Dinosaur(1)

Dinosaurs are a diverse group of animals that were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period (about 230 million years ago) until the end of the Cretaceous (about 65 million years ago) (IBM ThinkPad X41 Tablet battery). The extinction of most dinosaur species occurred during the Cretaceous–Tertiary extinction event. The fossil record indicates that birds evolved within theropod dinosaurs during the Jurassic period. Some of them survived the Cretaceous–Tertiary extinction event(IBM ThinkPad R51 battery), including the ancestors of all modern birds. Consequently, in modern classification systems, birds are considered a type of dinosaur—the only group of which that has survived to the present day.

Dinosaurs are a diverse and varied group of animals; birds, at over 9,000 species, are the most diverse group of vertebrate besides perciform fish(IBM ThinkPad R60 battery). Paleontologists have identified over 500 distinct genera and more than 1,000 different species of non-avian dinosaurs. Dinosaurs are represented on every continent by both extant species and fossil remains. Some dinosaurs are or were herbivorous, others carnivorous(IBM ThinkPad R50 battery). Some have been, and all extant dinosaurs are, bipedal, others quadrupedal, and others have been able to shift between these body postures. Many non-avian species developed elaborate skeletal modifications such as bony armor, horns or crests. Avian dinosaurs have been the planet's dominant flying vertebrate since the extinction of the pterosaurs(FUJITSU LIFEBOOK P2120 Battery). Although generally known for the large size of some species, most dinosaurs were human-sized or even smaller. Most groups of dinosaurs are known to have built nests and laid eggs.

The term "dinosaur" was coined in 1842 by the English paleontologist Richard Owen, and derives from Greek δεινός (deinos) "terrible, powerful(Fujitsu Lifebook S6120 battery), wondrous" + σαῦρος (sauros) "lizard". Through the first half of the 20th century, most of the scientific community believed dinosaurs to have been sluggish, unintelligent cold-blooded animals. Most research conducted since the 1970s, however, has indicated that dinosaurs were active animals with elevated metabolisms and numerous adaptations for social interaction(COMPAQ Presario R3000 Battery).

Since the first dinosaur fossils were recognized in the early 19th century, mounted dinosaur skeletons have been major attractions at museums around the world, and dinosaurs have become a part of world culture. They have been featured in best-selling books and films such as Jurassic Park(COMPAQ Presario 2100 Battery), and new discoveries are regularly covered by the media. In informal speech, the word "dinosaur" is used to describe things that are impractically large, obsolete, or bound for extinction, reflecting the outdated view that dinosaurs were maladapted monsters of the ancient world(COMPAQ Presario M2000 Battery).

Name

The taxon Dinosauria was formally named in 1842 by Sir Richard Owen, who used it to refer to the "distinct tribe or sub-order of Saurian Reptiles" that were then being recognized in England and around the world.:103 The term is derived from the Greek words δεινός (deinos meaning "terrible", "powerful", or "wondrous") (ASUS Eee PC 1000HE Battery)and σαῦρος (sauros meaning "lizard" or "reptile"). Though the taxonomic name has often been interpreted as a reference to dinosaurs' teeth, claws, and other fearsome characteristics, Owen intended it merely to evoke their size and majesty.

Modern definition(ASUS Eee PC 900 Battery)

Triceratops skeleton at the American Museum of Natural History in New York City

Formal definitions are written to correspond with scientific conceptions of dinosaurs that predate the modern use of phylogenetics. The continuity of meaning is intended to prevent confusion about what the term "dinosaur" means(ASUS A3000 Battery).

Under phylogenetic taxonomy, dinosaurs are usually defined as the group consisting of "Triceratops, Neornithes, their most recent common ancestor, and all descendants". It has also been suggested that Dinosauria be defined with respect to the MRCA of Megalosaurus and Iguanodon(ACER Aspire 5020 Battery), because these were two of the three genera cited by Richard Owen when he recognized the Dinosauria. Both definitions result in the same set of animals being defined as dinosaurs, that is "Dinosauria = Ornithischia + Saurischia", which encompasses theropods (mostly bipedal carnivores and birds), ankylosaurians (armored herbivorous quadrupeds) (ACER Aspire 3000 Battery), stegosaurians (plated herbivorous quadrupeds), ceratopsians (herbivorous quadrupeds with horns and frills), ornithopods (bipedal or quadrupedal herbivores including "duck-bills"), and presumptuously, sauropodomorphs (mostly large herbivorous quadrupeds with long necks and tails) (ACER Aspire 3020 Battery).

Many paleontologists note that the order in which sauropodomorphs and theropods diverged may omit sauropodomorphs from the definition for both saurischians and dinosaurs. To avoid the instability of Dinosauria, a more conservative definition of Dinosauria is defined with respect to four anchoring nodes(ACER Travelmate 2300 Battery): Triceratops horridus, Saltasaurus loricatus, and Passer domesticus, their most recent common ancestor, and all descendants. This "safer" definition can be expressed as "Dinosauria = Ornithischia + Sauropodomorpha + Theropoda"(Toshiba NB200 battery).

There is a wide consensus among paleontologists that birds are the descendants of theropod dinosaurs. Using the strict cladistical definition that all descendants of a single common ancestor must be included in a group for that group to be natural, birds would thus be dinosaurs and dinosaurs are, therefore, not extinct(Toshiba NB100 battery). Birds are classified by most paleontologists as belonging to the subgroup Maniraptora, which are coelurosaurs, which are theropods, which are saurischians, which are dinosaurs(Toshiba PA3641U-1BAS battery).

From the point of view of cladistics, birds are dinosaurs, but in ordinary speech the word "dinosaur" does not include birds. Additionally, referring to dinosaurs that are not birds as "non-avian dinosaurs" is cumbersome. For clarity, this article will use "dinosaur" as a synonym for "non-avian dinosaur". The term "non-avian dinosaur" will be used for emphasis as needed(Toshiba PA3640U-1BRS battery).

General description

Stegosaurus skeleton, Field Museum, Chicago

Using one of the above definitions, dinosaurs (aside from birds) can be generally described as terrestrial archosaurian reptiles with limbs held erect beneath the body, that existed from the Late Triassic (first appearing in the Carnian faunal stage) to the Late Cretaceous (going extinct at the end of the Maastrichtian) (Toshiba PA3640U-1BAS battery). Many prehistoric animals are popularly conceived of as dinosaurs, such as ichthyosaurs, mosasaurs, plesiosaurs, pterosaurs, and Dimetrodon, but are not classified scientifically as dinosaurs. Marine reptiles like ichthyosaurs, mosasaurs, and plesiosaurs were neither terrestrial nor archosaurs; pterosaurs were archosaurs but not terrestrial(Toshiba PA3591U-1BAS battery); and Dimetrodon was a Permian animal more closely related to mammals. Dinosaurs were the dominant terrestrial vertebrates of the Mesozoic, especially the Jurassic and Cretaceous. Other groups of animals were restricted in size and niches; mammals, for example(Toshiba PA3591U-1BRS battery), rarely exceeded the size of a cat, and were generally rodent-sized carnivores of small prey. One notable exception is Repenomamus giganticus, a triconodont weighing between 12 kilograms (26 lb) and 14 kilograms (31 lb) that is known to have eaten small dinosaurs like young Psittacosaurus(Toshiba PA3535U-1BAS battery).

Dinosaurs were an extremely varied group of animals; according to a 2006 study, over 500 dinosaur genera have been identified with certainty so far, and the total number of genera preserved in the fossil record has been estimated at around 1850, nearly 75% of which remain to be discovered(Toshiba PA3506U-1BRS battery). An earlier study predicted that about 3400 dinosaur genera existed, including many which would not have been preserved in the fossil record. As of September 17, 2008, 1047 different species of dinosaurs have been named. Some were herbivorous, others carnivorous. Some dinosaurs were bipeds, some were quadrupeds, and others(Toshiba PA3480U-1BRS battery), such as Ammosaurus and Iguanodon, could walk just as easily on two or four legs. Many had bony armor, or cranial modifications like horns and crests. Although known for large size, many dinosaurs were human-sized or smaller(Toshiba PA3478U-1BRS battery). Dinosaur remains have been found on every continent on Earth, including Antarctica. No non-avian dinosaurs are known to have lived in marine habitats or in aerial habitats, although it is possible some feathered non-avian theropods were flyers. There is also evidence that some spinosaurids had semi-aquatic habits(Toshiba PA3476U-1BRS battery).

Distinguishing anatomical features

While recent discoveries have made it more difficult to present a universally agreed-upon list of dinosaurs' distinguishing features, nearly all dinosaurs discovered so far share certain modifications to the ancestral archosaurian skeleton(Toshiba PA3475U-1BRS battery). Although some later groups of dinosaurs featured further modified versions of these traits, they are considered typical across Dinosauria; the earliest dinosaurs had them and passed them on to all their descendants. Such common features across a taxonomic group are called synapomorphies(Toshiba PA3383U-1BRS battery).

A detailed assessment of archosaur interrelations by S. Nesbitt confirmed or found the following 12 unambiguous synapomorphies, some previously known:

in the skull, a supratemporal fossa (excavation) is present in front of the supratemporal fenestra(Toshiba PA3356U-1BRS battery)

epipophyses present in anterior neck vertebrae (except atlas and axis)

apex of deltopectoral crest (a projection on which the deltopectoral muscles attach) located at or more than 30% down the length of the humerus (upper arm bone)

radius shorter than 80% of humerus length(Toshiba PA3395U-1BRS battery )

fourth trochanter (projection where the caudofemoralis muscle attaches) on the femur (thigh bone) is a sharp flange

fourth trochanter asymmetrical, with distal margin forming a steeper angle to the shaft

on the astragalus and calcaneum the proximal articular facet for fibula occupies less than 30% of the transverse width of the element(Toshiba PA3285U-1BRS battery )

exocciptials (bones at the back of the skull) do not meet along the midline on the floor of the endocranial cavity

proximal articular surfaces of the ischium with the ilium and the pubis separated by a large concave surface

cnemial crest on the tibia (shinbone) arcs anterolaterally(Toshiba PA3450U-1BRS battery)

distinct proximodistally oriented ridge present on the posterior face of the distal end of the tibia

Nesbitt found a number of further potential synampomorphies, and discouted a number of synapomorphies previously suggested. Some of these are also present in silesaurids, which Nesbitt recovered as a sister group to Dinosauria, including a large anterior trochanter(Toshiba PA3465U-1BRS battery), metatarsals II and IV of subequal length, reduced contact between ischium and pubis, the presence of a cenmial crest on the tibia and of an ascending process on the astragalus, and many others.

Edmontonia was an armored dinosaur of the group Ankylosauria

A variety of other skeletal features were shared by many dinosaurs. However, because they were either common to other groups of archosaurs or were not present in all early dinosaurs(Toshiba PA3285U-1BRS battery), these features are not considered to be synapomorphies. For example, as diapsid reptiles, dinosaurs ancestrally had two pairs of temporal fenestrae (openings in the skull behind the eyes), and as members of the diapsid group Archosauria, had additional openings in the snout and lower jaw. Additionally, several characteristics once thought to be synapomorphies are now known to have appeared before dinosaurs(Toshiba PA3399U-1BRS battery), or were absent in the earliest dinosaurs and independently evolved by different dinosaur groups. These include an elongated scapula, or shoulder blade; a sacrum composed of three or more fused vertebrae (three are found in some other archosaurs, but only two are found in Herrerasaurus);[11] and an acetabulum, or hip socket(Toshiba PA3399U-2BRS battery), with a hole at the center of its inside surface (closed in Saturnalia, for example). Another difficulty of determining distinctly dinosaurian features is that early dinosaurs and other archosaurs from the Late Triassic are often poorly known and were similar in many ways; these animals have sometimes been misidentified in the literature(Toshiba PA3534U-1BRS battery).

Hip joints and hindlimb postures

Dinosaurs stood erect in a manner similar to most modern mammals, but distinct from most other reptiles, whose limbs sprawl out to either side. Their posture was 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(Toshiba PA3535U-1BRS battery). Their erect posture enabled dinosaurs to breathe easily while moving, which likely permitted stamina and activity levels that surpassed those of "sprawling" reptiles. Erect limbs probably also helped support the evolution of large size by reducing bending stresses on limbs. Some non-dinosaurian archosaurs(Dell Studio 1457 battery), including rauisuchians, also had erect limbs but achieved this by a "pillar erect" configuration of the hip joint, where instead of having a projection from the femur insert on a socket on the hip, the upper pelvic bone was rotated to form an overhanging shelf(Dell Inspiron 1764 battery).

Natural history

Origins and early evolution

Main article: Evolution of dinosaurs

Marasuchus, a dinosaur-like ornithodiran

For a long time many scientists thought dinosaurs were polyphyletic with multiple groups of unrelated "dinosaurs" evolving due to similar pressures, but dinosaurs are now known to have formed a single group(Dell Inspiron 1564 battery). Dinosaurs diverged from their archosaur ancestors approximately 230 million years ago during the Middle to Late Triassic period, roughly 20 million years after the Permian–Triassic extinction event wiped out an estimated 95% of all life on Earth(Dell Inspiron 1464 battery). Radiometric dating of the rock formation that contained fossils from the early dinosaur genus Eoraptor establishes its presence in the fossil record at this time. Paleontologists believe Eoraptor resembles the common ancestor of all dinosaurs(Dell Inspiron 1320N battery); if this is true, its traits suggest that the first dinosaurs were small, bipedal predators. The discovery of primitive, dinosaur-like ornithodirans such as Marasuchus and Lagerpeton in Argentinian Middle Triassic strata supports this view; analysis of recovered fossils suggests that these animals were indeed small, bipedal predators(Dell Studio 1450 battery).

When dinosaurs appeared, terrestrial habitats were occupied by various types of basal archosaurs and therapsids, such as aetosaurs, cynodonts, dicynodonts, ornithosuchids, rauisuchias, and rhynchosaurs. Most of these other animals became extinct in the Triassic(Dell Inspiron 1320 battery), in one of two events. First, at about the boundary between the Carnian and Norian faunal stages (about 215 million years ago), dicynodonts and a variety of basal archosauromorphs, including the prolacertiforms and rhynchosaurs, became extinct. This was followed by the Triassic–Jurassic extinction event (about 200 million years ago) (Dell Latitude E6500 battery), that saw the end of most of the other groups of early archosaurs, like aetosaurs, ornithosuchids, phytosaurs, and rauisuchians. These losses left behind a land fauna of crocodylomorphs, dinosaurs, mammals, pterosaurians, and turtles.

The early forms Herrerasaurus (large), Eoraptor (small) and a Plateosaurus skull

The first few lines of primitive dinosaurs diversified through the Carnian and Norian stages of the Triassic(Dell Latitude E6400 battery), most likely by occupying the niches of groups that became extinct. Traditionally, dinosaurs were thought to have replaced the variety of other Triassic land animals by proving superior through a long period of competition. This now appears unlikely, for several reasons(Dell Studio 1737 battery). Dinosaurs do not show a pattern of steadily increasing in diversity and numbers, as would be predicted if they were competitively replacing other groups; instead, they were very rare through the Carnian, making up only 1–2% of individuals present in faunas. In the Norian, however(Dell Studio 1735 battery), after the extinction of several other groups, they became significant components of faunas, representing 50–90% of individuals. Also, what had been viewed as a key adaptation of dinosaurs, their erect stance, is now known to have been present in several contemporaneous groups that were not as successful (aetosaurs, ornithosuchids, rauisuchians, and some groups of crocodylomorphs) (Dell Vostro 1710 battery). Finally, the Late Triassic itself was a time of great upheaval in life, with shifts in plant life, marine life, and climate. Crurotarsans, today represented only by crocodilians but in the Late Triassic also encompassing such now-extinct groups as aetosaurs, phytosaurs(Dell Inspiron 1501 battery), ornithosuchians, and rauisuchians, were actually more diverse in the Late Triassic than dinosaurs, indicating that the survival of dinosaurs had more to do with luck than superiority.

Low diversification in the Cretaceous

Statistical analyses based on raw data suggest that dinosaurs diversified, that is, the number of species increased, in the Late Cretaceous(Dell Inspiron 6400 battery). However in July 2008 Graeme T. Lloyd et al. argued that this apparent diversification was an illusion caused by sampling bias, because Late Cretaceous rocks have been very heavily studied. Instead, they wrote(Dell Inspiron 6000 battery), dinosaurs underwent only two significant diversifications in the Late Cretaceous, the initial radiations of the euhadrosaurs and ceratopsians. In the Mid Cretaceous, the flowering angiosperm plants became a major part of terrestrial ecosystems, which had previous been dominated by gymnosperms such as conifers(Dell INSPIRON 1525 Battery). Dinosaur coprolites (fossilized dung) indicate that, while some ate angiosperms, most herbivorous dinosaurs mainly ate gymnosperms. Meanwhile herbivorous insects and mammals diversified rapidly to take advantage of the new type of plant food, while lizards, snakes, crocodilians and birds also diversified at the same time(Dell N3010 battery). Lloyd et al. suggest that dinosaurs' failure to diversify as ecosystems were changing doomed them to extinction.

Classification

Main article: Dinosaur classification

Dinosaurs (including birds) are archosaurs, like modern crocodilians. Archosaurs' diapsid skulls have two holes, called temporal fenestrae, located where the jaw muscles attach, and an additional antorbital fenestra in front of the eyes(Dell RM791 battery). Most reptiles (including birds) are diapsids; mammals, with only one temporal fenestra, are called synapsids; and turtles, with no temporal fenestra, are anapsids. Anatomically, dinosaurs share many other archosaur characteristics(Dell Latitude D620 Battery), including teeth that grow from sockets rather than as direct extensions of the jawbones. Within the archosaur group, dinosaurs are differentiated most noticeably by their gait. Dinosaur legs extend directly beneath the body, whereas the legs of lizards and crocodilians sprawl out to either side(Dell Inspiron E1505 Battery).

Collectively, dinosaurs are usually regarded as a superorder or an unranked clade. They are divided into two orders, Saurischia and Ornithischia, depending upon pelvic structure. Saurischia includes those taxa sharing a more recent common ancestor with birds than with Ornithischia(Dell INSPIRON 1420 Battery), while Ornithischia includes all taxa sharing a more recent common ancestor with Triceratops than with Saurischia. Saurischians ("lizard-hipped", from the Greek sauros (σαυρος) meaning "lizard" and ischion (ισχιον) meaning "hip joint") retained the hip structure of their ancestors, with a pubis bone directed cranially, or forward(HP PAVILION DV3000 Battery). This basic form was modified by rotating the pubis backward to varying degrees in several groups (Herrerasaurus, therizinosauroids, dromaeosaurids, and birds[14]). Saurischia includes the theropods (bipedal and mostly carnivores, except for birds) and sauropodomorphs (long-necked quadrupedal herbivores) ( HP PAVILION DV3 Battery).

By contrast, ornithischians ("bird-hipped", from the Greek ornitheios (ορνιθειος) meaning "of a bird" and ischion (ισχιον) meaning "hip joint") had a pelvis that superficially resembled a bird's pelvis: the pubis bone was oriented caudally (rear-pointing) Unlike birds(HP PAVILION DV2000 Battery), the ornithischian pubis also usually had an additional forward-pointing process. Ornithischia includes a variety of herbivores. (NB: the terms "lizard hip" and "bird hip" are misnomers – birds evolved from dinosaurs with "lizard hips".)

The following is a simplified classification of dinosaur families. A more detailed version can be found at List of dinosaur classifications(HP PAVILION DV2 Battery).

Dinosauria

Saurischia (theropods and sauropods)

†Herrerasaurians (early bipedal predators)

Theropods (all bipedal; most were carnivores)

†Coelophysoids (Coelophysis and close relatives)

†Ceratosaurians (Ceratosaurus and abelisaurids – the latter were important Late Cretaceous predators in southern continents) (Sony VAIO PCG-5K1L battery)

†Spinosauroids (long bodies; short arms; some with crocodile-like skulls and bony "sails" on their backs)

†Carnosaurians (Allosaurus and close relatives, like Carcharodontosaurus)

Coelurosaurians (diverse, with a range of body sizes and niches)

†Tyrannosauroids (small to gigantic, often with reduced forelimbs)

†Ornithomimosaurians ("ostrich-mimics"; mostly toothless; carnivores to possible herbivores) (Sony VAIO PCG-6W2L battery)

†Therizinosauroids (bipedal herbivores with large hand claws and small heads)

†Oviraptorosaurians (mostly toothless; their diet and lifestyle are uncertain)

†Dromaeosaurids (popularly known as "raptors"; bird-like carnivores)

†Troodontids (similar to dromaeosaurids, but more lightly built)

Avialans (flying dinosaurs, including modern birds: the only living dinosaurs) (Sony VAIO PCG-7112L battery)

†Sauropodomorphs (quadrupedal herbivores with small heads, long necks and tails, and elephant-like bodies)

†"Prosauropods" (early relatives of sauropods; small to quite large; some possibly omnivorous; bipeds and quadrupeds)

†Sauropods (very large, usually over 15 meters long [49 ft])

†Diplodocoids (skulls and tails elongated; teeth typically narrow and pencil-like)

†Macronarians (boxy skulls; spoon- or pencil-shaped teeth) (Sony VAIO PCG-8Z1L battery)

†Brachiosaurids (very long necks; forelimbs longer than hindlimbs)

†Titanosaurians (diverse; stocky, with wide hips; most common in the Late Cretaceous of southern continents)

†Ornithischians (diverse bipedal and quadrupedal herbivores)

†Heterodontosaurids (meter- or yard-scale herbivores or omnivores with prominent canine teeth) (Sony VAIO PCG-8Z2L battery)

†Thyreophorans (armored dinosaurs; mostly quadrupeds)

†Ankylosaurians (scutes as primary armor; some had club-like tails)

†Stegosaurians (spikes and plates as primary armor)

†Ornithopods (diverse, from meter- or yard-scale bipeds to 12-meter (39 ft) animals that could move as both bipeds and quadrupeds; evolved a method of chewing using skull flexibility and large numbers of teeth) (Sony VAIO PCG-8Y2L battery)

†Hadrosaurids ("duckbilled dinosaurs")

†Pachycephalosaurians ("bone-heads"; bipeds with domed or knobby growth on skulls)

†Ceratopsians (dinosaurs with horns and frills, although most early forms had only the beginnings of these features)

Evolution and paleobiogeography

Main article: Evolution of dinosaurs(Sony VAIO PCG-8Y1L battery)

Dinosaur evolution after the Triassic follows changes in vegetation and the location of continents. In the Late Triassic and Early Jurassic, the continents were connected as the single landmass Pangaea, and there was a worldwide dinosaur fauna mostly composed of coelophysoid carnivores and prosauropod herbivores(Sony VAIO PCG-7Z2L battery). Gymnosperm plants (particularly conifers), a potential food source, radiated in the Late Triassic. Prosauropods did not have sophisticated mechanisms for processing food in the mouth, and so must have employed other means of breaking down food farther along the digestive tract(Sony VAIO PCG-7Z1L battery). The general homogeneity of dinosaurian faunas continued into the Middle and Late Jurassic, where most localities had predators consisting of ceratosaurians, spinosauroids, and carnosaurians, and herbivores consisting of stegosaurian ornithischians and large sauropods(Sony VAIO PCG-7133L battery). Examples of this include the Morrison Formation of North America and Tendaguru Beds of Tanzania. Dinosaurs in China show some differences, with specialized sinraptorid theropods and unusual, long-necked sauropods like Mamenchisaurus. Ankylosaurians and ornithopods were also becoming more common(Sony VAIO PCG-7113L battery), but prosauropods had become extinct. Conifers and pteridophytes were the most common plants. Sauropods, like the earlier prosauropods, were not oral processors, but ornithischians were evolving various means of dealing with food in the mouth, including potential cheek-like organs to keep food in the mouth(Sony VAIO PCG-6W3L battery), and jaw motions to grind food. Another notable evolutionary event of the Jurassic was the appearance of true birds, descended from maniraptoran coelurosaurians.

By the Early Cretaceous and the ongoing breakup of Pangaea, dinosaurs were becoming strongly differentiated by landmass(Sony VAIO PCG-7111L battery). The earliest part of this time saw the spread of ankylosaurians, iguanodontians, and brachiosaurids through Europe, North America, and northern Africa. These were later supplemented or replaced in Africa by large spinosaurid and carcharodontosaurid theropods(Sony VAIO PCG-6W1L battery), and rebbachisaurid and titanosaurian sauropods, also found in South America. In Asia, maniraptoran coelurosaurians like dromaeosaurids, troodontids, and oviraptorosaurians became the common theropods, and ankylosaurids and early ceratopsians like Psittacosaurus became important herbivores(Sony VAIO PCG-6V1L battery). Meanwhile, Australia was home to a fauna of basal ankylosaurians, hypsilophodonts, and iguanodontians. The stegosaurians appear to have gone extinct at some point in the late Early Cretaceous or early Late Cretaceous. A major change in the Early Cretaceous, which would be amplified in the Late Cretaceous(Sony VAIO PCG-6S3L battery), was the evolution of flowering plants. At the same time, several groups of dinosaurian herbivores evolved more sophisticated ways to orally process food. Ceratopsians developed a method of slicing with teeth stacked on each other in batteries, and iguanodontians refined a method of grinding with tooth batteries(Sony VAIO PCG-6S2L battery), taken to its extreme in hadrosaurids. Some sauropods also evolved tooth batteries, best exemplified by the rebbachisaurid Nigersaurus.

There were three general dinosaur faunas in the Late Cretaceous. In the northern continents of North America and Asia(Sony VAIO PCG-5K2L battery), the major theropods were tyrannosaurids and various types of smaller maniraptoran theropods, with a predominantly ornithischian herbivore assemblage of hadrosaurids, ceratopsians, ankylosaurids, and pachycephalosaurians. In the southern continents that had made up the now-splitting Gondwana, abelisaurids were the common theropods(Sony VAIO PCG-5J2L battery), and titanosaurian sauropods the common herbivores. Finally, in Europe, dromaeosaurids, rhabdodontid iguanodontians, nodosaurid ankylosaurians, and titanosaurian sauropods were prevalent. Flowering plants were greatly radiating, with the first grasses appearing by the end of the Cretaceous(Sony VAIO PCG-5J1L battery). Grinding hadrosaurids and shearing ceratopsians became extremely diverse across North America and Asia. Theropods were also radiating as herbivores or omnivores, with therizinosaurians and ornithomimosaurians becoming common(Sony VAIO PCG-5G3L battery).

The Cretaceous–Tertiary extinction event, which occurred approximately 65 million years ago at the end of the Cretaceous period, caused the extinction of all dinosaurs except for the birds. Some other diapsid groups, such as crocodilians, lizards, snakes, sphenodontians, and choristoderans, also survived the event(Sony VAIO PCG-5G2L battery).

Paleobiology

Knowledge about dinosaurs is derived from a variety of fossil and non-fossil records, including fossilized bones, feces, trackways, gastroliths, feathers, impressions of skin, internal organs and soft tissues. Many fields of study contribute to our understanding of dinosaurs, including physics (especially biomechanics) (Sony VGP-BPS21A/B battery), chemistry, biology, and the earth sciences (of which paleontology is a sub-discipline). Two topics of particular interest and study have been dinosaur size and behavior.

Size

The sauropods were the largest dinosaurs. For much of the dinosaur era, the smallest sauropods were larger than anything else in their habitat(Sony VGP-BPS21/S battery), and the largest were an order of magnitude more massive than anything else that has since walked the Earth. Giant prehistoric mammals such as the Paraceratherium and the Columbian mammoth were dwarfed by the giant sauropods, and only a handful of modern aquatic animals approach or surpass them in size – most notably the blue whale(Sony VGP-BPS21B battery), which reaches up to 173000 kg (381000 lb) and over 30 meters (100 ft) in length. There are several proposed advantages for the large size of sauropods, including protection from predation, reduction of energy use, and longevity, but it may be that the most important advantage was dietary(Sony VGP-BPS21 battery). Large animals are more efficient at digestion than small animals, because food spends more time in their digestive systems. This also permits them to subsist on food with lower nutritive value than smaller animals. Sauropod remains are mostly found in rock formations interpreted as dry or seasonally dry(Sony VGN-FW31J battery), and the ability to eat large quantities of low-nutrient browse would have been advantageous in such environments.

Most dinosaurs, however, were much smaller than the giant sauropods. Current evidence suggests that dinosaur average size varied through the Triassic, early Jurassic, late Jurassic and Cretaceous periods. Theropod dinosaurs(Sony VGP-BPS13A/S battery), when sorted by estimated weight into categories based on order of magnitude, most often fall into the 100 to 1000 kilogram (220 to 2200 lb) category, whereas recent predatory carnivorans peak in the 10 to 100 kilogram (22 to 220 lb) category. The mode of dinosaur body masses is between one and ten metric tonnes(Sony VGP-BPS13B/S battery). This contrasts sharply with the size of Cenozoic mammals, estimated by the National Museum of Natural History as about 2 to 5 kilograms (5 to 10 lb).

Largest and smallest

Only a tiny percentage of animals ever fossilize, and most of these remain buried in the earth. Few of the specimens that are recovered are complete skeletons, and impressions of skin and other soft tissues are rare(Sony VGP-BPS13S battery). Rebuilding a complete skeleton by comparing the size and morphology of bones to those of similar, better-known species is an inexact art, and reconstructing the muscles and other organs of the living animal is, at best, a process of educated guesswork. As a result, scientists will probably never be certain of the largest and smallest dinosaurs(Sony VGP-BPS13AS battery).

The tallest and heaviest dinosaur known from good skeletons is Giraffatitan brancai (previously classified as a species of Brachiosaurus). Its remains were discovered in Tanzania between 1907–12. Bones from several similar-sized individuals were incorporated into the skeleton now mounted and on display at the Museum für Naturkunde Berlin(Sony PCGA-BP2EA battery); this mount is 12 meters (39 ft) tall and 22.5 meters (74 ft) long, and would have belonged to an animal that weighed between 30000 and 60000 kilograms (70000 and 130000 lb). The longest complete dinosaur is the 27-meter (89 ft) long Diplodocus, which was discovered in Wyoming in the United States and displayed in Pittsburgh's Carnegie Natural History Museum in 1907(Sony VGP-BPS13/B battery).

Comparative size of Eoraptor

There were larger dinosaurs, but knowledge of them is based entirely on a small number of fragmentary fossils. Most of the largest herbivorous specimens on record were all discovered in the 1970s or later, and include the massive Argentinosaurus, which may have weighed 80000 to 100000 kilograms (90 to 110 short tons) (Sony VGP-BPS13/S battery); some of the longest were the 33.5 meters (110 ft) long Diplodocus hallorum (formerly Seismosaurus) and the 33 meters (110 ft) long Supersaurus; and the tallest, the 18 meters (59 ft) tall Sauroposeidon, which could have reached a sixth-floor window. The heaviest and longest of them all may have been Amphicoelias fragillimus, known only from a now lost partial vertebral neural arch described in 1878(Sony VGP-BPS13A/B battery). Extrapolating from the illustration of this bone, the animal may have been 58 meters (190 ft) long and weighed over 120000 kg (260000 lb). The largest known carnivorous dinosaur was Spinosaurus, reaching a length of 16 to 18 meters (50 to 60 ft), and weighing in at 8150 kg (18000 lb) (Sony VGP-BPS13B/B battery). Other large meat-eaters included Giganotosaurus, Carcharodontosaurus and Tyrannosaurus.