Dinosaurs are a diverse group of animals of the clade Dinosauria. They first appeared during the Triassic period, approximately 230 million years ago, and became the dominant terrestrial vertebrates for 135 million years, from the beginning of the Jurassic (about 200 million years ago) until the end of the Cretaceous (65.5 million years ago) (Dell 1691P battery), when the Cretaceous–Paleogene extinction event led to the extinction of most dinosaur groups at the close of the Mesozoic era. The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic, and consequently they are considered a type of dinosaur in modern classification systems. Some birds survived the extinction event that occurred 65 million years ago, and continue the dinosaur lineage to the present day(Dell 310-6321 battery).
Dinosaurs are a varied group of animals from taxonomic, morphological and ecological standpoints. Birds, at over 9,000 living species, are the most diverse group of vertebrates besides perciform fish. Using fossil evidence, paleontologists have identified over 500 distinct genera and more than 1,000 different species of non-avian dinosaurs. (Dell 312-0068 battery) Dinosaurs are represented on every continent by both extant species and fossil remains. Some are herbivorous, others carnivorous. Most dinosaurs have been bipedal, though many extinct groups included quadrupedal species, and some were able to shift between these body postures. Many species possess elaborate display structures such as horns or crests(Dell 312-0078 battery), and some prehistoric groups developed skeletal modifications such as bony armor and spines. Birds have been the planet's dominant flying vertebrate since the extinction of the pterosaurs, and evidence suggests that egg laying and nest building is a trait shared by all dinosaurs. Many prehistoric dinosaurs were large animals—the largest sauropods could reach lengths of almost 60 meters (200 feet) and were several stories tall—and while many extinct theropods were quite large, a majority evolved very small sizes(Dell 312-0079 battery), especially among birds and other advanced groups.
Although the word dinosaur means "terrible lizard," the name is somewhat misleading, as dinosaurs are not lizards. Rather, they represent a separate group of reptiles with a distinct upright posture not found in lizards. Through the first half of the 20th century, before birds were recognized to be dinosaurs(Dell 312-0305 battery), most of the scientific community believed dinosaurs were sluggish and cold-blooded. Most research conducted since the 1970s, however, has indicated that ancient dinosaurs, particularly the carnivorous groups, were active animals with elevated metabolisms and numerous adaptations for social interaction.
Since the first dinosaur fossils were recognized in the early 19th century(Dell 312-0326 battery), mounted fossil dinosaur skeletons or replicas have been major attractions at museums around the world, and dinosaurs have become a part of world culture. Their diversity, the large sizes of some groups, and their seemingly monstrous and fantastic nature have captured the interest and imagination of the general public for over a century. They have been featured in best-selling books and films such as Jurassic Park(Dell 312-0518 battery), and new discoveries are regularly covered by the media.
The taxon Dinosauria was formally named in 1842 by paleontologist 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," "potent," or "fearfully great")(Dell 312-0566 battery) and σαῦρος (sauros, meaning "lizard" or "reptile").:103 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.
Triceratops horridus skeleton, American Museum of Natural History
Under phylogenetic taxonomy, dinosaurs are usually defined as the group consisting of "Triceratops, Neornithes [modern birds], their most recent common ancestor, and all descendants".(Dell 312-0585 battery)It has also been suggested that Dinosauria be defined with respect to the most recent common ancestor of Megalosaurus and Iguanodon, 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: "Dinosauria = Ornithischia + Saurischia"(Dell 312-0831 battery), encompassing theropods (mostly bipedal carnivores and birds), ankylosaurians (armored herbivorous quadrupeds), stegosaurians (plated herbivorous quadrupeds), ceratopsians (herbivorous quadrupeds with horns and frills), ornithopods (bipedal or quadrupedal herbivores including "duck-bills"), and, perhaps, sauropodomorphs (mostly large herbivorous quadrupeds with long necks and tails) (Dell BAT30WL battery).
The common House Sparrow (Passer domesticus) is often used to represent modern birds in definitions of the group Dinosauria
Many paleontologists note that the point at which sauropodomorphs and theropods diverged may omit sauropodomorphs from the definition for both saurischians and dinosaurs. To avoid instability, Dinosauria can be more conservatively defined with respect to four anchoring nodes: Triceratops horridus, Saltasaurus loricatus(Dell D6400 battery), and Passer domesticus, their most recent common ancestor, and all descendants. This "safer" definition can be expressed as "Dinosauria = Ornithischia + Sauropodomorpha + Theropoda".
There is near universal consensus among paleontologists that birds are the descendants of theropod dinosaurs. In traditional taxonomy, birds were considered a separate "class" which had evolved from dinosaurs(Dell HF674 battery). However, a majority of modern paleontologists reject the traditional style of classification in favor of phylogenetic nomenclature, which requires that all descendants of a single common ancestor must be included in a group for that group to be natural. Birds are thus considered by most modern scientists to be dinosaurs and dinosaurs are, therefore, not extinct(Dell N3010 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.
Using one of the above definitions, dinosaurs can be generally described as archosaurs with limbs held erect beneath the body. Many prehistoric animal groups are popularly conceived of as dinosaurs, such as ichthyosaurs, mosasaurs, plesiosaurs(Dell Inspiron N4010 battery), pterosaurs, and Dimetrodon, but are not classified scientifically as dinosaurs, and none had the erect limb posture characteristic of true dinosaurs. Dinosaurs were the dominant terrestrial vertebrates of the Mesozoic, especially the Jurassic and Cretaceous periods. Other groups of animals were restricted in size and niches; mammals, for example(Dell INSPIRON 1100 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. (Dell Inspiron 1200 battery)
Stegosaurus stenops skeleton, Field Museum
Dinosaurs have always been an extremely varied group of animals; according to a 2006 study, over 500 non-avialan 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. (Dell Inspiron 1420 battery) An earlier study predicted that about 3400 dinosaur genera existed, including many which would not have been preserved in the fossil record. By September 17, 2008, 1047 different species of dinosaurs had been named. Some are herbivorous, others carnivorous. While most dinosaurs have been bipeds, some prehistoric species were quadrupeds, and others, such as Ammosaurus and Iguanodon(Dell Inspiron 1464 battery), could walk just as easily on two or four legs. Cranial modifications like horns and crests are common among dinosaurs, and some extinct species had bony armor. Although known for large size, many Mesozoic dinosaurs were human-sized or smaller, and modern birds are generally very small in size. Dinosaurs today inhabit every continents, (Dell Inspiron 1564 battery) and fossils show that they had achieved global distribution by at least the early Jurassic period. Modern birds inhabit most available habitats, from terrestrial to marine, and there is evidence that some non-avialan dinosaurs (such as Microraptor) could fly or at least glide, and others, such as spinosaurids, had semi-aquatic habits.
Distinguishing anatomical features(Dell Inspiron 1764 battery)
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. Although some later groups of dinosaurs featured further modified versions of these traits(Dell Inspiron 1520 battery), 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.
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
epipophyses present in anterior neck vertebrae (except atlas and axis) (Dell Inspiron 1521 battery)
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
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(Dell inspiron 1525 battery)
on the astragalus and calcaneum the proximal articular facet for fibula occupies less than 30% of the transverse width of the element
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(Dell inspiron 1526 battery)
distinct proximodistally oriented ridge present on the posterior face of the distal end of the tibia
Nesbitt found a number of further potential synapomorphies, and discounted 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(Dell Inspiron 1720 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.
Diagram of a typical diapsid skull
Hip joints and hindlimb postures of typical reptiles (left), dinosaurs and mammals (middle), and rauisuchians (right)
A variety of other skeletal features are shared by dinosaurs. However, because they are either common to other groups of archosaurs or were not present in all early dinosaurs, these features are not considered to be synapomorphies(Dell Inspiron 2000 battery). For example, as diapsids, 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, or were absent in the earliest dinosaurs and independently evolved by different dinosaur groups(Dell INSPIRON 2600 battery). 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); and an acetabulum, or hip socket, with a hole at the center of its inside surface (closed in Saturnalia, for example). (Dell INSPIRON 3800 battery) 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.
Dinosaurs stand erect in a manner similar to most modern mammals, but distinct from most other reptiles, whose limbs sprawl out to either side. This posture is due to the development of a laterally facing recess in the pelvis (usually an open socket) (Dell INSPIRON 4000 battery) and a corresponding inwardly facing distinct head on the femur. Their erect posture enabled early 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. (Dell Inspiron 5000 battery) Some non-dinosaurian archosaurs, 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 500M battery)
Main article: Evolution of dinosaurs
Origins and early evolution
Skeleton of Marasuchus lilloensis, a dinosaur-like ornithodiran
The early forms Herrerasaurus (large), Eoraptor (small) and a Plateosaurus skull
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 5100 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 think that Eoraptor resembles the common ancestor of all dinosaurs; if this is true, its traits suggest that the first dinosaurs were small, bipedal predators. The discovery of primitive(Dell INSPIRON 510M battery), 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.
When dinosaurs appeared, terrestrial habitats were occupied by various types of archosaurs and therapsids, such as aetosaurs, cynodonts, dicynodonts, ornithosuchids, rauisuchians, and rhynchosaurs(Dell INSPIRON 6000 battery). Most of these other animals became extinct in the Triassic, 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 INSPIRON 600M 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 first few lines of early dinosaurs diversified through the Carnian and Norian stages of the Triassic, most likely by occupying the niches of the groups that became extinct(Dell Inspiron 6400 battery).
Evolution and paleobiogeography
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 early sauropodomorph herbivores. Gymnosperm plants (particularly conifers) (Dell INSPIRON 7000 battery), a potential food source, radiated in the Late Triassic. Early sauropodomorphs 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. The general homogeneity of dinosaurian faunas continued into the Middle and Late Jurassic, where most localities had predators consisting of ceratosaurians(Dell INSPIRON 700M battery), spinosauroids, and carnosaurians, and herbivores consisting of stegosaurian ornithischians and large sauropods. 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(Dell Inspiron 710m 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, and jaw motions to grind food(Dell INSPIRON 8200 battery). Another notable evolutionary event of the Jurassic was the appearance of true birds, descended from maniraptoran coelurosaurians.
Earth during the Jurassic, the period in which dinosaurs gained global distribution. The continents were in different locations from where they are today.
By the Early Cretaceous and the ongoing breakup of Pangaea, dinosaurs were becoming strongly differentiated by landmass. The earliest part of this time saw the spread of ankylosaurians, iguanodontians, and brachiosaurids through Europe, North America(Dell INSPIRON 8600 battery), and northern Africa. These were later supplemented or replaced in Africa by large spinosaurid and carcharodontosaurid theropods, and rebbachisaurid and titanosaurian sauropods, also found in South America. In Asia, maniraptoran coelurosaurians like dromaeosaurids, troodontids, and oviraptorosaurians became the common theropods(Dell INSPIRON 9100 battery), and ankylosaurids and early ceratopsians like Psittacosaurus became important herbivores. 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(Dell INSPIRON 9200 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, taken to its extreme in hadrosaurids. (Dell INSPIRON 9300 battery) 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, the major theropods were tyrannosaurids and various types of smaller maniraptoran theropods, with a predominantly ornithischian herbivore assemblage of hadrosaurids, ceratopsians, ankylosaurids(Dell Inspiron 9400 battery), and pachycephalosaurians. In the southern continents that had made up the now-splitting Gondwana, abelisaurids were the common theropods, and titanosaurian sauropods the common herbivores. Finally, in Europe, dromaeosaurids, rhabdodontid iguanodontians, nodosaurid ankylosaurians, and titanosaurian sauropods were prevalent. Flowering plants were greatly radiating, (Dell Inspiron E1505 battery) with the first grasses appearing by the end of the Cretaceous. 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. (Dell Inspiron E1705 battery)
The Cretaceous–Paleogene extinction event, which occurred approximately 65 million years ago at the end of the Cretaceous period, caused the extinction of all dinosaur groups except for the neornithine birds. Some other diapsid groups, such as crocodilians, sebecosuchians, turtles, lizards, snakes, sphenodontians, and choristoderans, also survived the event. (Dell Inspiron Mini 9 battery)
The surviving lineages of neornithine birds, including the ancestors of modern ratites, ducks and chickens, and a variety of waterbirds, diversified rapidly at the beginning of the Paleogene period, entering ecological niches left vacant by the extinction of Mesozoic dinosaur groups such as the arboreal enantiornithines, aquatic hesperornithines(Dell Latitude D400 battery), and even the larger terrestrial theropods (in the form of Gastornis, mihirungs, and "terror birds"). However, mammals were also rapidly diversifying during this time, and out-competed the neornithines for dominance of most terrestrial niches.
Main article: Dinosaur classification
Dinosaurs are archosaurs, like modern crocodilians. 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 STUDIO 1450 battery).
Collectively, dinosaurs as a clade are divided into two primary branches, Saurischia and Ornithischia. Saurischia includes those taxa sharing a more recent common ancestor with birds than with Ornithischia, while Ornithischia includes all taxa sharing a more recent common ancestor with Triceratops than with Saurischia(Dell Vostro 1400 battery). Anatomically, these two groups can be distinguished most noticeably by their pelvic structure. Early saurischians—"lizard-hipped&q
By contrast, ornithischians—"bird-hipped&q
The following is a simplified classification of dinosaur groups based on their evolutionary relationships, and organized based on the list of Mesozoic dinosaur species provided by Holtz (2008). A more detailed version can be found at Dinosaur classification. The cross (†) is used to signify groups with no living members(Dell XPS M1330 battery).
Saurischia ("lizard-hipped"; includes Theropoda and Sauropodomorpha)
Theropoda (all bipedal; most were carnivorous)
†Herrerasauria (early bipedal carnivores)
†Coelophysoidea (small, early theropods; includes Coelophysis and close relatives)
†Dilophosauridae (early crested and carnivorous theropods)
†Ceratosauria (generally elaborately horned, the dominant southern carnivores of the Cretaceous) (Dell XPS 1340 battery)
Tetanurae ("stiff tails"; includes most theropods)
†Megalosauroidea (early group of large carnivores including the semi-aquatic spinosaurids)
†Carnosauria (Allosaurus and close relatives, like Carcharodontosaurus)
Coelurosauria (feathered theropods, with a range of body sizes and niches)
†Compsognathidae (common early coelurosaurs with reduced forelimbs)
†Tyrannosauridae (Tyrannosaurus and close relatives; had reduced forelimbs) (Dell XPS M1530 battery)
†Ornithomimosauria ("ostrich-mimics"; mostly toothless; carnivores to possible herbivores)
†Alvarezsauroidea (small insectivores with reduced forelimbs each bearing one enlarged claw)
Maniraptora ("hand snatchers"; had long, slender arms and fingers)
†Therizinosauria (bipedal herbivores with large hand claws and small heads)
†Oviraptorosauria (mostly toothless; their diet and lifestyle are uncertain)
†Archaeopterygidae (small, winged theropods or primitive birds) (Dell XPS M170 battery)
†Deinonychosauria (small- to medium-sized; bird-like, with a distinctive toe claw)
Avialae (modern birds and extinct relatives)
†Scansoriopterygidae (small primitive avialans with long third fingers)
†Omnivoropterygidae (large, early short-tailed avialans)
†Confuciusornithidae (small toothless avialans)
†Enantiornithes (primitive tree-dwelling, flying avialans)
Euornithes (advanced flying birds)
†Yanornithiformes (toothed Cretaceous Chinese birds)
†Hesperornithes (specialized aquatic diving birds) (Dell XPS M1710 battery)
Aves (modern, beaked birds and their extinct relatives)
Several macronarian Sauropods: from left to right Camarasaurus, Brachiosaurus, Giraffatitan, and Euhelopus
†Sauropodomorpha (herbivores with small heads, long necks, long tails)
†Guaibasauridae (small, primitive, omnivorous sauropodomorphs)
†Plateosauridae (primitive, strictly bipedal "prosauropods")
†Riojasauridae (small, primitive sauropodomorphs)
†Massospondylidae (small, primitive sauropodomorphs) (Dell XPS M1730 battery)
†Sauropoda (very large and heavy, usually over 15 meters (49 feet) long; quadrupedal)
†Vulcanodontidae (primitive sauropods with pillar-like limbs)
†Eusauropoda ("true sauropods")
†Cetiosauridae ("whale reptiles")
†Turiasauria (European group of Jurassic and Cretaceous sauropods)
†Neosauropoda ("new sauropods")
†Diplodocoidea (skulls and tails elongated; teeth typically narrow and pencil-like)
†Macronaria (boxy skulls; spoon- or pencil-shaped teeth)
†Brachiosauridae (long-necked, long-armed macronarians) (Dell XPS M2010 battery)
†Titanosauria (diverse; stocky, with wide hips; most common in the Late Cretaceous of southern continents)
Various ornithopod dinosaurs and one heterodontosaurid. Far left: Camptosaurus, left: Iguanodon, center background: Shantungosaurus, center foreground: Dryosaurus, right: Corythosaurus, far right (small): Heterodontosaurus, far right (large) Tenontosaurus.
†Ornithischia ("bird-hipped"; diverse bipedal and quadrupedal herbivores) (Dell Latitude E5400 battery)
†Heterodontosauridae (small basal ornithopod herbivores/omnivores with prominent canine-like teeth)
†Thyreophora (armored dinosaurs; mostly quadrupeds)
†Ankylosauria (scutes as primary armor; some had club-like tails)
†Stegosauria (spikes and plates as primary armor)
†Neornithischia ("new ornithischians")
†Ornithopoda (various sizes; bipeds and quadrupeds; evolved a method of chewing using skull flexibility and numerous teeth) (Dell Latitude E5500 battery)
†Marginocephalia (characterized by a cranial growth)
†Pachycephalosauria (bipeds with domed or knobby growth on skulls)
†Ceratopsia (quadrupeds with frills; many also had horns)
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) (Dell Latitude E6400 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.
Main article: Dinosaur size
Scale diagram comparing the largest known dinosaurs in five major clades and a human
Current evidence suggests that dinosaur average size varied through the Triassic, early Jurassic, late Jurassic and Cretaceous periods. Theropod dinosaurs, when sorted by estimated weight into categories based on order of magnitude(Dell Latitude E6500 battery), 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. 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). (Dell Inspiron Mini 12 battery)
The sauropods were the largest and heaviest dinosaurs. For much of the dinosaur era, the smallest sauropods were larger than anything else in their habitat, 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 (the largest land mammal ever) (Dell XPS M140 battery) were dwarfed by the giant sauropods, and only modern whales approach or surpass them in size. 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(Dell XPS 13 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, and the ability to eat large quantities of low-nutrient browse would have been advantageous in such environments. (Dell XPS 16 battery)
Largest and smallest
Scientists will probably never be certain of the largest and smallest dinosaurs. This is because 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. Rebuilding a complete skeleton by comparing the size and morphology of bones to those of similar(Dell XPS 1640 battery), 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.
Comparative size of Giraffatitan
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(Dell XPS 1645 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(Dell XPS 1647 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); some of the longest were the 33.5 meters (Dell Latitude 131L battery) (110 ft) long Diplodocus hallorum (formerly Seismosaurus) and the 33 meters (108 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(Dell Latitude C400 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 (52 to 60 ft), and weighing in at 8150 kg (18000 lb). Other large meat-eaters included Giganotosaurus, Carcharodontosaurus and Tyrannosaurus. (Dell Latitude C500 battery)
Not including birds (Avialae), the smallest known dinosaurs were about the size of a pigeon. Not surprisingly, the smallest dinosaurs were theropods closely related to birds. Anchiornis, for example, had a total skeletal length of under 35 centimeters (1.1 ft). Anchiornis is currently the smallest dinosaur described from an adult specimen(Dell Latitude C510 battery), with an estimated weight of 110 grams. The smallest herbivorous non-avialan dinosaurs included Microceratus and Wannanosaurus, at about 60 cm (2 ft) long each.
A nesting ground of hadrosaur Maiasaura peeblesorum was discovered in 1978.
Many modern birds are highly social, often found living in flocks. There is general agreement that some behaviors which are common in birds, as well as in crocodiles (bird' closest living relatives), were also common among extinct dinosaur groups(Dell Latitude C540 battery). Interpretations of behavior in fossil species are generally based on the pose of skeletons and their habitat, computer simulations of their biomechanics, and comparisons with modern animals in similar ecological niches.
The first potential evidence for herding or flocking as a widespread behavior common to many dinosaur groups in addition to birds was the 1878 discovery of 31 Iguanodon bernissartensis(Dell Latitude C600 battery), ornithischians which were then thought to have perished together in Bernissart, Belgium, after they fell into a deep, flooded sinkhole and drowned. Other mass-death sites have been subsequently discovered. Those, along with multiple trackways, suggest that gregarious behavior was common in many early dinosaur species. Trackways of hundreds or even thousands of herbivores indicate that duck-bills (hadrosaurids) (Dell Latitude C610 battery) may have moved in great herds, like the American Bison or the African Springbok. Sauropod tracks document that these animals traveled in groups composed of several different species, at least in Oxfordshire, England, although there is not evidence for specific herd structures. Congregated into herds may have evolved for defense, for migratory purposes, or to provide protection for young(Dell Latitude C640 battery). There is evidence that many types of slow-growing dinosaurs, including various theropods, sauropods, ankylosaurians, ornithopods, and ceratopsians, formed aggregations of immature individuals. One example is a site in Inner Mongolia that has yielded the remains of over 20 Sinornithomimus, from one to seven years old. This assemblage is interpreted as a social group that was trapped in mud. (Dell Latitude C800 battery) The interpretation of dinosaurs as gregarious has also extended to depicting carnivorous theropods as pack hunters working together to bring down large prey. However, this lifestyle is uncommon among modern birds, crocodiles, and other reptiles, and the taphonomic evidence suggesting mammal-like pack hunting in such theropods as and Allosaurus can also be interpreted as the results of fatal disputes between feeding animals, as is seen in many modern diapsid predators. (Dell Latitude C810 battery)
Artist's rendering of two Centrosaurus apertus engaged in intra-specific combat.
The crests and frills of some dinosaurs, like the marginocephalians, theropods and lambeosaurines, may have been too fragile to be used for active defense, and so they were likely used for sexual or aggressive displays, though little is known about dinosaur mating and territorialism. Head wounds from bites suggest that theropods(Dell Latitude C840 battery), at least, engaged in active aggressive confrontations.
From a behavioral standpoint, one of the most valuable dinosaur fossils was discovered in the Gobi Desert in 1971. It included a Velociraptor attacking a Protoceratops, providing evidence that dinosaurs did indeed attack each other. Additional evidence for attacking live prey is the partially healed tail of an Edmontosaurus, a hadrosaurid dinosaur(Dell Latitude CPI battery); the tail is damaged in such a way that shows the animal was bitten by a tyrannosaur but survived. Cannibalism amongst some species of dinosaurs was confirmed by tooth marks found in Madagascar in 2003, involving the theropod Majungasaurus.
Comparisons between the scleral rings of dinosaurs and modern birds and reptiles have been used to infer daily activity patterns of dinosaurs(Dell Latitude CPX battery). Although it has been suggested that most dinosaurs were active during the day, these comparisons have shown that small predatory dinosaurs such as dromaeosaurids, Juravenator, and Megapnosaurus were likely nocturnal. Large and medium-sized herbivorous and omnivorous dinosaurs such as ceratopsians, sauropodomorphs, hadrosaurids, ornithomimosaurs may have been cathemeral(Dell Latitude D410 battery), active during short intervals throughout the day, although the small ornithischian Agilisaurus was inferred to be diurnal.
Based on current fossil evidence from dinosaurs such as Oryctodromeus, some ornithischian species seem to have led a partially fossorial (burrowing) lifestyle. Many modern birds are arboreal (tree climbing), and this was also true of many Mesozoic birds(Dell Latitude D420 battery), especially the enantiornithines. While some early bird-like species may have already been arboreal as well (including dromaeosaurids such as Microraptor) most non-avialan dinosaurs seem to have relied on land-based locomotion. A good understanding of how dinosaurs moved on the ground is key to models of dinosaur behavior; the science of biomechanics(Dell Latitude D430 battery), in particular, has provided significant insight in this area. For example, studies of the forces exerted by muscles and gravity on dinosaurs' skeletal structure have investigated how fast dinosaurs could run, whether diplodocids could create sonic booms via whip-like tail snapping, and whether sauropods could float. (Dell Latitude D500 battery)
Artists impression of a striking and unusual visual display in a Lambeosaurus magnicristatus
Modern birds are well known for communicating using primarily visual and auditory signals, and the wide diversity of visual display structures among fossil dinosaur groups suggests that visual communication has always been important to dinosaur biology(Dell Latitude D505 battery). However, the evolution of dinosaur vocalization is less certain. In 2008, paleontologist Phil Senter examined the evidence for vocalization in Mesozoic animal life, including dinosaurs. Senter found that, contrary to popular depictions of roaring dinosaurs in motion pictures, it is likely that most Mesozoic dinosaurs were not capable of creating any vocalizations (Dell Latitude D510 battery) (though the hollow crests of the lambeosaurines could have functioned as resonance chambers used for a wide range of vocalizations). To draw this conclusion, Senter studied the distribution of vocal organs in modern reptiles and birds. He found that vocal cords in the larynx probably evolved multiple times among reptiles, including crocodilians, which are able to produce guttural roars(Dell Latitude D520 battery). Birds, on the other hand, lack a larynx. Instead, bird calls are produced by the syrinx, a vocal organ found only in birds, and which is not related to the larynx, meaning it evolved independently from the vocal organs in reptiles. The syrinx depends on the air sac system in birds to function; specifically, it requires the presence of a clavicular air sac near the wishbone or collar bone(Dell Latitude D600 battery). This air sac leaves distinctive marks or opening on the bones, including a distinct opening in the upper arm bone (humerus). While extensive air sac systems are a unique characteristic of saurischian dinosaurs, the clavicular air sac necessary to vocalize does not appear in the fossil record until the enantiornithines (one exception, Aerosteon, probably evolved its clavicular air sac independently of birds for reasons other than vocalization). (Dell Latitude D610 battery)
The most primitive dinosaurs with evidence of a vocalizing syrinx are the enantironithine birds. Any bird-line archosaurs more primitive than this probably did not make vocal calls. Rather, several lines of evidence suggest that early dinosaurs used primarily visual communication, in the form of distinctive-looking (and possibly brightly colored) horns, frills, crests(Dell Latitude D620 battery), sails and feathers. This is similar to some modern reptile groups such as lizards, in which many forms are largely silent (though like dinosaurs they possess well-developed senses of hearing) but use complex coloration and display behaviors to communicate.
In addition, dinosaurs use other methods of producing sound for communication. Other animals, including other reptiles(Dell Latitude D630 battery), use a wide variety of non-vocal sound communication, including hissing, jaw grinding or clapping, use of environment (such as splashing), and wing beating (possible in winged maniraptoran dinosaurs).
Nest of a plover (Charadrius).
All dinosaurs lay amniotic eggs with hard shells made mostly of calcium carbonate. Eggs are usually laid in a nest. Most species create somewhat elaborate nests, which can be cups, domes, plates, beds scrapes, mounds, or burrows. (Dell Latitude D800 battery) Some species of modern bird have no nests; the cliff-nesting Common Guillemot lays its eggs on bare rock, and male Emperor Penguins keep eggs between their body and feet. Primitive birds and many non-avialan dinosaurs often lay eggs in communal nests, with males primarily incubating the eggs(Dell Latitude D810 battery). While modern birds have only one functional oviduct and lay one egg at a time, more primitive birds and dinosaurs had two oviducts, like crocodiles. Some non-avialan dinosaurs, such as Troodon, exhibited iterative laying, where the adult might lay a pair of eggs every one or two days, and then ensured simultaneous hatching by delaying brooding until all eggs were laid. (Dell Latitude D820 battery)
When laying eggs, females grow a special type of bone between the hard outer bone and the marrow of their limbs. This medullary bone, which is rich in calcium, is used to make eggshells. A discovery of features in a Tyrannosaurus rex skeleton provided evidence of medullary bone in extinct dinosaurs and, for the first time, allowed paleontologists to establish the sex of a fossil dinosaur specimen(Dell Latitude D830 battery). Further research has found medullary bone in the theropod Allosaurus and the ornithopod Tenontosaurus. Because the line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged from the line that led to Tenontosaurus very early in the evolution of dinosaurs, this suggests that dinosaurs in general produced medullary tissue. (Dell Latitude 2100 battery)
Fossilized egg of the oviraptorid Citipati, American Museum of Natural History
Another widespread trait among modern birds is parental care for young after hatching. Jack Horner's 1978 discovery of a Maiasaura ("good mother lizard") nesting ground in Montana demonstrated that parental care continued long after birth among ornithopods, suggesting this behavior might have been common to all dinosaurs. (Dell Latitude 2110 battery) There is also evidence that other non-theropod dinosaurs, like Patagonian titanosaurian sauropods (1997 discovery), also nested in large groups. A specimen of the Mongolian oviraptorid Citipati osmolskae was discovered in a chicken-like brooding position in 1993, which indicates that they had begun using an insulating layer of feathers to keep the eggs warm. (Dell Latitude E4300 battery) Parental care being a trait common to all dinosaurs is supported by other finds. For example, the fossilized remains of a grouping of Psittacosaurus has been found, consisting of one adult and 34 juveniles; in this case, the large number of juveniles may be due to communal nesting. Additionally, a dinosaur embryo (pertaining...