History UsefulNotes / Dinosaurs

* The K-Pg extinction, while indeed massive, was not the biggest in Earth's history. The [[http://en.wikipedia.org/wiki/Permian%E2%80%93Triassic_extinction_event Permian-Triassic extinction]] (also known as P-Tr or [[ExactlyWhatItSaysOnTheTin the Great Dying]]), the mass extinction that paved the way for the rise of the dinosaurs, wiped out well over ''90%'' of all life on Earth; it took 10 million years for life to recover to something resembling the pre P-Tr level of biodiversity[[note]]To put that in perspective, hominids have existed for less than 2.5 million years[[/note]]. Unlike the K-Pg extinction, the P-Tr's cause is actually still unknown.

* The K-Pg extinction was not the biggest in Earth's history. The [[http://en.wikipedia.org/wiki/Permian%E2%80%93Triassic_extinction_event Permian-Triassic extinction]] (also known as [[ExactlyWhatItSaysOnTheTin the Great Dying]]), the mass extinction that paved the way for the rise of the dinosaurs, wiped out well over ''90%'' of all life on Earth; it took 10 million years for life to recover to something resembling the pre P-Tr level of biodiversity[[note]]To put that in perspective, hominids have existed for less than 2.5 million years[[/note]].

* While long called the Cretaceous-Tertiary (K-T) boundary, this event is more properly called the Cretaceous-Paleogene (K-Pg) boundary. (The old term "Tertiary" has been abolished in favor of the more evenly spread Paleogene and Neogene Periods.)

* Dinosaurs didn't really go extinct, as birds still live today. Nonetheless, most dinosaur ''lineages did'' die out, including most bird groups. Most croc and mammal groups were also killed in the extinction, with only a few lineages surviving.
* Not all the surviving lineages are still alive today. Multituberculate mammals and champsosaurs survived the extinction but died later in the Cenozoic.

* Dinosaurs didn't really go extinct, as birds still live today. Nonetheless, most dinosaur ''lineages did'' die out, including most bird groups.
* In spite of being portrayed often as K-Pg survivors, most croc and mammal groups were also killed in the extinction, with only a few lineages surviving.

[[folder: Rocks Fall, Everyone Dies]]

During their time on Earth, the dinosaurs thrived in diverse terrestrial habitats, from swampy terrain and dense forests to open prairies and the driest of deserts. Some even weathered the harsh winter conditions of Antarctica and Australia (which were near the South Pole at the time). However, the dinosaurs did not survive the Cretaceous-Paleogene (formerly known as the Cretaceous-Tertiary) mass extinction event which wiped out roughly 65% of all living things, at least according to the fossil record. This is the event that inspires the trope PhlebotinumKilledTheDinosaurs, and if you read some of the hypotheses about how they died, they often run a lot like that trope.

* While long called the Cretaceous-Tertiary (K-T) extinction, this event is more properly called the Cretaceous-Paleogene (K-Pg) extinction. (The old term "Tertiary" has been abolished in favor of the more evenly spread Paleogene and Neogene Periods.)

Dinosaurs are divided into two different groups, and those two groups are ''not'' "meat-eaters" and "plant-eaters" (in the mid-eighties, some workers suggested placing all herbivorous dinosaurs in a group variously called Ornithischiformes or Phytodinosauria, but this was disproved within the next decade). In fact, some plant-eaters were more closely related to the meat-eaters than they were to other plant-eaters. In any case, this kind of grouping fails to account for those dinosaurs that were omnivorous, insectivorous, piscivorous etc., and is largely frowned upon within phylogenetic biology in any case, since in evolution a species can switch from one form of diet to another at the drop of a geological hat.

Actual dinosaurs, despite being astonishingly diverse in size, shape, and possibly even behaviour, are all restricted to the criteria mentioned below. Thus, pterosaurs (flying reptiles) and the sea-living Mesozoic reptiles, the ichthyosaurs, plesiosaurs, and mosasaurs, are not dinosaurs at all. Furthermore, dinosaurs always walked with their limbs tucked under their bodies and upright, just like humans, ostriches and elephants, and very unlike modern reptiles. Finally, if ''[[StockDinosaursNonDinosaurs Dimetrodon]]'' is ever described anywhere as a dinosaur, you have a right to get angry; not only was it a sail-backed synapsid (the group including mammals and their extinct kin) and more closely related to the mammals than to dinosaurs, but it lived at completely the wrong time. Most dinosaurs lived in the Mesozoic Era, (251-65 million years ago). ''Dimetrodon'' lived in the Permian period, (295-251 million years ago), when the dinosaurs hadn't even evolved yet.

The first group is known as the Saurischia, or “lizard-hipped” dinosaurs, which include all the meat-eating dinosaurs, or theropods, and those long-necked, plant-eating dinosaurs, the sauropodomorphs (which include the giant sauropods). Despite how vividly different a ''[[TyrannosaurusRex Tyrannosaurus]]'' looks to a ''[[StockDinosaursTrueDinosaurs Brachiosaurus]]'', the earliest theropods and sauropodomorphs looked very similar. One early saurischian, ''[[UsefulNotes/PrehistoricLifeDinosaurs Eoraptor]]'', has been considered a theropod, a sauropodomorph, or a basal saurischian that didn’t belong to either group.

The second group is known as the Ornithischia, or “bird-hipped” dinosaurs, and are mostly herbivorous dinosaurs, very different in body-shape both when compared with saurischians and when compared with each other. This group includes the ornithopods, such as the duck-billed hadrosaurs and ''[[StockDinosaursTrueDinosaurs Iguanodon]]'', the thyreophorans, such as the stegosaurs and ankylosaurs, and the marginocephalians, such as the thick-skulled pachycephalosaurs and horned and frilled ceratopsians.

Birds, considered as a distinct class of vertebrate in traditional systematics, are actually theropod dinosaurs, a hypothesis already proposed in the eighteenth century after the discovery of the famous ''[[StockDinosaursTrueDinosaurs Archaeopteryx]]'' (an animal with dinosaurian skeleton ''and'' feathered wings and tail) but rejected by most scientists for a long amount of time. The link between dinosaurs and birds through ''Archaeopteryx'' was resurrected again in the 1960s, and has been definitively proven only in the 1990's by the long list of feathered dinosaurs and early birds recently found in the fossil record, which show strong anatomical similarities.

Impressions of protofeathers and true feathers in the fossils make for good evidence, and blend into each other so seamlessly that telling apart bird-like dinosaurs (such as ''[[StockDinosaursTrueDinosaurs Velociraptor]]'', ''[[UsefulNotes/PrehistoricLifeDinosaurs Caudipteryx]]'', ''[[UsefulNotes/PrehistoricLifeDinosaurs Beipiaosaurus]]'', ''[[UsefulNotes/PrehistoricLifeDinosaurs Sinosauropteryx]]'') and dino-like birds (''[[UsefulNotes/PrehistoricLifeOtherExtinctCreatures Jeholornis]]'', ''[[UsefulNotes/PrehistoricLifeOtherExtinctCreatures Confuciusornis]]'' and so on) has become very difficult today.

Interestingly, some ornithischian dinosaurs like ''[[UsefulNotes/PrehistoricLifeDinosaurs Psittacosaurus]]'' and ''[[UsefulNotes/PrehistoricLifeDinosaurs Tianyulong]]'' have been discovered with quills or other structures that strongly resemble feathers, which has raised intriguing questions about what dinosaurs looked like. Some think all dinosaurs originally had some sort of covering at the start of their evolution; then this covering (perhaps some sort of hypothetical "down") was lost in some lineages, or at least modified into other specialized structures (the quills on ''Psittacosaurus'' or even the dorsal spines on the sauropod ''[[StockDinosaursTrueDinosaurs Diplodocus]]'' may have this origin). The fact that the closely-related pterosaurs have a covering made of a sort of hollow hair seems to confirm this hypothesis. All the same, it does not eliminate the chance that the pterosaurs stumbled across the same solution on their own. If so, it would have been an example of convergent evolution.

* Crocodilians are the closest living relatives to dinosaurs, and have therefore a lot more to do with birds than with lizards. This might be true about turtles as well, but this has yet to be confirmed.

[[folder:Triassic - A Brief History Of The Dinosaurs]]

In the midst of this inter-group conflict, roughly 230 million years ago, the early dinosaurs appeared. They were descended from tiny archosaurs such as the 1 ft-long ''[[UsefulNotes/PrehistoricLifeNonDinosaurianReptiles Lagosuchus]]'', but they did not make their impact felt until ten million years later, when they grew from small, unassuming bipeds to impressively large forms. The three main lineages were forged at this crucial time: the meat-eating theropods, the long-necked sauropodomorphs, and the plant-eating ornithischians, although at this stage they all looked like variations of the thin, elegant Theropods like ''[[StockDinosaursTrueDinosaurs Coelophysis]]'' and its relatives, and it's quite possible that all of them were omnivorous initially.

Even with all the similarly-adapted archosaurs, Dinosaurs were notably successful at this early stage, and by the end of the Triassic period they had diversified into some of the largest animals ever to appear on the land. Early sauropodomorphs in particular (traditionally called "prosauropods", which means "before the sauropods") reached lengths and heights never seen before, like ''[[StockDinosaursTrueDinosaurs Plateosaurus]]'', which could grow up to twenty feet long, and the even larger ''[[UsefulNotes/PrehistoricLifeDinosaurs Riojasaurus]]'' and ''[[UsefulNotes/PrehistoricLifeDinosaurs Melanorosaurus]]'', both elephant-sized and thirty feet long. On the other hand, predatory theropods remained generally small in the Triassic, with some exceptions such as ''Film/{{Gojira}}[[UsefulNotes/PrehistoricLifeDinosaurs saurus]]'' (whoever said that paleontologists had no sense of humour?), which could reach 15 ft in length.

Most ornithischians were still small plant-eaters, though even here, specialized new forms were emerging - such as the heterodontosaurids, creatures with large canine teeth that may have been used in mating disputes. As an aside, ''Heterodontosaurus'' is often speculated to be the male form of another heterodontosaurid species, and its tusks were thought to be weapons used against other males in mating disputes, rather like the tusks used by musk deer today. They are almost never talked about in pop culture, though the scientific interest for them sky-rocketed with the discovery of proto-feathers on the fossils of ''Tianyulong''. No one knows what became of the heterodontosaurids after their heyday in the Triassic and Early Jurassic periods, though the species ''Echinodon'', which lived during the Early Cretaceous period, implies that they may have survived quietly for millions of years long after the Jurassic.

[[folder:Jurassic - In The Middle Ages Of Dinosaur Times]]

The diversity of dinosaurs during this time was spectacular; the theropods had produced the dilophosaurids, the ceratosaurids and the Tetanurae, in addition to the coelophysids, who were holdovers from the Triassic. The coelophysids and dilophosaurids (who are sometimes lumped together into one group) did not last much further than the Early Jurassic, but the ceratosaurids did well enough. It was the Tetanurae, or 'stiff-tailed' theropods (the non-tetanuran theropods typically had much more flexible tails), who became overwhelmingly successful. In particular, the coelurosaurians, a subset of tetanurans who survived largely by being small and unobtrusive in their early days, took over the small predator roles during the Late Jurassic and some of these small coelurosaurs included the first bird-like dinosaurs.

The sauropods seemed, at this point, to have been pressurised by Nature into growing bigger and heavier, either because they were locked in an arms race with their predatory cousins (some of whom also became large and heavy) or because doing so allowed them to evolve larger guts to more fully digest their plant food (unlike ornithopods and modern herbivorous mammals, they could not chew their food and had to rely on swallowed stones to break down the tougher plant material). The sauropods also gave rise to new forms, many defined by the shape of their teeth and vertebrae - so, for example, the diplodocids had distinctly spoon-shaped teeth which would have allowed them to strip leaves from the branches. The sauropods, in fact, reached their heyday during the Jurassic period, and though they survived into the Cretaceous, they were never as widespread as they had been, at least not in the Northern Hemisphere. In the Cretaceous, they weren't as common as the ornithopods who largely supplanted them.

Meanwhile, the ornithischian dinosaurs were getting into their stride. The thyreophorans modified the scutes of their ancestral forms into new and unusual armour, the most distinctive of which was possessed by the stegosaurs. These animals had enlarged, flat plates arranged in rows along their backs. This arrangement was discovered to be correct in the mid twentieth century, since before them the fossils tended to be jumbled up and it was largely a matter of guesswork saying where on the animal they fitted. Some paleontologists thought they stuck out sideways like spikes, whereas others thought they were armour-plating that lay flat on the animal's flanks. No one could work out, however, why this supposed armour had grooves for blood vessels all over it, which surely wouldn't have made it much good for defence (why would a shield that bleeds ever evolve at all?). The correct arrangement was later discovered on a complete fossil of a ''Stegosaurus'', and ever since they have been portrayed as standing upright on their backs - not always, though, with complete accuracy.

The purpose of these plates is unclear, but the purpose of their tail spikes and shoulder spikes was arguably for self-defence against any animal that tried to hurt or kill the animal. Their cousins, the ankylosaurs, took the scutes and developed them into stronger armour all along their backs and even, in some cases, along their undersides. The species ''Ankylosaurus'' even had armour-plated eyelids, and a thickened block of bony tissue at the end of their tails to act as clubs in case the armour wasn't a good enough hint for some carnivores. They came into greater prominence during the Cretaceous period, when the stegosaurs died out.

* Many distinctive dinosaurs, such as the sauropods ''[[StockDinosaursTrueDinosaurs Brachiosaurus]]'' and ''[[StockDinosaursTrueDinosaurs Apatosaurus]]'', ''[[StockDinosaursTrueDinosaurs Stegosaurus]]'', ''[[StockDinosaursTrueDinosaurs Allosaurus]]'', and the earliest birds lived during the Jurassic, but others, such as ''[[TyrannosaurusRex Tyrannosaurus rex]]'', ''[[StockDinosaursTrueDinosaurs Triceratops]]'', and ''[[StockDinosaursTrueDinosaurs Velociraptor]]'' lived during the Cretaceous. So ''Tyrannosaurus'' never got to fight ''Stegosaurus'', {{Fantasia}}. In terms of geological time, ''Tyrannosaurus'' is closer in time to us than to ''Stegosaurus''!

[[folder:Cretaceous – Good Times with a DownerEnding]]

Dinosaurs continued to diversify further in the Cetaceous, but times were changing and things weren't what they used to be.

Among ornithischians, marginocephalians differentiated into two subgroups. The pachycephalosaurs remained small and bipedal but developed thick skulls for uncertain purposes, while the ceratopsians became progressively heavier, and quadrupedal. Their beaks became parrot-shaped, they evolved protrusions from their cheeks and a large "frill" from the backs of their heads, and some of the later ones had impressive sets of horns and spikes. The earliest ceratopsians, if you don't count creatures like ''Psittacosaurus'', were the leptoceratopsids & protoceratopsids. Later, these smallish (by dinosaur standards - by modern standards, some would be quite respectable in size) animals evolved into the large, rhino-resembling ceratopsids, including ''[[UsefulNotes/PrehistoricLifeDinosaurs Chasmosaurus]]'', ''[[StockDinosaursTrueDinosaurs Styracosaurus]]'', the ever-popular ''[[StockDinosaursTrueDinosaurs Triceratops]]'', and ''[[UsefulNotes/PrehistoricLifeDinosaurs Torosaurus]]'', though recently there have been discussions about whether or not ''Torosaurus'' and ''Triceratops'' are actually one species.

Strictly speaking, the split between pachycephalosaurs and ceratopsians is largely believed to have occurred in the Jurassic period, but fossils of Jurassic pachycephalosaurs have not yet been found. It is a bit of a mystery what the thick skulls of the pachycephalosaurs were used for. The impulse to say that they were used to defend the animal from predatory attack should not be indulged - the smaller creatures tend to have flatter skulls, admittedly, but the larger creatures have heads like bowling balls, which means that they have a very small surface area available for actually hitting anything. Try running into somebody while holding a bowling ball out in front of you at arm's length and you'll get some idea of how tricky this actually would be to pull off. Paleobiologists thesedays tend to believe that the skulls were used to butt the sides of rival pachycephalosaurs, perhaps over territorial or mating rights. Some pachycephalosaur skulls were surrounded by spikes and knobs of bone, and probably would have looked frightening in the eyes of a small predator or a rival, and an interesting little debate is going on over whether the spikier forms, like ''Stygimoloch'' and ''Dracorex'', are really different forms of the less spikier species - they may be child forms, or male-female forms, for example.

[[folder: RocksFallEveryoneDies]]

During their time on Earth, the dinosaurs thrived in diverse terrestrial habitats, from swampy terrain and dense forests to open prairies and the driest of deserts. Some even weathered the harsh winter conditions of Antarctica and Australia (which were near the South Pole at the time). However, the dinosaurs did not survive the Cretaceous-Paleogene (formerly known as the Cretaceous-Tertiary) mass extinction event which wiped out roughly 65% of all living things, at least according to the fossil record. This is that eponymous event which underscores the trope PhlebotinumKilledTheDinosaurs, and if you read some of the hypotheses about how they died, they often run a lot like that trope.

There have been several hypotheses in the past about this event, but most of them are bunk and the rest are on shaky ground at best. In the early days of palaeontology, when snobbery of the past was widespread and extinction more or less meant you were an inferior species, it was believed that the dinosaurs simply became TooDumbToLive, or to put it more scientifically, their craniums housed brains which were inefficient by dint of being too small, and so they were outwitted by the smaller but much cleverer mammals. Some have suggested that egg-devouring mammals were responsible, but there is no evidence that the mammals consumed eggs, at least not in bulk. Some suggested that the dinosaurs found it harder to supply their large bodies with oxygen, ignoring the fact that Late Cretaceous dinosaurs were pretty tame compared with the giganic sauropods which had dominated in the Jurassic (and which, as far as can be made out, never had breathing difficulties). Some have suggested that a nearby supernova caused cosmic rays to penetrate the atmosphere and destroy the dinosaurs, but a supernova that close to the Earth would have done considerably more than wipe out a few little reptiles on its surface, and in any case the claim is not justified by evidence.

* The Cretaceous spanned a long time. Not all dinosaurs that lived in the Cretaceous lived at the time of the Cretaceous-Paleogene mass extinction. Spinosaurids, for example, lived during the Early Cretaceous but went extinct long before the mass exinction. Even Late Cretaceous dinosaurs from ''near'' the end of the Cretaceous didn't all actually live to ''the'' end! (While two million years, for example, is short in geological terms, it's an immense time span for any living thing. Entire ecosystems can change and be replaced with new environments and organisms during that time.)
* Dinosaurs didn't really go exinct, as birds still live today. Nonetheless, most dinosaur ''lineages did'' die out, including most bird groups.

If you are interested in specific kinds of dinosaurs, just [[StockDinosaursTrueDinosaurs check]] [[UsefulNotes/PrehistoricLifeDinosaurs here]] some info.
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At first, you might think this rather puts a hole through our lovely little classification. How can a saurischian be a saurischian if it's got bird-like hips, a characteristic of ornithischians? The pelvic arrangement is not the only way to differentiate between the two groups. Luckily, ornithischians can still be kept separate from saurischians by another diagnostic: all ornithischians have a U-shaped protruding bone at the front of the lower jaw called a predentary bone, often ending with a beak, and most of them have a row of chewing teeth lined up either side of the face, giving it a certain "hollow cheek" look like that of a horse. This suggests that many ornithischians chewed their food, unlike saurischians. Also, saurischians have air sacs in the vertebrae that ornithischians lacked, air sacs which the birds inherited, and which help to keep birds lightweight when flying.

Dinosaurs are divided into two different groups, and those two groups are ''not'' "meat-eaters" and "plant-eaters" (at least not since TheEighties, anyway). In fact, some plant-eaters were more closely related to the meat-eaters than they were to other plant-eaters. In any case, this kind of grouping fails to account for those dinosaurs that were omnivorous, insectivorous, piscivorous etc., and is largely frowned upon within phylogenetic biology in any case, since in evolution a species can switch from one form of diet to another at the drop of a geological hat.

Dinosaurs are divided into two different groups, and those two groups are ''not'' "meat-eaters" and "plant-eaters". In fact, some plant-eaters were more closely related to the meat-eaters than they were to other plant-eaters. In any case, this kind of grouping fails to account for those dinosaurs that were omnivorous, insectivorous, piscivorous etc., and is largely frowned upon within phylogenetic biology in any case, since in evolution a species can switch from one form of diet to another at the drop of a geological hat.

* [[http://albertonykus.deviantart.com/art/Why-Birds-are-Dinosaurs-193639479 Birds are dinosaurs.]] Because there are still birds alive today, dinosaurs are [[ScienceMarchesOn technically not exinct]]! And ''we eat them'' all the time! Sometimes irony is ''[[LiteralMetaphor literally]]'' delicious.

The first group is known as the Saurischia, or “lizard-hipped” dinosaurs, which include all the meat-eating dinosaurs, or theropods, and those long-necked, plant-eating dinosaurs, the sauropodomorphs (which include the giant sauropods). Despite how vividly different a ''[[TyrannosaurusRex Tyrannosaurus]]'' looks to a ''[[StockDinosaursTrueDinosaurs Brachiosaurus]]'', the earliest theropods and sauropods looked very similar. One early saurischian, ''[[UsefulNotes/PrehistoricLifeDinosaurs Eoraptor]]'', has been considered a theropod, a sauropodomorph, or a basal saurischian that didn’t belong to either group.

Among ornithischians, marginocephalians differentiated into two subgroups. The pachycephalosaurs remained small and bipedal but developed thick skulls for uncertain purposes, while the ceratopsians became progressively heavier, and quadrupedal. Their beaks became parrot-shaped, they evolved protrusions from their cheeks and a large "frill" from the backs of their heads, and some of the later ones had impressive sets of horns and spikes. The earliest ceratopsians, if you don't count creatures like ''Psittacosaurus'', were the protoceratopsids. Later, these smallish (by dinosaur standards - by modern standards, some would be quite respectable in size) animals evolved into the large, rhino-resembling ceratopsids, including ''[[UsefulNotes/PrehistoricLifeDinosaurs Chasmosaurus]]'', ''[[StockDinosaursTrueDinosaurs Styracosaurus]]'', the ever-popular ''[[StockDinosaursTrueDinosaurs Triceratops]]'', and ''[[UsefulNotes/PrehistoricLifeDinosaurs Torosaurus]]'', though recently there have been discussions about whether or not ''Torosaurus'' and ''Triceratops'' are actually one species.

Even with all the similarly-adapted archosaurs, Dinosaurs were notably successful at this early stage, and by the end of the Triassic period they had diversified into some of the largest animals ever to appear on the land. Early sauropodomorphs in particular (traditionally called "prosauropods", which means "before the sauropods") reached lengths and heights never seen before, like ''[[StockDinosaursTrueDinosaurs Plateosaurus]]'', which could grow up to twenty feet long, and the even larger ''[[UsefulNotes/PrehistoricLifeDinosaurs Riojasaurus]]'' and ''[[UsefulNotes/PrehistoricLifeDinosaurs Melanorosaurus]]'', both elephant-sized and thirty feet long. On the other hand, predatory theropods remained generally small in the Triassic, with some exceptions such as ''[[{{Godzilla}} Gojira]][[UsefulNotes/PrehistoricLifeDinosaurs saurus]]'' (whoever said that paleontologists had no sense of humour?), which could reach 15 ft in length.

* [[http://albertonykus.deviantart.com/art/Why-Birds-are-Dinosaurs-193639479 Birds are dinosaurs.]] Because there are still birds alive today, dinosaurs are [[ScienceMarchesOn technically not exinct]]!

Note that, under this definition, birds are dinosaurs. Dinosaurs as thought of by the general public are actually a paraphyletic group - they exclude members that otherwise fit neatly in with the rest of the group according to descent and cousinship. In this case, the Dinosauria group excludes the clade Aves, which includes both ancient and modern birds. From this point on, when the word dinosaur is used, we mean it in this sense unless otherwise noted, this being what the average person will understand by the term.

* The Cretaceous spanned a long time. Not all dinosaurs that lived in the Cretaceous lived at the time of the Cretaceous-Paleogene mass extinction. Spinosaurids, for example, lived during the Early Cretaceous but went extinct long before the mass exinction.

This extinction event was not exclusive to the dinosaurs. It also hit many other animals, such as the flying pterosaurs, the sea reptiles, and the ammonites. Even the ones that survived had a tough time - many crocodilians and mammals died out as well, and it is anybody's guess whether their survival was due to adaptation or to sheer good fortune. The survivors, like the dinosaurs had done a hundred and forty million years before, got their chance to diversify once the competitors went extinct, and in the ensuing Cenozoic era, the mammal lineages were the most triumphant successors to the crown. They underwent an explosive evolution, taking up many of the ecological niches which the dinosaurs had formerly occupied and often becoming much larger than they were during the dinosaurs’ reign. Despite this, no land mammal managed to reach the record-size of the greatest sauropods, though the weight of some did come close.

Most ornithischians were still small plant-eaters, though even here, specialized new forms were emerging - such as the heterodontosaurids, creatures with large canine teeth that may have been used in mating disputes. As an aside, ''Heterodontosaurus'' is often speculated to be the male form of another heterodontosaurid species, and its tusks were thought to be weapons used against other males in mating disputes, rather like the tusks used by musk deer today. They are almost never talked about in pop culture, though the scientific interest for them sky-rocketed with the discovery of proto-feathers on the fossils of ''Tianyulong''. No one knows what became of the heterodontosaurids after their heyday in the Triassic and Early Jurassic periods, though the species ''Echinodon'', which lived during the Early Cretaceous period, implies that they may have survived quietly for millions of years long after the Triassic.

Many dinosaurs cared for their young in this way - it may have been what contributed to their success during the stable periods of the Mesozoic. Studies of their egg fossils, nests, and infant bones suggest that some dinosaurs, like ''Allosaurus'', started their lives as precocious babies, were guarded by their mothers for the first few years, and then left to grow up on their own or with the rest of the herd after that. One particularly heartwarming fossil shows an ''Oviraptor'' mother/father ([[PapaWolf likely the latter]]) roosting on its clutch of eggs, trying to protect them from a sandstorm which overwhelmed and fossilised its last act of parenthood for millions of years. This is all the more heartwarming because it also revealed that the previous suspicion that ''Oviraptor'' were egg-thieves was, in fact, based on a misunderstanding of a previous fossil.

Most ornithischians were still small plant-eaters, though even here, specialized new forms were emerging - such as the heterodontosaurids, creatures with large canine teeth that may have been used in mating disputes. As an aside, ''Heterodontosaurus'' is often speculated to be the male form of another heterodontosaurid species, and its tusks were thought to be weapons used against other males in mating disputes, rather like the tusks used by musk deer today. They are almost never talked about in pop culture, though the scientific interest for them sky-rocketed with the discovery of proto-feathers on the fossils of ''Fruitadens''. No one knows what became of the heterodontosaurids after their heyday in the Triassic and Early Jurassic periods, though the species ''Echinodon'', which lived during the Early Cretaceous period, implies that they may have survived quietly for millions of years long after the Triassic.

Actual dinosaurs, despite being astonishingly diverse in size, shape, and possibly even behaviour, are all restricted to the criteria mentioned below. Thus, pterosaurs (flying reptiles) and the sea-living Mesozoic reptiles, the ichthyosaurs, plesiosaurs, and mosasaurs, are not dinosaurs at all. Furthermore, dinosaurs always walked with their limbs tucked under their bodies and upright, just like humans, ostriches and elephants, and very unlike modern reptiles. Finally, if ''[[StockDinosaursNonDinosaurianReptiles Dimetrodon]]'' is ever described anywhere as a dinosaur, you have a right to get angry; not only was it a sail-backed synapsid (the group including mammals and their extinct kin) and more closely related to the mammals than to dinosaurs, but it lived at completely the wrong time. Most dinosaurs lived in the Mesozoic Era, (251-65 million years ago). ''Dimetrodon'' lived in the Permian period, (295-251 million years ago), when the dinosaurs hadn't even evolved yet.

Actual dinosaurs, despite being astonishingly diverse in size, shape, and possibly even behaviour, are all restricted to the criteria mentioned below. Thus, pterosaurs (flying reptiles) and the sea-living Mesozoic reptiles, the ichthyosaurs, plesiosaurs, pliosaurs, and mosasaurs, are not dinosaurs at all. Furthermore, dinosaurs always walked with their limbs tucked under their bodies and upright, just like humans, ostriches and elephants, and very unlike modern reptiles. Finally, if ''[[StockDinosaursNonDinosaurianReptiles Dimetrodon]]'' is ever described anywhere as a dinosaur, you have a right to get angry; not only was it a sail-backed synapsid (the group including mammals and their extinct kin) and more closely related to the mammals than to dinosaurs, but it lived at completely the wrong time. Most dinosaurs lived in the Mesozoic Era, (251-65 million years ago). ''Dimetrodon'' lived in the Permian period, (295-251 million years ago), when the dinosaurs hadn't even evolved yet.

* While successful, dinosaurs had to share the land with many other reptile groups during the Triassic, such as the dicynodonts, cynodonts, and several archosaurian relatives. They are, at this point, rather like rookie sports-players: showing promise, but unable to show themselves off on the pitch with all the other players getting in the way.

Many dinosaurs cared for their young in this way - it may have been what contributed to their success during the stable periods of the Mesozoic. Studies of their egg fossils, nests, and infant bones suggest that some dinosaurs, like ''Allosaurus'', started their lives as precocious babies, were guarded by their mothers for the first few years, and then left to grow up on their own or with the rest of the herd after that. One particularly heartwarming fossil shows an ''Oviraptor'' mother roosting on her clutch of eggs, trying to protect them from a sandstorm which overwhelmed and fossilised her last act of parenthood for millions of years. This is all the more heartwarming because it also revealed that the previous suspicion that Oviraptors were egg-thieves was, in fact, based on a misunderstanding of a previous fossil.

There is a tendency in popular culture to identify ''every'' prehistoric reptile as a dinosaur, especially if it is big, nasty-looking or just plain weird. This gets to the point that the word "dinosaur" is used as a synonym for "prehistoric critter". In the most extreme cases, [[DinosaursAreDragons legendary characters]] are called dinosaurs. It is important to point out that actual dinosaurs, despite being astonishingly diverse in size, shape and possibly even behaviour, are all restricted to the criteria mentioned below. Thus, pterosaurs (flying reptiles) and the sea-living Mesozoic reptiles, ichthyosaurs, plesiosaurs, and mosasaurs, are not dinosaurs at all. Furthermore, dinosaurs always walked with their limbs tucked under their bodies and upright, just like humans, ostriches and elephants, and very unlike modern reptiles. Finally, if ''[[StockDinosaursNonDinosaurianReptiles Dimetrodon]]'' is ever described anywhere as a dinosaur, you have a right to get angry; not only was it a sail-backed synapsid (the group including mammals and their extinct kin) and more closely related to the mammals than to dinosaurs, but it lived at completely the wrong time. Most dinosaurs lived in the Mesozoic Era, (251-65 million years ago). ''Dimetrodon'' lived in the Permian period, (295-251 million years ago), when the dinosaurs hadn't even evolved yet. Or in other words, that ''Dimetrodon'' is ''your'' uncle, not that of tonight's chicken dinner.

So '''what is a dinosaur?''' Well, time to get a bit technical. If we ask the phylogeneticists, a "dinosaur" is any species that falls under the group called Dinosauria. [[CaptainObvious A bit obvious]], but bear with me. The group Dinosauria is defined as “the latest common ancestor of ''Triceratops horridus'' or ''Iguanodon bernissartensis'' and ''Passer domesticus'' or ''Megalosaurus bucklandii'' and all of that ancestor’s descendants”. What distinguishes a member of this group from, say, other reptiles is a collection of features in the skeleton which all dinosaurs share, but which lizards and crocodilians don't. An upright stance, with the legs tucked under the body; two openings behind the eye socket as well as one opening between the eye socket and the nostril, this being the antorbital fenestra (some dinosaurs, such as ankylosaurs and pachycephalosaurs, later lost these holes in favor of thickened skulls); a hinge-like ankle joint; grasping forearms, at least in the earlier part of their evolutionary history; and a typically horizontal spine with a vertical hip-thigh attachment and a long counterbalancing tail, especially in the bipedal forms - all these can tell you if the thing you're looking at is a ''bona fide'' dinosaur.

Note that, if we're going to be accurate, birds are dinosaurs. However, “dinosaurs” as thought of by the general public are actually a paraphyletic group - they exclude members that otherwise fit neatly in with the other members. In this case the dinosaur group excludes the clade Aves, which includes both ancient and modern birds. From this point on, when the word dinosaur is used, we mean it in this sense unless otherwise noted, this being what the average person will understand by the term.

* [[TaxonomicTermConfusion Not everything big and dead is a dinosaur.]] Creatures often mislabeled "dinosaurs" in pop culture include pterosaurs (which were close relatives but not dinosaurs themselves), plesiosaurs (which were extinct marine reptiles), ichthyosaurs (also marine reptiles, that may or may not be related to plesiosaurs), mosasaurs (which were marine lizards), ''Dimetrodon'' and other non-mammal synapsids (which weren't even reptiles, but closer to mammals), and even some prehistoric mammals (which were really... [[ExactlyWhatItSaysOnTheTin mammals]]). "Dinosaur" refers to a very specific lineage of animals.

Dinosaurs are divided into two different groups. The first group is known as the Saurischia, or “lizard-hipped” dinosaurs, which include the meat-eating dinosaurs, or theropods, and those long-necked, plant-eating dinosaurs, the sauropodomorphs (which include the giant sauropods). Despite how vividly different a ''[[TyrannosaurusRex Tyrannosaurus]]'' and a ''[[StockDinosaursTrueDinosaurs Brachiosaurus]]'' look, the earliest theropods and sauropods looked very similar. One early saurischian, ''[[UsefulNotes/PrehistoricLifeDinosaurs Eoraptor]]'', has been considered a theropod, a sauropodomorph, or a basal saurischian that didn’t belong to either group.

The second group is known as the Ornithischia, or “bird-hipped” dinosaurs, and are mostly herbivorous dinosaurs, very different in body-shape both when compared with saurischians and when compared with each other. This group includes the ornithopods, such as the duck-billed hadrosaurs and ''[[StockDinosaursTrueDinosaurs Iguanodon]]'', the thyreophorans, such as the stegosaurs and ankylosaurs, and the marginocephalians, such as the thick-skulled pachycephalosaurs and horned and frilled ceratopsians. When the names of the Saurischia and the Ornithischia were first coined, they were differentiated chiefly by their pelvic arrangement, with the saurischian hip bones arranged more like a lizard hip, the pubis bone pointing forwards (but this is the case in crocs and mammals and turtles as well, it’s not really something special about lizards), and the ornithischian hip bones more like an avian hip, the pubis bone pointing backwards. (An adaptation designed to accomodate longer guts for the digestion of vegetation.)

Luckily, ornithischians can still be kept separate from saurischians by another diagnostic: all ornithischians have a U-shaped protruding bone at the front of the lower jaw called a predentary bone, often ending with a beak, and most of them have a row of chewing teeth lined up either side of the face, giving it a certain "hollow cheek" look. This suggests that many ornithischians chewed their food, unlike saurischians. Also, saurischians have air sacs in the vertebrae that ornithischians lacked.

Dinosaurs are close cousins to the crocodilians, both being archosaurs; they share many traits, such as alveolate teeth, solid skulls, and the tendency to develop the bipedal posture (it may appear odd, but modern crocodilians descend to bipedal, dinosaur-looking ancestors who returned in a four-legged body plan after their adaptation to water). Moreover, all quadrupedal dinosaurs descend directly or indirectly from bipedal dinosaurian ancestors. If chelonians (turtles and tortoises) are a basal archosaur offshot as seems following some recent researches, this would mean even they are closer to birds than to lizards.

Birds, considered as a distinct class of vertebrate in traditional systematics, are actually theropod dinosaurs, a hypothesis already proposed in the eighteenth century after the discovery of the famous ''[[StockDinosaursTrueDinosaurs Archaeopteryx]]'' (an animal with dinosaurian skeleton ''and'' feathered wings and tail) but rejected by most scientists for a long amount of time. The link between dinosaurs and bird through ''Archaeopteryx'' was resurrected again in the 1960s, and has been definitively proven only in the 1990's by the long list of feathered dinosaurs and early birds recently found in the fossil record, which show strong anatomical similarities - especially the presence of impression of protofeathers and true feathers; to the point that telling apart bird-like dinosaurs (such as ''[[StockDinosaursTrueDinosaurs Velociraptor]]'', ''[[UsefulNotes/PrehistoricLifeDinosaurs Caudipteryx]]'', ''[[UsefulNotes/PrehistoricLifeDinosaurs Beipiaosaurus]]'', ''[[UsefulNotes/PrehistoricLifeDinosaurs Sinosauropteryx]]'') and dino-like birds (''[[UsefulNotes/PrehistoricLifeOtherExtinctCreatures Jeholornis]]'', ''[[UsefulNotes/PrehistoricLifeOtherExtinctCreatures Confuciusornis]]'' and so on) has become very difficult today.

Interestingly, some ornithischian dinosaurs like ''[[UsefulNotes/PrehistoricLifeDinosaurs Psittacosaurus]]'' and ''[[UsefulNotes/PrehistoricLifeDinosaurs Tianyulong]]'' have been discovered with quills or other structures that strongly resemble feather structures, which has raised intriguing questions about dinosaurs’ appearance. Some think all dinosaurs originally had some sort of covering at the start of their evolution; then this covering (perhaps some sort of hypothetical "down") was lost in some lineages, or at least modified into other specialized structures (the quills on ''Psittacosaurus'' or even the dorsal spines on the sauropod ''[[StockDinosaursTrueDinosaurs Diplodocus]]'' may have this origin). The fact that the closely-related pterosaurs have a covering made of a sort of hollow hair seems to confirm this hypothesis. However, hair-like or feather-like structure may be evolved independently in these animal groups: if so, it will be an example of convergent evolution.

Dinosaurs dominated all the land environments of the Mesozoic era, the era which covers a vast geological timescale from 251 to 65.5 million years ago. And, unlike what many movies and illustrations show, not all dinosaurs lived at the same time. The earliest dinosaur forms, such as ''[[UsefulNotes/PrehistoricLifeDinosaurs Eoraptor]]'' and ''[[UsefulNotes/PrehistoricLifeDinosaurs Eocursor]]'', appeared during the middle of the Triassic period, the first of the three geological periods which make up the Mesozoic era, and for most of the Triassic period they were background detail. The Triassic wasn't a pleasant time for life - they were living in the wake of the greatest mass extinction of the Phanerozoic eon (the last five hundred million years, roughly speaking), which had wiped out almost 95% of all living species. The continents of the world had fused into one supercontinent, a giant landmass called Pangea, and vast deserts covered the innermost areas (this could be one reason why the Permian-Triassic mass extinction was so devastating). The surviving animals had to recover pretty quickly, and many animal classes battled it out. The mammal-like "reptiles" (that is, the dominant land vertebrate group before the mass-extinction, and were not actually reptiles at all) were the first to spread on the planet: they were mostly the herbivorous dicynodonts and the cynodonts (which include mammals and several mammal-like forms). However, they rapidly underwent the strong competition by the once-called "thecodonts", that is the first archosaur reptiles; the latter were particularly effective thanks to their water-conserving adaptations typical of most reptiles, differentiating in several groups such as rauisuchians, aetosaurs, phytosaurs and proterosuchids, and seizing control reducing the variety of the mammal-like reptiles, which in response decreased their size and evolved many special features that are typical of modern mammals today, humans included.

The early dinosaurs appeared roughly 230 million years ago, descended from tiny archosaurs such as the 1 ft-long ''[[UsefulNotes/PrehistoricLifeNonDinosaurianReptiles Lagosuchus]]'', but did not make their impact felt until ten million years later, when they grew from small, unassuming bipeds to impressively large forms. The three main lineages were forged at this crucial time: the meat-eating theropods, the long-necked sauropodomorphs, and the plant-eating ornithischians, although at this stage they all looked like variations of the thin, elegant Theropods like ''[[StockDinosaursTrueDinosaurs Coelophysis]]'' and its relatives, and it's quite possible that all of them were omnivorous initially. They were notable for taking the bipedal stance, which was also adopted by some triassic archosaurs related to modern crocodilians, such as ''[[UsefulNotes/PrehistoricLifeNonDinosaurianReptiles Ornithosuchus]]'' and the deceptively-dinosaurian ''[[UsefulNotes/PrehistoricLifeNonDinosaurianReptiles Effigia]]'', some of these were indeed mistaken for dinosaurs when first discovered. To obtain this result, early dinosaurs developed a horizontal backbone but vertical joints to the pelvis, which meant that their legs were tucked underneath their body to support their weight. This also permitted them an exceptionally good turn of speed, and with their long tails to serve as counterbalances, the dinosaurs had hit upon a good design feature which would serve them well again and again.

Dinosaurs were pretty successful even at this early stage - by the end of the Triassic period, they had diversified into some of the largest animals ever to appear on the land. Early sauropodomorphs in particular (traditionally called "prosauropods", which means "before the sauropods") reached lengths and heights never seen before, like ''[[StockDinosaursTrueDinosaurs Plateosaurus]]'', which could grow up to twenty feet long, and the even larger ''[[UsefulNotes/PrehistoricLifeDinosaurs Riojasaurus]]'' and ''[[UsefulNotes/PrehistoricLifeDinosaurs Melanorosaurus]]'', both elephant-sized and thirty feet long. On the other hand, predatory theropods remained generally small in the Triassic, with some exceptions such as ''[[{{Godzilla}} Gojira]][[UsefulNotes/PrehistoricLifeDinosaurs saurus]]'' (whoever said that paleontologists had no sense of humor?), which could reach 15 ft in length. Most ornithischians were still small plant-eaters, though even here, specialized new forms were emerging - such as the heterodontosaurids, creatures with large canine teeth that may have been used in mating disputes.

* While successful, dinosaurs had to share the land with many other reptile groups during the Triassic such as several crocodilian relatives.

Eventually, the Triassic-Jurassic mass extinction wiped out 20% of all life that had survived the Triassic, including many archosaurs, the dicynodonts, and most of the cynodonts, except the first tiny true mammals and some close mammal cousins. Somehow, the dinosaurs survived, and with no competitors to impede them the dinosaurs really began to diversify. Most of the distinctive families of dinosaurs appeared at some time during the Jurassic, and indeed many of them had their heyday during this period. The supercontinent of Pangea had broken up to form two landmasses, Laurasia and Gondwana, or the northern and southern supercontinents respectively, but there were also small islets and isolated geographical regions where dinosaur evolution could follow different pathways. The northern Laurasia consisted of North America, Europe, and most of Asia, while the southern Gondwana consisted of South America, Africa, India, Madagascar, Australia and Antarctica.

The diversity of dinosaurs during this time was broad; the theropods had produced the dilophosaurids, the ceratosaurs and the Tetanurae in addition to the coelophysids, holdovers from the Triassic. The coelophysids and dilophosaurids did not last much further than the early Jurassic, but the ceratosaurs did well and the Tetanurae, or 'stiff-tailed' theropods became spectacularly successful (the non-tetanuran theropods typically had much more flexible tails). In particular, the coelurosaurian tetanuran theropods took over the small predator roles during the Late Jurassic and some of these small coelurosaurs included the first birds and bird-like dinosaurs. The sauropods also gave rise to new forms, many defined by the shape of their teeth and vertebrae - so, for example, the diplodocids had distinctly spoon-shaped teeth which would have allowed them to strip leaves from the branches. The sauropods, in fact, reached their heyday during the Jurassic period, and though they survived into the Cretaceous, they weren't as common as the ornithopods.

Meanwhile, the ornithischian dinosaurs were getting into their stride. The thyreophorans modified the scutes of ancestral forms into new and unusual armor, the most distinctive of which was possessed by the stegosaurs. These animals had enlarged, flat plates arranged in rows along their backs. The purpose of these plates is unclear, but the purpose of their tail- and shoulder- spikes was arguably for self-defense against any animal that tried to hurt or kill the animal. Their cousins, the ankylosaurs, took the scutes and developed them into stronger armor all along their backs and even, in some cases, along their undersides. They came into greater prominence during the Cretaceous period, when the stegosaurs died out.

Besides the thyreophorans, there were the cerapods - the collective name for both the ornithopods and the marginocephalians. Ornithopods became more diverse during the Jurassic (''[[UsefulNotes/PrehistoricLifeDinosaurs Dryosaurus]]'' and ''[[UsefulNotes/PrehistoricLifeDinosaurs Camptosaurus]]'' were widespread at that time) but they really came into their own during the Cretaceous period. The marginocephalians possibly had late Jurassic roots, but they are better known for their Cretaceous forms, and originally evolved from very small ornithopod-like forms such as ''[[UsefulNotes/PrehistoricLifeDinosaurs Yinlong]]''.

* Many distinctive dinosaurs, such as the sauropods ''[[StockDinosaursTrueDinosaurs Brachiosaurus]]'' and ''[[StockDinosaursTrueDinosaurs Apatosaurus]]'', ''[[StockDinosaursTrueDinosaurs Stegosaurus]]'', ''[[StockDinosaursTrueDinosaurs Allosaurus]]'', and the earliest birds lived during the Jurassic, but others, such as ''[[TyrannosaurusRex Tyrannosaurus rex]]'', ''[[StockDinosaursTrueDinosaurs Triceratops]]'', and ''[[StockDinosaursTrueDinosaurs Velociraptor]]'' lived during the Cretaceous. So ''Tyrannosaurus'' never got to fight ''Stegosaurus''. In terms of geologic time, ''Tyrannosaurus'' is closer in time to us than to ''Stegosaurus''!

Dinosaurs continued to diversify further in the Cetaceous. Among ornithischians, marginocephalians differentiated into two subgroups. The pachycephalosaurs remained small and bipedal but developed thick skulls for uncertain purposes; the ceratopsians become progressively heavier and returned in a quadrupedal body-plan. Their beaks became parrot-shaped, they evolved protrusions from their cheeks and a large "frill" from the backs of their heads, such as the protoceratopsids; later, these smallish animals evolved in the large, rhino like ceratopsids, including ''[[UsefulNotes/PrehistoricLifeDinosaurs Chasmosaurus]]'', ''[[StockDinosaursTrueDinosaurs Styracosaurus]]'', the ever-popular ''[[StockDinosaursTrueDinosaurs Triceratops]]'', and ''[[UsefulNotes/PrehistoricLifeDinosaurs Torosaurus]]'', though recently there have been discussions about the synonymity of the latter with ''Triceratops''. (Note that the split between pachycephalosaurs and ceratopsians occurred in the Jurassic, but fossils of Jurassic pachycephalosaurs have not yet been found.) Ornithopods, meanwhile, included both small species such as ''[[UsefulNotes/PrehistoricLifeDinosaurs Hypsilophodon]]'' and huge animals, the most spectacular of which were the styracosternans, including hadrosaurs (some of the latter were the largest dinosaurs after the sauropods) and ''Iguanodon'', all of which had a specialized front foot with a thumb-spike (lost in hadrosaurids), three padded fingers to support the animal's weight during quadrupedal locomotion, and a flexible little finger for grasping food. Between these two extremes were middle-sized creatures like ''[[UsefulNotes/PrehistoricLifeDinosaurs Tenontosaurus]]'' and ''[[UsefulNotes/PrehistoricLifeDinosaurs Muttaburrasaurus]]''.

During their permanence on Earth, the dinosaurs thrived in diverse terrestrial habitats, from swampy terrain and dense forests to open prairies and the driest of deserts. Some even weathered the harsh winter conditions of Antarctica and Australia (which was near the South Pole at the time). However, the dinosaurs did not survive the Cretaceous-Paleogene mass extinction event which wiped out 65% of all living things. There have been several hypotheses in the past about this event, which run from very unlikely (such as egg-devouring mammals and even auto-destruction) to the most charming ones (like a supernova).

Here is the most likely hypothesis currently available. The extinction began with an increase in volcanic activity during the last few million years of the Cretaceous period, which would have introduced toxic gases and ash clouds into the atmosphere, interfering with the relatively stable weather conditions the dinosaurs had enjoyed all over the globe. Certainly, the fossil evidence suggests they were already in a state of decline (at least in North America) when the fateful asteroid/comet, about 65.5 million years ago, [[RocksFallEveryoneDies collided with the Gulf of Mexico]], producing the Chicxulub crater. This wouldn't have instantaneously wiped out all dinosaurs on Earth, but the collision would most likely have kicked up a huge cloud of dust and gas and have triggered a series of violent volcanic eruptions across the world. When the vast clouds blotted out the sun, most plant life was prevented from producing enough food from photosynthesis, as well as blocking out the sun's rays. This process would most likely have taken tens of thousands of years, perhaps even millions, but when it finished the dinosaurs were gone. The only surviving dinosaurs, technically speaking, are the neornithines, or modern-type birds.

This extinctions also hit many other animals as well, such as the flying pterosaurs, most sea reptiles, and ammonites. Many crocodilians and crocodile relatives as well as mammals also died as well, but other groups like freshwater crocodilians, lizards and turtles made their way in the following era - the Cenozoic. However, the animals which most benefited from the extinction event were the surviving mammals, which underwent an explosive evolution since that, substituting dinosaurs in their former ecological niches and often becoming much larger than they were during the dinosaurs’ reign. Despite this, no land mammal managed to reach the record-size of the greatest sauropods, though the weight of some did come close to things like ''Diplodocus''.

Main points:
* While long called the Cretaceous-Tertiary (K-T) extinction, this event is more properly called the Cretaceous-Paleogene (K-Pg) extinction. (The old term "Tertiary" has been abolished in favor of the more evenly spread Paleogene and Neogene Periods.)
* The Cretaceous spanned a long time. Not all dinosaurs that lived in the Cretaceous lived at the time of the Cretaceous-Paleogene mass extinction. Spinosaurids, for example, lived during the Early Cretaceous but went extinct long before the mass exinction.
* Dinosaurs didn't really go exinct, as birds still live today. Nonetheless, most dinosaur ''lineages did'' die out, including most bird groups.
* In spite of being portrayed often as K-Pg survivors, most croc and mammal groups were also killed in the extinction, with only a few lineages surviving.
* Not all the surviving lineages are still alive today. Multituberculate mammals and champsosaurs survived the extinction but died later in the Cenozoic.

* Many distinctive dinosaurs, such as the sauropods ''[[StockDinosaursTrueDinosaurs Brachiosaurus'' and ''[[StockDinosaursTrueDinosaurs Apatosaurus]]'', ''[[StockDinosaursTrueDinosaurs Stegosaurus]]'', ''[[StockDinosaursTrueDinosaurs Allosaurus]]'', and the earliest birds lived during the Jurassic, but others, such as ''[[TyrannosaurusRex Tyrannosaurus rex]]'', ''[[StockDinosaursTrueDinosaurs Triceratops]]'', and ''[[StockDinosaursTrueDinosaurs Velociraptor]]'' lived during the Cretaceous. So ''Tyrannosaurus'' never got to fight ''Stegosaurus''. In terms of geologic time, ''Tyrannosaurus'' is closer in time to us than to ''Stegosaurus''!

There is a tendency in popular culture to identify ''every'' prehistoric reptile as a dinosaur, especially if it is big, nasty-looking or just plain weird. This gets to the point that the word "dinosaur" is used as a synonym for "prehistoric critter". In the most extreme cases, [[DinosaursAreDragons legendary characters]] are called dinosaurs. It is important to point out that actual dinosaurs, despite being astonishingly diverse in size, shape and possibly even behaviour, are all restricted to the criteria mentioned below. Thus, pterosaurs (flying reptiles) and the sea-living Mesozoic reptiles, ichthyosaurs, plesiosaurs, and mosasaurs, are not dinosaurs at all. Furthermore, dinosaurs always walked with their limbs tucked under their bodies and upright, just like humans, ostriches and elephants, and very unlike modern reptiles. Finally, if ''[[StockDinosaurs Dimetrodon]]'' is ever described anywhere as a dinosaur, you have a right to get angry; not only was it a sail-backed synapsid (the group including mammals and their extinct kin) and more closely related to the mammals than to dinosaurs, but it lived at completely the wrong time. Most dinosaurs lived in the Mesozoic Era, (251-65 million years ago). ''Dimetrodon'' lived in the Permian period, (295-251 million years ago), when the dinosaurs hadn't even evolved yet. Or in other words, that ''Dimetrodon'' is ''your'' uncle, not that of tonight's chicken dinner.

Dinosaurs are divided into two different groups. The first group is known as the Saurischia, or “lizard-hipped” dinosaurs, which include the meat-eating dinosaurs, or theropods, and those long-necked, plant-eating dinosaurs, the sauropodomorphs (which include the giant sauropods). Despite how vividly different a ''[[TyrannosaurusRex Tyrannosaurus]]'' and a ''[[StockDinosaurs Brachiosaurus]]'' look, the earliest theropods and sauropods looked very similar. One early saurischian, ''[[UsefulNotes/PrehistoricLifeDinosaurs Eoraptor]]'', has been considered a theropod, a sauropodomorph, or a basal saurischian that didn’t belong to either group.

The second group is known as the Ornithischia, or “bird-hipped” dinosaurs, and are mostly herbivorous dinosaurs, very different in body-shape both when compared with saurischians and when compared with each other. This group includes the ornithopods, such as the duck-billed hadrosaurs and ''[[StockDinosaurs Iguanodon]]'', the thyreophorans, such as the stegosaurs and ankylosaurs, and the marginocephalians, such as the thick-skulled pachycephalosaurs and horned and frilled ceratopsians. When the names of the Saurischia and the Ornithischia were first coined, they were differentiated chiefly by their pelvic arrangement, with the saurischian hip bones arranged more like a lizard hip, the pubis bone pointing forwards (but this is the case in crocs and mammals and turtles as well, it’s not really something special about lizards), and the ornithischian hip bones more like an avian hip, the pubis bone pointing backwards. (An adaptation designed to accomodate longer guts for the digestion of vegetation.)

Birds, considered as a distinct class of vertebrate in traditional systematics, are actually theropod dinosaurs, a hypothesis already proposed in the eighteenth century after the discovery of the famous ''[[StockDinosaurs Archaeopteryx]]'' (an animal with dinosaurian skeleton ''and'' feathered wings and tail) but rejected by most scientists for a long amount of time. The link between dinosaurs and bird through ''Archaeopteryx'' was resurrected again in the 1960s, and has been definitively proven only in the 1990's by the long list of feathered dinosaurs and early birds recently found in the fossil record, which show strong anatomical similarities - especially the presence of impression of protofeathers and true feathers; to the point that telling apart bird-like dinosaurs (such as ''[[StockDinosaurs Velociraptor]]'', ''[[UsefulNotes/PrehistoricLifeDinosaurs Caudipteryx]]'', ''[[UsefulNotes/PrehistoricLifeDinosaurs Beipiaosaurus]]'', ''[[UsefulNotes/PrehistoricLifeDinosaurs Sinosauropteryx]]'') and dino-like birds (''[[UsefulNotes/PrehistoricLifeOtherExtinctCreatures Jeholornis]]'', ''[[UsefulNotes/PrehistoricLifeOtherExtinctCreatures Confuciusornis]]'' and so on) has become very difficult today.

Interestingly, some ornithischian dinosaurs like ''[[UsefulNotes/PrehistoricLifeDinosaurs Psittacosaurus]]'' and ''[[UsefulNotes/PrehistoricLifeDinosaurs Tianyulong]]'' have been discovered with quills or other structures that strongly resemble feather structures, which has raised intriguing questions about dinosaurs’ appearance. Some think all dinosaurs originally had some sort of covering at the start of their evolution; then this covering (perhaps some sort of hypothetical "down") was lost in some lineages, or at least modified into other specialized structures (the quills on ''Psittacosaurus'' or even the dorsal spines on the sauropod ''[[StockDinosaurs Diplodocus]]'' may have this origin). The fact that the closely-related pterosaurs have a covering made of a sort of hollow hair seems to confirm this hypothesis. However, hair-like or feather-like structure may be evolved independently in these animal groups: if so, it will be an example of convergent evolution.

The early dinosaurs appeared roughly 230 million years ago, descended from tiny archosaurs such as the 1 ft-long ''[[UsefulNotes/PrehistoricLifeNonDinosaurianReptiles Lagosuchus]]'', but did not make their impact felt until ten million years later, when they grew from small, unassuming bipeds to impressively large forms. The three main lineages were forged at this crucial time: the meat-eating theropods, the long-necked sauropodomorphs, and the plant-eating ornithischians, although at this stage they all looked like variations of the thin, elegant Theropods like ''[[UsefulNotes/PrehistoricLifeDinosaurs Coelophysis]]'' and its relatives, and it's quite possible that all of them were omnivorous initially. They were notable for taking the bipedal stance, which was also adopted by some triassic archosaurs related to modern crocodilians, such as ''[[UsefulNotes/PrehistoricLifeNonDinosaurianReptiles Ornithosuchus]]'' and the deceptively-dinosaurian ''[[UsefulNotes/PrehistoricLifeNonDinosaurianReptiles Effigia]]'', some of these were indeed mistaken for dinosaurs when first discovered. To obtain this result, early dinosaurs developed a horizontal backbone but vertical joints to the pelvis, which meant that their legs were tucked underneath their body to support their weight. This also permitted them an exceptionally good turn of speed, and with their long tails to serve as counterbalances, the dinosaurs had hit upon a good design feature which would serve them well again and again.

Dinosaurs were pretty successful even at this early stage - by the end of the Triassic period, they had diversified into some of the largest animals ever to appear on the land. Early sauropodomorphs in particular (traditionally called "prosauropods", which means "before the sauropods") reached lengths and heights never seen before, like ''[[StockDinosaurs Plateosaurus]]'', which could grow up to twenty feet long, and the even larger ''[[UsefulNotes/PrehistoricLifeDinosaurs Riojasaurus]]'' and ''[[UsefulNotes/PrehistoricLifeDinosaurs Melanorosaurus]]'', both elephant-sized and thirty feet long. On the other hand, predatory theropods remained generally small in the Triassic, with some exceptions such as ''[[{{Godzilla}} Gojira]][[UsefulNotes/PrehistoricLifeDinosaurs saurus]]'' (whoever said that paleontologists had no sense of humor?), which could reach 15 ft in length. Most ornithischians were still small plant-eaters, though even here, specialized new forms were emerging - such as the heterodontosaurids, creatures with large canine teeth that may have been used in mating disputes.

* Many distinctive dinosaurs, such as the sauropods ''[[StockDinosaurs Brachiosaurus]]'' and ''[[StockDinosaurs Apatosaurus]]'', ''[[StockDinosaurs Stegosaurus]]'', ''[[StockDinosaurs Allosaurus]]'', and the earliest birds lived during the Jurassic, but others, such as ''[[TyrannosaurusRex Tyrannosaurus rex]]'', ''[[StockDinosaurs Triceratops]]'', and ''[[StockDinosaurs Velociraptor]]'' lived during the Cretaceous. So ''Tyrannosaurus'' never got to fight ''Stegosaurus''. In terms of geologic time, ''Tyrannosaurus'' is closer in time to us than to ''Stegosaurus''!

Dinosaurs continued to diversify further in the Cetaceous. Among ornithischians, marginocephalians differentiated into two subgroups. The pachycephalosaurs remained small and bipedal but developed thick skulls for uncertain purposes; the ceratopsians become progressively heavier and returned in a quadrupedal body-plan. Their beaks became parrot-shaped, they evolved protrusions from their cheeks and a large "frill" from the backs of their heads, such as the protoceratopsids; later, these smallish animals evolved in the large, rhino like ceratopsids, including ''[[UsefulNotes/PrehistoricLifeDinosaurs Chasmosaurus]]'', ''[[StockDinosaurs Styracosaurus]]'', the ever-popular ''[[StockDinosaurs Triceratops]]'', and ''[[UsefulNotes/PrehistoricLifeDinosaurs Torosaurus]]'', though recently there have been discussions about the synonymity of the latter with ''Triceratops''. (Note that the split between pachycephalosaurs and ceratopsians occurred in the Jurassic, but fossils of Jurassic pachycephalosaurs have not yet been found.) Ornithopods, meanwhile, included both small species such as ''[[UsefulNotes/PrehistoricLifeDinosaurs Hypsilophodon]]'' and huge animals, the most spectacular of which were the styracosternans, including hadrosaurs (some of the latter were the largest dinosaurs after the sauropods) and ''Iguanodon'', all of which had a specialized front foot with a thumb-spike (lost in hadrosaurids), three padded fingers to support the animal's weight during quadrupedal locomotion, and a flexible little finger for grasping food. Between these two extremes were middle-sized creatures like ''[[UsefulNotes/PrehistoricLifeDinosaurs Tenontosaurus]]'' and ''[[UsefulNotes/PrehistoricLifeDinosaurs Muttaburrasaurus]]''.

This extinctions also hit many other animals as well, such as the flying pterosaurs, most sea reptiles, and ammonites. Many crocodilians and crocodile relatives as well as mammals also died as well, but other groups like freshwater crocodilians, lizards and turtles made their way in the following era - the Cenozoic. However, the animals which most benefited from the extinction event were the surviving mammals, which underwent an explosive evolution since that, substituting dinosaurs in their former ecological niches and often becoming much larger than they were during the dinosaurs’ reign. Despite this, no land mammal managed to reach the record-size of the greatest sauropods, though the weight of some did come close to things like ''[[StockDinosaurs Diplodocus]]''.

If you are interested in specific kinds of dinosaurs, just [[StockDinosaurs check]] [[UsefulNotes/PrehistoricLifeDinosaurs here]] some info.