The formal taxonomic name Dinosauria was coined in 1842 by the English anatomist and paleontologist Sir Richard Owen, who combined the Greek words deinos, meaning terrible or fearfully great, and sauros, meaning lizard or reptile. Under modern phylogenetic nomenclature, Dinosauria is defined as the crown clade comprising the most recent common ancestor of the bird-hipped dinosaur Triceratops and modern birds (or the lizard-hipped Megalosaurus), together with all of that ancestor's descendants.¹¹ Dinosaurs belong to the broader reptile group Archosauria, which also includes pterosaurs and crocodilians, and sit within the avemetatarsalian branch of archosaurs closer to birds than to alligators. Contrary to common misconception, prehistoric marine reptiles such as ichthyosaurs, plesiosaurs, and mosasaurs, as well as flying pterosaurs, lack the diagnostic skeletal features of the clade and are not dinosaurs.¹²¹³

Dinosaurs are distinguished from other archosaurs by several shared evolutionary traits, or synapomorphies, mostly centered on the pelvis and hindlimbs. Foremost among these is a perforate acetabulum, a hip socket with an open hole in its center bounded by the ilium, ischium, and pubis, rather than the solid cup of bone found in most other tetrapods.¹⁴ Into this open socket fitted an inwardly turned head of the femur, allowing the hindlimbs to swing in a vertical plane directly beneath the trunk.¹⁵ This parasagittal gait was supported by an enlarged sacrum consisting of three to five or more fused vertebrae attached to the pelvis, compared to the two sacral vertebrae typical of primitive reptiles.

This upright limb architecture fundamentally altered vertebrate locomotion on land. In sprawling reptiles, side-to-side flexion of the torso during walking compresses the lungs on alternating sides, restricting the animal's ability to breathe and run simultaneously. By placing the legs directly underneath the body, early dinosaurs decoupled locomotion from lateral trunk bending, permitting sustained, active movement. Additional skeletal traits common across the group include a reduced fourth and fifth digit on the hand, a foot resting primarily on three central weight-bearing toes, and modifications to the ankle joint that restricted rotation to a simple hinge-like motion.

In 1887 and 1888, the British paleontologist Harry Govier Seeley divided Dinosauria into two fundamental orders based on the arrangement of their pelvic bones: Saurischia and Ornithischia.¹⁶ In saurischians, meaning lizard-hipped, the pubis points downward and forward toward the ribcage, retaining the ancestral archosaurian configuration.¹⁷ In ornithischians, meaning bird-hipped, the pubis is rotated backward to run parallel to the ischium, creating room in the abdomen for an expanded digestive tract. In an example of convergent evolution, several advanced saurischian lineages—including the ancestors of modern birds—independently evolved a backward-pointing pubis millions of years later.

While Seeley's two-part division remained the universal consensus for nearly 130 years, a 2017 phylogenetic study led by Matthew Baron, David Norman, and Paul Barrett challenged this structure.¹⁸ Analyzing dozens of early dinosaur skeletons, Baron and his colleagues proposed that theropods were more closely related to ornithischians than to sauropodomorphs, grouping Theropoda and Ornithischia into a revived clade termed Ornithoscelida.¹⁹ Subsequent re-analyses by an international consortium led by Max Langer later in 2017 recovered the traditional Saurischia-Ornithischia split, noting that the competing arrangements differed by only a few evolutionary steps in computational datasets. Although the traditional division remains widely used, the debate highlighted significant anatomical overlap among the earliest Triassic forms, such as the enigmatic herbivore Chilesaurus.

Dinosaurs evolved in the wake of the Permian–Triassic extinction event (approximately 252 million years ago), which eradicated the majority of terrestrial vertebrate species.²⁶ Fragmentary fossils such as Nyasasaurus from Tanzania suggest that the dinosaur lineage may have diverged from other dinosauromorph archosaurs by the Middle Triassic, around 243 million years ago. The earliest unambiguous dinosaur skeletons come from the Carnian stage of the Late Triassic, dated to between 233 and 231 million years ago, in the Ischigualasto Formation of northwestern Argentina and the Santa Maria Formation of southern Brazil.²⁷ Early genera from these southern Pangaean basins—including the carnivores Herrerasaurus and Sanjuansaurus, and the small basal forms Eoraptor, Eodromaeus, and Panphagia—were relatively small, bipedal animals measuring one to six meters in length.²⁸

Throughout the Late Triassic, dinosaurs were a minor component of terrestrial ecosystems, outnumbered by crocodile-line archosaurs (pseudosuchians) and large synapsids. Their rise to ecological dominance was facilitated by two environmental crises: the Carnian Pluvial Episode around 232 million years ago, which coincided with the diversification of early sauropodomorphs, and the Triassic–Jurassic extinction event 201.3 million years ago. Triggered by massive volcanic eruptions in the Central Atlantic Magmatic Province as the supercontinent Pangaea began to rift apart, the end-Triassic extinction wiped out most large pseudosuchians and non-mammalian therapsids, leaving terrestrial niches open to dinosaurian radiation.

During the Jurassic period (201.3 to 145 million years ago), dinosaurs established global dominance across both northern Laurasia and southern Gondwana. Giant sauropods became the primary megaherbivores alongside stegosaurs, while megalosaurids and allosauroids served as apex predators. In the Late Jurassic, around 155 million years ago, small coelurosaurian theropods evolved powered or gliding flight, documented by fossils such as Archaeopteryx from Germany and Anchiornis from China.²⁹ As continental drift separated Pangaea into distinct landmasses during the Cretaceous period (145 to 66 million years ago), dinosaur faunas became increasingly provincial. Tyrannosaurids, ceratopsians, and hadrosaurids dominated North America and Asia, while abelisaurid theropods and titanosaurian sauropods prevailed across South America, Africa, Madagascar, and India.³⁰

Dinosaurs exhibited the widest body mass range of any group of terrestrial vertebrates in Earth's history. At the upper extreme, Cretaceous titanosaurs such as Argentinosaurus huinculensis and Patagotitan mayorum from Patagonia reached estimated lengths of 30 to 40 meters and masses of 50 to more than 70 metric tons, exceeding the weight of any land mammal by several orders of magnitude.³¹ Predatory theropods such as Tyrannosaurus and Spinosaurus reached lengths of 12 to 15 meters and masses of 6 to 8 metric tons. At the opposite extreme, small feathered paravians such as Microraptor and Anchiornis weighed between 100 and 1,000 grams, while the smallest living avian dinosaur, the bee hummingbird (Mellisuga helenae), measures 5 centimeters and weighs less than 2 grams.³²

For more than a century, paleontologists assumed that dinosaurs were ectothermic—cold-blooded animals reliant on environmental heat with slow metabolic rates similar to modern lizards and crocodiles.³³³⁴ Beginning in the late 1960s, researchers challenged this view using bone histology, predator-prey ratios, and paleolatitudinal distributions, including the presence of dinosaur fossils in polar regions.³⁵ Microscopic examination of dinosaur bones revealed dense networks of Haversian canals and fibrolamellar bone tissue indicative of rapid, continuous growth rates comparable to those of modern birds and mammals. Rather than uniform ectothermy or endothermy across the entire clade, evidence indicates a spectrum of metabolic strategies. Smaller coelurosaurian theropods and ornithopods likely maintained elevated, endothermic metabolic rates, whereas giant sauropods relied on gigantothermy (or inertial homeothermy), in which massive body volume prevented rapid fluctuations in core temperature without requiring high metabolic heat production.

Respiratory efficiency in saurischian dinosaurs was enhanced by an avian-style system of pulmonary air sacs connected to the lungs. In theropods and sauropods, extensions of these air sacs invaded the cervical and dorsal vertebrae, hollowing out the bones and leaving diagnostic pneumatic foramina. This flow-through respiratory architecture extracted oxygen far more efficiently than mammalian lungs while simultaneously lightening the massive skeletons of sauropods and large theropods.

Non-avian dinosaurs, along with pterosaurs, ammonites, mosasaurs, and plesiosaurs, vanished abruptly during the Cretaceous–Paleogene (K–Pg) extinction event 66 million years ago. In 1980, a team led by physicist Luis Alvarez and his son, geologist Walter Alvarez, proposed that the mass extinction was triggered by the impact of a bolide based on a global spike in the rare element iridium within the sedimentary clay marking the K–Pg boundary.³⁹⁴⁰ In the early 1990s, this hypothesis was corroborated by the identification of the 180-kilometer-wide Chicxulub impact crater buried beneath the Yucatán Peninsula and the Gulf of Mexico.

The impact of the 10-to-15-kilometer asteroid vaporized sulfate-rich gypsum and carbonate rocks, ejecting billions of tons of sulfur aerosols, soot, and pulverized rock into the stratosphere. Directly following an initial thermal pulse that ignited widespread wildfires and generated mega-tsunamis, the atmospheric ejecta blocked incoming solar radiation, plunging the planet into a prolonged impact winter.⁴¹ Global temperatures dropped sharply and photosynthesis in terrestrial plants and marine phytoplankton halted, collapsing food webs from the bottom up.

A longstanding scientific debate has examined whether the massive volcanic eruptions of the Deccan Traps in what is now India contributed to the extinction. The Deccan flood basalts erupted in multiple pulses over roughly 700,000 years spanning the K–Pg boundary, releasing vast quantities of carbon dioxide and sulfur dioxide.⁴² While researchers such as Gerta Keller have argued that Deccan volcanism was a primary driver of ecological stress prior to the impact, climate and habitat modeling studies—including a 2020 analysis by Alfio Alessandro Chiarenza and colleagues—concluded that the Chicxulub asteroid alone created global conditions uninhabitable for non-avian dinosaurs, whereas volcanic carbon dioxide emissions may have partially buffered the long-term cooling. Only small, ground-dwelling, seed-eating avian dinosaurs (Crown group Aves) survived the collapse of forest canopies and food chains, subsequently radiating in the Paleogene to produce the roughly 11,000 bird species alive today.

Although dinosaur fossils were encountered for millennia and occasionally interpreted in antiquity and medieval China as the bones of dragons or mythological giants, scientific study began in early 19th-century England. In 1824, William Buckland published the first scientific description of a non-avian dinosaur, the carnivorous Megalosaurus, based on a lower jaw with teeth from Oxfordshire. The following year, Gideon Mantell described the herbivorous Iguanodon from teeth and bones found in Sussex, and in 1832 he named the armored Hylaeosaurus. Recognizing that these three genera shared fused sacral vertebrae and upright limbs distinct from any living reptile, Richard Owen established Dinosauria in 1842.

During the late 19th century, the focus of paleontological fieldwork shifted to western North America, fueled by the intense personal and professional rivalry between Edward Drinker Cope of the Academy of Natural Sciences in Philadelphia and Othniel Charles Marsh of Yale University's Peabody Museum.⁴³ Known as the Bone Wars or the Great Dinosaur Rush (roughly 1877 to 1892), their competition across the Morrison Formation and other western rock units in Colorado, Wyoming, and Montana resulted in the discovery and description of 142 new dinosaur species, including Allosaurus, Apatosaurus, Diplodocus, Stegosaurus, and Triceratops. Despite employing bribery, espionage, and hasty publication practices that left behind numerous taxonomic synonyms, Cope and Marsh established the foundational collections of North American museums.

In the first half of the 20th century, major expeditions expanded the geographic reach of dinosaur paleontology—such as the German excavations at Tendaguru in East Africa (1909–1913) and the American Museum of Natural History's Central Asiatic Expeditions to Mongolia in the 1920s—even as theoretical views stagnated. Dinosaurs were widely regarded during this period as sluggish, tail-dragging evolutionary failures. This paradigm was overturned beginning in 1964 with John Ostrom's discovery of the agile, sickle-clawed dromaeosaurid Deinonychus antirrhopus in Montana.⁴⁴ Ostrom's 1969 anatomical monograph, combined with physiological studies by his student Robert T. Bakker, launched the "Dinosaur Renaissance" of the 1970s and 1980s, which recast dinosaurs as active, dynamic animals and revived Thomas Henry Huxley's 19th-century hypothesis that birds are the direct descendants of theropods.

In the 21st century, the rate of dinosaur discovery reached historic peaks, driven by intensive fieldwork in China, Argentina, Canada, the United States, Brazil, and Mongolia.⁴⁵ According to taxonomic compilations published in 2025, a total of 2,677 non-avialan dinosaur species had been named by the end of 2024, of which 1,383 were recognized as valid after accounting for synonyms and dubious taxa. Alongside traditional fieldwork, contemporary paleontology employs X-ray computed tomography (CT scanning) to reconstruct brain cavities and inner ear structures, finite element analysis to test bite forces and biomechanics, and scanning electron microscopy to identify fossilized melanosomes, allowing researchers to reconstruct the coloration and camouflage patterns of extinct feathered species.