Prehistoric Life/Other Extinct Creatures

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/wiki/Prehistoric Lifework

Amphibians

In paleontology, the word "Amphibian" has traditionally had a much broader meaning than that commonly attributed. Amphibians have been all Tetrapods ("four-limbed vertebrates") excluded "reptiles" and the latter's descendants, Birds-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" or "stegocephalians", but these terms shouldn't be used today, just like "thecodonts" for basal Archosaurs or "pelycosaurs" for basal Synapsids. This because they don't indicate any natural grouping of animals, but are instead catch-all words with little scientific significance in modern phylogenetic systematics. Labyrinthodonts means "labyrinth teeth", because many of these animals had convoluted, labyrinth-like internal patterns inside their teeth, but this doesn't interest us too much. 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 mammal-like "reptiles" and mesozoic mammals, they have not gained much attention in pop-culture (it seems only apes and monkeys were our ancestors in pop-consciousness...). Their apparently monotonous, uninteresting appearence may have contributed to this, or rather... just our mammalian pride generates a sort of "denial" towards our deep origins? Anyway, basal tetrapods weren't boring, uninteresting things: anything but. They were very diversified in shape, size and ecology; as we’ll se soon.

Hopping, crawling, and digging: Triadobatrachus, Karaurus, and Eocaecilia

• Lissamphibians have a rather mysterious story. Again, like lizards, snakes, birds, rodents and bats, their skeleton is way too fragile to fossilize well, and the reconstruction of their story has many holes within it. Anyway, we're pretty sure that, surprisingly, both frogs and salamanders originated in the Triassic period. Yes, they too did it. And they already resembled our modern froggies/salamanders (except some details). Triadobatrachus is often mentioned as "the first frog", lived in Triassic and was very frog-like except for having shorter hindlimbs, and it wasn't probably capable to leap yet. But Jurassic frogs were already virtually identical to ours. While salamanders haven't changed much from their apparition in the Triassic, with animals like Karaurus being already proper salamanders in every detail. Among prehistoric salamanders is also worth to be cited Andrias scheuchzeri, a very close relative of modern Giant Japanese Salamander (the largest modern lissamphibian); it has been one of the very first fossil animals ever described, in year 1726, when paleontology still didn't even exist yet as scientific field; because of its size (and scientific ignorance of those years), its human-sized skeleton was interpreted as belonging to a human dead during the Biblical Deluge! (please note Andrias just means man in Greek). Caecilians have the scantier fossil record among all lissamphibians: we don't even know when they appeared. But we're pretty sure the earliest ones still had limbs, just like snakes' ancestors.

Boomerang-heads, eel-bodies, and whip-tails: Diplocaulus, Phlegetontia, and Urocordylus

• As a whole, non-lissamphibian amphibians first appeared in the Devonian period (but we'll talk about these earliest forms in another paragraph), and encountered an enormous success, expecially in the Carboniferous, when immense swamps allowed them to spread widely on Earth. One of the most known Carboniferous amphibians is the tiny-limbed, eel-like Crassigyrinus. However, the most diversified group of amphibians living in the Coal Age were the Lepospondyls. Since paleo-amphibian classification is still a hard task for taxonomist, we’re not sure which affinities were among Lepospondyls and other amphibians. Some argue lepospondylan amphibians were the ancestor of modern amphibians, but still don't know from who the latter actually originated. Unlike many other groups, lepospondyls were generally small and uncospicous-looking: thus, they’re rarely portrayed in media. Some were similar to salamanders or newt, like Urocordylus; other lose their limbs altogether and became eel-like, such as Phlegetontia. On the other hand, one late-surviving lepospondylan (Early Permian) is one of the most frequently-depicted paleo-amphibians in artworks: Diplocaulus, thanks to its extremely widened triangular head that no other vertebrate has ever had (except for its closest relatives). No more than 3 ft long, Diplocaulus was quite small among prehistoric amphibians, and, except for its head, its body was similar to a stocky salamander. Its lateral “horns” are a true enigma. From a mean to better-plough the water during the swimming to a defensive device to prevent the animal to be swallowed whole by larger predators, every hypothesis has been made. We know for sure it was a mainly water-living animal, as its limbs were weak and its eyes are placed on the tip of its skull.

When amphibians were like crocs 1: Eryops, Cacops, and Platyhystrix

• Protoamphibians managed to flourish in the successive two periods as well, Permian and Triassic, conviving well with the rapidly-evolving reptiles and mammal-ancestors. However, In Early Permian Earth became more and more arid, with the disappearing of many ancient swamps. The water-loving lepospondylans underwent a serious chrisis, while other paleo-amphibian groups more adapted to a terrestrial lifestyle become widespread and diversfied. The most successful Permian / Triassic amphibians were the Temnospondyls. These are also the most commonly-portrayed paleo-amphibians in media, because they include some of the biggest and most spectacular kinds. The definitively most-depicted has always been Eryops. 10 ft long, it was similar to a shortened, armor-less alligator, with plump body, splayed limbs, short tail unapt to swim, and a wide, flat head with eyes and nostrils on the top and a very gatorish snout. Compared with crocodilians, Eryops had more teeth, more pointed but also more fragile; it could have been an ambush-predator of fish or smaller amphibians catched in water. When on land, the eryops could have become prey of the super-predator of its time, the famous Dimetrodon. The fact Eryops lived with the latter may explain its status as the archetypical "giant amphibian". Even though "Giant amphibian" is the term usually utilized in pop-media when referring to it or to other Temnospondyls, many members of the group were really giants: for example, Cacops was dog-sized. Also Early Permian, this one was much more terrestrial than Eryops, with stronger limbs and lateral eyes. This, along with a small armor over its back, makes Cacops deceptively reptile-looking; actually, reptiles descended from another totally different group of “amphibians” (see later). A close relative of Cacops was even more reptilian-looking: Platyhystrix (again from Early Permian) had a dorsal flat crest covered in skin, extremely similar to the one seen in the contemporary mammal-ancestors Dimetrodon and Edaphosaurus. A group of temnospondyls in the Late Permian were the Archegosaurs. Particulary similar to crocodiles and gharials with their long snout and dorsal armor, they were among the largest amphibians ever (the record-holder to date is Prionosuchus, 30 ft of length!),

When amphibians were like crocs 2: Mastodonsaurus, Gerrothorax, and Koolasuchus

• Temnospondylan amphibians managed to survive the huge mass extinction at the end of the Permian. In Early Triassic, they recovered fast and became very diversified again. Among them, the Trematosaurs were very similar to the earlier Archegosaurs, and also reached large size. However, the biggest and most famous Triassic amphibian is Mastodonsaurus (sometimes misspelled "Mastodontosaurus"), the first paleo-amphibian ever found, in the first half of the 1800s. The same bulk of a hippo, it had an even huger head, as long as a human, and with carnivorous teeth. One usually-ignored trait is a couple of lower teeth which were threaded through the perforated upper jaw when the mouth closed. With its bulky body and short tail, the mastodonsaur was probably slow-moving on land, and is usually thought a water-living ambush-predator like its predecessor Eryops. However, such giants were exceptions at their time: most other relatives were not bigger than modern amphibians. The most interesting is Gerrothorax; rather similar to the not-related Diplocaulus (but with much smaller lateral “horns”), it was a neotenic form which conserved gills the-whole-life, just like one modern kind of salamander, the Axolotl. In the Late Triassic, temnospondyls like Metoposaurus began to face the competition from aquatic archosaurs like the parasuchian Rutiodon, and became rarer and rarer, until they totally disappeared in the Late Triassic… at least, this is what was once believed. Recently found in Australia, Koolasuchus mysteriously managed to survive until the Early Cretaceous. 5 m long, it was an almost-fully aquatic animal similar to the modern Japanese Giant Salamander, with tiny limbs, robust tail for swimming, and a flattened head. Koolasuchus one has received some attention in recent popular media: it was portrayed both in Walking with Dinosaurs and in Disney's Dinosaurs (even though is named only the former).

Egg-shells, what an invention!: Proterogyrinus, Seymouria, and Diadectes

• Here we’re talking about Reptiliomorphs ("reptile-shaped"); that is, tetrapods much closer to reptiles than to frogs or Eryopses. They had the same variety in shape and size as the other paleoamphibians, but were generally more land-living than the others, and progressively gained those traits which allow us humans to remain costantly out of water without dehydratating: waterproof skin, efficient lungs and kidneys for storing water, but above all, amniotic eggs; that is, those oval, shelled things we use to eat for dinner, and that come to our mind when hearing the word "egg". Before that, eggs were just those tiny, soft marbles modern frogs still produce today. Among quasi-reptiles, the most mentioned has long been the Early Permian Seymouria, because it was once considered the missing-link between amphibians and reptiles; now is believed only a distant relative of Amniotes. Less-close to reptiles were the the Embolomers or Anthracosaurs, which were the dominant "amphibian" group in the Carboniferous, often (but not always) huge-sized. Examples: Eogyrinus, Proterogyrinus. Among the closest-to-reptiles reptiliomorphs is worth of mention the iguana-like Diadectes. From Early Permian, it was maybe one of the first herbivorous vertebrates ever appeared. In textbooks written in the 1990s is often heard the name Westlothiana, because was once considered the “first reptile ever appeared” (lived in Early Carboniferous, even before thr classic record-holder Hylonomus).

Limbs, what an invention!: Ichthyostega and Tiktaalik

• Along with Eryops, Ichthyostega has been the most famous paleoamphibian, but this time is a bit more justified: it has long had an enormous importance in paleontology indeed. Discovered in Greenland (still not the Grim Up North place we know today) and living in the Devonian Period, Ichthyostega has been the first four-limbed vertebrate known to science for almost a century: one of the icons of evolution thus, just like Archaeopteryx and horses. Today, many other "missing links" between fish and tetrapods are known to science: the most astonishing is Tiktaalik which was really a middle-way between a fish and an early "amphibian". Ichthyostega has often been described as a "fish with limbs", and with reason: its was still more fish-like than amphibian-like. Its 4 ft long body was streamlined like a fish; its head was smooth and very fish-like; its tail still retained a fin (albeit reduced); and its skin was, arguably, still covered with bony scales, just like fishes. But it had limbs instead of paired fins; very odd limbs to modern standards, since they had seven digits (all the other following tetrapods had only no more than five toes, a trait then inherited by reptiles-birds-mammals-humans). Expect to see it still mentioned as "the first land-living vertebrate". This is justified in works created some years ago, ex. Walking With Monsters, but not in the most recent ones. We now think it was completely aquatic and its limbs developed to move upon the bottom of swamps, rivers and lakes, since they would be too weak to support its bulk on land. And is very unlikey that it could emit loud screams as shown in Monsters, as well as laying frog-like eggs; not all amphibians are frogs, and Ichthyostega and its kin were far more fish-like than frog-like in Real Life.

Fish

"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. 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.

Lungs, what an invention! Prehistoric Coelacanth relatives, Prehistoric lungfish, and Eusthenopteron

• Sarcopterygians, aka “Lobe-finned fish”. Actually, considering these animals as fishes may appear rather arbitrary to some paleo-fans. Rather than creatures we'd normally call fish, they were a sorta middle-way between typical fish and amphibians, and many of them spent part of their life outside water, thus breaking the "fully-aquatic" criterium. They are traditionally divided in three groups: Actinists, Lungfish and Rhipidists. Actinists, better-known as Coelacanths from the common name of their only one two surviving species, are perhaps the most famous, and at the same time, the least amphibian-like: indeed, they were fully-aquatic, thus "true fish". They appeared in the Devonian Period, and were marine creatures that have remained virtually unmodified since 400 million years: but now they seriously risk to go extinct soon or later, only because they are not so cute as clownfish are. Lungfish were a bit closer to us: they really have lungs for breathing air other than classic gills, appeared in the Devonian as well and specialized themselves to muddy, marshy environment with very oxygen-poor waters: that's why their lifesaving airsacs evolved. Today are the most successful missing-links between fish and land vertebrates (although less than ten species are still alive today), and yet they are less-often-mentioned than Coelacanths. Finally, "rhipidists" are an artificial assemblage of not-related lobe-finned fish. Among them were the ancestors of mankind, as well as all land-living vertebrates ever existed, from dinosaurs to hamsters. Eusthenopteron has always been the stock rhipidist, probably because of the look of its tail that recalls Prongs of Poseidon; recently, the basically identical but far bigger Hyneria has gained some notoriety thanks to Walking With Monsters, portrayed as a Jaws-like villain. It's the only lobe-finned fish represented in that show, and with no mention at all about the role of lobe-finned fish as our-ancestors.

Bones, what an invention!: Xiphactinus, Lepidotes, and Leedsichthys

• Actinopterygians, aka “Ray-finned fish”. Or, more laconically, fish. They are by far the most diversified non-tetrapods today, compounding the 90% of our modern ichthyofauna, but only a small percentage of the pre-dinosaurian one. They appeared in the Devonian, but reached their immense today-success only at the Cretaceous, when they underwent an explosive evolution. From seahorses to puffers, from swordfish to ocean-sunfish, from piranhas to deep-sea anglers; almost all the most today-familiar fishie-kinds appeared only after the Cretaceous/Tertiary Rock-Falls-Everyone-Someone-Dies event. Among the few modern ray-finned fish which were already in life during the mosasaur/plesiosaur/ichthyosaur existence, there were herrings, sturgeons, gars and few, few others. There were also now-extinct guys as well in the Cretaceous: the most portrayed is the 15–20 ft long, bulldog-faced Xiphactinus, which competed successfully with its neighbouring mosasaurs in the "large predator" role. There was an even more awesome fish in the Jurassic sea: the whale-sized, filter-feeding Leedsicthys. Among other smaller (yet still intriguing) Mesozoic fish, we can mention: the "Saber toothed herring" Enchodus; the gar-like Aspidorhynchus; the stocky Dapedium; the herring-like Leptolepis; and, above all, the carp-like Lepidotes. The latter was a very successful genus, with dozens of species described which lived through most of the Mesozoic Era: this was the meal the fishing-dinosaur Baryonyx might have preferred for lunch, as shown by remains of Lepidotes within its ribcage.

Jaws, what an invention!: Acanthodians, Palaeoniscus, and Cheirolepis

• On the other hand, very few ray-finned fishes are known before the Triassic. Palaeoniscus and Cheirolepis are the most cited: their look was a sort of middle between a regular fish and a shark, but we'll understand later why. Some modern ray-finned fish have maintained this mixed look today: sturgeons are the most typical example. Also with the same, regular-fish/shark mixed look of the early rayfins, Acanthodes were only related with the latter. "Acanthode" means "spiky", and they indeed had plus many fin-spikes. They hadn't a great success compared to other groups, and became extinct at the end of the Permian, another victim of the most devastating mass-extinction of all times, that wiped out 90% of all living things. Despite this, Acanthodian fish were possibly among the most important fishes at all times. This because they probably were the very first vertebrates with jaws. This is not a trivial thing at all: thanks to this invention (made in the Silurian Period, just before the Devonian), fish as a whole started to be the most important large-sized animals in marine and inland waters, becoming active predators and outcompeting the so-called "Sea Scorpions" (see in the Invertebrates section) in this role. This role obligated fish to become more mobile and faster, thus giving them one day the capability to get out the water and to become human-ancestors (this thing is called "Pre-adaptation" in evolutionary terms).

Shark tales 1: Cladoselache, Stethacanthus, and Xenacanthus

• Not always Everything's Even Worse with Sharks, really. For many animal-lovers, they are among the most fascinating living things. And so is among paleo-fans. Prehistoric sharks were as diversified as modern shark are today, or rather, even more. But stop now. Things aren't always so simple as they seem. "Shark" is an ambiguous word: usually refers to some of the modern cartilaginous fish, expecially the most streamlined ones such as the Great White, but not, to say, the flattened rays and skates. But in paleontology, it usually refers to all cartilaginous fish, or at least, those belonging to the {{[http|//en.wikipedia.org/wiki/Elasmobranchii Elasmobranchian}} subgroup. The other main subgroup, the Holocephalians, are usually called Chimeras or, more confidentially, Rabbit-Fish. Cartilaginous fish (both sharks and chimeras) originated in the Devonian period. Chimeras have virtually not changed since then, and still retain today that mixed "shark/typical fish" appearence very common among Paleozoic fish. Sharks are often called "living fossils" because their anatomy has remained virtually unchanged since 400 million years, and yet they managed to survive the strong competiton both from the first Jurassic large sea-reptiles at first, and from the first Cretaceous ray-finned fish then.

Shark tales 2: Hybodus and Cretoxyrhina

• In the Devonian, Elasmobranchs were represented with generic forms such as Cladoselache, already with the familiar exposed gills, but with a terminal mouth, unlike most modern sharks. Actually all the fish-groups seen so far get their success for the first time in this period: Devonian is called with reason "the Fish Age" because it was the age in which fish were more diversified than every else. Many Paleozoic "sharks" are very cool-looking: let's give a look at the "Ironing Board" shark Stethacanthus, the eel-like freshwater sharks of the Xenacanthid family, and the weirdest of them all, the totally bizarre-toothed Helicoprion. In the Mesozoic, the dominant group was made of more modern-looking animals: among them, the "Switchblade Shark" Hybodus and the ray-like Ptychodus. However, the first true sharks appeared only in the Cretaceous, along with the first modern-looking bony fish; among them were also the first true rays/skates. Some sharks from that period were similar to Jaws-things. The "Ginsu Shark" Cretoxyrhina, for example, growed to the size of Spielberg's beast, and preyed upon marine reptiles. Despite this, the biggest and most famous prehistoric shark was still alive just 1.5 million years ago, when hominids were already existing: Carcharocles, better known as Megalodon.

Shark tales 3: Megalodon

• There's already a trope intentionally dedicated to it, but we'll add some paleontological information here. The "megalodont" is the largest fish known to science which could hunt large prey, but possibly not the largest shark ever; perhaps our modern Whale-Shark may get as large as it was. And we're unsure it really was the largest fish ever as often said: the aforementioned filter-feeder Leesdichthys might get larger. Many books have exagerrated the megalodont's size, to the point measures of 100 ft weren't rarely heard in media; if so, it would be as large as a blue whale... Adding material to Rule of Cool, its huge jaws have been sometimes depicted with six or more children inside, just to show how big they are. And Megalodon is not its scientific name, but only the surname: the correct way to call it is either Carcharocles megalodon or Carcharodon megalodon. It was probably similar in shape to an oversized Great White, but this still remains uncertain. Some scientists think it wasn't so close to the white shark; if so, its correct scientific name is Carcharocles megalodon. On the other hand, other paleontologists note the strong resemblance between the two sharks' jaws, and think the megalodon was a very close relative of the Great White. If so, they'd belong to the same genus, with the Great White being Carcharodon carcharias, and the Megalodont Carcharodon megalodon. And its teeth were indeed very similar to a White's, simple triangles with serrated edges but without those secondary points seen in some other modern shark species. These huge teeth have given it the famous-today second term of its scientific name: Megalodon means "big tooth" indeed. It's cool, that one of the largest carnivorous dinosaurs has also a reference to the Great White, because of its similarly serrated edge of its teeth: Carcharodontosaurus just means "White-Shark lizard". Another word about teeth: shark teeth are perhaps the most abundant vertebrate fossils, just as common as the famous Ammonites; yet ironically, their owners are much, much rarer in fossil record than most other fishes. Their cartilaginous skeletons don't usually fossilize, while their hard, enamel-rich teeth do very well. Indeed, many prehistoric sharks have been described only from one tooth.

Tough guys 1: Bothriolepis and Coccosteus

• “Placoderms” (“plated skin”) is the correct name for the “jawed armored fish”. They were the most numerous and diversified fish group living in the Fish-Golden-Age (the Devonian), but no one seems to have survived in the following period, Carboniferous. Placoderms' fossil abundance in devonian rocks might also be related to their main anatomical feature: a thick body armor made by large, articulated plates that covered the first half of their body. Placoderms and the so-called "Ostracoderms" (see further) are the only ancient armored fish known. But wait, we've said an inaccuracy. The classic fish scales we commonly know actually are a kind of body-armor, just as the plates of placoderms: only much lighter. They have the same basic bony structure, but are very diversified among fish groups. Scales of Teleosteans (aka the subgroup including almost all modern ray-finned fish) are thin laminae visible under their skin; those of sharks and some archaic rayfins (like the gar) are covered with enamel like teeth, and the shark's ones look just like minute teeth. This thing is quite interesting, as we'll see later. Placoderms are called "jawed armored fish" to separate them from the apparently similar, jawless Ostracoderms. We know several groups of placoderms, but the most relevant are the Antiarchs and the Arthrodires. Antiarchs had a singular anatomical feature: their pectoral fins had a very unlikely look among fish, resembling more crustacean legs than fins; the most well-known among them is Bothriolepis, one of the most common fossil fish in history, with 100 different species found on every continent. Arthrodires hadn't such crab-like legs, but in return they have very peculiar teeth, as we'll see later; Coccosteus it traditionally regarded as the prototypical arthrodiran.

Tough guys 2: Dunkleosteus (once called "Dinichthys")

• Bothriolepis, Coccosteus and most other placoderms were small predators of invertebrates or smaller fish... but not Dunkleosteus. Its name means "Dunkle's bone", but in older sources it used to be called "Dinicththys" ("fearsome fish"). But today nobody uses this name anymore. Just like the "Brontosaurus -> Apatosaurus" and "Diatryma -> Gastornis" examples, this is a pity for many long-standing paleofans: a really cool name deleted by Science Marches On and replaced by a really unexpressive one... This Devonian arthrodire was similar to Coccosteus, just overgrown: 30 ft long, the size of a killer-whale, it was the largest vertebrate known so far which lived before dinosaurs. Its size is even more astounding, if you think most other Devonian armoured fishes were salmon-sized. It is one of the most famous prehistoric "leviathans" along with Megalodon, Liopleurodon, Mosasaurus and Basilosaurus, and was the fiercest-looking among them, thanks to its armour, and also its strange-looking teeth, as we'll see later; no surprisingly, it is a staple in those not-so-common paleobooks which show also pre-dinosaurian fauna, always described as a "monstrous killing machine". Surprisingly, despite all this, Dunkleosteus has had very few apparitions in TV to date, much less than, to say, the not-so-impressive Elasmosaurus (another egregious case of Everything's Better with Dinosaurs: OK, elasmo wasn't a dino, but lived in the same age...) Perhaps the only relevant example in recent media is Sea Monsters, where Dunkleosteus was portrayed with cat eyes to make it even scarier-looking (while most drawings show rounded, fish-like pupils), with a non-proved cannibalistic attitude, and, to put the cherry on the cake, its hide was blood red. If you have watched that show, you'll remember those strange, blade-like teeth which cut like scissors (at last, we're talking about them); these are instead Truth in Television. Only... they weren't ordinary teeth; they were bony plates, the same kind of those which covered its forebody. And this is not an isolated case in the fish-world; remember we talked about sharks, and their enamel-covered, teeth-like scales? Finally we'll get it. At the start of fish evolution, scales and teeth were the same thing. Then, body scales were lost by land vertebrates, never to return: the so-called reptilian "scales" are a totally different thing, just horny thickening of the skin (usually) without bony core, just like our fingernails. But mouth scales remained, and how: and they allow us to chew our meals today. Our incisors, canines, premolars and molars are, really, the only remaining fish-scales we still have. Keep this in mind, every time you go to the dentist.

Tough guys 3: Cephalaspis and Pteraspis

• Ostracoderms (“armored skin”) is the traditional name for the “jaw-less armored fish”. Ostracoderms appeared about 480 million years ago, during the Ordovician Period, far before the other fish groups already seen. They become very successful in the following period, the Silurian, and managed to survive well in the Devonian along with the many new, jawed lineages already mentioned above. But stop now. Again, "ostracoderm" is an old, catch-all term which shouldn't be used anymore in a cladistic sense, but since is handy for us, we'll use it. They actually are made by several lineages which arose separately during fish evolution, but shared a similar body-plan. The most relevant are three: Heterostracians, Anaspids, and Osteostracians. The vaguely skate-like osteostracian Cephalaspis and the long-snouted heterostracian Pteraspis are the two most popular kinds in books and docus. Most other ostracoderms have their name ending in –“aspis” as well. The main ostracoderm subgroups differed each other mainly by body-shape and anatomical features, but they have a rather similar ecological role, so we don't get in detail about the single kinds. Ostracoderms are called "Jawless armored fish" because... guess. Their mouth was a simple opening without teeth or any grinding mechanism, making their feeding-style a filter-feeding and/or a food-sucking one, unlike their jawed successors. And they were small. Very small. If we'll put a modern-day grouper among them, it'll appear to them as a Great White Shark'll appear to us. Several species were not bigger than a human hand, and some were even shorter than a human "pinkie" finger! However, their most evident feature was their armor. This armor covered the whole body, and made a defense tougher than any human-created armor; it was made by the same hard bony material already seen in placoderms and modern fish (in the shape of scales). At this point is worth noting a thing: these fish didn't have a true skeleton inside yet, at least the meaning we usually intend for "skeleton". Their backbone was still a little more that a simple chord with some cartilage, but no bone: in fact, the first bony tissue even appeared among Vertebrates was outside the body, making de facto ostracoderms more similar to arthropods than to most modern backboned animals in this respect. The trend started reversing first with placoderms, which lost their posterior armour to be faster and more manouvrable (as needed by their hunting habits), but still had a cartilaginous skeleton inside. Sharks transformed their armour in a dense mesh of tooth-like bony scales, but still have no bone tissue in their internal skeleton (this means their nickname "cartilaginous fish" is not totally correct: they have bone, but only on their skin and within their teeth). Only Teleostomian fish (ray-finned + acanthodians + lobe-finned) make this work complete, developing bony ribs, bony vertebrae, bony girdles, and so on. Finally, the first land animals felt useless and heavy their old, scaly, fishy exoskeleton and lost it for good—even though some of their descendants re-built some kinds of body armor: turtles, crocs, ankylosaurs, glyptodonts, knights etc.

Our earliest origins 1: Cyclostomates

• If you get in your hands an old textbook, you'll probably read Ostracoderms were "the first fish ever", thus "the first vertebrates": actually it's untrue. Ostracoderms, indeed, were already very evolved animals. Practically, their only archaic feature was the jawless mouth that obligated them to eat only little items: all their other traits were as sophisticated as those of the other fishes. Particularly well-preserved fossil finds show us they had complex brains and very kin senses just like modern fish. An they had a whole-fishy shape, with all the classic fins (though less-developed than those of more recent fish-groups). And they weren't the ancestors of the other fish (and thus of amphibians, mammals, mankind etc.): rather, jawless armored fish went extinct at the end of the Devonian without leaving offspring. The "most primitive vertebrate" title belongs to even more primitive animals. Sadly, the common ancestors of all vertebrates are extremely poorly-known in paleontology: this because, being so ancient, they hadn't any sort of bony-covering, and thus they hardly fossilize; despite this, more-basal-than-ostracoderm vertebrates were possibly as abundant as the latter in Ordovician and Silurian seas, and maybe were successful even beyond the Devonian, perhaps until the Triassic. The amazing thing is, unlike armoured fish, some of the basal, unarmored vertebrate groups have survived until now. We’re talking about Hagfish and Lampreys. They didn't descend from ostracoderms which lost their armour, as said in old texts; they are far more archaic things, which resemble anything but a typical fish in shape (expecially the hagfish). Using the word "fish" for these animals may appear arbitrary to some paleo-fans, having no fins, no fish-shape, and in the case of hagfish, not even eyes And yet, they are very sophisticated critters nonetheless: their partially parasitic way-of-life towards the "proper fishes" needs specific adaptations, and also a larger size than ostracoderms: in fact, both hagfish and lamprey may reach 3 ft length or even more.

Our earliest origins 2: Haikouichthys

• However, their earliest ancestors were very different animals: they were really tiny creatures, even smaller than ostracoderms, and were almost surely filter-feeders just like the latter. The most known among these critters has been, until few years, the recently discovered Haikouichthys that lived in the Cambrian Period: that is, the very first age in which life on Earth began to really diversify. Haikouichthys was only 0.5 inches long, and its appearence was anything like a fish: a kinda "moving leaflet" without paired fins, maybe similar to the classic "amphioxus" (aka lancelet) so common in biology texts. However, it has recently been proposed it was only a vertebrate-relative just like the "lancelet". Haikouichthys appeared in Walking With Monsters, described as "the very first vertebrate, forerunner of all backboned animals in the future". That's all very well, since at that time it was considered as such. But... since it is a such non-spectacular character to show in a docu-drama like this... Rule of Cool does remedy all our problems: we see our alleged forerunner portrayed as a shoal animal swimming in the open sea, with high-developed swimming capabilities, and above all, with the same parasitic feeding behaviour of hagfishes. While in Real Life it was almost certainly a solitary, slow-moving bottom-dweller and an amphioxus-like filter-feeder, just like the living animal which resembles the common vertebrate ancestor more than anything else: the lamprey's larval stage, aka the "Ammocoetes".

Our earliest origins 3: Conodonts

• Still another group of pre-fishian vertebrates is known to science, perhaps even more enigmatic than Haikouichthys: the Conodonts. We known thousands and thousands of microscopic fossil "jaws" discovered everywhere from the Cambrian to Triassic terrains, attributed to them, but since few years ago, nothing from the rest of their body. In the past, scientist didn't even know if conodont remains pertained to vertebrate ancestor at all; recently, thanks to new discoveries, it has been found they were probably elongated, lamprey/hagfish-shaped critters: perhaps the ancestors of the latter? Conodonts are a prime example of the many still unresolved, intriguing mysteries of Paleontology.

Arthropods

When thinking about animal fossils, our mind usually goes on the pietrified 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 Ammonites and 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.

The first eyes: Trilobites

• There are things which are more important than others. Trilobites are among them. Their awesome fossil abundance, in fact, makes them "index fossils", that is, a hallmark to date a certain period of Earth's evolution. Trilobites are the hallmark of the Paleozoic Era, the era which preceeds the famous Mesozoic one. Trilobites lived in all the Paleozoic periods from Cambrian to Permian, and no one managed to survive the worst mass-extincion ever to make its way in the Triassic. Thus, fit perfectly the role of index fossils about Paleozoic. But wait. Trilobites are not crustaceans. There is the tendency in pop-culture to identify all aquatic arthropods as this; since crustaceans make the quasi-totality of them today, this is justified. But things in Prehistory, as always, are more complicated. There were many crustaceans in and after the Paleozoic, as we'll se later; but many others armored, spineless Paleozoic critters weren't. Trilobites are the most abundant of these, and lived mainly in the earliest periods of the Paleozoic: then, they'll be joined by crustaceans and chelicerates (see further), and their number began to fall. It's a bit useless to describe them: see the link in this entry to take a look. It's worth noting, however, they were among the very first animals to develope complex eyes, superficially similar to those of modern insects, made by many adjacent facets. Also note that, despite their completeness, we still don't know so much about their way-of-life; their soft tissues rarely have been preserved, making a problem for scientists. Trilobites were very diversified in look and arguably behavior, but most of them were bottom-dwellers and all were microphagous (ate very small items). In short, they were very similar in ecology to ostracoderms (the jawless armored fish). In fiction they rarely appear, just like all prehistoric invertebrates (authors typically prefer to show modern spineless animals in substitution); when this happens, they tend to look like those of the Phacopids subgroup. Don't expect to se Agnostids or Proetids, despite the former were among the first trilobites ever appeared, while the latter were the only ones which managed to survive until the end of the Permian.

Crabs and pseudo-crabs: Prehistoric Crustaceans and Prehistoric Xyphosurans

• Since Trilobites and Sea Scorpions (see further) are now extinct, we have today only two three remaining groups of marine arthropods: Crustaceans, Xiphosurans, and Pantopods But since the latter haven't almost left fossil record, we'll talk only about the first two. The only xiphosuran left today is deceptively called "Horseshoe Crab" (its correct name is "Limulus"). this might people think they are just another kind of crab, thus uninteresting guys; it's anything but. They in fact are not crustaceans at all, but rather primitive relatives of spiders and scorpions; but unlike the latter, they are always remained aquatic creatures. Their appearence quite reminds that of a large-headed, sword-tailed Trilobite: this is not an incidence, because the trilobite-like body-plan is the original one among all the most basal Arthropods ever (see "Cambrian Life"). And their larval stage is even more trilobite-looking. Limuluses are, in an extent, the arthropodian equivalents of the famous Coelacanth: classicaly mentioned as a prime example of "living fossils", because their anatomy has remained virtually unchanged since the early Paleozoic. Today there are very few species all very similar each other, but luckily they seem not to share the same, disheartening fate of the coelacanth (at least for now...). One useful note about our modern horse-shoed friend: it is not dangerous to humans at all as sometimes heard, its tail being totally harmless and lacking any venom: instead, it has a mechanical meaning, allowing the animal to move upon certain sandy soils, or overturn itself when upside down. Prehistoric crustaceans are far less interesting-looking: today they are enormously diversified per-se, from krill to the Japanese Giant Crab, from woodlice to barnacles (yes, these too are crustaceans). Their extinct equivalents were about the same groups we see nowadays, and ruled the same echological niches. Just like trilobites and xiphosuran, crustaceans' fossil record is huge thanks to their often-calcified exoskeleton. While pantopods have left few fossils just because they have got an unarmored body (a general rule among invertebrates).

Our distant enemies? Pterygotus and the Eurypterids

• Eurypterids, also known as Gigantostracians, are probably among the prehistoric critters with the most striking-sounding nickname: "Sea Scorpions"... just weren't scorpions. Even though scientist think they might be the ancestors of the true scorpions, those with the stingy thing on the tip of their... what? Tail?. "Marine scorpions", lived through most Paleozoic from Ordovician until the Great Permian/Triassic Extinction, nearly as long as the Trilobites. They are often cited as "the largest arthropods of all times", but this might depend to who you ask: modern Japanese Giant Crabs, with their gigantic legs, might be considered even larger, or at least, wider. And most eurypterids were much, much smaller than the 10 ft long Pterygotus—by far the most portrayed in media, for obvious reasons. Anyway, it seems the largest sea scorpions have been the heaviest marine arthropods nonetheless. And yet, if they were alive today, most eurypterids wouldn't really be scary-looking things. They'd look just like slender, paddle-legged, odd-looking lobsters: think a common grouper could gulp most of them with ease; and even Pterygotus could have even been appreciated as a delicacy by human gourmets. But at the times of their primeness (Ordovician and Silurian Periods) they were the largest predators in the seas (excluding giant cephalopods), while fish (still represented only by ostracoderms and few others) were still very small things. And they were Scorpions. Giant Scorpions. That's done. Here are the perfect guys for the Big Bad role: those cruel, monstrous things which had nothing better in their life than persecute our poor little (literally little) ancestors... But wait! Wait! Let's not forget a thing: Predators Are Not Mean, they are necessary to ol'Mother Nature's balance of life in our modern world. And so were they in the prehistoric one as well. Not only that: we might even have to thank sea scorpions for having existed! Preying upon our ancestors, it's possible they have unwillingly helped them to evolve their best traits we humans still retain, such as complex brains and fastness; going even further, one could say they made fish to get out of the water and become those four-limbed, land-loving animals from which we humans have originated. Keep this in mind, every time you crush a scorpion.

Out of water, at last! Palaeophonus and other non-insect land-arthropods

• The very first animals which made their first steps onto dryland weren't vertebrates, but Arthropods. It's easy to understand why. At the Silurian, vertebrates still were all fish-like and their fins weren't articulated structures which could make a leverage to substain the body constrasting the force of gravity; while Arthropods have had articulated legs since the Cambrian, 100 million years before. Thus, they were in clear advantage. The very first land arthropods weren't insects though; the latter have been a more recent appearence within Evolution. The first colonizer were the "Myriapods" (millipedes, centipedes and their extinct kin) and the "Chelicerates". The latter include, other than Spiders and Scorpions, the aforementioned "Horseshoe-crabs" and "Sea-Scorpions" which remained aquatic animals. We are not sure how arthropods managed to reach the land, but we know for sure that myriapods and "true" scorpions were already present in the Silurian, while the first known spiders appeared much later, only in the Carboniferous (contemporary to the very first reptiles). All these invertebrates were astonishingly similar to their today-descendants, to the point that the latter may be counted as real "living fossils". Most prehistoric land-living arthropods remained as small as they still are today, but some grew larger: expecially in the Carboniferous, and we'll discover why just in that period. Generally, Paleozoic land arthropods tend to be represented in a very generic way in fiction or documentaries, typically lived-interpreted by actual animals. For example, the series Walking With Monsters portrayed a land-scorpion in the Devonian (perhaps Palaeophonus), live-acted by a modern scorpion species. However, the same series has made perhaps the first example in TV of documentary-related arthropods in CGI. Other than trilobites and eurypterids, we can see the large, still semi-aquatic scorpion Brontoscorpio in the Silurian (imprecisely described as a "Sea-Scorpion", making to believe it was a Pterygotus relative...); in Carboniferous settings, the large, fully-terrestrial scorpion Pulmonoscorpius appears (unnamed), as well as the giant spider "Megarachne" - considered the largest spider ever at the time, but its fossil has later revealed actually to be an eurypterid; thus not a spider, and not even land-living. The latter has been perhaps the most awesome case of Rule of Cool within the entire Walking With series: with its creepy black venom, it destroys a nest full of young protoreptiles apparently for mere vengeance, and if it was not enough, it seems laughing sadistically upon its victims while the narrator saying THE ARTHROPODS ARE BACK! Not to mention those awe-inspiring "giant ants" in Beasts - these were discovered in the same Messel Pits in which the first bats have been discovered, but their behaviour as it appears in the docu is totally speculative, so relax. Nonetheless, the absolute stars of the series among CGI land-arthropods were two really big Carboniferous animals. See below.

Everything's better with Euras: Arthropleura and Meganeura

• Which animal do you prefer, the largest land arthropod ever known to science, or the largest flying insect ever known to science? It almost seems intentional they have similar-sounding names; actually Arthropleura means "articulated flanks", Meganeura "large wing-veins", thus being only an incidence. Both from the Carboniferous, they represent well the tendence towards gigantism among Arthropods in this age. They were not the only overgrown land invertebrates in their world (and many other arthropods at that time were normal-sized, let's not forget it). But both made surely the Up to Eleven example. And yet, in the following age, the Permian, land insects and millipedes returned as small as we were initially at their Silurian/Devonian origins, and remained such for all Mesozoic and Cenozoic, until today. Why just in the Carboniferous? The most credited theory trots out the almost-universally utilized fuel within the animal kingdom: Oxygen. Thanks to the extraordinary luxury of vegetation typical of that period, the vital gas increased its level more than every other time in Prehistory. And since size of land arthropods is severly limited by the oxygen abundance (because of their particular tracheal respiratory sistem), this was the only time ever in which insects and their kin managed to make the Big Creepy-Crawlies trope a Truth in Television one. The 7 ft long Arthropleura is the most odd-looking of the two: despite being a millipede-relative, it resembled more an elongated, land-living trilobite in shape, with its body dorsally flattened and wide-framed, long antennae and short legs. It was the "cow" of its habitat, the largest herbivore of its fauna, which grazed decomposing plant material, but thanks to its size and armour, it probaby had very few enemies when fully-grown: even giant amphibians (the most powerful predators at that time) rarely attacked it, according to our best guesses. The 3 ft wingspaned Meganeura, on the other hand, had a typical dragonfly-like appearence, and was arguably an astounding flier and a skilled aerial predator of smaller insects, just like its modern relatives. And it too had very few enemies: giant amphibians normally couldn't get catching giant dragonflies up to the canopy where they arguably passed most the time. In few words: both are two very, very cool guys. And yet, just like all prehistoric invertebrates, Arthropleura and Meganeura haven't traditionally received much attention by writers, due in part to Did Not Do the Research, in part because much, much Bigger Creepy Crawlies already exist in Fictionland for centuries. A curious thing is that Meganeura has traditionally received more attention than Arthropleura, despite its less-awesome size and look; but now this seems no longer true, in part thanks to the influence of "Walking With... - expecially "Prehistoric Park", which made Arthropleura the main animal character in the Bug House episode. Even though the most awesome scene is seen in Monsters, were an Arthropleura and an anthracosaur (reptiliomorph "amphibian") fight each other just like a cobra and a mongoose would in RealLife.

Thank you bug! PREHISTORIC INSECTS

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 larchs.

Remember Jurassic Park, 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 Truth in Television. 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 "Coal age", 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, later than scorpions and millipedes: they were still wingless as modern springtails and silverfish still are, but then the Carboniferous saw their success: the very first flying animals appeared, reaching large size up to 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.

Insect evolution has always been related with that of terrestrial plants, as we'll see better in the "Plants" section. 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, expecially 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 neornithan birds and placental mammals are the most today-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.

Cephalopods

Ammon's horns: Ammonites

• Is there anybody who has never seen those pietrified spiraled shells at least once in its life? Ammonites are probably the most iconic fossil invertebrate remains at all, thanks to their elegant shape and their extreme abundance in Mesozoic deposits, to the point they're used as index-fossils to identify Dinosaur-age-related rocks, just like Trilobites for the pre-dinosaurian ones. But wait... ammonites are not exclusive to Mesozoic! They appeared in the middle Paleozoic, but that's right, achieved their highlights in Mesozoic with kinds which are exclusive of this era: so, it works just the same. Before the first paleontologists were born, ammonites were already well-known to people, who asked themselves what the heck they are: Mother Nature's jokes, Pietrified snakes, "Ammon’s horns" (the meaning of their our-day name), or what? The very first human which understood their real nature was Leonardo da Vinci in XVI century, but wasn't believed: we had to expect the Founder-of-Paleontology, Cuvier, after see the true fossil's nature understood at the end of the XVIII. Sometimes is heard the modern-day Nautilus is the "modern ammonite": this is not true, as we'll see later: ammonites went totally extinct at the Cretaceous/Cenozoic extinction ever, the most famous victims (after real and alleged dinosaurs) of the most infamous (though not the worse) mass-extincion ever. Indeed, ammonites are very mysterious things, despite their abundance. We know almost nothing about the shape of their soft parts, since they don't fossilize usually: it seems, though, they were intermediate between the more basal Nautilus and the our more evolved octopusquids. Octopusquids have very complex eyes, almost identical to vertebrates, while Nautilus has one of the simplest eyes one can imagine, nothing but darkrooms without lens. And ammonites? Who knows... And their tentacles? Octopusquids have eight/ten with suckers, Nautilus more but lacking suckers. And ammonites? It seems were more nautilus-like in this respect. In drawings and models, however, expect to see Ammonites with octopusquid eyes, octopusquid tentacles and, last but not least, octopusquid ink (while Nautilus hasn't ink at all in Real Life). Despite their fame, Ammonites are very rare things in TV. Don't expect to hear more than a single line about them -if at all. Even Walking with Dinosaurs has dedicated only a brief cameo to them, in Jurassic "Cruel Seas". Indeed, for an unexplicable reason, ammonites are usually associated with Jurassic seas, despite they were just as common in Triassic and Cretaceous.... In dinosaur books, things are a bit better: ammonites here are frequent, but... don't expect to see single kinds identified, either (unless you've a technical or semi-technical work). And don't expect to see the so-called "heteromorph" ammonites, aka strange-looking species with uncoiled shells common in Cretaceous.

Stony arrows: Belemnites and extinct Coleoids

• Ammonites were not the only tentacled things in the Mesozoic: Belemnites were just as abundant. They have left many remains as the former, but these aren't so iconic as ammonites are. This is easy to understand: they haven't any elegant external shell, just a sort of smooth, sharp internal cuttlebone which made people think about the most awesome things, from pietrified arrows to devil's fingers. Despite this, we know a bit more about belemnites than to ammonites: they were close relatives to modern squid and cuttles, and they arguably had octopusquid eyes and certainly had ten tentacles - or, better, ten arms: the fussy guys tell us that the correct term for tentacles is "arms" while "tentacle" should be used only for those two long, thin things with a club-like end which are exclusive to cuttles/squids but not belemnites. If ammons are a rarity in TV, what about belemns? It they appear at all, they'll be identified as squids. On the other hand. if dealing with true octopusquids (more aptly called "new coleoideans"), they have a very scarce fossil record; most of them haven't any internal cuttlebone, often their only bodypart that fossilized is the "beaked mouth". And we already know how hard a soft-bodied animal fossilized. Thus, scientists are happy when founding complete remains of them. However, they lived during the whole Mesozoic Era (despite octopussies and argonauts seem a more recent thing), and Giant Squid -like forms already existed in the Cretaceous.

Finding Nemo: Orthoceras and the "Nautiloids"

• Despite their name, “Nautiloids” have nothing to do with Nemo's Nautilus, still less with this Nemo... "nautiloid" is a catch-all term including all the most archaic cephalopods, from which ammonites and belemnites and cuttlenites and squidenites and octop... ahem... derived in a direct or indirect way. Indeed, nautiloids were not exclusively Paleozoic things as it's easy to think: they too were thriving in the Mesozoic like their relatives. But don't expect to see any nautiloid in media outside pre-dinosaurian Age, nor expect to see them in the Devonian, Carboniferous or Permian periods; more commonly, they are shown in the Ordovician and Silurian seas, aka in the early Paleozoic. It's not necessary to be a genius to understand why: in these periods, Nautiloids were the top-predators of the seas (along with sea-scorpions), while since Devonian they were outcompeted by fish, followed by marine reptiles and cetaceans. Basal cephalopods began with long, straight shells but later these became more and more coiled, until their only still-living direct descendant, the aforementioned Nautilus (hence nautiloids, "pseudo-nautiluses"), a triumphant example of "living fossil" just as the iconic Coelacanth. Some of these shells are so well preserved that even their original colors are partially conserved (for example Orthoceras, the kind traditionally most portrayed, which had zigzaging lines on its cone-shaped shell). Indeed, the so-called "orthocones", aka cone-shelled nautiloids, are by far the most portrayed because their strange look of living tentacled-icecreams. The Walking With series has chosen the "giant orthocone" Cameroceras as the archetypical nautiloid, for obvious reasons. But most nautiloids weren't bigger than a common cuttlefish in Real Life. To compensate, the "giant orthocone" has been represented with its correct anatomy: simple darkroom-like eyes and more than ten sucker-less tentacles (oops... arms), instead of the octopusquid eyes and sucker-filled appendixes often-seen in paleo-art.

Other Invertebrates

There is not much to say about the portrayal of extinct non-arthropod / non-cephalopod invertebrates in media: they rarely appear even in books/documentaries, much less in Fictionland, and when they do, they are almost never named (except sometimes for the names of each group, but only in popular-science works). As an example, the original Disney's Fantasia showed several modern critters to symbolize the early evolution of invertebrates, but few or nothing among the really prehistoric ones. The Walking With series did the same: only modern medusae, 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.

Clams and Pseudo-clams: Rudists and Prehistoric Lingula

• Among non-cephalopod mollusks, Bivalves (clams and their kin), Gastropods (water/land snails and slugs), Scaphopods ("tusk-shells"), and Chitons, and "cap-shells", have been found in great number in fossil record from almost all ages (despite very few cap-shells are still living today, and since have remained unchanged since the Paleozoic, they deserve the title of "Living Fossils"). Most of these molluscs were similar to ours, but the Cretaceous “Rudists” were odd-shaped bivalves, with one half-shell much larger than the other, resembling an amphor with a lid above. However, the most abundant invertebrate group in Paleozoic deposits are a sort of pseudo-clams, the Brachiopods, which actually weren't even molluscs at all. They are still-living today, but are only a minor portion of the invertebrate fauna of the seas, and aren't known much among laymen. Some of them like the Lingula, have remained totally unchanged since 400 million years!

We love geometry: Cystoids, Blastoids, and Prehistoric Crinoids

• Echinoderms are extremely abundant in fossil record from Cambrian to Recent, because their hard internal "skeleton" fossilizes well (with one exception: holoturoids or "sea-cucumbers" which are soft-bodied). Other than our familiar groups, echinoids aka sea-urchins, asteroids (the starfish, not that asteroid!) and ophiuroids (bristle-stars), we have some now-extinct groups such as the Cystoids and the Blastoids (please note all these musical-sounding rhymes). But those far more common in Paleozoic fossil record are a now rare but still-living group, the fern-like Crinoids aka Sea-Lilies.

Odd relatives: Graptolites and Cothurnocystis

• Believe it or not, sea-urchins, sea-lilies and whatnot are among the closest relatives of vertebrates. But there is one now-extinct group that is even more unbelievably closer to us: Graptolites, so common in certain Paleozoic periods that are used like the more famous Trilobites as Index-Fossils. Graptolites were colonial animals more similar to the extremely more archaic cnidarians (jellies, corals etc.) in look, and their shape was awesomely diversified among species. If alive today, they'll resemble floating corals or something similar. Another group that is hard to believe to be close kin to vertebrates are the Homalozoans: vaguely resembling a cross between a fish, a crustacean and something else, they were once considered archaic protovertebrate, now they are believed to be closer to Echinoderms (if not echinoderms themselves). The most astonishing among them is the strongly asymmetrical Cothurnocystis.

Survival of the toughest: Prehistoric Sessile Invertebrates

• There is a general rule in Paleontology that no one living thing can escape: if you have hard portions within our body (shells, bones, armors etc.), you'll leave the memory of your importance in History of Life; it you have not these, you are probably destined to be forgotten forever. Sad, but true. This explains why so many modern relevant invertebrate groups are almost unknown in paleontology: for example, non-colonial cnidarians (medusae, sea-anemones) and several "worms" (annelids, nematodes, flatworms and so on). Who knows how many ancient important animal groups have actually existed in the Paleozoic and further, that we even know the existence... The odds do enhance however, if you are a colonial organism; if so, you probably have an external "skeleton" made of some sort of hard material (calcium carbonate, silicium, or simply horny matter like that of our hair and nails). Fortunately, many colonial groups are well-known in paleontology, and have had an unimaginable relevance not only for the evolution of life, but even for having building many portions of our planet. Their skeletons, fossilized and transformed in hard rock, have accumulated in million years and became our sedimentary rocks, from sandstone to mudstone. Naturally all creatures with something hard inside or outside have contributed to this (molluscs for example have had a great role as well). Among colonial organisms we've already seen the floating Graptolites; among those still-living, the most important have been three group of "sessile invertebrates" (those fixed to the bottom of seas and lakes): sponges, corals and the less-familiar bryozoans.

A treasure in the rocks: Foraminiferans, including Nummulites

• It may seem strange to you, but even microrganisms have left fossils, and a plenty of it. Of course these fossils do not receive much attention in media, but are of extreme interest among paleontologists. Again, the only-the-tough-ones-preserve rule also counts for single-celled Protozoans: pratically the only group which has left significative fossil record is the Foraminifers ("forams" for their friends), only because they have a sort of minute "shell" which covers their softer innerparts. But they have been very important for scientists in several ways. First, foraminifers have largely contributed to form sedimentary rocks like corals and molluscs: despite their minute size, they were so in high-numbers in ancient seas that their impact has been notable. Then, they have aided scientists to conferm the Rock Falls Everyone Dies thesis about non-avian dinosaur extinction. In rocks made before the mass-extinction forams abound, in those originated just after the extinction, they are almost missing (except few which managed to survive): a proof that the K/T extinction wasn't a slow journey to death, but a rapid cataclysm (geologically rapid, don’t forget it: it could be last 100,000 years, which is nothing in geology!). Third, they are inherently cool: some of them were not even microrganisms, would well visible to a naked eye, and reached even 6 cm of width: the latter are called Nummulites (from "nummus", "coin" in Latin). They were indeed small, round calcareous disks, and being exclusive to the Cenozoic, they are considered the best index-fossils for the Mammal-Age. Nummulites are expecially abundant in Egypt (still underwater at the time), to the point that... egyptian pyramids are made by the so-called "nummulite limestone", derived from fossilized nummulite shells melted together.

Once upon a time... CAMBRIAN ANIMALS

Cambrian. The first Paleozoic Period, in which the famous “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 ruthless rule are just some deposits from the Cambrian Period. : it almost seem Ol'Mother Nature has done this deliberately for ourselves.... The most famous and historically relevant is the Burgess Shale in British Columbia, but still others are also known (for example in 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, armored fish etc. appeared. Thus, is easy to imagine many of them were really bizarre-looking to our limited point of view. We still know very very few things about their lifestyle, but their appearence 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 all the members of the Cambrian Fauna: see the image here ^{[dead link]} for having an idea. It's immediately recognizable the bizarre shrimp-like animal in the center, by far the biggest creature in this fauna, and arguably the top predator. It is called Anomalocaris (meaning "bizarre shrimp" indeed), and was a distant relative of arthropods with no articulated legs but with the same composed eyes of the Trilobites. Of course it is the most portrayed Cambrian animal in documentaries and illustrations, classicaly mentioned as "the first prehistoric monster ever appeared on Earth". Actually, if alive today, the "terrible" anomalocarid would appear as a really narmy thing, a sort of 3 ft long, shell-less, pincer-less lobster, nothing dangerous for a tough-boned, tough-muscled, tough-skinned mammal we are in comparison. Nonetheless, anomalocarids were highly specialized predators, with a mouth build for prey around 1/12 to 1/6 of their own size.

However, at the anomalocaris’ time, every other organism was very small: The other creatures you see in the linked image are not longer than your hand, all possible prey for anomalocarids. Creatures like Hallucigenia and Pikaia were hunted by smaller predators, such as Opabinia and Anomalocaris saron. The later was the species shown in Walking With Monsters, though ridiculously oversized (6 ft.!?) and prone to attack it's own kind, despite the fact that neither its mouth nor grasps could injur an equally sized specimen in the way it was depicted, let alone possibly eating it.

Apart from Anomalocaris, we can mention at least other three invertebrates which are stock in drawings: Hallucigenia, Opabinia, and Pikaia. The first has a so strange look that its name means "hallucination-generator": it was a sort of "worm" with long paired spikes for uncertain purpose, and long softed paired legs: the reconstruction of Hallucigenia has had an astounding Science Marches On tangle for many years, and it still remains one of the most enigmatic Cambrian animal: nobody knowns exactly in which phylum it has to be placed.

Opabinia was related with 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 creatures mouth which was actually located behind the proboscis. But the most important find is the third guy, Pikaia: despite its rather insignificant appearence, it is the most well-known among vertebrate ancestor, a sort of prehistoric relative of our Lancelet (the closest vertebrate relative still-living today). 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 Walking With Monsters. In this Rule of Cool-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 anachronistical phacopid trilobite—remember that phacopids first evolved in the Ordovician, while Cambrian trilobites were very different-looking to the classic image we have when thinking about these animals. The absence of such awesome animals like Opabinia and Hallucigenia makes another egregious example of a missed opportunity, like the missing of the giant bird Argentavis and the “giant marsupials”.

Talking about Pikaia, this time its 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, absurdely-difficult names.

Plants

When thinking about fossils, we automatically think about animals. But also plants have left many remains, some of them just as spectacular than the animal ones (think about the Petrified Woods, the most famous being that in Arizona), other less-striking but even more significative, such as prints of leafs (very common in some deposits) and even the fossilized pollen which has allowed us to understand not only the composition of ancient floras, but even the climate they lived in. And, naturally, the aforementioned amber which has often caught insects inside, of course.

A flowery smell from Cretaceous: Prehistoric Magnoliophytans

• Dinosaur-Age-related vegetation wasn't so different to ours as commonly believed. Right, non-flowering plants were dominant at the time, but still today there are great extensions of dryland dominated by conifers - the siberian Taiga, not the Amazon, is the largest forest in our days. But not only because of that. If we have the chance to really Walking with Dinosaurs in the Cretaceous, we'll encounter many familiar critters. Most main groups of Angiosperms aka Flowering plants had already evolved: it has recently found that even grass populated the landscapes in which Triceratopses used to roam - though this doesn't justify at all the still-not-present grasslands so-common in Mesozoic Prehistoria. Most Cretaceous flowering plants were still trees then; most herbs have evolved later, despite they seem simpler-built. Some of the Cretaceous flowering trees have virtually unchanged since; the Magnolia tree it the prototypical example. Another plant often cited to be already living alongside dinosaurs is the Water Lily. But most trees we see in today-temperate settings, from oaks to apple-trees, from figs to vines were starting to evolve (though they became really widespread only after the mass-extinction). While grasslands only appeared in the Middle of the Mammal Age. The spread of grass is probably related with the global cooling/drying of Earth at the time, since grass is particularly well-adapted to cold, dry environment. Its success has been awesomely important for many of the today-most popular animals to evolve: if there had never been grass, elephants, lions and whatnot, simply, would not be here now. The evolution of large grazing herds of grass-eaters and their following predators would be not possible without this kind of vegetation, which to our limited knowledge, seems often the simplest, humblest thing one could imagine... We humans ourselves have to be grateful to grass for existing: remember that mankind evolution developed just thanks to the existence of grassy savannahs in Africa, while our closest relatives, chimps and gorillas, still are non-human "great apes" just for having been remained forest critters. Not to mention the matchless relevance grasses have in a more direct way for us: cereals, forage, hay, straw, bamboo, bread, pizza, hay fever... two-thirds of mankind food is still made of few kinds of cultivated grasses. Keep this in mind, every time you uproot some grass.

Dinosaur-tree: Prehistoric Ginkgophytes

• When hearing the Stock Phrase "Living Fossil", our mind goes automatically to moving guys: the Coelacanth, the Tuatara, the Horseshoe Crab... It's easy to forget that living fossils exist even in the a-bit-disregarded plant word. The Ginkgo biloba is the most-often cited example, and with reason: it's the only species of its whole group to have survived until today: it's hard to believe its ancient kin was one of the dominant group of landplant during the whole Mesozoic era. But wait... isn't ginkgo a normal-looking flowering plant? Indeed it looks like one of these... but hey, Not Broadleaf Plants are Angiosperms, as we'll see soon. Once, Ginkgo and its ancestors were put together with pines, firs and sequoias in the catch-all group called Gymnosperms (aka all non-flowering seedplants). But Science Marches On, and if you'll still use this term, expect somebody deleting your sentence.

A resiny smell from Jurassic: Prehistoric Pinophytes

• Really? Did pines, firs and spruces live alongside Jurassic Brontosaurs and Camptosaurs? And were they even their main food? About the latter we're not sure; but about the former, yes, they did. At least, pine and fir ancestors, still non-adapted to cold climates. Prehistoria is always a warm place to pass some vacation, and pine-looking trees seem a bit out-of-place there for us folks... but Real Life Is Always Different When Talking About Paleontology. But wait, we've not finished. Since in common thought conifer = pine/fir, some paleoartists tend to litterally feed brontosaurs and camptosaurs with modern conifers; expect thus to see Camarasaurus with a mouthful of spruce-needles, or Albertosaurus knocking down some poor pinetrees during its hunt-rush for a tasty hypacrosaur. This may be Truth In Art, right, but the most widespread conifer in Mesozoic pertained, rather, to other conifer kinds, many of them still-living today: yews, plum-yews, yellow-woods, monkey-puzzles, and, the more striking of all, Sequoia trees. If you think brachiosaurs and titanosaurs were the real giants of their world, think again: a fully-grown Giraffatitan, next to an ancient redwood, would be as tall as a beer can would be next to a fully-grown man.

Palms, or not? Prehistoric Cycads, Prehistoric Bennettitales, and Prehistoric Seed-ferns

• These are the plants we usually associate with the idea of Prehistory (along with true ferns and lycopods, see later). They were very palm-looking, and the still-living Cycads are often confused with the latter in Real Life: however, true palms started to appear only at the end of the Cretaceous, thus Diplodocus whip-tail would never become twisted on palm-branches. On the other hand, cycads were perhaps the most abundant seed-producing plant in the Mesozoic, along with their close (and often confused with them) relatives, the Cycadeoids or Bennettitals. However, an ever more ancient group of seed plants was still more archaic-looking. These are called Pteridosperms, aka "seed ferns": they resembled ferns in shape, only they produced seeds for spreading their kind unlike the latter. One seed fern, the Triassic Glossopteris, has ben often mentioned in textbooks because it has been an historical proof for the Pangea theory. Remains of it have been discovered in Permian rocks both in Africa and in South America, India, Australia and even Antarctica: only the supercontinent thesis could explain why Glossopteris took roots in all these landmasses without swimming. Another famous Permian critter, the near-reptile Mesosaurus, has been the subject of the same matter, since it too was discovered in all these continents (easier to understand if we think it was a small freshwater swimmer, thus too weak to navigate in open oceans). Both seed ferns and pseudo-cycads went extinct before the Cenozoic, while cycads have managed to reach our day and embellish our cities.

The Mesozoic undergrowth: Prehistoric Pteridophytes

• Horsetails and True Ferns are the today-most common archaic-looking plants. Watch one of them and your mind could travel back in time down to the Edaphosaurus days and even further. You'll note at this point that most archaic plants are either fern-looking, or palm-looking. This is not mere case: this "bodyplan" is the most ancient among terrestrial plants, and all the others - from the pine-like to grass-like - are simple evolutions of the latter. These spore-reproducing critters were already thriving in the Carboniferus, the Golden Age of Plants, but they have never been dominant compared to other groups: they have, rather, played the undergrowth role, and still play this today: but today they suffer the concurrence of modern herb-shaped floweringplants. This doens't mean, however, that ferns and horsetails have always been small things: take a look to the aptly named tree ferns, arguably one of the favourite food of large veggiesaurs, and still widespread in the original vegetation of New Zealand and part of Australia—it seems the Land Down Under and its little sister really are an endless source of living fossils: not only the platypus or the tuatara. Even equisetuses (the horsetails) have had some 30 ft tall members in their family, and some overgrown guys are still-living today: this, for example.

The Paleozoic overgrowth: Lycopodiophytes

• However, the most striking-looking among prehistoric plants are maybe those which dominated the Carboniferous world. 100 ft tall or more, these plants, if alive today, would resemble odd-looking trees, but were actually archaic spore-reproducing critters. But wait, they were not ferns, nor were they even close fern relatives. They were even more primitive plants: the Giant Lycopods. Lycopods are still-living today, but now they are nothing but tiny herb-like greens; in the Coal-Age, though, lycopods thrived in the widespread swamps with several species very different-looking among each other. Lepidodendron and Sigillaria are the iconic members of the group. Dinosaur-related vegetation was not such a strange-looking world, after all: while yes, Carboniferus was really a different world than ours. Imagine a wet landscape full of scaly-trunked "trees" with no more than one or two big branches on which Meganeura dragonflies used to perch like birds; a world in which every storm was enough to make those tough-looking plants to fall down with extreme ease, creating a dense undergrowth in which man-sized yet inoffensive Arthropleuras crawled in the undergrowth eating the abundant dead plant matter like armored cattle. Prehistoric Park has recreated in TV that weird and wonderful world to our pleasure - and the other sequel Monsters as well, but the Prehistoric Park one is far more fascinating and also more realistic. Sadly, this world has disappeared in the Permian, when Earth became to be cooler and drier, but has left to us one legacy: tons and tons of fossil coal we burn today. No other age has gifted to us so much coal, just because no other age has had a similar lush of green. But there is another reason: since giant lycopods were not only fragile things but also grew much faster than our seed-trees, they produced an enormous quantity of decaying plant matter during the about 50 million years of the Carboniferous. In short, if we managed to begin the Industrial Revolution, we have to thank Carboniferous vegetation. Keep this in mind, every time you burn some coal.

The first shoot: Cooksonia

• Carboniferous forests were not the very first ones in Earth's history: some tree-like plants had already existed in the preceeding period, the Devonian, and most were already shaped like their descendants (lycopods, tree-ferns etc.), for example Archaeopteris (not Archaeopteryx!!!). But the deepest origin of land vegetations go even before that. in the Silurian Period, when fish started to get their jaws, and scorpions get their first airbreath, the very first aquatic plants began to colonize dryland: Cooksonia is the most known. They were small, fragile-looking greens still partially submerged in water, but they did already have the same basic structure of Jurassic redwood trees or modern beeches: they had internal fiber which made their body more resistent, with erect "branches"; a thin covering of cere which prevented their dry-exposed parts to dry under the sun: and they were the first vascular plants, that is, plants with inner conducts in which lymph flows, making their metabolism faster and more efficient. Sadly, we still know very few things about plant groups even more primitive than these (many of them are not even plant in modern taxonomy): their non-vascular body was usually soft and didn't fossilize well - yes, not even plants manage to escape to the fatal rule of only-the-tough-ones-preserve. Thus, natural history of mosses, liverworts and hornworts still remains an enigma, as well as that of Green algae and several other kinds of organisms collectively called "Algae" in traditional biology that are not classified as true plants since many years. Not even fungi and lichens escaped this fate: they are virtually unknown in fossil record, but we know at least they were already present alongside the first terrestrial plant in the Devonian Period. It's the logic which tell us fungi were already there at the time: they have always played a crucial role in land ecosystems as the main decomposing organisms. Thus it's easy to think if there weren't fungi at the Devonian, dead plant matter from that age would have been accumulated in huge quantities without decomposing, literally stuffing dry lands with tons and tons of trunks, leaves and so on: maybe...some of the latter will be still-present today!

The origin of Life

Animals, or plants?: Ediacara Biota

• It is sometimes said that multicellular organisms appeared at the "Cambrian Explosion". Actually, multicellular animals appeared then, but this doesn't mean all Pre-Cambrian forms of life were one-celled like modern "protozoans" and bacterians. We have the improperly-called “Ediacaran fauna”, which lived just before the beginning of the Paleozoic Era. Several macroscopic organisms lived then, but they are so different than even those of the Cambrian that we don't know for sure if they are plants or animals: more probably, they were neither. The distinction bewteen animals and plants is so fixed in our common sense that it's difficult to imagine a world were eterotrophic and autotrophic beings were still not distinguished each other. The fate of the Ediacar critters at the end of the Archeozoic (aka Pre-Cambrian) Era is just as mysterious as that of many Cambrian creatures: they really disappeared, or were the common ancestor of all Paleozoic --> Mesozoic --> Cenozoic --> Neozoic forms of life, ourselves included? It'll remain for long one of the greatest mystery in Paleontology.

The first Earthlings: Prehistoric Stromatolites

• However, many unicellular creatures have left their track in Archeozoic rocks: obviously they are micro-fossils, thus not visible to a naked eye, but they are of immense importance, because they are the most ancient forms of life known to science. The most relevant are the so-called blue-green algae (actually a kind of bacterians), which since 2,700 million years ago have created (and still do create today) the so-called “Stromatolites”: that is, rocky concretions made from the accumulation of huge numbers of microrganisms along with inorganic matter, all cemented together. Cyanobacterians (the true name of blue-green algae) have had a keystone role for the whole life: they were the very first organisms to produce oxygen as a waste-prodoct of their photosyntesis, and widespread it in waters and the air. Before that, organisms made only fermentation (like modern yeast) and were anaerobical, they didn't consume oxygen for their vital necessities: its only after the apparition of photosyntetical beings that aerobical organism could appear and become the ancestor of the future multi-cellular organisms (remember that plants do make respiration as well, just like animals). Today, bacteria still have a keystone role in our ecosystems: they regulate the whole thing, still producing most oxygen today, recycling nutrients in the soil, making associations with other organisms (humans included) often allowing them to live, "eating" the petroleum we reverse on the sea, fermenting bread and beer, making antibiotics and so on. Yes, there aren't only disease-bearers, among bacteria: if you and me are here, we have to thank bacteria more than every other living being! But how the first cells appeared? Talking about this issue, we completely get out the field of Paleontology: this still remains mainly speculation and phylosophy, even though biologists are making great effort to find the answer.