History UsefulNotes / Dinosaurs

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

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 ''Chasmosaurus'', ''Styracosaurus'', the ever-popular ''Triceratops'', and ''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 ''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 ''Tenontosaurus'' and ''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 animal groups 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.

If you are interested in specific kinds of dinosaurs, just check [[StockDinosaurs here]] some infos.

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 ''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 ''Tyrannosaurus'' and a ''Brachiosaurus'' look, the earliest theropods and sauropods looked very similar. One early saurischian, ''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 ''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.)

That much most textbooks and children's books will tell you, but the curious thing is that birds are theropod saurischians, not ornithischians, even though you'd expect some sort of bird-like hip arrangement to appear in their ancestors. So how did theropods bring about the bird group if they had the lizard hip, not the bird hip? Fortunately, this is quickly resolved: ornithischians are not the only dinosaurs to have bird-like hips. Many maniraptorans, those theropod dinosaurs which include birds as well as the theropods most closely related to birds (and are therefore what most people refer to when they talk about bird-like dinosaurs), have a bird-like hip arrangement, which appears to be a side effect of some adopting a knee-driven running style, and of others being weird herbivorous theropods. Interestingly, the most basal ornithischians, such as ''Pisanosaurus'', hadn't yet evolved the backwards-pointing pubis.

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.

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 ''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 ''Velociraptor'', ''Caudipteryx'', ''Beipaosaurus'', ''Sinosauropteryx'') and dino-like birds (''Shenzhouraptor'', ''Confuciusornis'' and so on) has become very difficult today.

Interestingly, some ornithischian dinosaurs like ''Psittacosaurus'' and ''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 ''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.

* Crocodilians are the closest living relatives to dinosaurs, and have therefore a lot more to do with birds than with lizards.

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 ''Eoraptor'' and ''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 ''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 ''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 ''Ornithosuchus'' and the deceptively-dinosaurian ''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 ''Plateosaurus'', which could grow up to twenty feet long, and the even larger ''Riojasaurus'' and ''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]]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.

Besides the thyreophorans, there were the cerapods - the collective name for both the ornithopods and the marginocephalians. Ornithopods became more diverse during the Jurassic (''Dryosaurus'' and ''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 ''Yinlong''.

* Many distinctive dinosaurs, such as the sauropods ''Brachiosaurus'' and ''Apatosaurus'', ''Stegosaurus'', ''Allosaurus'', and the earliest birds lived during the Jurassic, but others, such as TyrannosaurusRex, ''Triceratops'', and ''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 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 ''Plateosaurus'', which could grow up to twenty feet long, and the even larger ''Riojasaurus'' and ''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 ''Gojirasaurus'', 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.

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 ''Dimetrodon'' is ever described anywhere as a dinosaur; 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.

[[VindicatedByHistory Dinosaurs]] are now a pretty big hit in popular culture. Just go see the TropesaurusIndex, and you'll find links to the legacy that the dinosaurs left behind. Yes, that includes the bird index, too.

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 had air sacs in the vertebrae that ornithischians lack.

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 the Supernova).

Here is the most likely theory 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, 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.

Interestingly, some ornithischian dinosaurs like ''Psittacosaurus'' and ''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 those on the sauropod ''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.

Here is the most likely theory 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, 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, possibly because they were high in the food chain and therefore vulnerable to major catastrophic change. The only surviving dinosaurs, technically speaking, are the neornithines, or modern-type birds.

* 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.

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, 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 ''Dimetrodon'' is ever described anywhere as a dinosaur; 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.

* 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), and even some prehistoric mammals (which were really... [[ExactlyWhatItSaysOnTheTin mammals]]).

That much most textbooks and children's books will tell you, but the curious thing is that birds are theropod saurischians, not ornithischians, even though you'd expect some sort of bird-like hip arrangement to appear in their ancestors. So how did theropods bring about the bird group if they had the lizard hip, not the bird hip? Fortunately, this is quickly resolved: ornithischians are not the only dinosaurs to have bird-like hips. Many maniraptorans, those theropod dinosaurs which include birds as well as the theropods most closely related to birds (and are therefore what most people refer to when they talk about bird-like dinosaurs), have a bird-like hip arrangement, which appears to be a side effect of some adopting a knee-driven running style, and of others being weird herbivorous theropods. Interestingly, the most basal ornithsichians, such as ''Pisanosaurus'', hadn't yet evolved the backwards-pointing pubis.

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. The earliest dinosaur forms, such as ''Eoraptor'' and ''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 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). 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.

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 ''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.

The curious thing is that birds are theropod saurischians, not ornithischians, even though you'd expect some sort of bird-like hip arrangement to appear in their ancestors. So how did theropods bring about the bird group if they had the lizard hip, not the bird hip? Fortunately, this is quickly resolved: ornithischians are not the only dinosaurs to have bird-like hips. Many maniraptorans, those theropod dinosaurs which include birds as well as the theropods most closely related to birds (and are therefore what most people refer to when they talk about bird-like dinosaurs), have a bird-like hip arrangement, which appears to be a side effect of them adopting a knee-driven running style.

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 had air sacs in the vertebrae that ornithischians lack.

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. The earliest dinosaur forms, such as ''Eoraptor'' and ''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, whose real name is Therapsids) were the first to spread on the planet: they were mostly the herbivorous dicynodonts and the very mammal-like cynodonts (the ancestors of mammals). However, they rapidly underwent the strong competition by the once-called "thecodonts", that is the first archosaur reptiles; the latter were particularly effective, 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 ''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 Sauropods, and the plant-eating Ornithischians, although at this stage they all looked like variations of the thin, elegant Theropods like ''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 ''Ornithosuchus'' and the deceptively-dinosaurian ''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. Sauropod ancestors in particular (traditionally called "prosauropods", which means "before the sauropods") reached lenghts and heights never seen before, like ''Plateosaurus'', which could grow up to twenty feet long, and the even larger ''Riojasaurus'' and ''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 ''Gojirasaurus'', 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 Heterodontosaurs, creatures with large canine teeth that may have been used in mating disputes.

Eventually, the Triassic-Jurassic mass extinction wiped out 20% of all life that had survived the Triassic, including most of the other archosaurs, the dicynodonts, and the cynodonts which gave rise to the first tiny true mammals. 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 branched out from the basal Coelophysids to produce the Dilophosaurids, the Ceratosaurids and the Tetanurae. The Coelophysids and Dilophosaurids did not last much further than the early Jurassic, but the Ceratosaurids did well and the remaining Tetanurae, or 'stiff-tailed' Theropods became spectacularly successful (the Ceratosaurids typically had much more flexible tails, among other distinguishing features). 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 later Ornithopods.

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. 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 armour 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, though some fossils indicate that they survived up to the early Cretaceous.

Besides the Thyreophorans, there were the Cerapods - the collective name for both the Ornithopods and the Marginocephalians. Ornithopods became more diverse during the Jurassic (''Dryosaurus'' and ''Camptosaurus'' were widespread at that time) but they really came into their own during the Cretaceous period. Ornithopods were either obligate or facultative bipedal, and were armour-lacking unlike Stegosaurs and Ankylosaurs; they included both small species such as ''Hypsilophodon'' or ''Leaellynasaura'', and huge animals, the most spectacular of which were the Hadrosaurs in the late Cretaceous (some of the latter were the largest dinosaurs after the sauropods) and ''Iguanodon'', which also had a specialised front foot with a thumb-spike, 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 ''Tenontosaurus'' and ''Ouranosaurus''. 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. They differentiated in two subgroups. The pachycephalosaurs remained small and bipedal but developed thick head for uncertain purpose; 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 ''Chasmosaurus'', ''Styracosaurus'', the ever-popular ''Triceratops'', and ''Torosaurus'', though recently there have been discussions about the synonymity of the latter with ''Triceratops''.

[[folder:Cretaceous - Towards The End Of The Dinosaurs]]

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-Tertiary mass extinction event which wiped out 65% of all living things. There have been several theories 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 the Supernova).

Here is the most likely theory 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 when the fateful asteroid/comet, about 65.5 million years ago, 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. An ice age settled when the vast clouds blotted out the sun, preventing most plant life 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, possibly because they were high in the food chain and therefore vulnerable to major catastrophic change. The only surviving dinosaurs, technically speaking, are the birds.

This extinctions also hit many other animals as well, such as the flying pterosaurs, most sea-reptiles, and ammonites, while other groups like crocodilians, lizards and turtles made their way in the following era - the Cenozoic. However, the animal group which most benefited to the extinction event was the mammals, which underwent an explosive evolution since that, substituting dinosaurs in their former ecological niches and often becoming gigantic, even though not reaching the records obtained by the largest dinosaurs.

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, 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 Dimetrodon is ever described anywhere as a dinosaur; not only was it a sail-backed synapsid and more closely related to the mammal-like reptiles than to dinosaurs, but it lived at completely the wrong time. Dinosaurs all lived in the Mesozoic Era, (251-65 million years ago). Dimetrodon and its relatives lived in the Permian period, (295-251 million years ago), when the dinosaurs hadn't even evolved yet.

So '''what is a dinosaur?''' Well, time to get a bit technical. If we ask the taxonomists, a "dinosaur" is any species that falls under the group called Dinosauria. [[CaptainObvious A bit obvious]], but bear with me. What distinguishes a dinosaur from, say, other reptiles is a collection of features in the skeleton which all dinosaurs share, but which snakes, 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 favour 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 subclade Aves, which includes both ancient and modern birds. While modern birds admittedly lack the long counterbalancing tail and have reduced forearms and skull openings, they otherwise fit the criteria mentioned above, especially when you consider their evolutionary history. From this point on, when the word dinosaur is used, we mean it in the paraphyletic sense, this being what the average person will understand by the term.

[[folder:Two Sides Of A Dinosaur War]]

Surprisingly, when we talk about dinosaurs we are actually talking about two different (albeit closely related) groups of animals, each with its own anatomical features which distinguish it from the other. The first group is known as the Saurischia, or 'lizard-hipped' dinosaurs, which include the meat-eating dinosaurs, or Theropods, and those immense, long-necked, plant-eating dinosaurs, the Sauropods. Despite how vividly different these two types look, their ancestry is so tight that at times it's hard to say which evolved from which, or even if that's a meaningful question to pursue.

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 iguanodontids, the Thyreophorans, such as the stegosaurs and ankylosaurs, and the Marginocephalians, such as the thick-skulled pachycephalosaurs and horned ceratopsians. The distinction between the two groups, the Saurischia and the Ornithischia, is based chiefly on their pelvic arrangement, with the saurischian hip bones arranged more like a reptilian hip, and the ornithischian hip bones more like an avian hip.

The curious thing is that birds evolved from saurischians, not ornithischians, and so you'd expect some sort of bird-like hip arrangement to appear in their dinosaur ancestors. But their ancestors were saurischians, not ornithischians. So how did Theropods bring about the bird group if they had the lizard hip, not the bird hip? Fortunately, this is quickly resolved: ornithiscians are not the only dinosaurs to have bird-like hips. Maniraptorans, those Theropod dinosaurs which look a lot like birds (and are therefore what most people refer to when they talk about bird-like dinosaurs), have a bird-like hip arrangement, and an interesting question would be how (and why) this hip modification evolved.

Luckily, Ornithischians can still be kept separate from them 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 battery of chewing teeth lined up either side of the face, giving it a certain ''hollow cheek'' look. This ability to chew sets them apart from the Sauropods, who had to rely on swallowed stones called gastroliths to pulp up the food for them in their stomachs. Theropods likewise couldn't chew their food and had to rely on their sharp teeth and claws to butcher the food before swallowing the chunks whole. Some of the best dinosaur fossils include individuals who died while doing this; a TyrannosaurusRex fossil was once found with a Triceratops leg bone caught in its throat.

Dinosaurs are close cousins to the crocodilians, both being archosaurs; they shared many traits, such as alveolate teeth, solid skulls, and the tendence 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.

Birds, considered as a distinct class of vertebrate in traditional systematics, actually evolved directly from Theropod dinosaurs, a theory already proposed in the eighteenth century after the discover of the famous ''Archaeopteryx'' (an animal with dinosaurian skeleton ''and'' feathered wing 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 ''Velociraptor'', ''Caudipteryx'', ''Beipaosaurus'', ''Sinosauropteryx'') and dino-like birds (''Shenzhouraptor'', ''Confuciusornis'' and so on) is a rather subjective argument today.

Interestingly, some Ornithischian dinosaurs like ''Psittacosaurus'' and ''Tianyulong'' have been discovered with quills or other structures that strongly resemble feather structures, which has raised intriguing questions about dinosaurs appearence. Some think all dinosaurs originally had some sort of covering at the start of their evolution; then this covering (perhaps some sort of hypotetical "down") was lost in larger specimens to avoid overheating (much like hairless elephants or hippos today), or at least modified in specialized structures
(the quills on ''Psittacosaurus'' or even those on the sauropod ''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 theory. 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".

Birds, considered as a distinct class of vertebrate in traditional systematics, actually evolved directly from Theropod dinosaurs, a theory already proposed in the eighteenth century after the discover of the famous ''Archaeopteryx'' (an animal with dinosaurian skeleton ''and'' feathered wing 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 ''Velociraptor'', ''Caudipteryx'', ''Beipaosaurus'', ''Sinosauropteryx'') and dino-like bird (''Ichthyornis, ''Confuciusornis'' and so on) is a rather subjective argument today.

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. Sauropod ancestors in particular (traditionally called "prosauropods", which means "before the sauropods") reached lenghts and heights never seen before, like ''Plateosaurus'', which could grow up to twenty feet long, and the even larger ''Riojasaurus'' and ''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 ''Gojirasaurus'', 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 Heterodontosaurs, creatures with large canine teeth that may have been used in mating disputes, and the early Thyreophorans, like ''Scelidosaurus'', which had small armoured scutes growing along its back.

Note that, if we're going to be accurate, dinosaurs are actually a paraphyletic group - they exclude members that would (some might argue ''should'') otherwise fit neatly in with the other members. In this case the dinosaur group excludes the subclade Aves, which includes both ancient and modern birds. While birds admittedly lack the long counterbalancing tail and have reduced forearms and skull openings, they otherwise fit the criteria mentioned above, especially when you consider their evolutionary history. From this point on, when the word dinosaur is used, we mean it in the exclusive sense, this being what the average person will understand by the term.

[[VindicatedByHistory Dinosaurs]] are now a pretty big hit in popular culture. Just go see the TropesaurusIndex, and you'll find links to the legacy that the dinosaurs left behind. Yes, that includes the bird index, too.
[[/folder]]

[[VindicatedByHistory]]
Dinosaurs are now a pretty big hit in popular culture. Just go see the TropesaurusIndex, and you'll find links to the legacy that the dinosaurs left behind. Yes, that includes the bird index, too.

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, 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 Dimetrodon is ever described anywhere as a dinosaur; not only was it a sail-backed synapsid and more closely related to the mammal-like reptiles than to dinosaurs, but it lived at completely the wrong time. Dinosaurs all lived in the Mesozoic Era (251-65 million years ago). Dimetrodon and its relatives lived in the Permian period, (295-251 million years ago).

Surprisingly, when we talk about dinosaurs we are actually talking about two different (albeit closely related) groups of animals, each with its own anatomical features which distinguish it from the other. The first group is known as the Saurischia, or 'lizard-hipped' dinosaurs, which include the meat-eating dinosaurs, or Theropods, and those immense, long-necked, plant-eating dinosaurs, the Sauropods. Despite how vividly different these two types look, their ancestry is so tight that at times it's hard to say which evolved from which, or even if that's a meaningful question to pursue. The second group is known as the Ornithischia, or 'bird-hipped' dinosaurs, and are mostly herbivorous dinosaurs, very different in body-shape among each other but always with toothed jaws ending with a sort of bill. This group includes the Ornithopods, such as the duck-billed hadrosaurs and iguanodontids, the Thyreophorans, such as the stegosaurs and ankylosaurs, and the Marginocephalians, such as the thick-skulled pachycephalosaurs and horned ceratopsians. The distinction between the two groups, the Saurischia and the Ornithischia, is based chiefly on their pelvic arrangement, with the saurischian hip bones arranged more like a reptilian hip, and the ornithischian hip bones more like an avian hip. The curious thing is that birds evolved from Saurischians, not Ornithischians.

Firstly, '''what is a dinosaur?''' Well, time to get a bit technical. If we ask the taxonomists, a "dinosaur" is any species that falls under the group called Dinosauria. [[CaptainObvious A bit obvious]], but bear with me. What distinguishes a dinosaur from, say, other reptiles is a collection of features in the skeleton which all dinosaurs share, but which snakes, 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 favour 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.

There is a tendancy 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 above. Thus, pterosaurs (flying reptiles) and the sea-living Mesozoic reptiles, 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 Dimetrodon is ever described anywhere as a dinosaur; not only was it a sail-backed synapsid and more closely related to the mammal-like reptiles than to dinosaurs, but it lived at completely the wrong time. Dinosaurs all lived in the Mesozoic Era (251-65 million years ago). Dimetrodon and its relatives lived in the Permian period, (295-251 million years ago).

Firstly, what is a dinosaur? Well, time to get a bit technical. If we ask the taxonomists, a "dinosaur" is any species that falls under the group called Dinosauria. [[CaptainObvious A bit obvious]], but bear with me. What distinguishes a dinosaur from, say, other reptiles is a collection of features in the skeleton which all dinosaurs share, but which snakes, 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 favour 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.

'''A Brief History of the Dinosaurs'''

Dinosaurs (or non-Avian dinosaurs if you want to exclude birds) 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. The earliest dinosaur forms, such as ''Eoraptor'' and ''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, whose real name is Therapsids) were the first to spread on the planet: they were mostly the herbivorous dicynodonts and the very mammal-like cynodonts (the ancestors of mammals). However, they rapidly underwent the strong competition by the once-called "thecodonts", that is the first archosaur reptiles; the latter were particularly effective, 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.

This extinctions also hit many other animals as well, such as the flying pterosaurs, most sea-reptiles, and ammonites, while other groups like crocodilians, lizards and turtles made their way in the following era - the Cenozoic. However, the animal group which most benefited to the extinction event was the mammals, which underwent an explosive evolution since that, substituting dinosaurs in their former ecological niches and often becoming gigantic, even though not reaching the records obtained by the largest dinosaurs.

A brief summary of dinosaurs, for those of us who wish to address a few problems where SomewhereAPalaeontologistIsCrying.

Firstly, what is a dinosaur? Well, if we follow the cladistic taxonomy, a "dinosaur" is any species pertaining to the group called Dinosauria; however, if we count the traditional sense, dinosaurs result to be a paraphyletic group which excludes the subclade Aves (to say, ancient and modern birds). Here we follow the second meaning (being this the common sense).

There is the tendence in pop-consciousness to identify ''every'' prehistoric reptile as a dinosaur (expecially those with large size and unusual appearence by modern standards), and sometime "dinosaur" is even synonymized with "prehistoric critter", and reaches the point to include even [[DinosaursAreDragons legendary characters]]. Actual dinosaurs, despite being astonishingly diverse in size, shape and ecology, were all land-living; thus, pterosaurs (flying reptiles) and sea-living Mesozoic animals were not dinosaurs at all. Furthermore, dinosaurs always walked with their limbs upright just like humans, ostriches and elephants, and unlike modern reptiles; and they all lived in the Mesozoic Era (220-65 million years ago).

Firstly, what is a dinosaur? Well, if we follow the cladistic taxonomy, a "dinosaur" is every species pertaining to the group called Dinosauria; however, if we count the traditional sense, dinosaurs result to be a paraphyletic group which excludes the subclade Aves (to say, ancient and modern birds). Here we follow the second meaning (being this the common one).

There is the tendence in pop-consciousness to identify ''every'' prehistoric reptile as a dinosaur (expecially those with large size and unusual appearence by modern standards), and sometime "dinosaur" is even synonymized with "prehistoric critter", and reaches the point to include even [[DinosaursAreDragons legendary characters]]. Actual dinosaurs, despite being astonishingly diverse in size, shape and ecology, were all land-living; thus, pterosaurs (flying reptiles) and sea-living animals which shared the dinosaurs' world were not dinosaurs at all. Furthermore, dinosaurs always walked with their limbs upright just like humans, ostriches and elephants, unlike modern reptiles; and they all lived in the Mesozoic Era (220-65 million years ago).

Surprisingly, when we talk about dinosaurs we are actually talking about two different (albeit closely related) groups of animals, each with its own anatomical features which distinguish it from the other. The first group is known as the Saurischia, or 'lizard-hipped' dinosaurs, which include the meat-eating dinosaurs, or Theropods, and those immense, long-necked, plant-eating dinosaurs, the Sauropods. Despite how vividly different these two types look, their ancestry is so tight that at times it's hard to say which evolved from which, or even if that's a meaningful question to pursue. The second group is known as the Ornithischia, or 'bird-hipped' dinosaurs, and are mostly herbivorous dinosaurs, very different in body-shape among each other but always with toothed jaws ending with a sort of bill. This group includes the Ornithopods, such as the duck-billed hadrosaurs and iguanodontids, the Thyreophorans, such as the stegosaurs and ankylosaurs, and the Marginocephalians, such as the thick-skulled pachycephalosaurs and horned ceratopsians. The distinction between the two groups, the Saurischia and the Ornithischia, is based chiefly on their pelvic arrangement, with the saurischian hip bones arranged more like a reptilian hip, and the ornithischian hip bones more like an avian hip. Ironically, it is generally believed that birds evolved from Saurischians, not Ornithischians.

Dinosaurs are close cousins to the crocodilians, both being archosaurs; they shared many traits, such as alveolate teeth, solid skulls, and the tendence to develop the bipedal posture (it may appear odd to you, 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.

Birds, considered as a distinct class of vertebrate in traditional systematics, actually evolved directly from Theropod dinosaurs, a theory already proposed in the eighteenth century after the discover of the famous ''Archaeopteryx'', an animal with dinosaurian skeleton ''and'' feathered wing 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 ''Velociraptor'', ''Caudipteryx'', ''Beipaosaurus'', ''Sinosauropteryx'') and dino-like bird (''Ichthyornis, ''Confuciusornis'' and so on) is a rather subjective argument today.

Dinosaurs (or non-Avian dinosaurs if you want to exclude birds) 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. The earliest dinosaur forms, such as ''Eoraptor'' and ''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, whose real name is Therapsids) were the first to spread on the planet: they were mostly the herbivorous dicynodonts and the very mammal-like cynodonts (the ancestors of mammals). However, they rapidly underwent the strong competition from the once-called "thecodonts", that is the first archosaur reptiles; the latter were particularly effective, differentiating in several groups such as rauisuchians, aetosaurs, phytosaurs and proterosuchids, and seizing control, reducing the variety of the mammal-like reptiles which decreased their size in response 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 ''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 Sauropods, and the plant-eating Ornithischians, although at this stage they all looked like variations of the thin, elegant Theropods like ''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 ''Ornithosuchus'' and the deceptively-dinosaurian ''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. T

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. Sauropod ancestors in particular (traditionally called "prosauropods" which means "before the sauropods") reached lenghts and heights never seen before, like ''Plateosaurus'', which could grow up to twenty feet long, and the even larger ''Riojasaurus'' and ''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 ''Gojirasaurus'', 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 Heterodontosaurs, creatures with large canine teeth that may have been used in mating disputes, and the early Thyreophorans, like ''Scelidosaurus'', which had small armoured scutes growing along its back.

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. The purpose of these plates is unclear, but the purpose of their tail and shoulder spikes was obviously to self-defend 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. They came into greater prominence during the Cretaceous period, when the stegosaurs died out, though some fossils indicate that they survived up to the early Cretaceous.

Besides the Thyreophorans, there were the Cerapods - the collective name for both the Ornithopods and the Marginocephalians. Ornithopods became more diverse during the Jurassic (''Dryosaurus'' and ''Camptosaurus'' were widespread at that time) but they really came into their own during the Cretaceous period. Ornithopods were either obligate or facultative bipedal, were armour-lacking unlike Stegosaurs and Ankylosaurs, and included both small species such as ''Hypsilophodon'' or ''Leaellynasaura'', and huge animals, the most spectacular of which were the Hadrosaurs in the late Cretaceous (some of the latter were the largest dinosaurs before the sauropods) and ''Iguanodon'', which also had a specialised front foot with a thumb-spike, 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 ''Tenontosaurus'' and ''Ouranosaurus''. 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. They differentiated in two subgroups. The pachycephalosaurs remained small and bipedal but developed thick head for uncertain purpose; 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 ''Chasmosaurus'', ''Styracosaurus'', the ever-popular ''Triceratops'', and ''Torosaurus'', though recently there have been discussions about the synonymity of the latter with ''Triceratops''.

Firstly, what is a dinosaur? Well, surprisingly, when we talk about dinosaurs we are actually talking about two different groups of animals, each with its own anatomical features which distinguish it from the other. The first group is known as the Saurischia, or 'lizard-hipped' dinosaurs, which include the meat-eating dinosaurs, or Theropods, and those immense, long-necked, plant-eating dinosaurs, the Sauropods. Despite how vividly different these two types look, their ancestry is so tight that at times it's hard to say which evolved from which, or even if that's a meaningful question to pursue. The second group is known as the Ornithischia, or 'bird-hipped' dinosaurs, and are mostly herbivorous dinosaurs. This group includes the Ornithopods, such as the duck-billed hadrosaurs and iguanodontids, the Thyreophorans, such as the stegosaurs and ankylosaurs, and the Marginocephalians, such as the thick-skulled pachycephalosaurs and horned ceratopsians. The distinction between the two groups, the Saurischia and the Ornithischia, is based chiefly on their pelvic arrangement, with the saurischian hip bones arranged more like a reptilian hip, and the ornithischian hip bones more like an avian hip. Despite this distinction, it is generally believed that birds evolved from Saurischians, not Ornithischians.

Dinosaurs are close cousins to the crocodilians, both being archosaurs, and the modern class of animals, the birds, evolved from Theropod dinosaurs, as has been proven by the long list of feathered dinosaurs and early birds so far found in the fossil record, which show strong anatomical similarities - especially the presence of protofeathers and true feathers in some species like Archaeopteryx, Velociraptor, Caudipteryx, Beipaosaurus, Sinosauropteryx, Ichthyornis and Confuciousornis. Bizarrely, some Ornithischian dinosaurs like Psittacosaurus have been discovered with quills that strongly resemble feather structures, which has raised intriguing questions about the ancestry of birds. Did they all really evolve from Theropods, or could some birds have evolved from these plant-eating dinosaurs? Was this a case of convergent evolution, and if so, how can we be sure which birds evolved from which dinosaurs? So far, the fossil evidence isn't strong enough to support the hypothesis of multiple bird ancestries, but if more turn up then it could become a viable theory.

Dinosaurs (or non-Avian dinosaurs if you want to exclude birds) dominated the Mesozoic era, the era which covers a vast geological timescale from 251 to 65.5 million years ago. The earliest dinosaur forms, such as Eoraptor and Eocursor, appeared during 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 archosaur reptiles were particularly effective, with rauisuchians, aetosaurs and the proterosuchids seizing control, but they were up against the mammal-like reptiles, mostly the dicynodonts and cynodonts, and for the most part were battling against each other.

The early dinosaurs appeared roughly 230 million years ago, 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 Sauropods, and the plant-eating Ornithischians, although at this stage they all looked like variations of the thin, elegant Theropods like Coelophysis and its relatives, and it's quite possible that all of them were omnivorous initially. They were notable for taking the bipedal stance often seen in Rauisuchians and the deceptively-dinosaurian Effigia, with 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. They weren't unique in this respect, however - animals which were only distantly related, like Effigia, also had such a design plan, and indeed were often mistaken for dinosaurs when first discovered.

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, like the monstrous Plateosaurus, which could grow up to thirty feet long, and the large predatory Gojirasaurus, which could also reach a similar size. Most Ornithischians were still small plant-eaters, though even here, strange new forms were emerging - such as the Heterodontosaurs, creatures with large canine teeth that may have been used in mating disputes, and the early Thyreophorans, like Scelidosaurus, which had small armoured scutes growing along its back.

Eventually, the Triassic-Jurassic mass extinction wiped out 20% of all life that had survived the Triassic, including most of the other archosaurs, the dicynodonts, and the cynodonts. 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.

Meanwhile, the plant-eating 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. The purpose of these plates is unclear, but the purpose of their tail spikes was obviously to attack 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. They came into greater prominence during the Cretaceous period, when the stegosaurs died out, though some fossils indicate that they survived up to the early Cretaceous.

Besides the Thyreophorans, there were the Cerapods - the collective name for both the Ornithopods and the Marginocephalians. These also became more diverse during the Jurassic, but really came into their own during the Cretaceous period. The Ornithopods include the smaller species, such as Hypsilophodon or Leallynasaura, and the larger animals, the most spectacular of which were the Hadrosaurs in the late Cretaceous. Between these two extremes were moderate-sized creatures like Tenontosaurus and Iguanodon, the latter of which also had a specialised front foot with a thumb-spike, three padded fingers to support the animal's weight during quadrupedal locomotion, and a flexible little finger for grasping food. The Marginocephalians possibly had late Jurassic roots, but they are better known for their Cretaceous forms, and originally evolved from the smaller primitive Ornithopods. Their heads grew thicker, their beaks sharper, they evolved protrusions from their jaws, skulls and the backs of their heads and, from this design, produced the bone-headed pachycephalosaurs, the crested protoceratopsids and the large horned ceratopsians, including Chasmosaurus, Styracosaurus and the ever-popular Triceratops, though recently there have been discussions about its synonymity with Torosaurus.

The dinosaurs thrived in diverse 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. However, the dinosaurs did not survive the Cretaceous-Tertiary mass extinction event which wiped out 65% of all living things. The extinction likely 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 when the fateful comet, about 65.5 million years ago, collided with the Gulf of Mexico, producing the Chicxulub crater. This wouldn't have instantaneously wiped out the dinosaurs, 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. An ice age settled when the vast clouds blotted out the sun, preventing most plant life 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, possibly because they were high in the food chain and therefore vulnerable to major catastrophic change. The only surviving dinosaurs, technically speaking, are the birds.

Meanwhile, the plant-eating 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. The purpose of these plates is unclear, but the purpose of their tail spikes was obviously used to attack 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. They came into greater prominence during the Cretaceous period, when the stegosaurs died out, though some fossils indicate that they survived up to the early Cretaceous.

Besides the Thyreophorans, there were the Cerapods - the collective name for both the Ornithopods and the Marginocephalians. These also became more diverse during the Jurassic, but really came into their own during the Cretaceous period. The Ornithopods include the smaller species, such as Hypsiolophodon or Leallynasaura, and the larger animals, the most spectacular of which were the Hadrosaurs in the late Cretaceous. Between these two extremes were moderate-sized creatures like Tenontosaurus and Iguanodon, the latter of which also had a specialised front foot with a thumb-spike, three padded fingers to support the animal's weight during quadrupedal locomotion, and a flexible little finger for grasping food. The Marginocephalians possibly had late Jurassic roots, but they are better known for their Cretaceous forms, and originally evolved from the smaller primitive Ornithopods. Their heads grew thicker, their beaks sharper, they evolved protrusions from their jaws, skulls and the backs of their heads and, from this design, produced the bone-headed pachycephalosaurs, the crested protoceratopsids and the large horned ceratopsians, including Chasmosaurus, Styracosaurus and the ever-popular Triceratops, though recently there have been discussions about its synonymity with Torosaurus.

The dinosaurs thrived in diverse 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. However, the dinosaurs did not survive the Cretaceous-Tertiary mass extinction event which wiped out 65% of all living things. The extinction likely 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 when the fateful comet, about 65.5 million years ago, collided with the Gulf of Mexico, producing the Chicxulub crater. This wouldn't have instantaneously wiped out the dinosaurs, 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. An ice age settled when the vast clouds blotted out the sun, preventing most plant life from producing enough food from photosynthesis, as well as blocking out the sun's rays. This process would mot likely have taken tens of thousands of years, perhaps even millions, but when it finished the dinosaurs were gone, possibly because they were high in the food chain and therefore vulnerable to major catastrophic change. The only surviving dinosaurs, technically speaking, are the birds.