Dinosauria

Dinosauria is a clade of terrestrial archosaurs that thrived from roughly 240 million years ago until the end of the Cretaceous 66 million years ago. Its members, commonly known as dinosaurs, are collectively the most familiar prehistoric animals to the general public, although they also include the clade Aves or modern birds. During the entirety of the Jurassic and Cretaceous periods dinosaurs were the dominant land animals, while Aves was left as the only surviving dinosaur group after the K/Pg Extinction Event.

Today birds are the second most diverse group of vertebrates, second only to perciform fish, as they number about 10 000 species - to put that into perspective, around 5500 mammal species are known. Counting by known species, dinosaurs are therefore the most diverse terrestrial vertebrates in our planet's history.

Discovery
Dinosaur remains have been known for millenia, though only in the 1820s was it actually realized what these bones were. From 1815-1824, British scientist William Buckland collected many dinosaur remains and became the first person to publish a scientific description of a dinosaur - that dinosaur being Megalosaurus. The study of dinosaurs soon began gaining more attention and in 1842 paleontologist Richard Owen coined the word "dinosaur". Soon afterwards the first dinosaurs were discovered in America and the discovery of evolution helped shed light on how these creatures had appeared and what they were.

The rivalry between Othniel Charles Marsh and Edward Drinker Cope sparked the "Bone Wars", during which the two scientists competed against each other, trying to find, name and describe the most fossil species - and the highlight was dinosaurs. This intense rivarly led to increased interest in palaeontology, and by 1900 the search for dinosaur remains had expanded all over the world.

Classification
Dinosaurs are classified into two major groups, Saurischia and Ornithischia. Saurischia contains the clades*  Sauropodomorpha and Theropoda, including Aves; while Ornithischia contains the clades Thyreophora, Ornithopoda and Marginocephalia. Most modern-day paleontologists agree that Sauropodomorpha contains the paraphyletic group previously known as "Prosauropoda" while a few continue to classify Prosauropoda as an entirely separate clade. Similarly, there is heated debate regarding the classification of Aves as part of the Coelurosauria in Theropoda, with paleontologists such as Alan Feduccia asserting that birds are descended from earlier gliding archosaurs such as Longisquama insignis.

The lower classification of Ornithischia is in somewhat less turmoil. Anatomical analyses suggest a close relation between Ornithopoda and Marginocephalia; with Thyreophora being more basal and not particularly closely related to either of the former clades. Ornithopoda is among the largest extinct dinosaur clades in terms of species diversity, while Marginocephalia is arguably the most diverse in terms of body form. Marginocephalia contains two distinct divisions: the Ceratopsia, or horned dinosaurs, which includes such familiar species as Triceratops horridus, and the Pachycephalosauria.

Dinosaurs are placed within the Archosauria, an informal clade of reptile-like organisms which also includes the flying pterosaurs and the pseudosuchians which include modern-day crocodilians. Pterosauria is thought to be relatively closely related to Dinosauria, and its members are commonly misidentified as dinosaurs in the media. Dinosauria and Pterosauria are placed to gether in the clade Avemetatarsalia, alternatively known as Ornithodira or Panaves. Also of note is the fact that modern-day crocodilians, despite their vague similarity to lizards and tuataras, are not closely related to either, and despite the word dinosaur translating to "terrible lizard" or "terrible reptile", lizards, being squamates, are not closely related to dinosaurs.

Phylogenetic hierarchy

 * Clade Dinosauria
 * Clade Ornithischia
 * Genus Eocursor
 * Genus Fabrosaurus
 * Genus Pisanosaurus
 * Genus Taveirosaurus
 * Genus Trimucrodon
 * Family Heterodontosauridae
 * Clade Genasauria
 * Genus Lesothosaurus
 * Clade Neornithischia
 * Genus Agilisaurus
 * Genus Alocodon
 * Genus Ferganocephale
 * Genus Gongbusaurus
 * Genus Hexinlusaurus
 * Genus Nanosaurus
 * Genus Othnielia
 * Genus Othnielosaurus
 * Genus Phyllodon
 * Genus Stormbergia
 * Genus Xiaosaurus
 * Clade Cerapoda
 * Clade Marginocephalia
 * Clade Ornithopoda
 * Clade Thyreophora
 * Genus Emausaurus
 * Genus Scutellosaurus
 * Family Scelidosauridae
 * Clade Eurypoda
 * Clade Ankylosauria
 * Clade Stegosauria
 * Clade Saurischia
 * Family Herrerasauridae
 * Clade Eusaurischia
 * Genus Eoraptor
 * Clade Sauropodomorpha
 * Genus Arcusaurus
 * Genus Asylosaurus
 * Genus Efraasia
 * Genus Nambalia
 * Genus Pampadromaeus
 * Genus Pantydraco
 * Genus Plateosauravus
 * Genus Ruehelia
 * Genus Thecodontosaurus
 * Clade Plateosauria
 * Clade Sauropoda
 * Clade Theropoda
 * Genus Daemonosaurus
 * Genus Eodromaeus
 * Genus Protoavis
 * Genus Tawa
 * Clade Neotheropoda
 * Genus Zupaysaurus
 * Superfamily Coelophysoidea
 * Family Dilophosauridae
 * Clade Averostra
 * Clade Ceratosauria
 * Clade Tetanurae (includes Aves)

Fossil Record
While non-avian dinosaurs are famous for being much larger than most modern animals, it is often the smallest specimens which leave behind the most complete remains. Individuals of Epidexipteryx hui, less than 30 cm long, have been preserved in exquisite detail, complete with impressions of their long tail feathers, while the stomach contents of at least one individual Compsognathus longipes were left intact, allowing a unique insight into the species' diet and, by extension, the diets of similar, related theropods. However, large dinosaurs may also leave behind complete or near-complete remains, a prime example being "Sue", a Tyrannosaurus rex whose fossil skeleton is roughly 97% complete. By contrast, other species of dinosaurs are known only from fossilized skin impressions, footprints, or individual bones. Yet such poorly known species may include the largest of all dinosaurs: Amphicoelias fragillimus, known from an individual vertebra, may have reached 60 metres in length, while the ichnogenus Parabrontopodus might include animals that exceeded 120 tons in weight.

Dinosaur fossil remains have been uncovered on every continent, including Antarctica, and in addition to bones include fossilized teeth, gastroliths, coprolites, footprints, skin and feather impressions, and eggs and nests. Like most prehistoric animals, the taxonomic relations between dinosaur species and clades is determined by anatomical features, particularly in the skull, pelvic area, and structure of the vertebrae. Genetic evidence in modern-day birds has also played a role in the classification of theropod dinosaur clades. Some evidence of dinosaur behaviour has also been preserved, including dinosaur nesting sites, migrating herds, and hunting groups*.

Natural history
The oldest definitive fossil dinosaurs are bipedal, theropod-like forms dating to the Middle Triassic (Carnian stage) of Argentina, although Nyasasaurus parringtoni of the Anisian stage of Tanzania may have been a dinosaur as well and predated both South American forms by roughly 15 million years. By the late Carnian to early Norian stage of the Triassic, a clear division between Sauropodomorpha and Theropoda was evident, with the earliest sauropodomorphs being small, bipedal and likely omnivorous species, and the earliest theropods slender, fast-running generalist predators. Most early dinosaurs appear to have long necks and tails and five-fingered hands, though by the end of the Triassic some sauropodomorphs, including Mussaurus patagonicus, had evolved relatively short necks, and the coelophysoid theropods had evolved four-fingered hands.

Dinosaurs began to diversify in the Lower Jurassic. Sauropodomorphs increased in number, diversity, and distribution, represented in North America by Anchisaurus species; in Europe by Gyposaurus species; in eastern Asia by Lufengosaurus magnus, and in South Africa by Massospondylus carinatus and later by Vulcanodon karibaensis. The latter is notable in that it is among the earliest known true sauropods as well as one of the smallest. Coelophysoids persisted into the early Jurassic, but were soon overshadowed by the evolution of the larger dilophosaurids, which may have been basal averostrans; i.e. the common ancestors of ceratosaurs and tetanuran theropods. Berberosaurus liassicus and Cryolophosaurus ellioti, the earliest definitive ceratosaurian and tetanuran theropods, respectively, also evolved during this time period. The most significant diversification of dinosaurs during the Lower Jurassic, however, was the advent of the Thryeophora, at first represented by small, lightly armoured species like Scutellosaurus lawleri and Scelidosaurus harrisoni. Potential early ornithopods are also known from the Lower Jurassic.

By the middle of the Jurassic period, coelophysoid and dilophosaurid theropods appear to have become extinct, along with all sauropodomorphs outside Sauropoda. The reason for their extinction is unknown, but they appear to have been replaced by megalosaurid and ceratosaurian theropods in North America and Europe and by the carnosaurian sinraptorids in Asia. Another new family of theropods, the abelisaurids, also saw its origins in the middle Jurassic. Thryeophorans and ornithopods took the role of the extinct prosauropods, while true sauropods gradually began to increase in size, producing such large forms as Barapasaurus tagorei and the more derived cetiosaurids and macronarians. Among ornithischians, stegosaurs first emerged during the middle Jurassic period, along with dryosaurid ornithopods, just as the last members of the Heterodontosauridae went extinct.

The Upper Jurassic period saw another bloom in dinosaur diversity. The diplodocoid sauropods, which included several of the longest animals ever to live, originially evolved in the middle Jurassic period, but diversified significantly both in number and range. The macronarians gave rise to such well-known species as Brachiosaurus altithorax and Giraffatitan brancai (more commonly known as Brachiosaurus brancai),  while the cetiosaurids began to decline. Megalosaurid theropods remained apex predators in North America, but were accompanied by the newly evolved allosaurids, close relatives of the Asian sinraptorids. North America's largest predator during this time was either the megalosaurid Torvosaurus tanneri or the allosaurid* Saurophaganax maximus. Entirely new theropod forms also emerged during the Upper Jurassic, including the spinosaurids, likely related to the megalosaurids, and the Coelurosauria; a tetanuran clade that emerged either from Carnosauria or a more basal group such as Dilophosauridae. Ornithischians began to diversify, with iguanodont ornithopods and ankylosaurs evolving and stegosaurians diversifying.

The end of the Jurassic was marked by a mass extinction which saw a dramatic reduction in the number of dinosaur species. Cetiosaurids went extinct completely during this time, as did all macronarians outside Brachiosauridae. Diplodocoids declined sharply, but a few species would ultimately survive until the middle to Upper Cretaceous. Megalosaurid, ceratosaurid, and sinraptorid theropods were also casualties of this extinction event. While allosaurid theropods became extinct, they also gave rise to the Carcharodontosauridae at the end of the Jurassic, a clade which would survive until the very end of the Cretaceous. Stegosaurs, too, survived the extinction event as a group, but were far less ecologically successful during the Cretaceous.

Dinosaurs recovered rapidly during the Lower Cretaceous. The maniraptoran coelurosaurs, represented in the Jurassic by only a few species such as Archaeopteryx lithographica, diversified significantly. The Dromaeosauridae, a group which played a minor ecological role in the Jurassic, gave rise to a greater diversity of species, many of which were formidable, large-brained predators. Many paleontologists have speculated dromaeosaurids to be pack-hunting animals, although there is no direct evidence of this. Notable Lower Cretaceous dromaeosaurids included Utahraptor ostrommaysorum and Dromaeosauroides bornholmensis, both of which were among the largest members of the family. Birds began to spread from their point of origin in Europe, with species such as Sapeornis chaoyangensis and Confuciusornis sanctus eventually reaching eastern Asia. Entirely new forms of coelurosaurs such as the ornithomimosaurs and oviraptorosaurs were among the few dinosaur lineages to originate in the Cretaceous. Ceratosaurs, in the form of the abelisaurids, began their recovery in South America, while the carnosaurs reemerged as apex predators in Asia (Fukuiraptor kitadaniensis) and North America (Acrocanthosaurus atokensis). The diplodocoids persisted into the Lower Cretaceous, represented by at least one genus in South America, as did the brachiosaurids, which were represented by Sauroposeidon proteles in North America. Most Cretaceous sauropods, however, were titanosaurs, another macronarian group.

The middle of the Cretaceous saw a noteworthy increase in the ecological importance of coelurosaurs. Dromaeosaurids diversified, giving rise to the North American Deinonychus antirrhopus, a relatively well-known species, along with a variety of smaller species, including the gliding Microraptor and Sinornithosaurus spp.. The related troodontids flourished in China and Mongolia, giving rise to the bird-like Sinovenator changii and Sinornithoides youngi. True birds continued to expand their range, in some cases beginning to compete with small, insect-hunting pterosaurs for food, and the oviraptorosaurs produced the small caudipterids which may have filled a similar ecological niche to modern pheasants or wild turkey. The bizzare therizinosaurs and the tyrannosaurs increased in size and distribution during the middle Cretaceous, with the latter group giving rise to the largest known feathered dinosaur, Yutyrannus huali. The coelurosaurs were not the only group of theropods to prosper during the middle Cretaceous, however. The carnosaurs, represented by carcharodontosaurids and megaraptorans, quickly took over as apex predators of the southern continents, where many became specialist sauropod hunters. The carnosaurs were accompanied by abelisaurids and the semi-aquatic spinosaurids, a group closely related to and possibly descended from, the megalosaurids. The reason why the southern continents were capable of supporting such a diversity of large theropods, while the apex predators in the northern hemisphere were almost exclusively tyrannosaurs, remains a mystery.

Despite the extinction of the Stegosauridae, Diplodocoidea, and Brachiosauridae, during the Middle Cretaceous, the Upper Cretaceous was the heyday of dinosaurs as a group. In the northern continents, the tyrannosaurids, at first represented by medium-sized species such as Appalachiosaurus montgomeriensis, soon became utterly undisputed as the apex predators, while the maniraptorans appear to have experienced a general downwards trend in terms of size. Therizinosaurs along with ornithomimosaurs flourished, particularly in Asia, and birds continued their increase in ecological importance; in some places, displacing pterosaur populations. Such familiar avian groups as Anseriformes, or waterfowl, can in fact trace their origins back to the upper Cretaceous. The southern continents were shared by abelisaurids, carnosaurs, and spinosaurs, all of which reached their largest size and greatest diversity during the Cenomanian to Turonian stages (between 100 and 90 million years ago). These theropods seem to have declined during the Turonian; fossil remains of all three groups are less common following the Coniacian stage. Despite this, the abelisaurids, at least, appear to have been represented in Europe during the Santonian by at least one species; the megaraptorans, meanwhile, appear to have made a near-complete recovery by the Campanian. The Campanian also appears to have been the heyday for the tyrannosaurs and the non-avian maniraptorans in terms of diversity of species, although by the Maastrichtian the tyrannosaurs seemingly declined to just a few species, such as Tyrannosaurus rex and Tarbosaurus bataar. Sauropods likewise thrived during the Upper Cretaceous; being represented almost exclusively by the titanosaurs. The largest forms hailed from the Americas and Africa*; however smaller forms existed in Madagascar, India, Australia* and Europe.

The ornithischians, however, enjoyed the greatest success of any dinosaur group during the Upper Cretaceous. Ornithopods reached a truly unprecedented level of ecological significance in the form of the hadrosaurs; a group most notable for the distinctive head crests on many of its member species. Hadrosaurs were the most common large herbivores in the northern continents during the Upper Cretaceous, and perhaps the most diverse. They produced a number of relatively well-known species, including Parasaurolophus walkeri, Lambeosaurus lambei, and Corythosaurus casuarius. The marginocephalians likewise flourished during the Upper Cretaceous. Ceratopsians exploded into a rich diversity of species ranging from the pig-like Protoceratops andrewsi to massive species which boasted prominent horns and crests, including Triceratops horridus; a species which vies with Tyrannosaurus rex for the title of best-known non-avian dinosaur. The other marginocephalian group, the pacycephalosaurs, were somewhat less prominent than the hadrosaurs and ceratopsians, but nevertheless they were represented by a noteworthy number of species in eastern Asia and western North America, particularly during the Campanian and Maastrichtian stages.

The end of the Cretaceous period was punctuated with a mass extinction event that caused the extinction of nearly 70% of all animal life on Earth; including all non-avian dinosaurs. This extinction event is often credited with providing early mammals the opportunity to take over as the dominant land animals; however, it also gave birds an opportunity to diversify that they took full advantage of.

Paleobiology & Evolution
Due to the extremely varied and, in many cases, comprehensive fossil evidence, we have an unusually solid understanding of dinosaur biology. In addition to the fossil remains, knowledge of dinosaurs also comes from other fields of study including biomechanics, zoology, and chemistry. The topics of most interest to both the general public and a number of paleontologists have been dinosaur size and behaviour; however dinosaur anatomy and physiology have also been subject to much scientific research among the paleontological community.

Anatomy
Dinosaurs are a remarkably varied group of animals, and, as previously mentioned, are the most diverse group of vertebrates in the planet's history; however, they share a number of anatomical features that serve to distinguish them from other archosaurs. Skeletal features, for obvious reasons, the most familiar aspect of dinosaur anatomy; however, we also currently have knowledge of soft tissue including skin, respritory and cardiovascular systems. This section is a very general summary of dinosaur anatomy; more information can be found on the pages for each individual species.

Like other diapsids, dinosaurs possess two pairs of temporal fenestrae as well as smaller openings in the snout and lower jaw; a feature shared with other archosaurs. All dinosaurs have a supratemporal fossa, or excavation, immediately in front of the supratemporal fenestra; a feature not shared with non-dinosaurian archosaurs. The radius (one of the lower arm bones) is less than 80% of the length of the humerus and the deltopectoral crest is located 30% or more of the length down the humerus. The Ornithischia and Saurischia are distinguished from one another based on the structure of the pelvis. In ornithischians the ilium is vaguely sickle-shaped; the pubis and the ischium are both alligned backwards, toward the cloacal opening. Saurischians, meanwhile, possess an elliptical ilium with a dumbell-shaped pubis that extends down and forwards, away from the cloacal opening.

The cnemial crest, a ridgelike prominence located at the head of the tibia, arcs anterolaterally, meaning it curves to the front and the outer side. The posterior face of the tibia also possesses a  distinct proximodistally oriented (vertical) ridge.

Unlike most other reptiles, all dinosaurs stand fully erect (yet their posture is distinct from the pillar-erect posture of rauisuchians such as Prestosuchus chiniquensis). This posture is due to the development of a laterally facing recess in the pelvis (usually an open socket) and a corresponding inwardly facing distinct head on the femur.

Some non-avian dinosaurs had pneumatised bones, or bones that contained air-filled hollowings. Such air sacs would have served to pump air into the dinosaurs' inflexible lungs. Such a respiratory system may have assisited in regulating body temperature and increased the animal's stamina (as it does for modern-day monitor lizards).

Physiology
Like modern-day birds and crocodilians, non-avian dinosaurs presumably possessed four-chambered hearts. Due to the fact that many species of dinosaurs possessed long necks, they would have required high blood pressure in order to deliver blood to their heads. In order to deal with the fact that vertebrate lungs are sensitive to high blood pressure, dinosaurs probably had two-part circulation. This hypothesis appears to be supported by the fossil remains of an individual Thescelosaurus neglectus named "Willo", apparently containing the remnants of a four-chambered heart and an aorta. The authors interpreted the structure of the heart as indicating an elevated metabolic rate for T. neglectus, rather than reptilian ectothermy. Critics of the study have argued that the mass is not the heart of an animal, but rather a concentration of silicate minerals gradually deposited in the chest cavity after the animal died. They point out that the aorta appears to be oddly structured, at its narrowest at the point where it contacts the heart.

As mentioned in the previous section,

Extinction
Main article: K/Pg Extinction Event