Dinosauria

Phylogenetically, Dinosauria is defined as the most recent common ancestor of Triceratops and modern birds, along with all its descendants. Its diagnostic skeletal traits include a fully open hip socket (acetabulum), an inturned femoral head that allows for an erect, parasagittal limb posture, and an elongated deltopectoral crest on the humerus. These features appeared in small bipedal ancestors during the Middle Triassic. They provided the biomechanical foundation that eventually allowed certain lineages to reach sizes unmatched by any other terrestrial vertebrates.

The clade diverged early into two primary branches. Saurischia, or "lizard-hipped" dinosaurs, retained an ancestral forward-pointing pubis. This group includes theropods, birds, and the long-necked sauropodomorphs. Ornithischia, or "bird-hipped" dinosaurs, is not the lineage that led to modern birds. Its name refers to a backward-rotated pubis that evolved independently. Ornithischians were primarily herbivores and included armoured, horned, and duck-billed varieties.

Dinosaurs were the dominant terrestrial vertebrates for approximately 165 million years. While non-avian lineages went extinct at the Cretaceous–Paleogene boundary 66 million years ago due to the Chicxulub impact, avian theropods survived. They eventually radiated into the roughly 11,000 species of birds alive today, which means that humans still coexist with members of the dinosaur clade.

Researchers infer dinosaur behaviour from fossil trackways, bonebeds, nesting sites, brain endocasts, and bone growth rings. There is a general consensus that most groups were endothermic or nearly so, a theory supported by bone histology. Evidence also suggests many lineages provided parental care and engaged in complex social behaviours, ranging from herd movements in sauropods to potential pack-hunting in certain theropods. Areas of ongoing debate include the thermal physiology of giant sauropods, the extent of feathering outside Coelurosauria, and whether the largest predators were solitary ambushers or coordinated hunters.

• Megalosaurus bucklandii (OUMNH J.13505) — first scientifically described dinosaur, William Buckland, 1824. Oxford University Museum.
• Iguanodon bernissartensis Bernissart specimens (RBINS) — ~38 articulated skeletons recovered from a Belgian coal mine, 1878. Royal Belgian Institute of Natural Sciences.
• Sue (FMNH PR 2081, Tyrannosaurus rex) — the most complete large theropod ever found. Field Museum, Chicago.
• Berlin Archaeopteryx (HMN 1880) — feathered transitional fossil, Museum für Naturkunde, Berlin.

The biggest ongoing debate of the last decade is the Ornithoscelida hypothesis (Baron, Norman & Barrett, 2017), which proposed re-grouping ornithischians with theropods to the exclusion of sauropodomorphs. Subsequent re-analyses have largely (but not unanimously) rebutted it, and the traditional Saurischia–Ornithischia split remains the working consensus. Other live questions: how widespread feathering was outside coelurosaurs (some ornithischian "filaments" are contested), the precise pace of the K-Pg extinction (gradual decline vs sudden impact-driven), and whether some "dinosauromorphs" like Nyasasaurus push the clade's origin into the Anisian.

Dinosaurs have been shorthand for "deep time" since Owen coined the name and the Crystal Palace concrete sculptures opened in 1854. Modern visual culture — Jurassic Park (1993), Walking with Dinosaurs (1999), Prehistoric Planet (2022) — has progressively replaced the post-war "tail-dragging swamp lizard" image with active, often-feathered, ecologically diverse animals. Public familiarity is now broad enough that the T. rex–Triceratops matchup is one of the most recognisable cultural icons on Earth.

• saurischia
• ornithischia
• theropoda
• sauropodomorpha

• Benton, M. J. (2014). Vertebrate Palaeontology (4th ed.). Wiley-Blackwell.
• Brusatte, S. L. (2018). The Rise and Fall of the Dinosaurs. William Morrow.
• Baron, M. G., Norman, D. B., & Barrett, P. M. (2017). A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature, 543, 501–506.
• Padian, K. (2004). Basal Avialae. In The Dinosauria (2nd ed.), Weishampel, Dodson & Osmólska (eds.). University of California Press.
• Langer, M. C., et al. (2010). The origin and early evolution of dinosaurs. Biological Reviews, 85(1), 55–110.