History UsefulNotes / PrehistoricLifeOtherExtinctCreatures

No other animal group has had a greater importance in paleontology than molluscs. Their fossils are extremely abundant, to the point that many rocky formations are mainly made of cemented mollusc shells. Among molluscs cephalopods deserve a mention apart, being much more "evolved" than the others. Together with arthropods, cephalopods are the extinct invertebrates you have more chances to see in media - at least, the documentary ones: you [[SmallTaxonomyPools hardly can see]] a trilobite, an ammonite, a sea scorpion or a nautiloid in Fictionland. If it happens, they would be simple "ambient critters", and good luck if the animal [[NoNameGiven is mentioned]].

* Extinct cephalopods have given fuel to many legends, even before paleontology itself was "invented" by [[http:/en.wikipedia.org/wiki/Georges_Cuvier Georges Cuvier]] at the end of the 1700 century. Before that, those strange things today called fossils were believed to be Nature's jokes, the Earth's flowers, or other things (Only Creator/LeonardoDaVinci recognized correctly their nature, but his discovery was long ignored). And then, there are more specific legend about cephalopod fossils. [[UsefulNotes/StockDinosaursNonDinosaurs Ammonites]] were believed stony horns ("ammonite" comes from Amun, an often ram-horned/headed Ancient Egyptian deity), or petrified snakes. [[note]] Some had fun to sculpt a snake-head on the shell's extremity to make it like a snake! [[/note]] The less-known belemnites (technically the belemnoids), with their straight pointed shape, were believed stony arrows, or even the Devil's fingers! Belemnites were cephalopods living in the Mesozoic era together with ammonites, and probably gave rise to squid. Like ammonites, only their shell is usually preserved. This shell was ''inside'' the animal's body and invisible in life; belemnites would resemble simple squid or cuttlefish if alive today. Their lifestyle was more active than ammonites, and probably they were able to do the same things modern squid do (spraying ink, swimming using the lateral "fins", catching preys with their suckers, seeing images with their eyes). True modern squid became widespread in the Cretaceous, and the first octopuses (''[[http://en.wikipedia.org/wiki/Proteroctopus Proteroctopus]]'' for example) were already around in the Middle Jurassic; these mollusks (called "coleoids") usually have no shell inside, or at least only a remnant of shell (ex. the famous cuttle-bone). Some Cretaceous squid were as large as a modern giant squid.

* Some fossil animals' names recall literary characters, rather than mythical things: nautiloids (literally "similar to the Nautilus") are so-called from the modern ''[[http://en.wikipedia.org/wiki/Nautilus Nautilus]]'', whose name (meaning "sailor" in Greek) incidentally recalls [[Creator/JulesVerne Nemo's submarine]] (we're talking about the human captain, NOT the cartoon fish!). Actually, nautiloids are an artificial assemblage of all cephalopods which were neither ammonoids (ammonites) nor coleoids (octopodes, squids, belemnites etc.). They include both the modern nautilus and the most ancient cephalopods, widespread in the Paleozoic. Their traits were probably like those of the nautilus: their shell was divided in sections which could be filled alternately with water or with air like in a submarine. This allowed them to regulate their buoyancy (ammonites had a similar inner-shell anatomy and probably did the same). As the nautilus' eyes are simple chambers lacking any lens and incapable of forming images, this was probably true for other nautiloids as well. They should have had dozen of sucker-less tentacles, and were probably slow swimmers. However, many Paleozoic nautiloids had straight conical shells, not curly like the nautilus; ''Orthoceras'' ("straight horn") is the classic example. Its shell shows even traces of its original colors. ''Rhizoceras'' ("root horn") had a slightly curved but still conical shell. ''Rayonnoceras'' was more similar to ''Orthoceras'', but was longer than a human and lived in the Carboniferous. The ''Walking With'' series has popularized another straight-shelled nautiloid, the Ordovician ''[[UsefulNotes/StockDinosaursNonDinosaurs Cameroceras]]'' (named the "giant orthocone"), chosen as the prototypical Paleozoic cephalopod only because it [[BiggerIsBetter was 10m long]] and the biggest known nautiloid so far (and one of the largest Paleozoic animals at all).

There is not much to say about the portrayal of extinct non-arthropod / non-cephalopod invertebrates in media: they [[SeldomSeenSpecies rarely appear]] even in books/documentaries, much less in Fictionland, and when they do, they are almost [[NoNameGiven never named]] (except sometimes for the names of each group, but only in popular science works). As an example, the original Disney's ''Disney/{{Fantasia}}'' showed several ''modern'' critters to symbolize the early evolution of invertebrates, but [[SmallTaxonomyPools few or nothing]] among the ''really'' prehistoric ones. The ''Walking With'' series did the same: only modern jellyfish, sponges and sea urchins appear, all live-acted. Indeed, many modern invertebrate groups have populated our seas since the Cambrian Period, but many others are extinct today. Among modern land arthropods expect to see dragonflies, scorpions, centipedes, spiders, cockroaches, beetles, and sometimes crickets. They would be oversized: [[BigCreepyCrawlies roaches the size of a rat, scorpions as big as cats]], and so on. Finally, let's not forget a staple in every Paleozoic or Mesozoic sea: a Jellyfish, used as the symbol of the most ancient Forms of Life.

'''Clams and Pseudo-clams:''' [[http://en.wikipedia.org/wiki/Rudist Rudists]] and prehistoric ''[[http://en.wikipedia.org/wiki/Lingula Lingula]]''

* Gastropods, scaphopods, bivalves, brachiopods, chitons, monoplacophorans: [[RiddleMeThis who's the intruder?]] All these invertebrate groups were "shellfish", and all but one were molluscs: brachiopods weren't. They were only distant mollusc relatives, and more related with the coral-like bryozoans (see "sessile invertebrates" below). As a whole, molluscs and brachiopods are extremely abundant in fossil record of all ages, from early Paleozoic up to the modern era. However, while molluscs are still a dominant group in modern seas, brachiopods are nearly extinct today, and thus cites as "living fossils". The most representative is probably ''Lingula'' ("small tongue"), a clam-like, filter-feeding animal that lives anchored to the sea floor with a fleshy protrusion (the "tongue"); prehistoric ''Lingula'' have been identical to the modern one since the Early Ordovician! Among extinct non-cephalopod molluscs are worthy of note the Rudists like ''Hippurites''. Exclusively Cretaceous, these clams are notable both for their often great size (some were as tall as a human), and for their unique shape. They can be described as giant cups with a lid: the lower valve (the one attached to the sea bed) was conical and much bigger than the flat upper valve. Like ''Lingula'', rudists were probably almost motionless creatures that filter-fed on tiny organisms.

When thinking about animal fossils, our mind usually goes on the petrified bones of dinosaurs. But dinosaurs in paleontology are ''extremely rare finds'' compared to other vertebrate groups, such as sea reptiles, Cenozoic mammals and fish. And vertebrates as a whole are in turn only a ''very small'' part of the total. Indeed, more than 90% of animal fossils that Earth left to us are from invertebrates. Some invertebrate groups like [[UsefulNotes/StockDinosaursNonDinosaurs ammonites and trilobites]] are so common they're objects of collection by many paleo fans of modest means, while it's rare for dinosaur bones to be traded like this — some trade of dinosaur bones does exist, but it's highly debated if it's the right thing to do, since dino fossils are such a rarity.

When thinking about animal fossils, our mind usually goes on the petrified bones of dinosaurs. But dinosaurs in paleontology are ''extremely rare finds'' compared to other vertebrate groups, such as sea reptiles, Cenozoic mammals and fish. And yet, vertebrates as a whole are in turn only a ''very small'' part of the total. Indeed, more than 90% animal fossils that Earth left to us are from invertebrates. Some invertebrate groups like [[UsefulNotes/StockDinosaursNonDinosaurs ammonites]] and [[UsefulNotes/StockDinosaursNonDinosaurs trilobites]] are so common they're object of collection by many paleo fans; while it's ''unlikely'' dinosaur bones will receive this trade — although some trade of dinosaur bones do exist as well, but it's highly debated if it's a right thing to do, since dino fossils are such a rarity.

* Today, crustaceans are by far the most abundant water arthropods. Their prehistoric relatives were like the modern ones, and have always been an important group. Examples of Mesozoic crustaceans include the shrimp ''Aeger'', the crab ''Eryon'', and the enigmatic Thylacocephalans. However, in the Paleozoic another group of aquatic arthropods were even more diversified: the early chelicerates, more related with ''spiders'' than to crabs. The most spectacular water chelicerates were the so-called sea scorpions (see the following paragraph); the smaller xiphosurans were just as abundant. Like the coelacanth and the nautilus, they have classically been mentioned as "living fossils". Indeed, the modern ''Limulus'' (the horseshoe crab) is just the only surviving xiphosuran, and the only surviving aquatic chelicerate (except for the little-known sea spiders or Pantopods). All the other chelicerates became terrestrial, forming the arachnids (true spiders, true scorpions, whip-scorpions, pseudoscorpions, soliphugans, harvestmen, ticks, mites and others). Xiphosura means "sword tail". Contrary to what is believed, the "sword" at the end of the horseshoe crab is not venomous or dangerous at all (you could probably lightly prick your foot if you step on it, but that's pretty much it), is only a mechanical device to overturn again the animal when capsized. Many extinct xiphosurans were identical to our modern ''Limulus'', and probably behaved the same; our horseshoe crab lives mainly in the bottom of the seas like trilobites, feed only on small items, but comes ashore to lay its eggs. Curiously, its young are very similar to an adult trilobite.

* When talking about evolution, the superpredators are usually described as a sort of foe that try to destroy our distant ancestors, almost as their precise purpose was to delete Man's modern presence on Earth. Just see ''Series/WalkingWithMonsters'' for an JustForFun/{{egregious}} example. When the portrayals are about Mesozoic life this treatment is typically reserved to dinosaurs (see [[UsefulNotes/PrehistoricLifeNonDinosaurianReptiles The origin of Mammals]]); in Paleozoic settings, however, the classic choice has been the eurypterids (also called Gigantostracians, "giant shellfish"), better known as the [[BigCreepyCrawlies sea scorpions]]. They were indeed related with modern scorpions (and maybe their ancestors), but technically they ''were not'' scorpions, and not all were marine (some became freshwater dwellers). They hadn't the venomous sting, and resembled more slender lobsters than scorpions. Their had big composed eyes like insects, scorpion-like pincers, and their rear pair of legs were flattened and used to swim; they arguably lived both in the bottom and in open waters. We don't know if they came on land to lay their eggs. Eurypterids were active predators, and the biggest ones were among the apex predators especially in the Silurian period; in the following Devonian they were outcompeted by jawed fish like ''[[UsefulNotes/StockDinosaursNonDinosaurs Dunkleosteus]]'', but managed nonetheless to survive until the Permian. ''Eurypterus'' ("broad wing"), the namesake of the group, is the most common eurypterid in fossil record; but the most famous is ''[[UsefulNotes/StockDinosaursNonDinosaurs Pterygotus]]'' ("the winged one"). It was one of the biggest gigantostracians (the length of a human), and one of the biggest arthropods of all times along with the extinct millipede ''[[UsefulNotes/StockDinosaursNonDinosaurs Arthropleura]]'' and the modern Giant Japanese Crab. ''Pterygotus'' is the sea scorpion traditionally most portrayed in media, [[BiggerIsBetter because of its size]] of course. Ironically, however, its even larger cousin, ''[[http://en.wikipedia.org/wiki/Jaekelopterus Jaekelopterus]]'' was the largest known eurypterid, and yet has still not appeared in broad-audience media.

* Within the evolution of animal life, it is universally agreed that arthropods and vertebrates have been the two animal groups which reached the best results. Cephalopod molluscs, too, are very complex creatures; but they ''never'' managed to come on land. Arthropods and vertebrates did that, but it was the former which made the first step on dry land, in the Silurian period. Vertebrates joined them only later in the Devonian. Even when out of the liquid element arthropods and vertebrates have continued to co-exist and to co-evolve, and this competition has made both more and more perfected. It's actually unfair to declare arthropods have been the vertebrates' ''worst enemies'', and that the latter had to fight a "war" against spider-scorpions-insects (as said in the preface of ''[[Series/WalkingWithMonsters Monsters]]''); indeed, arthropods have unwillingly ''helped'' us vertebrates to became those successful beings we are. Among the very first land arthropods were the first TRUE scorpions and the first myriapods (millipedes and centipedes). Critters such as ''Palaeophonus'' were already identical to a modern scorpion; the same about the earliest myriapods like ''Euphoberia''. Spiders appeared a bit later, in the Carboniferous (ex. ''Arthrolycosa''); the first non-insect hexapods (hexapod = six legs) evolved in the Devonian (''Rhyniella''), but the first winged TRUE insects took their first flight in the Carboniferous forests: they were the very first flying animals ever, and the ''only'' flyers until pterosaurs made their appearance in the Triassic, followed by birds and finally bats. In the Carboniferous, land arthropods became often huge; two in particular have become a staple in paleo-books and documentaries: ''[[UsefulNotes/StockDinosaursNonDinosaurs Arthropleura]]'' and ''[[UsefulNotes/StockDinosaursNonDinosaurs Meganeura]]''. They have even made some appearances in fiction, too. [[note]]Curiously, the more impressive giant millipede ''Arthropleura'' has entered Fictionland only recently, while the giant dragonfly Meganeura can be also seen in older fictional works. [[/note]] ''Walking With'' has also popularized other critters: the huge true scorpions ''[[http://en.wikipedia/org/wiki/Brontoscorpio Brontoscorpio]]'' from Silurian and ''[[http://en.wikipedia/org/wiki/Pulmonoscorpius Pulmonoscorpius]]'' from Carboniferous, the alleged giant spider ''Megarachne'' (it was actually a spider-looking freshwater eurypterid, also from Carboniferous) and the Early Cenozoic giant ants seen in ''Beasts''. All, more or less, affected by RuleOfCool in the show.

Insects. The most abundant, diversified, biomechanically efficient, and last but not least, by far the most studied animals by modern fauna zoologists. Sadly, in paleontology this is not the same. Their fossil record is ''extremely'' poor, for understandable reasons: their tiny body isn't precisely the most adapt to turn into stone, and their terrestrial habitat doesn't help either - most fossil animals discovered so far were aquatic indeed, just because water helps a lot the process of fossilization. However, if we know something more about the ancient relatives of modern insects, we have to thank pines, firs, spruces and larches. Remember ''Film/JurassicPark'', and those fossilized mosquitoes in amber from which dinosaurian DNA was extracted? The DNA extraction thing was obviously fictional, but the amber thing itself is TruthInTelevision. Insects preserved in amber are perhaps the most marvelous fossil a palaeontologist could wish in its life. Not only they are perfectly preserved in every detail, included external anatomy and ''even color'' (an almost unique example among fossils); they have their original tissues preserved, single cells included. And they can provide an extraordinary clue to understand the entire ecosystem in which they lived in indirect ways. Unfortunately, most insects fossilized in amber come from the Cenozoic era (the Mammal Age), a period in which insect were already similar to their modern relatives. But we still know little about their Mesozoic ancestors, still less about the Paleozoic ones - except for those living in the Carboniferous (such as the aforementioned ''Meganeura'') because many of them did preserve well in coal. Anyway... we know some things with a good grade of certainty. The first insects appeared in the Devonian [[note]]Technically these were the first hexapods, since springtails are no longer considered proper insects by modern entomologists: however, we follow the traditional view because it's more convenient.[[/note]], later than scorpions and millipedes: they were still wingless as modern [[http://en.wikipedia.org/wiki/Springtail springtails]] and [[http://en.wikipedia.org/wiki/Silverfish silverfish]] still are, but then the Carboniferous saw their success: the ''very first'' flying animals appeared, among them the now-extinct Palaeodictiopterans with ''six'' wings. In this period they reached large size up to the four-winged ''[[UsefulNotes/StockDinosaursNonDinosaurs Meganeura]]'' and starting their radiation destined to continue even today: not only dragonflies, but also cockroaches, grasshoppers and beetles appeared first in the Carboniferous. Other groups began their history in the Triassic: moths, wasps, flies and true bugs appeared at that time or a bit later, as well as parasitic kinds like fleas and lice. [[http://en.wikipedia.org/wiki/Insect_evolution Insect evolution]] has always been related with that of terrestrial plants. It's worth noting, however, that this relationship has always been ''far more strict'' than one may think: biologists talk about a veritable co-evolution between insects and seed-producing plants, especially the flowering ones (angiosperms). This partnership reached its climax in the Cretaceous, when flowering plants became the new dominant group, just because of the relationship with two new kinds of insects barely appeared: the pollinators and the social ones. The former include butterflies, bees, wasps, flies and even some beetles, while the latter include ants, bees, wasps, and termites. Both ensembles began to affect dramatically their ecosystem, conditioning indirectly the evolution of ''all'' the other terrestrial animals, dinosaurs included. Many paleontologists think if modern birds and placental mammals are today the most diversified land vertebrates, they have to thank the insect-plant mutualism which has created well-suited habitats for their (initial) small size and eating versatility. Think about those birds and bats who feed only upon nectar, anteaters and pangolins which feed upon nothing but social insects, or the infinite insectivorous/"angiospermivorous" modern animals. And think about all the plant-related products we humans utilize today. Keep this in mind, every time you crush a bug.

* However, ostracoderms were ''not'' the most primitive fish; they were already highly evolved animals, with complex brains, fins, and keen senses (''Cephalaspis'' seemingly even shows electric sensors!). Their anatomy is unusually well-known because the inner portion of their head shows imprints of the brain, nerves, inner ear, and other soft tissues.[[note]] Ostracoderms and the other jawless fish shared a curious anatomical trait: one single nostril on their head (all jawed vertebrates share two paired nasal openings like us humans).[[/note]] Actually ostracoderms descended from unarmoured fish, among them the very first fish that appeared. Sadly, as soft tissues don't usually fossilize, they are virtually unknown to science. Three exceptions are ''Haikouichthys'' ("Haikou's fish"), ''Yunnanozoon'' ("Yunnan's animal": Yunnan is one of the southernmost province of China), and ''Myllokunmingia'' (Kunming is the capital city of Yunnan); all from the Cambrian period and found in China, they were similar to the famous invertebrate lancelet, or to the larval stage of the modern lamprey (the so-called ammocoetes). ''Haikouichthys'' was a tiny animal the size of a human nail, and was probably a harmless filter feeder. Incredibly, ''Series/WalkingWithMonsters'' managed to transform even this inconspicuous critter in a "terror": here, a shoal of ''Haikouichthys'' is seen feeding on the flesh of a wounded ''Anomalocaris'' like modern lampreys and hagfish would do. Actually, lampreys and hagfish (aka the cyclostomates) are highly evolved parasitic animals capable of feeding on large prey despite their [[LampreyMouth lacking of jaws]]; their evolution is very poorly known. Finally, we could not forget the enigmatic conodonts. These tiny fossils similar to toothed jaws (conodont means "cone tooth") have been a real headache for scientists (invertebrates? early vertebrates? worms?). Today we know they belonged to early jawless fishes that lived during the whole Paleozoic, but their lifestyle is still uncertain; they may have been the ancestors of the lampreys.

* Welcome to the Aspis family. Most "ostracoderms" ("shelled skin") have this suffix. Despite this, they ''don't'' form a true taxonomic group: every armored fish devoid of jaws is traditionally called with this name, but they are actually distinct lineages, some closer to jawed fish than to other ostracoderms. Compared with placoderms, ostracoderms' armor was more complete, covering the whole body, and formed a true shell at the head level. Despite their badass look they were very harmless creatures; their size ranged from that of a thumbnail up to a human hand, and with their jawless mouths they could only have feed on tiny food items like algae, small invertebrates, etc. They often fell victim to predators like the eurypterids ("sea scorpions"), cephalopods and jawed fish that were strong enough to get through their thick armored skin. One successful ostracoderm lineage is the Osteostracans ("bony shell"), whose prototype is the flat-headed ''[[UsefulNotes/StockDinosaursNonDinosaurs Cephalaspis]]'' ("head shield") but include also ''Hemicyclaspis'' ("semicircle shield") among the others. Another is the Anaspids ("with no shield"), more streamlined and with a lighter armor: ''[[http://en.wikipedia.org/wiki/Birkenia Birkenia]]'' was a typical example. The Heterostracans ("different shell") had often long snouts; ''[[UsefulNotes/StockDinosaursNonDinosaurs Pteraspis]]'' ("winged shield") is their prototype. ''Drepanaspis'' was a round-shaped example of this group; ''Arandaspis'' and ''Astraspis'' were related to the heterostracans. Finally, the Thelodonts (named after their namesake ''[[http://en.wikipedia.org/wiki/Thelodus Thelodus]]''), which where the most closely-related to jawed fish (and may have been their ancestors).

"Fish" is a catch-all word containing all non-tetrapod vertebrates; that is, all backboned animals which are ''not only'' fully aquatic, but descend from fully aquatic ancestors as well. Ichthyosaurs, plesiosaurs, mosasaurs and dolphins aren't fish, just because they ''did'' descend from land-living creatures (except they are... in the same way birds are dinosaurs, but in this case even US humans are fish!). There are only two groups of fish which are still successful today: sharks and ray-finned fish. Not so in Prehistory, as you'll get soon.

* Sarcopterygians, aka "lobe-finned fish" or "lobefins". They should better be called proto-amphibians rather than fish: their blood circulation is double like land vertebrates, not simple like typical fish; their fins are bony and fleshy, and their nostrils are connected with their mouth (those of fish aren't). And some can even ''breathe air'' like us, other than absorbing oxygen in water with the usual gills. The latter are meaningfully called lungfish, they have one or two true lungs which originally were simple protrusions of the digestive tube. Lungfish are very few today: the most archaic is the Australian one, ''Neoceratodus'', with robust paired "fins". Its name means "new Ceratodus": ''Ceratodus'' ("horned tooth") is the most abundant prehistoric lungfish, and was virtually identical to ''Neoceratodus''. The other modern lungfish are African ''Protopterus'' and South American ''Lepidosiren''. Among the very first Paleozoic lungfishes there were ''Dipterus'' ("two wings") and ''Scaumenacia''; the first one was already in the Devonian. However, the most famous modern lobe-finned fish is ''[[http://en.wikipedia.org/wiki/Latimeria Latimeria]]'', the only surviving coelacanth. Prehistoric coelacanths, too, were virtually identical to their modern descendents: one of the most common in fossil record is ''Macropoma'' from the Mesozoic; the namesake of the group ''Coelacanthus'' ("hollow spine") lived earlier in the Paleozoic. ''Mawsonia'' was Cretaceous and the biggest of the group, 12 ft long and one of the largest freshwater fish ever (and a possible prey of ''[[UsefulNotes/StockDinosaursTrueDinosaurs Spinosaurus]]''). Unlike lungfish, coelacanths (also known as actinists) have been exclusively aquatic: unable to breathe air, they are the most "fishy" among the lobe-finned fish. It's worthy to note that during the Dinosaur Age lungfish and coelacanths were very abundant: coelacanths swam in great numbers in the sea, while lungfish lived in oxygen-poor swamps, sometimes obligated to walk out of water when the water pools dried out just like modern lungfish.

'''Fish crawled onto land:''' ''[[http://en.wikipedia.org/wiki/Osteolepis Osteolepis]]'', ''[[http://en.wikipedia.org/wiki/Holoptychius Holoptychius]]'', ''[[http://en.wikipedia.org/wiki/Panderichthys Panderichthys]]'', ''[[http://en.wikipedia.org/wiki/Tiktaalik Tiktaalik]]'', and ''[[http://en.wikipedia.org/wiki/Acanthostega Acanthostega]]''

* Even though more closely related with US than with true fish, lungfish and coelacanths were less close to terrestrial vertebrates than some fossil lobefins, the basal tetrapodomorphs (called "rhipidists" or "Osteolepidotes" in older sources). Among them, there was the common ancestor of all tetrapods aka land vertebrates, but we don't know which was really ''the'' ancestor. The traditionally most cited is ''[[UsefulNotes/StockDinosaursNonDinosaurs Eusthenopteron]]'': other relatives of it included ''Holoptychius'' and ''Osteolepis'', both with a more normal-looking, non-"trident" tailfin. In the 1990s/2000s some new animals were discovered, which appear evolutionarily in the middle between a ''Eusthenopteron'' and an ''Ichthyostega'': excellent examples are ''Panderichthys'', ''Tiktaalik'', and ''Acanthostega''. The latest one was very similar to the ichthyostega, but had ''eight'' digits on each "hand" instead of seven.

* Actinopterygians, aka "ray-finned fish" or "rayfins". Or, more simply, "the fish". They are by far the most common fish today (95% of all fish species!), but in the Dinosaur Age they were only one of the several fish groups roaming the seas and fresh waters. The most evolved rayfins, the Teleostei, became widespread only in the Cretaceous. Almost all modern ray-finned fish are teleosts. However, most familiar teleostans appeared only after the K-Pg extinction, in the Age of Mammals. There were no goldfishes, swordfishes, seahorses, piranhas, ocean sunfishes or deep sea anglers in the Cretaceous (only herrings, tarpons, and a few others). However, non-teleostan rayfins were already common in the Mesozoic, among them gars, sturgeons and their relatives. And then, there were things such as ''Leedsichthys'' ("Leeds fish"), a Jurassic fish with no modern relatives that was ''as big as a humpback whale'', maybe the largest fish of all times. [[note]] Maybe not bigger than a modern whale shark however, and with a toothless mouth which indicates an inoffensive filter-feeding attitude like the latter [[/note]] Another non-teleostan fish was ''Lepidotes'' ("the scaly one"). It was one of the most common fish, with more than 100 species that lived during the whole Mesozoic timespan. Similar to a carp, it was actually not related with any modern fish: its primitiveness is revealed by its heavy scales similar to an armor. These scales are sometimes found inside the rib cages of other animals, e.g. the fishing dinosaur ''[[UsefulNotes/StockDinosaursTrueDinosaurs Baryonyx]]''. Since the Cretaceous all these early forms have been outcompeted by teleosts, which were more agile thanks to their lighter scales. One of the most common of them was ''Leptolepis'' ("thin scale"), an ancient herring-like fish . Another, ''Enchodus'' is nicknamed the "saber-toothed herring" but was not a clupeiform (the herring group). Perhaps the most famous Cretaceous rayfin is ''[[UsefulNotes/StockDinosaursNonDinosaurs Xiphactinus]]'': also similar to a herring but with teeth, it was 5-6m long (like a great white shark), and a voracious predator in competition with the giant sea reptiles of the time. ''Xiphactinus'' is most famous for a fossil in which a 4m-long specimen was preserved [[BigEater with a 2m-long fish in its gut.]] Among the Mammal-Age rayfins, the most common in fossil record is ''Knightia'', an ancient true herring.

'''The first fish with the fishbone:''' ''[[http://en.wikipedia.org/wiki/Palaeoniscum Palaeoniscum]]'', ''[[http://en.wikipedia.org/wiki/Cheirolepis Cheirolepis]]'', ''[[http://en.wikipedia.org/wiki/Acanthodes Acanthodes]]'', and ''[[http://en.wikipedia.org/wiki/Climatius Climatius]]''

* Together, ray-finned fish and lobe-finned fish make the traditional Osteichthyes, aka bony fish. Indeed, a bony skeleton was their main invention - all the earlier fish groups had cartilaginous skeletons, while their bones were only ''outside'' their body in the form of scales and teeth. Bony fish (both lobefins and rayfins) appeared in the Paleozoic era, 200 million years before the first dinosaur.[[note]]Once, ichthyologists called the most primitive rayfins "Chondrostei" (including the modern sturgeon and paddlefish), and those in the middle between them and the Teleostei were called "Holostei": the only surviving "holosteans" are the gars (aka the Lepisosteids) and the bowfin (''Amia calva'').[[/note]] Among the earliest ray-finned fish, ''Palaeoniscum'' (traditionally called "Palaeoniscus") and the Devonian ''Cheirolepis'' ("hand scale") resembled a cross between a shark and a modern bony fish: mouth placed downwards and asymmetrical caudal fin like the sharks'; fins, opercula, and smooth scales like the bony fish's. Curiously some extant archaic rayfins like the sturgeon still preserve this [[MixAndMatchCritter mixed look]]. In the Paleozoic there was also a unique fish group that went totally extinct before the Mesozoic: the acanthodians (so-called from their namesake ''Acanthodes'', "the spiny one"). They were externally similar to the earliest ray-finned fishes above, but with a cartilaginous inner skeleton like a shark. Their name means "spiky" from their spiny fins, and have been nicknamed "spiny sharks", but they were actually closer to bony fish than to sharks. Among the other acanthodians we can mention ''Climatius''. Together, sharks, acanthodians, bony fish and placoderms (see below) have one thing in common: jaws. See also "Tough Guys" below.

* If you ask a paleontologist what are the most common vertebrate fossils, (s)he'll answer you "shark teeth". However, these teeth are usually found isolated - ironically, the remaining skeletons are among the rarest fossil finds (sharks' cartilage don't fossilize well, unlike bone). Even though sharks made only a small percentage of the modern fish species (about 5%), they were a very successful group in the past, even more diversified than they are today. But stop a moment. What is a shark, exactly? In common sense, sharks are things like the great white or the bull shark; but zoologists often use this word to indicate every cartilaginous fish, or more technically, every Chondrichthyian. Palaeontologists usually give the same meaning to "shark", too. So, the first ever sharks appeared in the Devonian period 400 million years ago, and since then have changed very little: sharks are often cited as "living fossils". More precisely, the modern kinds of shark appeared in the Cretaceous: earlier sharks were only distantly related with them. Examples are ''Cladoselache'', ''Stethacanthus'', and ''Xenacanthus'' (once also called ''Pleuracanthus''), all from the Devonian period, and also a little known modern fish, the deep sea chimaera (also called ratfish or rabbitfish). The latter has preserved to this day the mixed shark/bony fish anatomy of the acanthodians and the earliest rayfins. ''Cladoselache'' ("branch shark") was more like a true shark, with uncovered gills and tooth-like scales covered in enamel (its fossils have preserved prints of soft tissues), but its mouth was strangely placed at the top of its head like the modern unrelated whale-shark. ''Xenacanthus'' and ''Stethacanthus'' were more bizarre. The former had an eel-body and a long filament protruding from its head (its name means "strange spine"); the latter is nicknamed "ironing-board shark" from its flat head prominence (not a dorsal fin) with many dentines on the top. Maybe only males had these things, which resemble the much smaller tubercle of the male chimaera (if so, they would have been courtship devices). But the perhaps weirdest "shark" ever is ''Helicoprion'' ("helix saw"), the "whorl-toothed shark", whose teeth were placed in a spiral line inside its mouth! It now appears that both ''Helicoprion'' and ''Stethacanthus'' may actually belong to the chimaera lineage, thus not proper sharks. Other early chondrichthyans, ''Falcatus'', ''Harpagofutator'', and many others, also showed strange prominences above their heads.

'''Shark Tales:''' ''[[http://en.wikipedia.org/wiki/Hybodus Hybodus]]'', ''[[http://en.wikipedia.org/wiki/Ptychodus Ptychodus]]'', ''[[http://en.wikipedia.org/wiki/Cretoxyrhina Cretoxyrhina]]'', and prehistoric Rays

* Triassic and Jurassic sharks were still primitive, but with a more modern look. One of the most common was ''Hybodus'' ("humped tooth"), whose shape recalls a typical predatory shark but with small "horns" on its head, like the modern bull-head shark. This is the shark portrayed in the "Walking With" series in the two episodes in which ''[[UsefulNotes/StockDinosaursNonDinosaurs Liopleurodon]]'' is the main character, acting in both as a minor predator. Other relatives became flatter and rather similar to a ray: ''Ptychodus'' is one example. Both ''Hybodus'' and ''Ptychodus'' belonged to the same group: the Hybodont sharks, more evolved than the ones above but more primitive than most our-day sharks. If we imagine to swim in Cretaceous waters, however, we'll see sharks virtually identical to the modern ones. ''Cretoxyrhina'' (nicknamed "the Ginsu Shark") was very similar to a great white shark. It was one of the top predators of the Late Cretaceous North American inland sea, in competition with [[UsefulNotes/StockDinosaursNonDinosaurs mosasaurs, plesiosaurs]], and the bony fish ''Xiphactinus'' above. But we could also encounter another kind of fish which little resembles a shark, but technically ''is'' a true shark: rays and skates. They appeared in the Cretaceous, in the same time of the commonly intended "sharks", and also little changed their anatomy during the times.

* The Devonian is rightly called "the Fish Age". Almost every main fish group was present in Devonian waters: lobe-finned fish, ray-finned fish, cartilaginous fish, acanthodians, and several jawless fish (see further). But the perhaps most typical Devonian fish group is another: the Placoderms. Their name means "skin with plates", and are nicknamed "armored fish"; their scales were notably thick and broad and covered their forebody like a true armor. However, this armor was still flexible and didn't make a "shell", making these fish more agile than their armored predecessors, ostracoderms; unlike the latter, they were active predators. There were two main placoderm groups: antiarchs and arthrodires. The former had bizarre pectoral fins more similar to crustacean legs than fins, and lived near the bottom of the seas: ''Bothriolepis'' and ''Pterichthyodes'' are the most known example. The latter had strange scissor-like teeth for cutting meat, and probably lived in more open waters. Ex. ''Coccosteus'' and ''[[UsefulNotes/StockDinosaursNonDinosaurs Dunkleosteus]]''. Another interesting placoderm is ''Lunaspis'', with its crescent-shaped body. There is a curious thing about the evolutionary origin of jaws and teeth: the former arose from the first pair of ''gills'' of the earliest jawless fish; while the teeth had the same origin of ''scales''. After all, modern sharks still have enamel-covered scales on their body, the same shape of their teeth (only smaller). [[note]] The reptilian and bird scales have nothing to do with fish scales: the latter are pieces of bone covered or not with enamel; the reptilian ones are simple horny protrusion of the skin, like our nails. [[/note]] In a sense, you could even say teeth are the ''only'' fish scales we human still preserve.

* Welcome to the Aspis family. Most "ostracoderms" ("shelled skin") have this suffix. Despite this, they ''don't'' make a real fish group: every armored fish devoid of jaws is traditionally called with this name, but they are actually distinct lineages, some closer to jawed fish than to other ostracoderms. Compared with placoderms, ostracoderms' armor was more complete, covering the whole body, and formed a true shell at the head level. Despite their badass look they were very harmless creatures; their size was from a thumbnail up to a human hand, and with their jawless mouths they could only have feed on tiny food items like algae, small invertebrates, etc... They often fell victim to predators like the eurypterids ("sea scorpions"), cephalopods and jawed fish that were strong enough to go through their thick armored skin. One successful ostracoderm lineage is the Osteostracans ("bony shell"), whose prototype is the flat-headed ''[[UsefulNotes/StockDinosaursNonDinosaurs Cephalaspis]]'' ("head shield") but include also ''Hemicyclaspis'' ("semicircle shield") among the others. Another is the Anaspids ("with no shield"), more streamlined and with a lighter armor: ''[[http://en.wikipedia.org/wiki/Birkenia Birkenia]]'' was a typical representant. The Heterostracans ("different shell") had often long snouts; ''[[UsefulNotes/StockDinosaursNonDinosaurs Pteraspis]]'' ("winged shield") is their prototype. ''Drepanaspis'' was a round-shaped example of them; ''Arandaspis'' and ''Astraspis'' were related with the heterostracans. Finally, the Thelodonts (named after their namesake ''[[http://en.wikipedia.org/wiki/Thelodus Thelodus]]''), which where the most closely related with jawed fish (and maybe their ancestors).

* However, ostracoderms were ''not'' the most primitive fish; they were already very evolved animals, with complex brains, fins, and keen senses (''Cephalaspis'' seemingly shows even electric sensors!) Their anatomy is unusually well-known because the inner portion of their head shows the prints of the brain, nerves, inner ear, and other soft tissues. [[note]]Ostracoderms and the other jawless fish shared a curious anatomical trait: one single nostril on their head (all jawed vertebrates share two paired nasal openings like us humans).[[/note]] Actually ostracoderms descended from unarmoured fish, among them the very first fish appeared. Sadly, as soft tissue don't usually fossilize, they are virtually unknown by science. Three exceptions are ''Haikouichthys'' (improperly "Haikou's fish"), ''Yunnanozoon'' ("Yunnan's animal": Yunnan is one of the southernmost province of China), and ''Myllokunmingia'' (Kunming is the capital city of Yunnan); all from the Cambrian period and found in China, they were similar to the famous invertebrate lancelet, or also to the larval stage of the modern lamprey (the so-called ammocoetes). ''Haikouichthys'' was a tiny animal the size of a human nail, and was probably a harmless filter feeder. Incredibly, ''Series/WalkingWithMonsters'' managed to transform even this unconspicuous critter in a "terror": here, a shoal of ''Haikouichthys'' is seen feeding on the flesh of a wounded ''Anomalocaris'' like modern lampreys and hagfish would do. Actually, lampreys and hagfish (aka the cyclostomates) are very evolved parasitic animals capable to feed on large items despite their [[LampreyMouth lacking of jaws]]; their evolution is very poorly known. Finally, we could not forget the enigmatic conodonts. These tiny fossils similar to toothed jaws (conodont means "cone tooth") have been a real headache for scientists (invertebrates? early vertebrates? worms?). Today we know they belonged to early jawless fishes that lived during the whole Paleozoic, but their lifestyle is still uncertain; maybe they were the ancestors of the lampreys.

A study by Yale University revealed the Tully Monster's true identity, and it was the least expected identity. They noticed the remains of a primitive notochord in the creature (meaning it was a vertebrate), and the weird lumps turned out to be ''eyestalks''. Using these, and other parts of the fossil, they came to the conclusion that it was a jawless "fish", closely related to the lamprey.

In paleontology, the word "amphibian" has traditionally had a much broader meaning than how it's commonly used. Amphibians were all [[http://en.wikipedia.org/wiki/Tetrapoda tetrapods]] (four-limbed vertebrates) excluding amniotes (reptiles + mammals). Today, even scientists tend to restrict the world to indicate only modern frogs, salamanders, caecilians, and their common ancestors. If you want to use "amphibians" in its former, broader sense, you have to call modern groups "lissamphibians". We'll use here amphibian in the old, wider meaning because it's much handier to say this rather than "basal tetrapod" every time we refer to non-frog, non-salamander, and non-caecilian animals.

The critters in this page pertain to groups of "biota" (= living beings) that have by old-fashioned biologists been quoted as "lower-ranking" than the so-called "higher animals" (mammals and birds). [[note]] Reptiles may get placed in either one category or the other depending on scientific age and their subgroup. In the past, all reptiles were usually labeled as "lower animals", but [[ScienceMarchesOn today]] the closest-to-mammals/birds ones (dinosaurs, pterosaurs, and sometimes therapsids and crocodilians) tend to be upgraded, while non-croc living reptiles and their ancestors tend to be still considered more similar to "lower animals" (while sea-reptiles, traditionally ranked as "lower", tend increasingly to be upgraded as well in recent years, though to a lesser degree than dinosaurs and pterosaurs).[[/note]]

However, modern scientists do not accept such an unfairly "racistic" distinction. Furthermore, "lower animals" often show biological traits and behaviours traditionally considered typical of mammals and birds: social attitudes, parental care, intelligence, even some "warm-blood" abilities. On the other hand, in popular media they may be treated as stupid/unfeeling brutes even to this day. Documentaries not excluded: within the ''[[Series/WalkingWithDinosaurs Walking With]]'' series, for example, ''Series/WalkingWithDinosaurs'' and ''Series/WalkingWithBeasts'' show dinosaurs and mammals as smart/social/caring creatures, while the meaningfully-named ''[[Series/WalkingWithMonsters Walking With]] [[PrehistoricMonster Monsters]]'' and ''[[SeaMonster Sea Monsters]]'' focus mainly to non-dino reptiles and to all the animal groups listed below, with only animals with bones shown as being smart/social/caring (ArtisticLicenseBiology).

We humans should be more grateful to extinct "lower beasts" however: several of them gave rise to many modern animals, and some did that to Mankind itself (yes, there were not only APES among our ancestors!). And don't think they were boring, uninteresting creatures: anything but, as you'll see soon.

In paleontology, the word "amphibian" has traditionally had a much broader meaning than that commonly attributed. Amphibians have been all [[http://en.wikipedia.org/wiki/Tetrapoda tetrapods]] (four-limbed vertebrates) excluded amniotes (reptiles + mammals). Today, even scientists tend to restrict the world to indicate only modern frogs, salamanders, caecilians, and their common ancestors. If you want to use "amphibians" in its former, broader sense, you have to call modern groups "lissamphibians". We'll use here amphibian in the old, wider meaning because it's far much handier to say this rather than "basal tetrapod" every time we refer to non-frog, non-salamander, and non-caecilian animals.

Lissamphibians excluded, prehistoric amphibians are traditionally called "labyrinthodonts" ("labyrinth tooth") or "stegocephalians" ("roof-head"), but these terms shouldn't be used today, just like "thecodonts" for basal archosaurs or "pelycosaurs" for basal synapsids. They don't indicate any natural grouping of animals, but are instead catch-all words with little scientific significance [[ScienceMarchesOn in modern phylogenetic systematics]]. Labyrinthodonts means labyrinth-teeth because many of these animals had convoluted, labyrinth-like internal patterns inside their teeth, but this isn't a fundamental thing. Their importance was much, much greater than this and lies upon another aspect. They were, simply, the links between fish and truly terrestrial vertebrates, a keystone group for mankind's evolution. And yet, just like synapsids and Mesozoic mammals, they have not gained much attention in pop culture. If they appear at all in fictional works, they'll be simply described as "giant amphibians".

* The only real amphibians in modern taxonomy, lissamphibians (frogs + salamanders + caecilians) appeared in the Triassic together with the first dinosaurs, but their deepest origins and their relationship with extinct amphibian groups are still unclear. The first frogs (''Triadobatrachus'', meaning the "triassic frog", is the most cited example) were already like the modern ones, only with shorter hind legs less adapted for hopping. Frogs have possibly been the most evolved amphibians ever, with their complex social behaviours and the ability to produce loud vocal sounds (this ability is atypical for a non-mammal / non-archosaur vertebrate). On the other hand, extinct salamanders have retained the primitive crawling shape of early amphibians. It's curious the case of ''[[http://en.wikipedia.org/wiki/Andrias_scheuchzeri Andrias scheuchzeri]]'' (a close relative of the modern Japanese Giant Salamander) which was initially believed a ''man dead during the Biblic Great Deluge''! ("Andrias" just means "man" in Greek). About the legless worm-like caecilians, they are as little known in palaeontology as they are in RealLife. Their fossils are extremely rare, but they are believed to have had small limbs at the start of their evolution (ex. ''Eocaecilia'', "dawn caecilian").

* This guy deserves his own section. ''Beelzebufo'' was an enormous relative of modern horned frogs that lived in Madagascar during the Cretaceous period, alongside the herbivorous crocodile ''[[UsefulNotes/PrehistoricLifeNonDinosaurianReptiles Simosuchus]]''. It could grow up to sixteen inches in length, and weight over nine pounds, making it larger than any modern frog, including the already gigantic goliath frog of Africa. It's also known as the "devil frog" (which is basically what its name means). Given the eating habits of its modern relatives (which will devour mice) this amphibian could have easily eaten baby dinosaurs. With that said, it still would have lived in fear of large carnivorous dinosaurs such as ''[[UsefulNotes/PrehistoricLifeLargeTheropods Majungasaurus]]'' that also lived in the area.

* Lepospondyls were among the most numerous and diversified "amphibians" in the Carboniferous. They could have been the closest relatives of lissamphibians, and perhaps their ancestors. Usually with long bodies and weak limbs, they lived mainly in water or in soil. Some were like salamanders, ex. the whip-tailed ''Urocordylus''; other were limbless and eel-like, ex. ''Phlegetontia''. But the most interesting one is certainly ''Diplocaulus'', who can be found on the UsefulNotes/StockDinosaursNonDinosaurs page.

* The most successful and diversified group of prehistoric amphibians, temnospondyls are often described in documentary media as "croc-like": however, their jaws and teeth were smaller and weaker than a crocodile's, but they should nonetheless be efficient predators. The most famous is the 8 ft long, alligator-like ''[[UsefulNotes/StockDinosaursNonDinosaurs Eryops]]'' from Early Permian North America (the Dimetrodon's place), but there's also ''Prionosuchus'' ("saw crocodile") from Early Permian Brazil, which resembled an enormous gharial and one reported specimen is estimated to have grown up to ''thirty feet'', making it the largest known temnospondyl. Other Permian relatives were more terrestrial: the dissorophids ''Cacops'' and ''Platyhystrix'' had an armor on their back to protect them from land predators. ''Cacops'' is remarkable for its large head respect to the body and stronger limbs than the more water-living ''Eryops''; ''Platyhystrix'' ("flat porcupine") was similar, but with striking dorsal spines very similar to a ''Dimetrodon'''s or an ''Edaphosaurus'', maybe sustaining a "sail". Even though not a temnospondyl is also worthy of note ''Crassigyrinus scoticus'' ("Scottish fat tadpole"), as an example of how paleo-amphibians were diversified to each other: living in the Carboniferous, it was a very specialized water-loving animal with tiny limbs, just like what happened to some modern salamanders (the olm, the mudpuppy, the amphiumas, the sirens).

* Temnospondyls survived well the huge Permian mass extinction and made their way in the Triassic: only competition with croc relatives at the end of the period caused their decline and their nearly total extinction before the Jurassic. The most famous Triassic amphibian has been the huge massive ''[[UsefulNotes/StockDinosaursNonDinosaurs Mastodonsaurus]]''; very large but much more slender was ''Trematosaurus'', more similar to a gharial in shape. ''[[http://en.wikipedia.org/wiki/Aphaneramma Aphaneramma]]'' looked like the latter, though was not as big: it was a rare example of a ''marine'' amphibian, an unexisting fact in the modern amphibian world made exclusively of freshwater or dryland creatures. ''Gerrothorax'' ("wicker chest") was much smaller than the mastodonsaur, and a bit similar to the unrelated arrow-headed ''Diplocaulus''. Interestigly, it shows neoteny: that is, adults retained the external gills of their larval stage, like the modern [[http://en.wikipedia.org/wiki/Axolotl axolotl]]). Another known neotenic paleoamphibian was the salamander-like ''[[http://en.wikipedia.org/wiki/Branchiosaurus Branchiosaurus]]'' ("gill lizard", nothing to do with the dinosaur ''[[UsefulNotes/StockDinosaursTrueDinosaurs Brachiosaurus]]'', "arm lizard"). Few temnospondyls reached the Late Triassic: among them, the North American ''Metoposaurus'' had eyes put more frontally on its head than its earlier relatives, and was able to see ''[[UsefulNotes/StockDinosaursTrueDinosaurs Coelophysis]]'' in RealLife. In 1997 a new temnospondyl was unexpectedly discovered in Cretaceous terrains: ''Koolasuchus'' ("Koola's croc") was probably an isolated Australian late survivor which managed to resist the competition with freshwater reptiles. It shows both in Series/WalkingWithDinosaurs and in Disney's ''Disney/{{Dinosaur}}'' (rather incorrectly in the latter, in truth).

'''Close to become reptiles:''' ''[[http://en.wikipedia.org/wiki/Proterogyrinus Proterogyrinus]]'', ''[[http://en.wikipedia.org/wiki/Westlothiana Westlothiana]]'', and ''[[http://en.wikipedia.org/wiki/Diadectes Diadectes]]''

* The "amphibians" cited here are collectively called reptiliomorphs ("reptile-shaped"), were more related to amniotes (reptiles + mammals) than to every amphibian mentioned above: indeed, many of them were once variably classified as reptiles or amphibians depending on the scientist. More terrestrial than the latter, they progressively evolved the typical traits of amniotes, but we don't know when these traits appeared exactly: waterproof skin, water-storing lungs and kidneys, and egg shells. The latest invention was crucial for vertebrate evolution: embryos inside shelled eggs were able to develop and to hatch out of water, and the descendants of the reptiliomorphs became able to survive in arid environments. Traditionally the most known reptiliomorph has been ''[[UsefulNotes/StockDinosaursNonDinosaurs Seymouria]]'', but well-known are also the Anthracosaurs or Embolomers, the biggest "amphibians" in the Carboniferous (some reached 9m in length!): ''Eogyrinus'' ("dawn tadpole") and the croc-sized ''Proterogyrinus'' ("first tadpole") are among them. Also in the Carboniferous lived ''Westlothiana lizziae'', a tiny lizard-like animal that was briefly considered "the first reptile" in the 1990s (and familiarly nicknamed [[ALizardNamedLiz "Lizzie"]]). Last example, the Early Permian ''Diadectes'' ("crosswise biter"): similar to an iguana, it was possibly one of the first land vertebrates that evolved (partial or total) vegetarianism.

Cambrian. The first Paleozoic period, in which the famous [[http://en.wikipedia.org/wiki/Cambrian_explosion Cambrian explosion]] of life happened. The less known thing is, we humans are ''unbelievably lucky'' if we know that remote event. At that time, animals just were starting to achieve hard parts in their body, and we already know that, usually, soft-bodied organisms do not preserve at all. The astounding luck is, one of the greatest exceptions of this rule are ''just some deposits from the Cambrian period.'' : it almost seem Ol'Mother Nature [[BecauseDestinySaysSo has done this deliberately for ourselves...]]. The most famous and historically relevant is the [[http://en.wikipedia.org/wiki/Burgess_Shale Burgess Shale]] in British Columbia (first found in early 1900 by Charles Walcott), but still others are also known (for example that of Chengjiang, China). For obvious reason, our curiosity now reaches the top: which were the first animals (not counting protozoans) that thrived in our oceans? Well, the answer is not simple: we can divide them in two ensembles. One is made from those groups either still alive today or extinct several ages after the Cambrian: among the former, most non-arthropod / non-cephalopod invertebrate groups already seen; among the latter, the trilobites. We'll talk here about the second ensemble: many Cambrian invertebrates were indeed ''exclusive of the Cambrian'' and didn't survive long enough, not even to reach the following period, Ordovician - in which the most famous Paleozoic critters: sea scorpions, nautiloids, ostracoderms etc. appeared. Thus, is easy to imagine many of them are classically viewed as RealLife examples of OurMonstersAreWeird by paleo-fanatics. We still know very very few things about their lifestyle, but their appearance is extraordinarily well-known, because these Cambrian deposits ''have preserved soft bodies''; not only that, they have preserved them very well! It would be too long to mention [[http://en.wikipedia.org/wiki/Fossils_of_the_Burgess_Shale all the members]] of the Cambrian fauna: see [[http://www.trilobites.info/triloclass.htm the image here]] for having an idea. The large guy in the center is immediately recognisable, by far the biggest creature in this fauna: ''[[UsefulNotes/StockDinosaursNonDinosaurs Anomalocaris]]''. Of course it is the most portrayed Cambrian animal in documentaries and illustrations, classically mentioned as "[[PrehistoricMonster the first prehistoric monster ever appeared on Earth]]". Actually, if alive today, the "terrible" anomalocaridid would appear simply as a sort of "lobster" 3 ft long, shell-less and pincer-less... nothing dangerous for a tough-boned, tough-muscled, tough-skinned mammal we are in comparison. Apart from ''Anomalocaris'' we can mention other three invertebrates which are stock in drawings: ''[[http://en.wikipedia.org/wiki/Hallucigenia Hallucigenia]]'', ''[[http://en.wikipedia.org/wiki/Opabinia Opabinia]]'', and ''[[http://en.wikipedia.org/wiki/Pikaia Pikaia]]''. The first has a name that means "hallucination generator"; it was a sort of "worm" with long paired spikes for uncertain purpose, and long soft paired legs. The reconstruction of ''Hallucigenia'' has had an astounding ScienceMarchesOn tangle for many years: it was believed ''overturned'', with the dorsal spikes believed actually legs, and the legs pointing ''upwards'' ending with one small ''mouth'' each! Nobody knowns exactly in which phylum it has to be placed: maybe it was a Lobopod, aka a distant arthropod relative. ''Opabinia'' was related to ''Anomalocaris'', but had an even weirder look: maybe no other fossil animal resembles a fiction-related extraterrestrial thing more than ''Opabinia''. It had ''five eyes'' put in circular fashion on its head, and a pincer-like grasp at the end of a long, flexible proboscis, often mistaken for the mouth which was actually located behind the proboscis. When it was first described, many paleontologists didn't believed its describer was serious and openly laughed at it! But the most important find is the third guy, ''Pikaia'': despite its rather insignificant slug-like appearance, it is the most well-known vertebrate ancestor, a sort of prehistoric relative of our [[http://en.wikipedia.org/wiki/Lancelet lancelet]] (the closest extant vertebrate relative). This relevance has made ''Pikaia'' one of the unofficial symbols of evolution, just like the ur-amphibian ''Ichthyostega'', the ur-bird ''Archaeopteryx'' and the ur-horse ''Eohippus''. But wait... have you see these critters in TV at least once? A hard thing, even if you watched ''Series/WalkingWithMonsters''. In this RuleOfCool-filled show, the ''only real'' Cambrian invertebrate to appear is... Guess what? Well, the superpredator ''Anomalocaris'' of course! The other two invertebrate guys shown up are... a modern jellyfish and an [[AnachronismStew anachronistical phacopid trilobite]] - remember that phacopids first evolved in the Ordovician, while Cambrian trilobites looked very differently to the classic image we have when thinking about these animals. The absence of such awesome animals like ''Opabinia'' and ''Hallucigenia'' -- and still others, like the multi-tentacled ''[[http://en.wikipedia.org/wiki/Wiwaxia Wiwaxia]]'', the trilobite-like ''[[http://en.wikipedia.org/wiki/Marrella Marrella]]'', the caterpillar-looking ''[[http://en.wikipedia.org/wiki/Aysheaia Aysheaia]]'', the lobster-looking ''[[http://en.wikipedia.org/wiki/Sidneyia Sidneyia]]'', the "hairy worm" ''[[http://en.wikipedia.org/wiki/Canadia Canadia]]'', etc. -- makes another egregious example of a [[TheyWastedAPerfectlyGoodPlot missed opportunity]]. Talking about ''Pikaia'', this time it's missing is no problem for us: the aforementioned proto-vertebrate ''Haikouichthys'' did perform its role. Still another thing: if you watch the list of creatures from the Burgess Shale, you'll note almost all animals (the main exception being ''Anomalocaris'') have uncommonly short scientific names, most of them ending in '''-a'''. A rather amusing thing to read, and - let's face it - a ''true'' oasis of happiness among so many other unutterable, absurdly-difficult names.

* If you ask a paleontologist what are the most common vertebrate fossils, (s)he'll answer you "shark teeth". However, these teeth are usually found isolated - ironically, the remaining skeletons are among the rarest fossil finds (sharks' cartilage don't fossilize well, unlike bone). Even though sharks made only a small percentage of the modern fish species (about 5%), they were a very successful group in the past, even more diversified than they are today. But stop a moment. What is a shark, exactly? In common sense, sharks are things like the great white or the bull shark; but zoologists often use this word to indicate every cartilaginous fish, or more technically, every Chondrichthyian. Palaeontologists usually give the same meaning to "shark", too. So, the first ever sharks appeared in the Devonian period 400 million years ago, and since then have changed very little: sharks are often cited as "living fossils". More precisely, the modern kinds of shark appeared in the Cretaceous: earlier sharks were only distantly related with them. Examples are ''Cladoselache'', ''Stethacanthus'', and ''Xenacanthus'' (once also called, ''Pleuracanthus''), all from the Devonian period, and also a little known modern fish, the deep sea chimaera (also called ratfish or rabbitfish). The latter has preserved to this day the mixed shark/bony fish anatomy of the acanthodians and the earliest rayfins. ''Cladoselache'' ("branch shark") was more like a true shark, with uncovered gills and tooth-like scales covered in enamel (its fossils have preserved prints of soft tissues), but its mouth was strangely placed at the top of its head like the modern unrelated whale-shark. ''Xenacanthus'' and ''Stethacanthus'' were more bizarre. The former had an eel-body and a long filament protruding from its head (its name means "strange spine"); the latter is nicknamed "ironing-board shark" from its flat head prominence (not a dorsal fin) with many dentines on the top. Maybe only males had these things, which resemble the much smaller tubercle of the male chimaera (if so, they would have been courtship devices). But the perhaps weirdest "shark" ever is ''Helicoprion'' ("helix saw"), the "whorl-toothed shark", whose teeth were placed in a spiral line inside its mouth! It now appears that both ''Helicoprion'' and ''Stethacanthus'' may actually belong to the chimaera lineage, thus not proper sharks. Other early chondrichthyans, ''Falcatus'', ''Harpagofutator'', and many others, also showed strange prominences above their heads.

'''The earliest "sharks":''' ''[[http://en.wikipedia.org/wiki/Cladoselache Cladoselache]]'', ''[[http://en.wikipedia.org/wiki/Stethacanthus Stethacanthus]]'', ''[[http://en.wikipedia.org/wiki/Xenacanthus Xenacanthus]]'', ''[[http://en.wikipedia.org/wiki/Pleuracanthus Pleuracanthus]]'', and ''[[http://en.wikipedia.org/wiki/Helicoprion Helicoprion]]''

* If you ask a paleontologist what are the most common vertebrate fossils, (s)he'll answer you "shark teeth". However, these teeth are usually found isolated - ironically, the remaining skeletons are among the rarest fossil finds (sharks' cartilage don't fossilize well, unlike bone). Even though sharks made only a small percentage of the modern fish species (about 5%), they were a very successful group in the past, even more diversified than they are today. But stop a moment. What is a shark, exactly? In common sense, sharks are things like the great white or the bull shark; but zoologists often use this word to indicate every cartilaginous fish, or more technically, every Chondrichthyian. Palaeontologists usually give the same meaning to "shark", too. So, the first ever sharks appeared in the Devonian period 400 million years ago, and since then have changed very little: sharks are often cited as "living fossils". More precisely, the modern kinds of shark appeared in the Cretaceous: earlier sharks were only distantly related with them. Examples are ''Cladoselache'', ''Stethacanthus'', ''Xenacanthus'', ''Pleuracanthus'' (all from the Devonian period), and also a little known modern fish, the deep sea chimaera (also called ratfish or rabbitfish). The latter has preserved to this day the mixed shark/bony fish anatomy of the acanthodians and the earliest rayfins. ''Cladoselache'' ("branch shark") was more like a true shark, with uncovered gills and tooth-like scales covered in enamel (its fossils have preserved prints of soft tissues), but its mouth was strangely placed at the top of its head like the modern unrelated whale-shark. ''Xenacanthus'' and ''Stethacanthus'' were more bizarre. The former had an eel-body and a long filament protruding from its head (its name means "strange spine"); the latter is nicknamed "ironing-board shark" from its flat head prominence (not a dorsal fin) with many dentines on the top. Maybe only males had these things, which resemble the much smaller tubercle of the male chimaera (if so, they would have been courtship devices). But the perhaps weirdest "shark" ever is ''Helicoprion'' ("helix saw"), the "whorl-toothed shark", whose teeth were placed in a spiral line inside its mouth! It now appears that both ''Helicoprion'' and ''Stethacanthus'' may actually belong to the chimaera lineage, thus not proper sharks. Other early chondrichthyans, ''Falcatus'', ''Harpagofutator'', and many others, also showed strange prominences above their heads.

'''The earliest "sharks":''' ''[[http://en.wikipedia.org/wiki/Cladoselache Cladoselache]]'', ''[[http://en.wikipedia.org/wiki/Stethacanthus Stethacanthus]]'', ''[[http://en.wikipedia.org/wiki/Xenacanthus Xenacanthus]]'', and ''[[http://en.wikipedia.org/wiki/Helicoprion Helicoprion]]''

* If you ask a paleontologist what are the most common vertebrate fossils, (s)he'll answer you "shark teeth". However, these teeth are usually found isolated - ironically, the remaining skeletons are among the rarest fossil finds (sharks' cartilage don't fossilize well, unlike bone). Even though sharks made only a small percentage of the modern fish species (about 5%), they were a very successful group in the past, even more diversified than they are today. But stop a moment. What is a shark, exactly? In common sense, sharks are things like the great white or the bull shark; but zoologists often use this word to indicate every cartilaginous fish, or more technically, every Chondrichthyian. Palaeontologists usually give the same meaning to "shark", too. So, the first ever sharks appeared in the Devonian period 400 million years ago, and since then have changed very little: sharks are often cited as "living fossils". More precisely, the modern kinds of shark appeared in the Cretaceous: earlier sharks were only distantly related with them. Examples are ''Cladoselache'', ''Stethacanthus'', and ''Xenacanthus'' (all from the Devonian period), and also a little known modern fish, the deep sea chimaera (also called ratfish or rabbitfish). The latter has preserved to this day the mixed shark/bony fish anatomy of the acanthodians and the earliest rayfins. ''Cladoselache'' ("branch shark") was more like a true shark, with uncovered gills and tooth-like scales covered in enamel (its fossils have preserved prints of soft tissues), but its mouth was strangely placed at the top of its head like the modern unrelated whale-shark. ''Xenacanthus'' and ''Stethacanthus'' were more bizarre. The former had an eel-body and a long filament protruding from its head (its name means "strange spine"); the latter is nicknamed "ironing-board shark" from its flat head prominence (not a dorsal fin) with many dentines on the top. Maybe only males had these things, which resemble the much smaller tubercle of the male chimaera (if so, they would have been courtship devices). But the perhaps weirdest "shark" ever is ''Helicoprion'' ("helix saw"), the "whorl-toothed shark", whose teeth were placed in a spiral line inside its mouth! It now appears that both ''Helicoprion'' and ''Stethacanthus'' may actually belong to the chimaera lineage, thus not proper sharks. Other early chondrichthyans, ''Falcatus'', ''Harpagofutator'', and many others, also showed strange prominences above their heads.

* These are the Mesozoic seed plants that more resemble the ones seen in the fictional Dinosaur Age. They ''did'' resemble palm trees, but were NOT related at all with them. Cycads are the only ones still living, often used to embellish our cities. Bennettitales or cycadeoids ("pseudo-cycads") went extinct at the end of the Mesozoic. These two groups were very abundant at the dinosaur times, but were already present before the Triassic (as well as the little-known Cordaitales, maybe the ancestors of the conifers). About pteridosperms (the "seed ferns"), they are so called because of their external look, but were ''not'' true ferns: ferns do reproduce with spores. Seed ferns were among the very first seed plants ever appeared, in the Devonian, and survived until the Cretaceous. One of them, the Triassic ''[[http://en.wikipedia.org/wiki/Glossopteris Glossopteris]]'', has been used to demonstrate the Pangaea hypothesis: its fossils have been found in every southern continent, showing landmasses were still united at the start of the Dinosaur Age (see also ''[[UsefulNotes/PrehistoricLifeNonDinosaurianReptiles Mesosaurus]]'' in another page).

Cambrian. The first Paleozoic period, in which the famous [[http://en.wikipedia.org/wiki/Cambrian_explosion Cambrian explosion]] of life happened. The less known thing is, we humans are ''unbelievably lucky'' if we know that remote event. At that time, animals just were starting to achieve hard parts in their body, and we already know that, usually, soft-bodied organisms do not preserve at all. The astounding luck is, one of the greatest exceptions of this rule are ''just some deposits from the Cambrian period.'' : it almost seem Ol'Mother Nature [[BecauseDestinySaysSo has done this deliberately for ourselves...]]. The most famous and historically relevant is the [[http://en.wikipedia.org/wiki/Burgess_Shale Burgess Shale]] in British Columbia (first found in early 1900 by Charles Walcott), but still others are also known (for example that of Chengjiang, China). For obvious reason, our curiosity now reaches the top: which were the first animals (not counting protozoans) that thrived in our oceans? Well, the answer is not simple: we can divide them in two ensembles. One is made from those groups either still alive today or extinct several ages after the Cambrian: among the former, most non-arthropod / non-cephalopod invertebrate groups already seen; among the latter, the trilobites. We'll talk here about the second ensemble: many Cambrian invertebrates were indeed ''exclusive of the Cambrian'' and didn't survive long enough, not even to reach the following period, Ordovician - in which the most famous Paleozoic critters: sea scorpions, nautiloids, ostracoderms etc. appeared. Thus, is easy to imagine many of them are classically viewed as RealLife examples of OurMonstersAreWeird by paleo-fanatics. We still know very very few things about their lifestyle, but their appearance is extraordinarily well-known, because these Cambrian deposits ''have preserved soft bodies''; not only that, they have preserved them very well! It would be too long to mention [[http://en.wikipedia.org/wiki/Fossils_of_the_Burgess_Shale all the members]] of the Cambrian fauna: see [[http://www.trilobites.info/triloclass.htm the image here]] for having an idea. The large guy in the center is immediately recognisable, by far the biggest creature in this fauna: ''[[UsefulNotes/StockDinosaursNonDinosaurs Anomalocaris]]''. Of course it is the most portrayed Cambrian animal in documentaries and illustrations, classically mentioned as "[[PrehistoricMonster the first prehistoric monster ever appeared on Earth]]". Actually, if alive today, the "terrible" anomalocaridid would appear simply as a sort of "lobster" 3 ft long, shell-less and pincer-less... nothing dangerous for a tough-boned, tough-muscled, tough-skinned mammal we are in comparison. Apart from ''Anomalocaris'' we can mention other three invertebrates which are stock in drawings: ''[[http://en.wikipedia.org/wiki/Hallucigenia Hallucigenia]]'', ''[[http://en.wikipedia.org/wiki/Opabinia Opabinia]]'', and ''[[http://en.wikipedia.org/wiki/Pikaia Pikaia]]''. The first has a name that means "hallucination generator"; it was a sort of "worm" with long paired spikes for uncertain purpose, and long soft paired legs. The reconstruction of ''Hallucigenia'' has had an astounding ScienceMarchesOn tangle for many years: it was believed ''overturned'', with the dorsal spikes believed actually legs, and the legs pointing ''upwards'' ending with one small ''mouth'' each! Nobody knowns exactly in which phylum it has to be placed: maybe it was a Lobopod, aka a distant arthropod relative. ''Opabinia'' was related to ''Anomalocaris'', but had an even weirder look: maybe no other fossil animal resembles a fiction-related extraterrestrial thing more than ''Opabinia''. It had ''five eyes'' put in circular fashion on its head, and a pincer-like grasp at the end of a long, flexible proboscis, often mistaken for the mouth which was actually located behind the proboscis. When it was first described, many paleontologists didn't believed its describer was serious and openly laughed at it! But the most important find is the third guy, ''Pikaia'': despite its rather insignificant slug-like appearance, it is the most well-known vertebrate ancestor, a sort of prehistoric relative of our [[http://en.wikipedia.org/wiki/Lancelet lancelet]] (the closest extant vertebrate relative). This relevance has made ''Pikaia'' one of the unofficial symbols of evolution, just like the ur-amphibian ''Ichthyostega'', the ur-bird ''Archaeopteryx'' and the ur-horse ''Eohippus''. But wait... have you see these critters in TV at least once? A hard thing, even if you watched ''Series/WalkingWithMonsters''. In this RuleOfCool-filled show, the ''only real'' Cambrian invertebrate to appear is... Guess what? Well, the superpredator ''Anomalocaris'' of course! The other two invertebrate guys shown up are... a modern jellyfish and an [[AnachronismStew anachronistical phacopid trilobite]] - remember that phacopids first evolved in the Ordovician, while Cambrian trilobites looked very differently to the classic image we have when thinking about these animals. The absence of such awesome animals like ''Opabinia'' and ''Hallucigenia'' -- and still others, like the multi-tentacled ''[[http://en.wikipedia.org/wiki/Wiwaxia Wiwaxia]]'', the trilobite-like ''[[http://en.wikipedia.org/wiki/Marrella Marrella]]'', the caterpillar-looking ''[[http://en.wikipedia.org/wiki/Aysheaia Aysheaia]]'', the lobster-looking ''[[http://en.wikipedia.org/wiki/Sidneyia Sidneyia]]'', the "hairy worm" ''[[http://en.wikipedia.org/wiki/Canadia Canadia]]'', etc. -- makes another egregious example of a missed opportunity. Talking about ''Pikaia'', this time it's missing is no problem for us: the aforementioned proto-vertebrate ''Haikouichthys'' did perform its role. Still another thing: if you watch the list of creatures from the Burgess Shale, you'll note almost all animals (the main exception being ''Anomalocaris'') have uncommonly short scientific names, most of them ending in '''-a'''. A rather amusing thing to read, and - let's face it - a ''true'' oasis of happiness among so many other unutterable, absurdly-difficult names.

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* [[https://en.wikipedia.org/wiki/RNA_world The RNA world]] about as far as we can look back. The RNA world is a well founded hypothesis - it has enough evidence to be generally believed, but insufficient to be considered proven. In the modern world, DNA codes genes. Genes are copied to messenger RNA. Messenger RNA is read by ribosomes to create proteins. Almost all the functions of the cell are performed by proteins, including duplicating DNA, copying DNA to RNA, and most of the ribosome is protein. This works great, but how could it start? DNA can't replicate without proteins, and proteins can't be made without the instructions in DNA. However, RNA can both store information ''and'' catalyse reactions. Although it would be grossly inefficient by modern standards, life based on RNA with no DNA and no proteins is quite plausible. The RNA world hypothesis is that such RNA life existed, and is ancestral to modern life. Evidence for this is functional RNA (RNA which does stuff directly, rather than simply being instructions to ribosomes) performs just a few cellular functions, but very central critical ones. Most central is that RNA forms the reactive core of the ribosomes.

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* Triassic and Jurassic sharks were still primitive, but with a more modern look. One of the most common was ''Hybodus'' ("humped tooth"), whose shape recalls a typical predatory shark but with small "horns" on its head, like the modern bull-head shark. This is the shark portrayed in the "Walking With" series in the two episodes in which ''[[UsefulNotes/StockDinosaursNonDinosaurs Liopleurodon]]'' is the main character, acting in both as a minor predator. Other relatives became flatter and similar to a ray: ''Ptychodus'' is one example. If we imagine to swim in Cretaceous waters, however, we'll see sharks virtually identical to the modern ones. ''Cretoxyrhina'' (nicknamed "the Ginsu Shark") was very similar to a great white shark. It was one of the top predators of the Late Cretaceous North American inland sea, in competition with [[UsefulNotes/StockDinosaursNonDinosaurs mosasaurs, plesiosaurs]], and the bony fish ''Xiphactinus'' above. But we could also encounter another kind of fish which little resembles a shark, but technically ''is'' a true shark: rays and skates. They appeared in the Cretaceous, in the same time of the commonly intended "sharks", and also little changed their anatomy during the times.

* Triassic and Jurassic sharks were still primitive, but with a more modern look. One of the most common was ''Hybodus'', whose shape recalls a typical predatory shark but with small "horns" on its head, like the modern bull-head shark. This is the shark portrayed in the "Walking With" series in the two episodes in which ''[[UsefulNotes/StockDinosaursNonDinosaurs Liopleurodon]]'' is the main character, acting in both as a minor predator. Other relatives became flatter and similar to a ray: ''Ptychodus'' is one example. If we imagine to swim in Cretaceous waters, however, we'll see sharks virtually identical to the modern ones. ''Cretoxyrhina'' (nicknamed "the Ginsu Shark") was very similar to a great white shark. It was one of the top predators of the Late Cretaceous North American inland sea, in competition with [[UsefulNotes/StockDinosaursNonDinosaurs mosasaurs, plesiosaurs]], and the bony fish ''Xiphactinus'' above. But we could also encounter another kind of fish which little resembles a shark, but technically ''is'' a true shark: rays and skates. They appeared in the Cretaceous, in the same time of the commonly intended "sharks", and also little changed their anatomy during the times.

* Triassic and Jurassic sharks were still primitive, but with a more modern look. One of the most common was ''Hybodus'', whose shape recalls a typical predatory shark but with small "horns" on its head, like the modern bull-head shark. Other relatives became flatter and similar to a ray: ''Ptychodus'' is one example. If we imagine to swim in Cretaceous waters, however, we'll see sharks virtually identical to the modern ones. ''Cretoxyrhina'' (nicknamed "the Ginsu Shark") was very similar to a great white shark. It was one of the top predators of the Late Cretaceous North American inland sea, in competition with [[UsefulNotes/StockDinosaursNonDinosaurs mosasaurs, plesiosaurs]], and the bony fish ''Xiphactinus'' above. But we could also encounter another kind of fish which little resembles a shark, but technically ''is'' a true shark: rays and skates. They appeared in the Cretaceous, in the same time of the commonly intended "sharks", and also little changed their anatomy during the times.

* If you ask a paleontologist what are the most common vertebrate fossils, (s)he'll answer you "shark teeth". However, these teeth are usually found isolated - ironically, the remaining skeletons are among the rarest fossil finds (sharks' cartilage don't fossilize well, unlike bone). Even though sharks made only a small percentage of the modern fish species (about 5%), they were a very successful group in the past, even more diversified than they are today. But stop a moment. What is a shark, exactly? In common sense, sharks are things like the great white or the bull shark; but zoologists often use this word to indicate every cartilaginous fish, or more technically, every Chondrichthyian. Palaeontologists usually give the same meaning to "shark", too. So, the first ever sharks appeared in the Devonian period 400 million years ago, and since then have changed very little: sharks are often cited as "living fossils". More precisely, the modern kinds of shark appeared in the Cretaceous: earlier sharks were only distantly related with them. Examples are ''Cladoselache'', ''Stethacanthus'', and ''Xenacanthus'' (all from the Devonian period), and also a little known modern fish, the deep sea chimaera (also called ratfish or rabbitfish). The latter has preserved to this day the mixed shark/bony fish anatomy of the acanthodians and the earliest rayfins. ''Cladoselache'' ("branch shark") was more like a true shark, with uncovered gills and tooth-like scales covered in enamel (its fossils have preserved prints of soft tissues), but its mouth was strangely placed at the top of its head like the modern unrelated whale-shark. ''Xenacanthus'' and ''Stethacanthus'' were more bizarre. The former had an eel-body and a long filament protruding from its head (its name means "strange spine"); the latter is nicknamed "ironing-board shark" from its flat head prominence with many dentines on the top. Maybe only males had these things, which resemble the much smaller tubercle of the male chimaera (if so, they would have been courtship devices). But the perhaps weirdest "shark" ever is ''Helicoprion'' ("helix saw"), whose teeth were placed in a spiral line inside its mouth![[note]]It now appears that both ''Helicoprion'' and ''Stethacanthus'' may actually belong to the chimaera lineage.[[/note]]. Other early chondrichthyans, ''Falcatus'', ''Harpagofutator'', and many others, also showed strange prominences above their heads.

'''Shark Tales:''' ''[[http://en.wikipedia.org/wiki/Hybodus Hybodus]]'', ''[[http://en.wikipedia.org/wiki/Ptychodus Ptychodus]]'', and ''[[http://en.wikipedia.org/wiki/Cretoxyrhina Cretoxyrhina]]''

* Even though more closely related with US than with true fish, lungfish and coelacanths were less close to terrestrial vertebrates than some fossil lobefins, the basal tetrapodomorphs (called "rhipidists" or "Osteolepidotes" in older sources). Among them, there was the common ancestor of all tetrapods aka land vertebrates, but we don't know which was really ''the'' ancestor. The traditionally most cited is ''[[UsefulNotes/StockDinosaursNonDinosaurs Eusthenopteron]]'': other relatives of it included ''[[http://en.wikipedia.org/wiki/Holoptychius Holoptychius]]'' and ''Osteolepis'', both with a more normal-looking, non-"trident" tailfin. In the 1990s/2000s some new animals were discovered, which appear evolutionarily in the middle between a ''Eusthenopteron'' and an ''Ichthyostega'': excellent examples are ''Panderichthys'', ''Tiktaalik'', and ''Acanthostega''. The latest one was very similar to the ichthyostega, but had ''eight'' digits on each "hand" instead of seven.

'''Tough guys:''' ''[[http://en.wikipedia.org/wiki/Bothriolepis Bothriolepis]]'', ''[[http://en.wikipedia.org/wiki/Coccosteus Coccosteus]]'', ''[[http://en.wikipedia.org/wiki/Lunaspis Lunaspis]]'', and the other Placoderms

'''Fish crawled onto land:''' ''[[http://en.wikipedia.org/wiki/Osteolepis Osteolepis]]'', ''[[http://en.wikipedia.org/wiki/Panderichthys Panderichthys]]'', ''[[http://en.wikipedia.org/wiki/Tiktaalik Tiktaalik]]'', and ''[[http://en.wikipedia.org/wiki/Acanthostega Acanthostega]]''

* Welcome to the Aspis family. Most "ostracoderms" ("shelled skin") have this suffix. Despite this, they ''don't'' make a real fish group: every armored fish devoid of jaws is traditionally called with this name, but they are actually distinct lineages, some closer to jawed fish than to other ostracoderms. Compared with placoderms, ostracoderms' armor was more complete, covering the whole body, and formed a true shell at the head level. Despite their badass look they were very harmless creatures; their size was from a thumbnail up to a human hand, and with their jawless mouths they could only have feed on tiny food items like algae, small invertebrates, etc... They often fell victim to predators like the eurypterids ("sea scorpions"), cephalopods and jawed fish that were strong enough to go through their thick armored skin. One successful ostracoderm lineage is the Osteostracans ("bony shell"), whose prototype is the flat-headed ''[[UsefulNotes/StockDinosaursNonDinosaurs Cephalaspis]]'' ("head shield") but include also ''Hemicyclaspis'' ("semicircle shield") among the others. Another is the Anaspids ("with no shield"), more streamlined and with a lighter armor: ''[[http://en.wikipedia.org/wiki/Birkenia Birkenia]]'' was a typical representant. The Heterostracans ("different shell") had often long snouts; ''[[UsefulNotes/StockDinosaursNonDinosaurs Pteraspis]]'' ("winged shield") is their prototype. ''Drepanaspis'' was a round-shaped example of them; ''Arandaspis'' and ''Astraspis'' were related with the heterostracans. Finally, the Thelodonts, which where the most closely related with jawed fish (and maybe their ancestors).

'''The Aspis family:''' ''[[http://en.wikipedia.org/wiki/Hemicyclaspis Hemicyclaspis]]'', ''[[http://en.wikipedia.org/wiki/Arandaspis Arandaspis]]'', ''[[http://en.wikipedia.org/wiki/Drepanaspis Drepanaspis]]'', and the other "Ostracoderms"

* Welcome to the Aspis family. Most "ostracoderms" ("shelled skin") have this suffix. Despite this, they ''don't'' make a real fish group: every armored fish devoid of jaws is traditionally called with this name, but they are actually distinct lineages, some closer to jawed fish than to other ostracoderms. Compared with placoderms, ostracoderms' armor was more complete, covering the whole body, and formed a true shell at the head level. Despite their badass look they were very harmless creatures; their size was from a thumbnail up to a human hand, and with their jawless mouths they could only have feed on tiny food items like algae, small invertebrates, etc... They often fell victim to predators like the eurypterids ("sea scorpions"), cephalopods and jawed fish that were strong enough to go through their thick armored skin. One successful ostracoderm lineage is the osteostracans ("bony shell"), whose prototype is the flat-headed ''[[UsefulNotes/StockDinosaursNonDinosaurs Cephalaspis]]'' ("head shield") but include also ''Hemicyclaspis'' ("semicircle shield") among the others. Another is the anaspids ("with no shield"), more streamlined and with a lighter armor: ''[[http://en.wikipedia.org/wiki/Birkenia Birkenia]]'' was a typical representant. The heterostracans ("different shell") had often long snouts; ''[[UsefulNotes/StockDinosaursNonDinosaurs Pteraspis]]'' ("winged shield") is their prototype, ''Drepanaspis'' was a round-shaped example of them. Finally, the Thelodonts, which where the most closely related with jawed fish (and maybe their ancestors).