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The world of the dinosaurs.

A very brief summary about dinosaurs, for those of us who wish to address a few problems with Artistic License – Paleontology.

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    What Is A Dinosaur, Exactly? 

There is a tendency in popular culture to identify every prehistoric reptile, or even animal, 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 creatures are called dinosaurs.

Actual dinosaurs, despite being astonishingly diverse in size, shape, and probably 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 (pterosaurs are closely related to dinosaurs, but themselves are not, and mosasaurs are actually closer to monitor lizards and snakes than they are to any dinosaur). 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 (and therefore humans) than to dinosaurs, but it lived at completely the wrong time. Most dinosaurs lived during the Mesozoic Era, (252-66 million years ago). Dimetrodon lived during the Permian period, (299-252 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 us. 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:

  • An upright stance, with the legs tucked directly under the body.
  • Two openings behind the eye socket as well as one opening between the eye socket and the nostril. This is called 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.
  • Feathers possibly being the default integument of dinosaurs (with certain groups reverting to the ancestral scales). Note that the pycnofibers of pterosaurs may be related to feathers, as are the quills on some ornithischians.
  • A typically horizontal spine with a vertical hip-thigh attachment and a long counterbalancing tail, especially in the bipedal forms. The brachiosaurids and therizinosaurs are some of the few groups to subvert this.

All these can tell you if the thing you're looking at is a bona fide dinosaur. Forms that have most of these traits, but not all of them, are probably non-dinosaurian dinosauromorphs.

Note that, under this definition, birds are dinosaurs. Dinosaurs as thought of by the general public are actually a paraphyletic group - they exclude members (in this case birds) that otherwise fit neatly in with the rest of the group according to descent and cousinship.

Main points:
  • Not everything big and dead is a dinosaur. Creatures often mislabelled "dinosaurs" in pop culture include pterosaurs (which were close relatives, but not dinosaurs themselves), plesiosaurs and pliosaurs (which were extinct marine reptiles), ichthyosaurs (also marine reptiles that may or may not be closely related to plesiosaurs), mosasaurs (which were marine lizards), Dimetrodon and other non-mammal synapsids (which weren't even reptiles, but closer to mammals), and even some prehistoric mammals (which were really... mammals). "Dinosaur" refers to a very specific lineage of animals; often with the definition "Megalosaurus bucklandii, Iguanodon bernissartensis, the LCA (Last Common Ancestor) of both genera, and all descendants thereof" or "Passer domesticus, Triceratops horridus, the LCA of both genera and all descendants thereof".
  • Birds are dinosaurs. Because there are over 10,000 described species of birds alive today, dinosaurs are therefore not extinct! And we eat them all the time! Sometimes irony is literally delicious.

All clear? Right. On to the next topic.

    The Dinosaur Family Tree 

Dinosaurs are divided into two main 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 known 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 to lizards), and the ornithischian hip bones more like an avian hip, the pubis bone pointing backwards. This latter was possibly an adaptation designed to accommodate longer guts for the digestion of vegetation, which is notoriously hard to digest compared to meat.

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. Bird-like hips therefore evolved independently in theropods several times. 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 (this is probably a reason why no ornithischian ever reached the size of the largest sauropods, and no bat reaches even close to the size of the largest flying bird).

Main points:

  • Although originally classified based on their hip arrangement (ornithischians had backwards-pointing hips, saurischians have forward-pointing hips), the most basal ornithischians still had forward-pointing pubis bones and some groups of saurischians have backwards-pointing pubis bones.
  • Birds are saurischian (lizard-hipped) dinosaurs, not ornithischian (bird-hipped) dinosaurs.
  • There are a few other differences that help distinguish between the two groups.

    My Cousin Is Also An Alligator 

Dinosaurs are close cousins to the crocodilians, both being archosaurs; they share many traits, such as alveolate teeth, more solid skulls, antorbital fenestrae (which was lost in crocodilians), a four-chambered heart, 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, endothermic (warm-blooded) ancestors which returned to a four-legged, ectothermic (cold-blooded) body plan after their adaptation to live in 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. Turtles might be next closest living relative of dinosaurs (although still quite distant), but the placement of turtles among the reptile tree of life is ongoing and controversial. note 

Even more closely related to dinosaurs than the crocodilians are the pterosaurs. Pterosaurs were the first vertebrates to evolve powered flight and they did so with a membrane of skin connecting the hindlimbs to the fourth finger, more akin to the wings of bats than the wings of birds. Like saurischian dinosaurs, pterosaurs had hollow bones, both as a weight saving adaptation and to meet the high oxygen demands of powered flight, though they likely evolved them independently from each other. Pterosaurs were covered in hairlike filaments called pycnofibers, which may share a common origin with dinosaurian feathers. Like dinosaurs, they almost certainly were warm blooded, but unlike them adopted a quadrupedal posture while on the ground. This was advantageous for them because the same muscles they used for flying were also used for launching themselves off the ground, unlike in birds where they launch off the ground using their hindlimbs which otherwise weigh themselves down in flight. This may have allowed pterosaurs to reach enormous sizes unseen in any other flying animal, with the largest members having a wingspan as wide as a school bus is long. This also meant that pterosaurs had quite scrawny hindlimbs, and certainly weren't capable of snatching things from the ground with their hindlimbs like a hawk. Unfortunately, the pterosaurs perished during the K-Pg extinction event 66 million years ago and left no living decendants. Despite their close relation, it is important to stress that pterosaurs were NOT dinosaurs.

Closer to dinosaurs than even the pterosaurs, we see a lot of small, carnivorous non-dinosaurian archosaurs like Lagosuchus and Marasuchus which you would be hard pressed to tell apart from generic small theropod dinosaurs. The major outlier of these close dino relatives are the silesaurids, which started as carnivores but later evolved into herbivores. Unlike the other close relatives of dinosaurs, silesaurs were facultative bipeds, running on two legs but otherwise spending most of their time on all fours. It has been suggested in recent times that silesaurs were actually dinosaurs, specifically they were a grade leading to the ornithischian dinosaursnote , though that hypothesis is still far from being proven. If true, Pisanosaurus would be a transitional form, as it has been lumped in with the silesaurs in some studies, and with the ornithischians in others. These forms died out in the Triassic-Jurassic mass extinction, leaving only the crocodylomorphs, pterosaurs and dinosaurs as the sole surviving archosaurs.

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. During this time birds were thought to have descended from "thecodonts" (a now rejected "wastebasket" grouping of Triassic reptiles) and any similarities between birds and dinosaurs were a product of convergent evolution. The link between dinosaurs and birds through Archaeopteryx was resurrected again in the 1960s during a period known as the Dinosaur Renaissance, and has been definitively proven only in the 1990s by the long list of feathered dinosaurs and early birds recently found in the fossil record, which show strong anatomical similarities.

Impressions of 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 or even before; 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 or at least add evidence to 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. The more recently discovered Kulindadromeus of Siberia has hair-like covering of several varieties, the most advanced of which resemble stage-3 theropod feathers, strengthening the idea that feathers are at least basal to dinosaurs.

Main points:

  • Crocodilians are the closest living relatives to dinosaurs, and have therefore a lot more to do with birds than with lizards. This might be true about turtles as well, an idea which has yet to be confirmed.
  • Pterosaurs were more closely related to dinosaurs than crocodilans are, and were the first flying vertebrates. Still, they were not dinosaurs.
  • At least some dinosaur lineages, particularly theropods, had feathers or feather-like structures. Whether such structures originated at the base of Dinosauria or were independently acquired in different lineages remains to be seen, but fossilized ornithscians bearing feather-like structures may link feathers to Dinosauria as a whole. Pterosaur pycnofibers may push the evolution of feathers even earlier than that, to at least the common ancestor of dinosaurs and pterosaurs.

    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 for most of the Mesozoic era, the era which covers a vast geological timescale from 252.2 to 66 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 (the Permian/Triassic) of the Phanerozoic eon (the last five hundred and forty million years, roughly speaking), which had wiped out approximately 96% of all living species. The continents of the world had fused into one supercontinent, a giant landmass called Pangaea, 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" (members of the group Synapsida, which mammals belong), 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 eventually 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 or more specialized 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 rather similar; thin, elegant bipeds 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 or defence. 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:

  • While successful early on, dinosaurs had to share the land with many other groups during the Triassic, such as the dicynodonts, cynodonts, and several archosaurian relatives. They are, at this point, rather like rookie sports-players: showing promise, but unable to show themselves off on the pitch with all the other players getting in the way.
  • Although rather new, their hip design and water-storage adaptations allowed dinosaurs to persist through the Triassic/Jurassic extinction, and this allows them to explode in diversity.

    Jurassic - In the Middle Ages of Dinosaur Times 

Eventually, the Triassic-Jurassic mass extinction changed history. For one thing, it wiped out around 70% of all life that had survived the Triassic, including most 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, with 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 Pangaea was breaking up to form two giant landmasses Laurasia and Gondwana, or the northern and southern supercontinents respectively. The two landmasses were separated for most of their existence by the ancient Tethys Sea. 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 and Madagascar 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 ceratosaurs and the tetanurans, 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 ceratosaurs 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. Even the famous lineage of the tyrannosaurs, like Tyrannosaurus itself, started off here in the form of much smaller, primitive forms like Guanlong, Stokesosaurus and Juratyrant. The earliest paravians and maniraptorans (Anchiornis, Eosinopteryx, Archaeopteryx, Scansoriopteryx) appeared in the Jurassic, as well. Possible therizinosaur remains are dated from early in the Jurassic, and it's possible that the group had a ghost lineage that appeared even earlier, in the Triassic.

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, arguably 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 probably 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. Ankylosaurs during the Jurassic were relatively small and rare; 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 in North America 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 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:

  • After their competitors were largely obliterated by the Triassic-Jurassic mass extinction event, the dinosaurs diversified and enjoyed their Golden Age, the only period in history when they were truly successful from start to finish.
  • Many distinctive dinosaurs, such as the sauropods Brachiosaurus and Apatosaurus, Stegosaurus, Allosaurus, and the earliest birds lived during the Jurassic, but others, such as Tyrannosaurus rex, Triceratops, and Velociraptor lived during the Cretaceous. So Tyrannosaurus never got to fight Stegosaurus, Fantasia to the contrary. In terms of geological time, Tyrannosaurus is closer in time to us than to Stegosaurus!
  • The coelurosaurs (basal forms, tyrannosauroids and paravians, specifically) evolved at either this point or some time during the Late Triassic.

    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.

During the first half of the Cretaceous, carnosaurs and spinosaurs were the dominant terrestrial predators, with genera such as Giganotosaurus, Spinosaurus, Oxalaia and Carcharodontosaurus being some of the largest land carnivores to ever exist. But at the same time, ceratosaurs and coelurosaurs continued to diversify as the end of the Jurassic weeded out the more generalist forms. By the end of the Cretaceous carnosaurs and spinosaurs were more or less already extinct while abelisaurs (a type of ceratosaur) and megaraptorans (probably a type of coelurosaur, we don't know for sure yet) shared the apex predator title in the the southern continents (though abelisaurs made it to Europe) while tyrannosaurs (a type of coelurosaur) became the apex predators of Asia and North America. Maniraptorans began to dominate smaller carnivore and herbivore niches (as well as some larger ones), particularly birds, which by the Late Cretaceous had outed pterosaurs as the most common aerial vertebrates. Many bizarre forms which had first appeared in the Jurassic radiated outward, like the bird-like oviraptorosaurs, the pot-bellied therizinosaurus, and the one-fingered alvarezsaurs.

Sauropods were no longer as widespread and common as they had been in the Jurassic, particularly in Asia and North America, being slowly replaced by ornithischians as the dominant land herbivores. By the end of the Early Cretaceous only one group had survived, the titanosaurs. However, the titanosaurs were arguably the most impressive sauropods, as they included some of the largest land animals to ever walk the Earth. Species such as Puertasaurus, Argentinosaurus and Futalognkosaurus have all been suggested to reach or more a hundred feet in length and more than seventy tonnes in weight. During the Cretaceous, there was a slow, but steady trend towards smaller sizes in sauropods as well as the evolution of scaly armour, possibly as additional protection against predators. By the end of the Late Cretaceous, most sauropods were "only" around forty to fifty feet in length or less, although the species Alamosaurus, a contemporary of Tyrannosaurus in time and place, was one of the largest known species, weighing over seventy tonnes.

Among ornithischians, marginocephalians differentiated into two subgroups. The pachycephalosaurs remained small and bipedal but developed thick skulls probably used in combat and/or display, 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. Small ceratopsians lacking big spikes and frills still remained around until the very end in forms like Leptoceratops however.

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 or shoving against each other, 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. It has also been suggested the smaller, flat-headed pachycephalosaurs are in fact the young stage of the dome-headed forms.

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. Their ability to chew likely contributed to their success alongside ceratopsids in the Late Cretaceous, with both groups having structures known as "dental batteries", which were basically tightly-packed rows of slicing or grinding teeth that were constantly replaced when worn down. Hadrosaurs included some of the largest ornithischians, forms such as Edmontosaurus and Shantungosaurus possibly approached fifteen tonnes in weight. Fossil nests and eggs show that at least 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 (more likely the latter) roosting on its clutch of eggs, trying to protect them from a sandstorm which overwhelmed and fossilized 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 in the same position.

     The Boundary Event: Rocks Fall, Everyone Dies 

During their time on Earth, the dinosaurs thrived in every imaginable terrestrial habitat, from swampy terrain and dense forests to open prairies and the driest of deserts. Some even weathered the harsh winter conditions of Antarctica, Australia (which were near the South Pole at the time) and Alaska, all of which would have experienced six months of complete darkness each year. Some dinosaurs even took to the sea in the form of birds, such as the flightless hesperornithes and tern-like Ichthyornis. However, the dinosaurs did not survive the mass extinction event at the end of the Cretaceous period, known as the Cretaceous-Paleogene extinction eventnote , which wiped out roughly 75% 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 unintelligent, 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 contemporary mammals consumed eggs, at least not enough to effect dinosaur populations significantly. Some suggested that the dinosaurs found it harder to supply their large bodies with oxygen, ignoring the fact that most 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, causing noticeable global warming in a geologically short span of time. 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), or at the very least would have been vulnerable to additional change to their environment when the fateful meteor, about 66 million years ago, collided with the Gulf of Mexico, producing the Chicxulub crater.

The impact would have released the energy equivalent to a hundred teratons of TNT (roughly two million times more powerful than the detonation of the Tsar Bomba, or more than a thousand times more than every nuclear weapon ever created used at once) and carved out a hole in the planet over a hundred miles wide and two miles deep. The amount of thermal radiation generated by the blast would have instantly incinerated anything within a 1000-mile radius. As it struck the planet, it would caused an earthquake felt across the world, creating worldwide tsunamis more than five kilometres high. The ejecta would have started to rain down as superheated debris and rock thrown up by the meteor would start to obey the effects of gravity. A blast front radiated in all directions from the impact, a wave of hot death that would have incinerated almost everything in its path, which would have been the entire planet.

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 severely 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, perhaps even millions of years, to end, but when it was finished, the non-avian dinosaurs were gone. Any that potentially survived even all that would have certainly become extinct soon after, as their populations dropped below the threshold that would allow them to recover and sustain genetic diversity. 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 (particularly terrestrial forms) and mammals died out as well, and it is anybody's guess whether their survival was due to adaptation or to sheer luck. In short, everything over fifty pounds in weight and/or wasn't able to survive on scant resources were doomed. 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. Birds (and by extension theropod dinosaurs) gave it a good shot at first, evolving many large forms, but eventually they were out-competed in most habitats by mammals. 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 almost come close.

Main points:

  • While long known to geologists as the Cretaceous-Tertiary (K-T) boundary, this event is more properly called the Cretaceous-Paleogene (K-Pg) boundary. (The old term "Tertiary" has been abolished in favor of the more evenly spread Paleogene and Neogene Periods.)
  • The Cretaceous spanned a long time. Not all dinosaurs that lived in the Cretaceous lived at the time of the Cretaceous-Paleogene mass extinction. Spinosaurids, for example, lived during the Early Cretaceous but went extinct long before the mass extinction. Even Late Cretaceous dinosaurs from near the end of the Cretaceous didn't all actually live to the end! (While two million years, for example, is short in geological terms, it's an immense time span for any living thing. Entire ecosystems can change and be replaced with new environments and organisms during that time.)
  • Dinosaurs didn't really go extinct, as birds still live today. Nonetheless, most dinosaur lineages did die out, including most bird groups. Most croc and mammal groups were also killed in the extinction, with only a few lineages surviving, and even some surviving lineages (multituberculate mammals and champsosaurs) died out later in the Cenozoic.
  • The K-Pg was the first mass extinction to be discovered and is still the most popular, but it wasn't the biggest in Earth's history. The Permian-Triassic extinction (also known as P-Tr or the Great Dying) paved the way for the rise of the dinosaurs by wiping out well over 90% of all life on Earth; it took 10 million years for life to recover to something resembling the pre P-Tr level of biodiversity.note  Unlike the K-Pg extinction, the P-Tr's cause is actually still unknown.

     Other Useful Notes 
Alright, so after a brief recap, we'll continue with the notes;
  • The word "dinosaur" applies to a specific group of animals all sharing the defining common traits.
  • Dinosaurs were highly diverse animals, varying wildly in morphology, niche and ecological function.
  • The chicken you eat for dinner and the eagle you see in America's national emblem are both dinosaurs.
  • Crocodilians are the closest living relatives to dinosaurs (And therefore modern birds as well).
  • The P-Tr extinction was an incredibly damaging time for life on Earth, killing off around 96% of life.
  • Dimetrodon, Pteranodon or Plesiosaurus are not dinosaurs; Pteranodon is the closest relative to dinosaurs of the three, but is still not a dinosaur proper.
  • Smilodon and Mammut aren't dinosaurs, either.
  • The K/T extinction is properly the K/Pg extinction.
  • The K/Pg occurred 66 million years ago, and not 65 or 64.

Alright, now onto the more dinosaur-related notes. These are all more related to the media representation of dinosaurs and how "accurate" they can be. Buckle up the kids; this may shatter beliefs they hold dear.

  • Velociraptor was covered in pennaceous feathers, and not leathery skin or pebbly scales. This is due to a forearm found in 2006 that preserved quill knobs (a dead ringer of pennaceous feathers). Jurassic Park popularized the "pebbly, giant Velociraptor" image, and it's stuck since.
  • Although Tyrannosaurus was a part of a group of dinosaurs that did have feathers, recent discoveries of skin-impressions show that T. rex (and most large theropods) was mostly scaled, probably with only sparse feathers located on the dorsum (neck and back). This is similar to how elephants stay cool (having sparse hairs is actually better than having no hairs to stay cool). Skin-impressions of Albertosaurus and ''Tarbosaurus shows that most large theropods had were scaly.
  • Velociraptor was not likely to have had wolf-like pack behaviour. The most likely group interaction Velociraptor had were probably more like a Komodo dragon or bird of prey than wolves. For context, Komodo dragons are just as likely to cannibalize each other as they are consume the prey item. In fact, studies suggest that tyrannosaurids were more likely to live in social groups.
  • Large herbivorous dinosaurs were probably even more dangerous than the predatory ones; predators have to worry about an injury crippling their hunting abilities, whilst prey just have to worry about living; so they can be extremely aggressivenote .
  • For the love of Tyrannosaurus, sauropods did not chew the food they consumed. Jurassic Park got this wrong, as well; and it snowballed. Hard. Sauropods swallowed gizzard stones and used the stones and their gut to aid the digestion of plants, but they ate those plants whole. Chewing wasn't even a thing for dinosaurs until the ornithopods started doing it.
    • Similarly, dinosaurs that did chew (like hadrosaurs and ceratopsians) didn't chew like mammals did by moving their lower jaw side to side, instead moving the jaws front to back.
  • Tyrannosaurus wasn't the only predatory dinosaur. This is forgivable for early works with dinosaurs.

Dinosaurs are now a pretty big hit in popular culture — after all, they were the subject of the American Cyclic National Fascination during The '90s and it seems they are here to stay. 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.