped caudal fin other than the four paired “flippers”. A strange thing is, unlike modern sharks, the backbone curved downwards at the tail level, and filled the lower lobe of the caudal fin, not the upper one.
Their eyes were noticeably large for good vision; most portraits show ichthyosaurs with round pupils and no eyelids like a typical fish. Skull nasal openings were just in front of the eyes, but the nostrils could have been on the head-top. Their skin was smooth and hydrodinamic like a dolphin, as shown in fossil prints. The mouth was usually filled with acute teeth: most ichthyos were fish-eaters, but also ammonites and other shellfish were in their menu. We don’t know what amount of time they passed underwater: maybe they could extract some oxygen directly from the water like modern sea-turtles, but certainly did breath regularly like every reptile. The resemblance with dolphins means artists classically show ichthyosaurs jumping out of water in a dolphinish style, but this is not proven. Unusually for extinct reptiles, “Fish-lizards” often escape the fate to be described as “monsters”… at least in modern docu-media. Originally, ichthyosaurs were depicted more crocodile- or mosasaur-like, with no caudal or pectoral fins. The famous “ichthyosaur” in Verne’s novel is just based on this early interpretation. Several other fictional ichthyosaurs have then been inspired by the original.
Today, more updated ichthyosaurs are regular sights in dino-books. They’re very useful to show evolutive mechanisms, making a classic example of "convergent evolution" with fish and cetaceans. On the other hand, they are rarely seen in recent dino-stories, much less than the long-necked plesiosaurs. Maybe they are not that exotic-looking, or just not impressive enough to attract writers’ intrerest. The species shown is always Ichthyosaurus, because was the first discovered (in the 1810s. in England, before the “first known dinosaurs”, and the prototype of the group. Being only 8/10 ft long in Real Life, expect to see it oversized and over-scary. And never mind that some other ichthyos (Temnodontosaurus, Cymbospondylus, Shonisaurus), being 25 ft long or more, could be very apt for the role. The absence of Shonisaurus is particularly strange: as large as a sperm-whale, it could be the biggest known sea-reptile.
Sea-serpents: Mosasaurs *
The most recent group of Mesozoic sea-reptiles, Mosasaurs lived worldwide in the Late Cretaceous, at the end of the Age of Dinosaurs. They replaced Ichthyosaurs and coexisting with the last Plesiosaurs. While Ichthyos and Plesios were not closely related with any modern reptile, Mosasaurs are the only prehistoric animals which literally deserve the title of "giant lizards". They belong to the Squamates, the group containing modern lizards and snakes , and were closely related to modern snakes and monitor-lizards. Indeed, due to their elongated shape, they have often been compared with the legendary "sea-serpent", and often depicted with a speculative dragon-like crest running along their back in much old paleo-art.
Descended from monitor-like animals, Mosasaurs often reached gigantic sizes, but exaggerations tend to be common. Some source talk about 20 m long animals, even though most giant mosasaurs were probably no more than 10 m long. With their slender bodies, they were also less-heavy than the robust plesiosaurs and pliosaurs. To be more hydrodinamic, they could have lost the original lizard-scales and developed a smooth skin-texture, like ichthyosaurs and plesiosaurs. Their tail was long and laterally-flattened: unlike plesiosaurs, they swum in a crocodilian-manner swinging their tail side-to-side. Since many modern snakes and lizards are ovoviviparous (that is, produce eggs that hatch inside the mother’s body), this could be true also for mosasaurs. In this case, they hadn’t to come ashore to reproduce, and maybe lived entirely in water. As their limbs were fin-like, this could be a proof. All marine-reptiles described here obtained their flipper-like limbs in the same way of modern cetaceans, embedding their original digits in one single fleshy mass, and enormously multiplying the number of phalanxes (ichthyosaurs took this to an extreme).
Mosasaurs’ head was similar to modern lizards but with a longer snout. Like the latter, they’d have had fleshy lips. Like modern snakes, their mouth had a notably loose hinge between the jaws: this allowed mosasaurs to swallow big items without tearing them in pieces. The teeth were conical, the upper ones placed in two rows on each half-jaw, again like modern snakes and monitors. According to stomach contents, Mosasaurs were very generalist feeders: fish, sharks, squids, pterosaurs, early birds like Hesperornis and even smaller mosasaurs have been found. . We don’t known if mosasaurs had a forked tongue and ever-open eyes like many modern squamates, nor if they had heat-sensors like some boas and rattlesnakes: these things usually don’t preserve in fossil record.
Like pliosaurs and ichthyosaurs, Mosasaurs are a staple in documentaristic media, but are not-so-common in stories. Most “giant leviathans” with huge jaws seen in fiction tend to be generic monsters a-la-Verne, rather than precise kinds of sea-reptiles. And don’t exclude to see mosasaurs confused with sharks. Tylosaurus and the namesake Mosasaurus are the stock members of the mosasaur family: needless to say, they're among the largest, up to 10-15 m long. The former was found during the “Bone Wars” in USA. The latter has a much more fascinating story. Found in the Netherlands near the Mosa river at the end of the 1700 (hence its name), Mosasaurus was not only the first sea reptile ever discovered, but the second fossil recognized by science as belonging to a Mesozoic reptile, after Pterodactylus. See also Prehistoric Life.
A turtle outside a snake's body: Archelon *
The only other marine reptile which has appeared in fiction more than once, Archelon lived in the same Late Cretaceous inland shallow sea which once covered the Great Plains. Discovered at the start of the XX century, it shared its habitat with Elasmosaurus, Tylosaurus, and the flying Pteranodon: its size and armor made adult Archelons virtually unattackable by predators. (Even though in WWD a dead archelon is shown killed by a giant mosasaur, but the latter was oversized).
Not all Mesozoic reptiles were exotic critters, however. Archelon was just what it seems: just a sea-turtle. But it fits perfectly the subtrope “Everything was huge at dinosaur times”: it’s the largest known fossil turtle--6 m/20 ft long and weighing some tons, Archelon was 2-3 times bigger than the biggest modern turtle (the Leatherback turtle, confirming Turtle Power is Truth in Television. However, it was not the ancestor of modern sea-turtles: it pertained to a different lineage, the Protostegids, which went extinct along the other giant reptiles at the end of the Cretaceous. Its name is a Portmanteau of “arche” (primeval) and “chelon” (turtle).
As turtles and tortoises have virtually remained unchanged since their first apparition in the Triassic, Archelon had the same traits seen in modern chelonians: beaked jaws, forelimbs transformed in strong flippers (with multiple phalanxes as usual), weaker hind-flippers and short tail. However, its armor was lighter than most modern turtles, and maybe the shell was leathery instead of horny. The modern turtle which mostly resembles an archelon could just be the Leatherback. As modern species of sea turtles eat very different items (some eat shellfishes, others seaweed, and some jellyfishes), we don’t know what were the archelon’s preferences. Certainly it came ashore to lay its eggs like its relatives.
Among the rare Archelon appearences in fiction, the most remembered is in Harryhausen’s One Million Years BC. The turtle is the first animal cavemen encounter in the island, upsized in such a degree to be as big as a house. Surprisingly, many viewers think it was live-acted by a Real Life turtle, but it too is Stop-Motion like most other animals here.
Other sea reptiles
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At least Pterosaurs and the above-mentioned seagoing animals are from the same Mesozoic time-period. Don't even get us started on how Synapsids (commonly named "mammal-like reptiles") are sometimes labeled dinosaurs.
Synapsids in media
Most synapsids lived well before the apparition of the first dinosaur; indeed, synapsids were the very first large land vertebrates and diversified much during their permanence on Earth, until most of them got wiped out in the Permian mass extinction. In the Brave New World that followed, the few surviving non-mammalian species were outcompeted by archosaurs, the group containing dinosaurs, pterosaurs and crocodilians.
Synapsids had an extraordinary relevance in the history of evolution because they were the ancestors of mammals and thus of mankind itself, and yet they have not gained popularity like that of the dinosaurs, probably because of their relatively small size compared to things like T. rex or Sauropods.
Technically they are not even "reptiles".
Hairy lizard, or naked mammal? Dimetrodon **
Lived in North America 280 million years ago, in the Permian period (just before the Triassic one). It is the only mammal-like "reptile" whose popularity matches that of the stock dinosaurs, thanks to its mohawk-esque crest (sail) on its back. Its iconic status among mammal-like reptiles is partially justified by its fossil abundance--dozens of specimens are known, juveniles included. As one could expect at this point, it was discovered in North America during the US Bone-Wars, in the second half of the XIX century.
Being a very early mammal-ancestor, Dimetrodon has still a lizardy shape, with long tail, long body, splayed legs, and skull with a small braincase. Fossil prints show a lizard-like gait. Other traits, on the other hand, were quite mammalian: a laterally-flattened trunk (not wider-than-taller like most modern reptiles); a solid skull with one single pair of temporal opening placed near the maxillary hinge (the so-called “synapsid” condition also seen in mammals, us humans included); and differently-shaped teeth--“Dimetrodon” just means “teeth with two lengths”. Even though all teeth were conical, the anterior ones were small and crammed together like incisors, while the longest teeth were in the place mammals have usually their canines. Behind them, the posterior teeth were small and not apt for chewing food unlike our molars, but are somewhat analogue to those seen in primitive insectivorous mammals. More evolved synapsids like Cynognathus achieved a clearly mammalian anatomy, with more erect limbs, shorter tails, larger brains, and teeth very similar to mammals (see Prehistoric Life).
Its “sail” substained by elongated vertebral spines has always been an headache for scientists. The classic theory consider it a thermoregulating device. Put against the solar rays it could have captures much heat like a solar panel; if put parallel to them, it was more like a radiator, dispersing heat. Considering its desertic habitat, this hyp still remains a good one. Other theories are mating or threat display, inter-specific identification, and so on. Maybe the sail served for all these purposes. Among external features, Dimetrodon could also have had some sparse hair, hints of auricles and maybe even proto-milk glands. These things are totally unsure, and given its primitiveness, are unlikely. The coloration is totally speculative--living in harsh habitat, it should be brownish like modern desert mammals, but its sail could have been vividly coloured and/or able to change colors for display purpose. Sadly, synapsid soft tissues are virtually unknown. No eggs or nests are known from the dimetrodont, and we don’t know if it was oviparous, or viviparous like modern mammals (if the latter is true, viviparity was achieved independently).
Dimetrodon is usually described as the top-predator of its time, shown hunting early “amphibians” like Eryops, Diplocaulus, Seymouria etc., as well as what could be called its Non-Identical Twin, Edaphosaurus. Its crest, its (apparently) reptilian look and the meat-eating attitude makes the dimetrodon a predestined victim of Dinosaurs Are Dragons and Prehistoric Monster both in Fiction and in docu-media. However, if compared with other famous prehistoric animals, Dimetrodon could appear rather narmy in comparison. If we imagine a battle against a Tyrannosaurus/Deinosuchus/Smilodon/Mosasaur, the primitive and relatively small dimetrodont would always result the loser--this could also be true when put against modern predators (lions, kodiak-bears, Nile crocs etc), as well as most ancient and modern giant herbivores. But in Permian landscapes, Dimetrodon was still faster and more powerful than every other land animal, definitively debunking the Narm thing.
Although Dimetrodon is more closely related to you than to any dinosaur, and predated the first dinosaur by at least a country mile of geologic time, it is often mixed with dinosaurs in toy collections just because it looks cool. In movies and comics, it may even show up living with Cavemen. Expect to see it with a giant iguana-like look and scaly skin. Actually, scales are a strict reptilian thing, and Dimetrodon hide was probably naked like modern hairless mammals, with some hardened fish-like belly scales left over from its amphibian ancestry. Its shape makes the dimetro the most abused animal within the Slurpasaur trope. For example, in the 1970 film version of A Journey to The Center of The Earth some Caribbean iguanas with a ridicolous crest attacked on their backs live-act Dimetrodons , which of course attack the humans.
Sorry, these aren't here. If you're looking for Ophiacodon, Sphenacodon, Cotylorhynchus, Estemmenosuchus, Anteosaurus, Titanosuchus, Dicynodon, Thrinaxodon, and others, see here.
Even prehistoric mammals, are sometimes mislabeled dinosaurs. Colloquially, this is often true of fish as well, or any prehistoric-looking creature, such as the Coelacanth.
Mammals in media
Among mammals, those living in the Ice Age are the most portrayed, because they lived along with the most iconic hominid species.
One and the same? The Woolly Mammoth and the American Mastodon ***
Mammoths and Mastodons often show up in anything dealing with prehistory, though usually associated with the Ice Ages thankfully. The species definitely most portrayed of these is Mammuthus primigenius, better-known as the Woolly Mammoth. Probably the most iconic non-dino prehistoric animal of all, thanks to the countless, extraordinarily well-preserved known specimens with soft tissues, which make it perhaps the only prehistoric animal almost as scientifically well-known as a still-living animal.
A common misconception about the Woolly Mammoth is saying it was larger than modern elephants: actually the 'woolly' was the same size as its tropical, 21st century cousins--perhaps this is due to the confusion with other mammoth species that were a bit larger, like the Emperor Mammoth and Columbian Mammoth). Also note that only males had the typical huge, curly tusks: the females' tusks were not that different from those of modern elephants. As preserved fossil hair are often reddish-brown, some depictions show woollies with this color: actually, this is due to a chemical change since 10.000 years ago. When alive they were blackish, as seen in the Walking With series.
It's worth noting that mammoths, scientifically speaking, are just another type of elephant, since they belong to the same phylogenetic branch. An Asian Elephant is slightly more closely related to mammoths than to his more distant African modern relative (thus mammoths weren't the direct ancestors of elephants as heard sometimes). On the other hand, the Mastodon is not a true elephant but just a distant relative of both modern pachyderms and mammoths -- its scientific name, Mammut americanum, is misleading). For other extinct elephant relatives, see Prehistoric Life.
Smilodon, aka the "Saber-toothed Tiger" ***
Saber-tooth cats, with their distinct fangs, are just as iconic in pop-culture. There were many species of them, but the only saber-toothed cats you'll ever likely see are Smilodon fatalis and Smilodon populator, which is larger but has a less awesome name.
Although "saber-tooths" belongs to the "cat family", they are in a separate branch of that clade compared to modern felines; thus, the popular denomination "Sabertooth tiger" is not correct at all. The "tiger" thing means that Smilodon is often heard roaring just like an actual tiger or a lion, though only the big cats of the genus Panthera (that is, Lion, Tiger, Jaguar and Leopard) could make roars thanks to the structure of their larynxes unique of this group. This depiction is, however, vindicated by science, as the structure of the small bones in its mouth are set up for making a sort of roar.
It will probably use its sabers for every conceivable task, like slaying herbivores the size of the mastodons or Megatherium with a single stab, despite the fact that most real sabertooths (as well as their relatives, the scimitartooths and dirktooths) had relatively delicate fangs that could not safely be used for stabbing. Instead, they probably slashed out the throats of prey from below. Oh, and it'll be stripy, like a tiger, which -- in S. populator 's case -- isn't completely impossible, given that it lived on grasslands.
Expect to see Smilodon heavily interacting with humans, as our ancestors' main predator: in Real Life other carnivores such as prehistoric lions were probably more important predators. And expect to see it living alongside woolly mammoths. Even though they were contemporary, their habitat in Real Life was largely different, with Smilodons preferring warmer climates. And, naturally, don't exclude seeing saber-toothed cats somehow living alongside dinosaurs, and in the worst scenario, fighting against a T. rex.
- Appears in the third Volvic bottled water ad.
Other prehistoric mammals
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Other extinct creatures
Sorry, these aren't here. If you're looking for Scansoriopterygids, Enantiorns, Neorns, Teratornis, Crocodylomorphs, Protosuchus, Hallopus, Pristichampsus, Mekosuchines, Bavarisaurus, Megalania, Postosuchus, Rutiodon, Euparkeria, Erythrosuchus, Kuehneosaurus, Sharovipteryx, Longisquama, Scutosaurus, Procolophon, Eudibamus, Triadobatrachus, Karaurus, Eocaecilia, Eryops, Cacops, Platyhystrix, Ichthyostega, Tiktaalik, Coelacanths, Eusthenopteron, Lungfish, Acanthodes, Palaeoniscus, Cheirolepis, Megalodon, Haikouichthys, Trilobites, Pterygotus, Ammonites, Orthoceras, Rudists, Lingula, Graptolites, Cothurnocystis, and others, see here, here or here.
- ↑ dinosaurs were
- ↑ The normally-sized ones
- ↑ Oddly, it too was initially classified as Pterodactylus.
- ↑ An astounding thing is, short-crested pteranodonts are almost never shown even in documentary media!
- ↑ Compared to body size pterosaurs were even lighter than birds, having totally hollow bones like straws. For example, the prehistoric vulture Argentavis had a similar wingspan of the pteranodont, but is extimated 80 kg.
- ↑ It’s not to exclude that only males had the “fin”. If so, this thing could have been for display.
- ↑ It’s worth noting that the terrestrial hyp was already postulated just after the first find, but initially not demostrated.
- ↑ Note that Pteranodon bones have been discovered in the belly of a plesiosaur fossil, but they come from a small, female or juvenile pterosaur, not a gigantic male
- ↑ The fictional length 80ft/25m was based on misidentified bone fragments; in reality, the species maxed out at 25ft/7m long
- ↑ For some reason, unlike dinosaurs and pterosaurs, marine reptiles are almost-always shown with dull colors even in modern portraits. However, they could have been very colorful like modern tropical seagoing animals.
- ↑ Never forget snakes are legless lizards just as birds are flying dinosaurs
- ↑ This doesn’t necessarily mean they were cannibals, though: the preyed mosasaurs could be from different species than their predators. After all, modern orcas do eat smaller dolphins