History UsefulNotes / PrehistoricLifeOtherExtinctCreatures

* But what ''was'' this thing? Found in 1966, and very common in the Carboniferous Mazon Creek Beds of Illinois, it looked like nothing paleontologists had ever seen before. It had a jaw like a crab claw, the body plan of a cuttlefish, and two weird stalked lumps on both sides of its body. The only thing anyone could classify this as was as an invertebrate, probably some kind of worm. [[ScienceMarchesOn Until 2016]]. A study by Yale University revealed the Tully Monster's true identity, and it was the last one anyone expected. 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 concluded that it was a jawless "fish", closely related to the lamprey.

* Fictional media would typically have us believe the trees from the Mesozoic were all palm-shaped. If non-palmlike trees appear, they usually have the shape of an angiosperm. Pines, firs, spruces, larches, cypresses and cedars are usually unseen in Fictionland... possibly because are usually associated with ''cold'' in the writers' minds (while OneMillionBC is ''always'' a very hot world filled with volcanoes). In RealLife things were very different. Conifers were among the dominant plant groups for the whole Mesozoic, and a common food source for camarasaurs, camptosaurs, centrosaurs, cetiosaurs, chasmosaurs, corythosaurs, and so on. However, there weren't just pines and firs [[note]]or rather, their ancestors were adapted to a warmer climate than the modern ones are[[/note]] at the time: more common were some kinds of conifers which are rare or extinct today. For example, the araucarians (monkey puzzles), the podocarps, the yews, and the most spectacular of all, the sequoia[[note]]One sequoia was nicknamed "Mammoth Tree" for its size, but a mammoth would be a midget next to its green namesake![[/note]]. If you think sauropods were the real titans of the Jurassic, think again: a ''[[UsefulNotes/StockDinosaursTrueDinosaurs Giraffatitan]]'' near a redwood would look like a house cat next to a full-grown man.

* This is a merciless law even in palaeontology. With rare exception, only hard parts of organisms' bodies fossilize: bones, shells, armor, jaws, teeth, and whatnot. Many surviving invertebrate groups which don't have such tough elements have left little if any fossil record. For examples, most "worms" (annelids aka segmented worms, nematodes aka round worms, platyhelmintes aka flatworms, nemertheans aka ribbonworms, the microscopic rotifers aka "wheel-animals", the equally tiny tardigrades aka "water-bears", which thankfully have a few fossils in amber from the Americas, and many others) are virtually unknown in paleontology, and their evolution can only be guessed at. This is also true for coelenterates - that is, the cnidarians (jellyfish, sea anemones, hydrae, siphonophores) and the ctenophores (comb jellies). At least, the totally soft ones; fortunately, corals (which are also cnidarians) build tough external skeletons that fossilize well, and their extinct relatives are well-known from the start of the Paleozoic. The same is true of the unfamiliar bryozoans (moss animals), very similar to corals, but more closely related to the brachiopods. Finally, the enigmatic sponges (which many zoologists scarcely consider real animals) are also common fossils. They too have had an inner "skeleton" made of limestone (and sometimes glass-like silicon or a horny material, but these usually don't fossilize); one extinct group of possible sponge relatives were the [[http://en.wikipedia.org/wiki/Archaeocyatha archaeocyathans]], which became the planet's first reef-building animals in the Early Cambrian.

* This is a merciless law even in palaeontology. With rare exception, only hard parts of organisms' bodies fossilize: bones, shells, armor, jaws, teeth, and whatnot. Many surviving invertebrate groups which don't have such tough elements have left little if any fossil record. For examples, most "worms" (annelids aka segmented worms, nematodes aka round worms, platyhelmintes aka flatworms, nemertheans aka ribbonworms, the microscopic rotifers aka "wheel-animals", the equally tiny tardigrades aka "water-bears", and many others) are virtually unknown in paleontology, and their evolution can only be guessed at. This is also true for coelenterates - that is, the cnidarians (jellyfish, sea anemones, hydrae, siphonophores) and the ctenophores (comb jellies). At least, the totally soft ones; fortunately, corals (which are also cnidarians) build tough external skeletons that fossilize well, and their extinct relatives are well-known from the start of the Paleozoic. The same is true of the unfamiliar bryozoans (moss animals), very similar to corals, but more closely related to the brachiopods. Finally, the enigmatic sponges (which many zoologists scarcely consider real animals) are also common fossils. They too have had an inner "skeleton" made of limestone (and sometimes glass-like silicon or a horny material, but these usually don't fossilize); one extinct group of possible sponge relatives were the [[http://en.wikipedia.org/wiki/Archaeocyatha archaeocyathans]], which became the planet's first reef-building animals in the Early Cambrian.

* 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 bullhead 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 derived than the ones above but more primitive than most our-day sharks. If we imagine swimming in Cretaceous waters, however, we'll see sharks virtually identical to 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 might also encounter other kinds of fish which little resemble sharks, but technically ''are'' true sharks: rays and skates. They appeared in the Cretaceous, during the same time as those fish commonly called sharks, and have also little changed their anatomy since then.

* If you ask a paleontologist what are the most common vertebrate fossils, s/he'll answer "shark teeth". However, these teeth are usually found isolated - ironically, the remaining skeletons are among the rarest fossil finds (sharks' cartilage doesn't fossilize well, unlike bone). Even though sharks make up only a small percentage of 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. 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 to 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 front of its head like the modern, unrelated whale shark. ''Xenacanthus'' and ''Stethacanthus'' were more bizarre. The former had an eel-like 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. Possibly 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 arranged in a spiral coil 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.

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 composed mostly of fossilized 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're most likely to see in media - at least, documentary media; you'll [[SmallTaxonomyPools rarely see]] a trilobite, ammonite, sea scorpion or nautiloid in Fictionland. If it happens, they'll be simple "ambient critters", and good luck if the animal [[NoNameGiven is mentioned]]. There isn't much to say about the portrayal of extinct non-arthropod/non-cephalopod invertebrates in media: they rarely appear even in books and 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 ''WesternAnimation/{{Fantasia}}'' showed several ''modern'' critters to symbolize the early evolution of invertebrates, but [[SmallTaxonomyPools few or no]] ''truly'' prehistoric ones. The ''Walking With'' series did the same: only modern jellyfish, sponges and sea urchins appear, all live-action. 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 will 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.

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 composed mostly of fossilized 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're most likely to see in media - at least, documentary media; you'll [[SmallTaxonomyPools rarely see]] a trilobite, ammonite, sea scorpion or nautiloid in Fictionland. If it happens, they'll be simple "ambient critters", and good luck if the animal [[NoNameGiven is mentioned]]. There isn't much to say about the portrayal of extinct non-arthropod/non-cephalopod invertebrates in media: they [[SeldomSeenSpecies rarely appear]] even in books and 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 ''WesternAnimation/{{Fantasia}}'' showed several ''modern'' critters to symbolize the early evolution of invertebrates, but [[SmallTaxonomyPools few or no]] ''truly'' prehistoric ones. The ''Walking With'' series did the same: only modern jellyfish, sponges and sea urchins appear, all live-action. 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 will 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.

* Prior to 1956, the general consensus was that complex life didn't really start appearing until the Cambrian explosion, but a landmark discovery in England that year proved just how wrong that was when the 560-million-year-old organism ''[[https://en.wikipedia.org/wiki/Charnia Charnia]]'' was found. Since then, a myriad of different creatures have been discovered from the so-called Ediacaran period that existed between the [[https://en.wikipedia.org/wiki/Cryogenian Cryogenian Glaciation]] and the Cambrian explosion. Many experts think they were [[OurMonstersAreWeird neither plants nor animals]], but something else entirely. Notable groups include the uniquely tri-radial [[https://en.wikipedia.org/wiki/Trilobozoa Trilobozoa]], the ribbed oval "bags" knows as [[https://en.wikipedia.org/wiki/Proarticulata Proarticulata]], the fern- or sea pen-like [[https://en.wikipedia.org/wiki/Rangeomorph Rangeomorpha]] of which ''Charnia'' was a member, and the, um... [[BuffySpeak "fleshy spiral"]] ''[[https://en.wikipedia.org/wiki/Eoandromeda Eoandromeda]]''. But despite all these Ediacaran Biota (often improperly called "Ediacaran Fauna") being wildly different in appearance, they do have things in common that define life from this era: they almost universally exhibit [[https://en.wikipedia.org/wiki/Glide_reflection glide reflection]] in their body plan, they appear completely unrelated to modern animals/plants/fungi as mentioned above, they almost completely disappeared at the end of the Ediacaran period, and they were arguably completely sessile, probably either filtering nutrients out of the water or "growing" on top of microbial mats on which they fed on through "roots". This unique aspect has caused the group to sometimes be referred to as "the Garden of Ediacara". More of what we would call traditional animals also started to appear toward the end of the period, such as the slug-esque ''[[https://en.wikipedia.org/wiki/Kimberella Kimberella]]'' (which might also have been one of the first creatures capable of locomotion), the arthropod/trilobite like ''[[https://en.wikipedia.org/wiki/Spriggina Spriggina]]'' and the Cnidarian/jellyfish relative known as ''[[https://en.wikipedia.org/wiki/Haootia Haootia]]'', which was rather similar to the modern freshwater [[https://en.wikipedia.org/wiki/Hydra_viridis Green Hydra]].

* One of the worst errors in popular media (sometimes even documentary media) is to portray ''grasslands'' [[MisplacedVegetation in the Mesozoic]]. Arguably, writers think grass is the simplest kind of plant ever... thus, the first ever to appear on Earth. As seen above, grass are actually some of the ''most derived'' plants and among the latest to become widespread on our planet. In the Dinosaur Age, the dominant small land plants were much, much more primitive: ferns, their close relatives, and horsetails. These are collectively called pteridophytes, and their modern descendants still make up a wide portion of the undergrowth in many forests. In the Mesozoic, ferns and horsetails already made up much forest undergrowth, but also formed true prairies where trees were absent. Only in the Age of Mammals were fern prairies definitively replaced by grasslands. But don't think ferns and horsetails were always small: in Prehistoria there were also giant horsetails and tree ferns, both deceptively similar to trees. In some places, they still live today: the 10m tall ''[[http://en.wikipedia.org/wiki/Equisetum_giganteum Equisetum giganteum]]'' is found in tropical landscapes. And tree ferns are still present in Australia and New Zealand - to the point that in the latter country they have become a national symbol, just like the kiwi bird. Indeed, the LandDownUnder and its neighbor archipelago are a real mine of "living fossils", not limited to the platypus, the echidna, the kiwi, or the tuatara.

* These conifer-looking trees were a common sight in the Paleozoic era from the late Devonian period up to the early Permian period, with the Carboniferous period being the peak of their diversity. Despite their looks, however, ''Calamites'' were actually distant relatives of modern horsetails, and thus, like with other early plants like it, were strictly tied to swampy environments. Their trunks were usually hollow and resemble modern bamboo with vertical growth lines from the base to the canopy, and each segment contained rows of branches with 25 needle-shaped leaves each. They were known to reproduce by directly cloning themselves from underground root structures called rhizomes, the only plant of its age to even do so, which allowed them to remain anchored to loose wet ground and spread rapidly. Their resemblance to Christmas trees lead to many paleontologists joking about if a time traveler were to spend the Christmas holidays in the Carboniferous, ''Calamites'' are the way to go.

* Sometimes it seems paleontologists have fun making cryptic jokes with scientific names. While the traditional "first bird" is called ''[[UsefulNotes/StockDinosaursTrueDinosaurs Archaeopteryx]]'', one of the first land plants to ever appear is the almost-homonymous ''Archaeopteris''. But wait, ''Archaeopteryx'' means "ancient wing", ''Archaeopteris'' means "ancient fern". This plant lived in the Devonian (before the Carboniferous), just when the proto-amphibian ''Ichthyostega'' made the first step on dry land; it was one of the first terrestrial plants to develop to the size of a tree. Since at the time land animals were very few and mostly carnivorous or detritivorous, ''Archaeopteris'' and its relatives were able to spread worldwide, but never far from water, just like amphibians. Their appearence was like that of a tree fern: indeed, the fern's shape is considered one of the most primitive body plans among terrestrial plants. However, plants went on land ''before'' the amphibians. In the Silurian period (before the Devonian) there were already some aquatic plants (ex. the Psilophytes) emerging out of water; ''[[http://en.wikipedia.org/wiki/Cooksonia Cooksonia]]'' is an often-cited example. These tiny plants still kept their roots underwater, but their "branches" grew above the water's surface, capturing extra light. Terrestrial plants are actually very evolved organisms and have worked hard to develop adaptations for surviving outside the liquid element - among them, a vascular system (that is, tiny vessels for the flow of fluids), waterproof "skin", and fibers to make their stalks more robust against gravity.

* Some fossil animals' names recall literary characters, rather than mythical things: nautiloids (literally "similar to the Nautilus") are named for 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 likely 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 even shows 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 in general).

* Gastropods, scaphopods, bivalves, brachiopods, chitons, monoplacophorans: [[RiddleMeThis who's the intruder?]] All these invertebrate groups were "shellfish", and all but one were molluscs: the exception is the brachiopods. They were only distant mollusc relatives, and more closely related to the coral-like bryozoans (see "sessile invertebrates" below). As a whole, molluscs and brachiopods are extremely abundant in fossil records of all ages, from the 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 cited 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 modern ones since the Early Ordovician! Among extinct non-cephalopod molluscs worthy of note are 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.

* Foraminifers ("forams" for friends) are ''really'' a treasure in the rock, in all senses. They have helped confirm the deep impact theory of dinosaur extinction, their shells have contributed to the formation of sedimentary rocks around the world, and they are cool in their own right, with their immense variety of shapes. The biggest forams, the nummulites, look often like literal coins emerging from the rocks (''nummulus'' means "little coin" in Latin); they are used as index fossils for the Cenozoic era, and also created the material for the [[BuildLikeAnEgyptian Egyptian pyramids]]! But wait: what are actually the foraminifers? Originally, scientists thought they were molluscs or mollusc-like critters; today we know they weren't even true animals. They were protozoans, aka single-celled organisms with animal-like traits. Most other one-celled "animals" have left very little if any fossil material; the ancestors of ''Amoeba'', ''Paramecium'', ''Noctiluca'', ''Euglena'', ''Vorticella'', and so on are a mystery. Even the radiolarians: their shells are siliceous, and silicon usually dissolves before fossilising. However, the [[http://en.wikipedia.org/wiki/Tintinnid Tintinnids]] (little-known ciliates today) have left noteworthy remains of their "shells" in the fossil record.

* The Cambrian. The first period of the Paleozoic era, in which the famous [[http://en.wikipedia.org/wiki/Cambrian_explosion Cambrian explosion]] of life happened. The less famous fact is that we're ''unbelievably lucky'' to know anything about this remote event. At that time, animals just were starting to achieve hard parts in their bodies, and we already know that, usually, soft-bodied organisms don't fossilize at all. By astounding luck, one of the greatest exceptions to this rule are some deposits from the Cambrian period: it almost seems the natural world [[BecauseDestinySaysSo did this deliberately for us]]. The most famous and historically relevant is the [[http://en.wikipedia.org/wiki/Burgess_Shale Burgess Shale]] in British Columbia (first found in 1909 by Charles Walcott), but others are known (for example that of Chengjiang, China). For obvious reasons, our first question is: which were the first animals (not counting protozoans) that thrived in our oceans? Well, the answer isn't simple, but we can divide them into two groupings. One is made of those clades either still alive today or that went extinct several ages after the Cambrian: among the former, most non-arthropod/non-cephalopod invertebrate groups already mentioned; among the latter, the trilobites. We'll talk here about the second grouping: many Cambrian invertebrates were indeed ''exclusively of the Cambrian'' and didn't survive long, not even reaching the following period, the Ordovician - in which the most famous Paleozoic critters, sea scorpions, nautiloids, ostracoderms, etc. appeared. Thus, many of them are classically viewed as RealLife examples of OurMonstersAreWeird by paleo-fanatics. We still know very very little about their lifestyles, 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 take 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]] to get an idea. The large guy in the center is immediately recognizable, by far the biggest creature in this fauna: ''[[UsefulNotes/StockDinosaursNonDinosaurs Anomalocaris]]''. Of course it is the most portrayed Cambrian animal in documentaries and illustrations, often called "[[SeaMonster the first sea monster to appear on Earth]]". Actually, if alive today, the "terrible" anomalocaridid would look vaguely like a lobster, 3 ft long, shell-less and pincer-less... nothing dangerous for a tough-boned, tough-muscled, tough-skinned mammal such as a human.

* Apart from ''Anomalocaris'', the three probably most-portrayed Cambrian animals were very different from each other: ''[[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 on its back, of uncertain purpose, and long soft paired legs. The reconstruction of ''Hallucigenia'' was an astounding ScienceMarchesOn tangle for many years: it was first reconstructed ''upside-down'', with the dorsal spikes believed to be legs, and the legs pointing upwards and thought to each end with a small mouth! Its taxonomy is uncertain, but it's tentatively classified as a lobopod, a distant relative of arthropods. ''Opabinia'' was related to ''Anomalocaris'', but even weirder-looking; perhaps no other fossil animal more resembles a fictional space alien. It had ''five eyes'' placed in circular fashion on its head, and a pincer 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 relative of vertebrates). 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''. Interestingly, while most of these organisms appearances in the media were only limited to documentaries, ''Hallucigenia'' particularly had gained a sudden fame when it revealed to be one of the main plot device in ''Manga/AttackOnTitan''. So much so that almost every [=YouTube=] video that makes mention of it would have its comment section filled with people quoting the series.

* But wait... have you sees these critters on TV at least once? Unlikely, 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 invertebrates that show up are... a modern jellyfish and an [[AnachronismStew anachronistical phacopid trilobite]] - remember that phacopids first evolved in the Ordovician, while Cambrian trilobites looked very different from 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-like ''[[http://en.wikipedia.org/wiki/Aysheaia Aysheaia]]'', the slug-like ''[[http://en.wikipedia.org/wiki/Amiskwia Amiskwia]]'', the shrimp-like ''[[http://en.wikipedia.org/wiki/Waptia Waptia]]'', the lobster-like ''[[http://en.wikipedia.org/wiki/Sidneyia Sidneyia]]'', the jellyfish-like ''[[http://en.wikipedia.org/wiki/Eldonia Eldonia]]'', the "hairy worm" ''[[http://en.wikipedia.org/wiki/Canadia_(annelid) Canadia]]'', etc. - is another egregious example of a [[TheyWastedAPerfectlyGoodPlot missed opportunity]]. Speaking of ''Pikaia'', this time its absence is less of a problem: the aforementioned proto-vertebrate ''Haikouichthys'' filled its role. And another thing: if you read 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'''. Rather amusing to read, and - let's face it - a ''true'' oasis of happiness among so many other unutterable, absurdly-difficult names.

* The first insects appeared in the Devonian. 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. Anyway, they appeared later than scorpions and millipedes. They were still wingless, like modern [[http://en.wikipedia.org/wiki/Springtail springtails]] and [[http://en.wikipedia.org/wiki/Silverfish silverfish]], 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 sizes - the four-winged ''[[UsefulNotes/StockDinosaursNonDinosaurs Meganeura]]'' had a wingspan of more than two feet - and started their radiations, which 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 intertwined 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).

* Actinopterygians, aka "ray-finned fish" or "rayfins". Or, more simply, "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 several fish groups roaming the seas and fresh waters. The most derived rayfins, the Teleostei, became widespread only in the Cretaceous. Almost all modern ray-finned fish are teleosts. However, most familiar teleosts 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 may have ''as big as a humpback whale'' and the largest fish of all time. [[note]]Size estimates vary, of course; it may not have been bigger than a modern whale shark, and had a toothless mouth which indicates an inoffensive filter-feeding lifestyle 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 over the course of the Mesozoic. Similar to a carp, it was actually not related with any modern fish: its primitiveness is revealed by its heavy armor-like scales. 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 clades 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 (comparable to 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 the fossil record is ''Knightia'', an ancient true herring.

* If you ask a paleontologist what are the most common vertebrate fossils, s/he'll answer "shark teeth". However, these teeth are usually found isolated - ironically, the remaining skeletons are among the rarest fossil finds (sharks' cartilage doesn't fossilize well, unlike bone). Even though sharks make up only a small percentage of 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. 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 to 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 front of its head like the modern, unrelated whale shark. ''Xenacanthus'' and ''Stethacanthus'' were more bizarre. The former had an eel-like 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. Possibly 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 arranged in a spiral coil 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 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 was another one entirely: the Placoderms. Their name means "skin with plates", and they're nicknamed "armored fish"; their scales were notably thick and broad and covered their forebody like true armor. However, this armor was still flexible and didn't form 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 the fins of other fish, and lived near the bottom of the seas: ''Bothriolepis'' and ''Pterichthyodes'' are the best-known examples. 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 teeth had the same origin as ''scales''. After all, modern sharks still have enamel-covered scales on their body, the same shape as their teeth (only smaller). [[note]]Reptilian and bird scales have nothing to do with fish scales: the latter are pieces of bone covered or not with enamel; reptilian ones are simple horny protrusions of the skin, like our nails.[[/note]] In a sense, you could even say teeth are the ''only'' fish scales we human still preserve.

* 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 unarmored 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.

* But what ''was'' this thing? Found in 1966, and very common in the Carboniferous Mazon Creek Beds of Illinois, it looked like nothing paleontologists had ever seen before. It had a jaw like a crab claw, the body plan of a cuttlefish, and two weird stalked lumps on both sides of its body. The only thing anyone could classify this as was as an invertebrate, probably some kind of worm. [[ScienceMarchesOn Until 2016]]. A study by Yale University revealed the Tully Monster's true identity, and it was the last one anyone expected. 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.

[[folder:Plants]]

* This species was first described in 1843, and for the longest time experts couldn't decide ''what'' it was. Many thought it was a conifer tree (its name means "First yew"), others thought it was a giant aquatic alga. It wasn't until 2007 that scientists confirmed it was a [[FungusHumongous giant terrestrial fungus]]! Since this thing grew up to 8 meters, it would've been the tallest organism of its time. Many are still confused as to how such an organism could grow like that without photosynthesis.

* Actinopterygians, aka "ray-finned fish" or "rayfins". Or, more simply, "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 several fish groups roaming the seas and fresh waters. The most derived rayfins, the Teleostei, became widespread only in the Cretaceous. Almost all modern ray-finned fish are teleosts. However, most familiar teleosts 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 may have ''as big as a humpback whale'' and the largest fish of all time. [[note]]Size estimates vary, of course; it may not have been bigger than a modern whale shark, and had a toothless mouth which indicates an inoffensive filter-feeding lifestyle 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 over the course of the the Mesozoic. Similar to a carp, it was actually not related with any modern fish: its primitiveness is revealed by its heavy armor-like scales. 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 clades 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 (comparable to 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 the fossil record is ''Knightia'', an ancient true herring.

* Apart from ''Anomalocaris'', the three probably most-portrayed Cambrian animals were very different from each other: ''[[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 on its back, of uncertain purpose, and long soft paired legs. The reconstruction of ''Hallucigenia'' was an astounding ScienceMarchesOn tangle for many years: it was first reconstructed ''upside-down'', with the dorsal spikes believed to be legs, and the legs pointing upwards and thought to each end with a small mouth! Its taxonomy is uncertain, but it's tentatively classified as a lobopod, a distant relative of arthropods. ''Opabinia'' was related to ''Anomalocaris'', but even weirder-looking; perhaps no other fossil animal more resembles a fictional space alien. It had ''five eyes'' placed in circular fashion on its head, and a pincer 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 relative of vertebrates). 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''.