What Is A Dinosaur, Exactly?
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 when the word "dinosaur" is used as a synonym for "prehistoric monster". In the most extreme cases, legendary characters are called dinosaurs. 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 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 evolutionary 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. 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, a rattlesnake, is a collection of features in the skeleton which all dinosaurs share, but which lizards and crocodilians don't. These include:
The Dinosaur Family Tree
Dinosaurs are divided into two different groups, and those two groups are not "meat-eaters" and "plant-eaters". note 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. 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 Tyrannosaurus looks to a Brachiosaurus, the earliest theropods and sauropodomorphs 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, mammals and turtles as well, so it is not something unique about lizards), and the ornithischian hip bones more like an avian hip, the pubis bone pointing backwards. This latter was possibly an adaptation designed to accomodate longer guts for the digestion of vegetation, which is notoriously hard to digest. 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. 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? One answer is that the pelvis of all members of a group is similar in a variety of details, not merely whether the pubis points forward or back. And 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. Main points:
My Cousin Is Also An Alligator
Dinosaurs are close cousins to the crocodilians, both being archosaurs; they share many traits, such as alveolate teeth, solid skulls, diapsid openings, and a bipedal posture, though this was later replaced with a semi-erect gait in crocodilians. It may appear odd, but modern crocodilians descended from bipedal, dinosaur-looking ancestors which returned to a four-legged body plan after their adaptation to water. This kind of back-and-forth evolving is quite common in nature, with cetaceans, ostriches, seals, flightless beetles and turtles being good examples of animals that evolved out of one adaptation and then evolved back into it again. Birds, considered as a distinct class of vertebrate in traditional systematics, are actually theropod dinosaurs, a hypothesis already proposed in the nineteenth 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 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 Velociraptor, Caudipteryx, Beipiaosaurus, Sinosauropteryx) and dino-like birds (Jeholornis, 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 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 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. Main points:
Triassic - A Brief History of the Dinosaurs
And now, the history. We've got a lot to cover - ninety thousand times more dinosaur history than human history, for a start. 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", which had ruled in waves during the Permian period, were the first to spread on the planet, and it looked like they were set to rule it all again. There were two groups of these, mostly the herbivorous dicynodonts and the cynodonts. Mammals evolved from the latter set; the former died out without leaving any descendants. However, both groups quickly found themselves under stiff competition against the first archosaur reptiles, from which the dinosaurs would emerge. The archosaurs were particularly effective thanks to their water-conserving adaptations, which were typical of most reptiles. Quickly, the archosaurs divided into several groups, such as rauisuchians, aetosaurs, phytosaurs and proterosuchids, and by seizing control they reduced the variety of the mammal-like reptiles, which survived only through their smaller species. Later, cynodonts evolved many special features that are typical of modern mammals today, humans included. 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 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 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. Indeed, some of these were mistaken for dinosaurs when they were first discovered. To walk in the upright stance, 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. 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 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 (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. They are almost never talked about in pop culture, though the scientific interest in them sky-rocketed with the discovery of feather-like structures 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. Main points:
Jurassic - In the Middle Ages of Dinosaur Times
Eventually, the Triassic-Jurassic mass extinction changed history. For one thing, it wiped out 20% of all life that had survived the Triassic, including many archosaurs, the dicynodonts, and most of the cynodonts. Somehow, the dinosaurs survived. No one yet knows why. One of the advantages of being a surviving lineage of a mass extinction event, however, is that, most of the competition being dead, you have plenty of room for evolutionary diversity, and with fewer 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 was breaking up to form two giant landmasses Laurasia and Gondwana, or the northern and southern supercontinents respectively. There were also small islets and isolated geographical regions where dinosaur evolution could follow different pathways, such as the scattered islands of flooded Europe and the isolated subcontinent of India when it broke off later from Gondwana. The northern Laurasia consisted of North America, Europe, and most of Asia, while the southern Gondwana consisted of South America, Africa, India (before it broke away later on in that period), Madagascar, Australia and Antarctica. 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 later 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 pressed 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. The purpose of these plates is unclear, but the purpose of their tail spikes and shoulder spikes was arguably for self-defense against any animal that tried to hurt or kill them. 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. 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, possibly because of the spread of the newly-evolved flowering plants which had appeared not long before, or because they coped better with the changing climate as sea levels rose worldwide. The marginocephalians possibly had Late Jurassic roots, but like the ornithopods they are better known for their Cretaceous forms, and originally evolved from very small ornithopod-like forms such as Yinlong. Main points:
Cretaceous – Good Times with a Downer Ending
Dinosaurs continued to diversify further in the Cretaceous, 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 Chasmosaurus, Styracosaurus, the ever-popular Triceratops, and 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 these days 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. Ornithopods, meanwhile, included both small species such as Hypsilophodon and huge species, the most spectacular of which were the styracosternans. These included the hadrosaurs, giant four-legged beasts with duck-like bills for cropping plants over a wide area, and iguanodontids like Iguanodon, all of which had a specialized 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 hadrosaurs do not have the thumb spike, and it was probably used for decoration or feeding rather than for defence. Between these two extremes were middle-sized creatures like Tenontosaurus and Muttaburrasaurus. Hadrosaurs were the more successful of the styracosternans, and came with a broad variety of crests and nasal passages decorating the skulls, which were probably used to tell each other apart in case one hadrosaur made the mistake of trying to mate with another hadrosaur from a different species. The styracosternans as a whole were successful, perhaps because of their chewing teeth, which could pulp up even the toughest plant material. Fossil nests and eggs show that hadrosaurs were devoted mothers, like many modern birds are, and looked after their babies until they were old enough to fend for themselves. Maiasaura is probably the best known hadrosaur whose fossils include evidence of maternal care - indeed, Maiasaura means "good mother lizard". 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 a short time, and then left to grow up on their own, often forming herds with other young dinos of their own kind. One particularly heartwarming fossil shows an Oviraptor mother/father (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.
The Boundary Event: 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 mass extinction event at the end of the Cretaceous period which wiped out roughly 65% of all living things, at least according to the fossil record. This is the event that inspires the trope Phlebotinum Killed the Dinosaurs, 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 Too Dumb to Live, 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 gigantic sauropods which had dominated in the Jurassic (and which, as far as can be made out, never had breathing difficulties). A further theory—one that didn't get fully debunked until the 1980s—claimed that the dinosaurs simply ran out of genetic diversity and were no longer capable of evolving to suit their climate. This was proven wrong by the fact that the fossil record remains just as rich and diverse right up to their extinction, and there were even signs that the dinosaurs were adapting quite well to the climate changes of their day. 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. 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. This interfered with the relatively stable weather conditions the dinosaurs had enjoyed all over the globe (most of the Mesozoic era was comparatively stable, at least when compared with the turbulent climate changes of most of the Cenozoic era). Certainly, the fossil evidence suggests they were already in a state of decline (at least in North America) when the fateful meteor, 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 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, and so vegetarians suddenly found themselves being forced into the ultimate crash diet. When their numbers fell, the carnivores in turn found their food in short supply and would have declined. Megafauna - particularly big animals - would have been the first to go, since their large bodies required a correspondingly large amount of food to fuel it. This process would most likely have taken tens of thousands of years, perhaps even millions of years, to end, but when it was finished, the dinosaurs were gone. The only surviving dinosaurs, technically speaking, are the neornithines, or modern-type birds. This extinction event was not exclusive to the dinosaurs. It also hit many other animals, such as the flying pterosaurs, the plesiosaurs, the mosasaurs, 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. Main points:Dinosaurs are now a pretty big hit in popular culture. Just go see the Tropesaurus Index, and you'll find links to the legacy that the dinosaurs left behind. Yes, that includes the bird index, too. If you are interested in specific kinds of dinosaurs, just check here for some info.