Dinosaurs traditionally have been placed in the reptilian subclass Diapsida,
based on the fact that these reptiles have two pairs of temporal openings in their skull.
As diapsids, dinosaurs are
also grouped with the crocodilians, thecodonts, and pterosaurs, all of which have socketed teeth and a number of other features in common
(sometimes referred to as "archosaurian reptiles"). In recent years it has been suggested that dinosaurs be ranked as a class of their own, comparable to the classes Mammalia and Aves. That idea has not
yet been universally accepted however and at the present time, the term Dinosauria is not used as a taxonomic category to include all dinosaurs. Instead, they are classified in their two orders, as either Saurischia or Ornithischia.
Unfortunately, no universally accepted classification of dinosaurs exists. Fossil remains are often difficult to interpret, especially when only a few fragmentary specimens of a type have been found. Moreover, classifications may be constructed to serve different purposes that require different categories or organization.
Occasionally, for example, the Sauropodomorpha have been divided into more or fewer lower-rank categories (e.g., families, subfamilies); but the twofold division into the infraorders Sauropoda and Prosauropoda has stood the test of time and has been followed here.
Likewise, previous classifications divided the suborder Theropoda into two infraorders, the Carnosauria and the Coelurosauria. The former included all the larger animals and the latter all the smaller kinds. That arrangement did recognize certain distinctive anatomic features such as large heads and short necks in the Carnosauria and small heads and long necks in the Coelurosauria. But great numbers of theropod discoveries around the world in the past several decades have blurred those anatomic distinctions and reduced the importance of size as a diagnostic criterion. Accordingly, infraordinal categories are not always used in current classifications of the Theropoda; sometimes only family groupings are listed. In the classification adopted here, the theropods are divided into two infraorders, the Ceratosauria and the Tetanurae. The tetanuran theropods are further divided into certain subcategories—Coelurosauria, Ornithomimosauria, Maniraptora,
Segnosauria, and Carnosauria—that are at a higher level than the families of this infraorder. It must be noted, however, that evolutionary affinities among all the theropod types are still being analyzed, and experts have not reached full agreement on a formal classification.
Within the order Ornithischia, two distinct subdivisions are generally given equal rank, currently as the suborders Cerapoda and Thyreophora.
A final example is the recently discovered Scutellosaurus, which has been assigned by some to the Fabrosauridae (Ornithopoda) and by others to the Stegosauria. Scutellosaurus might well represent an evolutionary link between the ornithopods and the later stegosaurs or ankylosaurs. Since its affinities are still unclear, it has here been tentatively placed with the Stegosauria.
The reptile-hipped dinosaurs.
All the reptile-hipped herbivorous dinosaurs. Late Triassic to Late Cretaceous.
Facultative bipeds; primitive forerunners of sauropods. Late Triassic to Early Jurassic.
Primitive prosauropods including Anchisaurus and Plateosaurus. Late Triassic to Early Jurassic.
Advanced prosauropods such as Melanorosaurus and Riojasaurus; probably includes the sauropod ancestry. Late Triassic to Early Jurassic.
Large to gigantic obligatory quadrupeds; all herbivorous. Early Jurassic to Late Cretaceous.
Primitive and poorly studied sauropods such as Cetiosaurus; specimens mostly from the Old World; vertebrae well-excavated to lighten bone weight. Early to Late Jurassic.
More advanced and better-known large sauropods; highly excavated vertebrae. Diplodocus and Apatosaurus (Brontosaurus) are the best known. Late Jurassic to Late Cretaceous.
Largest of all the sauropods; greatly elongated necks and highly excavated vertebrae; front legs longer than back legs. Brachiosaurus is the most famous. Middle Jurassic to Late Cretaceous.
Advanced sauropods, primarily from the southern continents. Examples are Titanosaurus and Alamosaurus. Early to Late Cretaceous.
Moderate-size sauropods; relatively short necks and tails for this infraorder; teeth spatulate
or spoon-shaped. Camarasaurus and Morosaurus are typical. Late Jurassic to Late Cretaceous.
Primitive carnivorous dinosaurs resembling theropods; known only from South America. Classification is uncertain(?). Name bearer is Staurikosaurus. Middle to Late Triassic.
All the carnivorous dinosaurs except the staurikosaurs; obligatory bipeds. Late Triassic to Late Cretaceous.
Small to medium-size, hollow-boned carnivores. Late Triassic to Late Jurassic. Includes the primitive theropods such as Ceratosaurus, Coelophysis, and Syntarsus.
All other theropods are grouped here into the five subcategories Coelurosauria, Ornithomimosauria, Maniraptora (previously Deinonychosauria), Segnosauria, and Carnosauria. Late Triassic to Late Cretaceous.
Family Compsognathidae (Coelurosauria)
Advanced; smallest of the theropods; all known specimens 2-fingered. Name bearer is Compsognathus. Late Jurassic.
Family Oviraptoridae (Maniraptora)
Small, toothless theropods with an odd skull form; perhaps related to the ornithomimids. Oviraptor is the best-known example. Late Cretaceous.
Family Dromaeosauridae (Maniraptora)
Includes Deinonychus and Velociraptor. Early to Late Cretaceous.
Family Troodontidae (Maniraptora)
Less specialized in foot structure, but close to the dromaeosaurids. Name bearer is Troodon. Late Cretaceous.
Family Megalosauridae (Carnosauria)
Primitive large theropods; often 4-fingered. Megalosaurus is the best known. Early Jurassic to Late Cretaceous.
Family Allosauridae (Carnosauria)
More advanced large theropods; all 3-fingered except for Ceratosaurus. Best known is Allosaurus. Late Jurassic to Late Cretaceous.
Family Tyrannosauridae (Carnosauria)
Largest of the theropods and the most advanced; all with just 2 fingers. Examples are Tyrannosaurus, Tarbosaurus, and Albertosaurus. Late Cretaceous.
The bird-hipped, herbivorous dinosaurs; characterized by the diagnostic predentary
bone of the jaw.
Facultatively bipedal ornithischian dinosaurs plus the horned ceratopsian forms. Late Triassic to Late Cretaceous.
Earliest and most primitive of the ornithopods; small and often hollow-boned. Best known is Fabrosaurus. Late Triassic to Early Cretaceous.
More advanced small ornithopods, with the beginnings of specialized dentition. Best examples are Heterodontosaurus and the primitive Pisanosaurus. Late Triassic to Late Jurassic.
More advanced small to medium-size ornithopods, with only a suggestion of
specialized dentition. Hypsilophodon and Thescelosaurus are examples. Late Jurassic to Late Cretaceous.
Medium to large ornithopods, with the first stages of specialized grinding dentition. Iguanodon and Camptosaurus are the best known. Late Jurassic to Late Cretaceous.
The duck-billed ornithopods, with highly specialized grinding dentition; medium
to large size. Edmontosaurus, Corythosaurus, and Lambeosaurus are well known. Late Cretaceous.
The dome-headed ornithischians; closely related to the ornithopods; usually with
a massively thick bony skull roof; bipedal. Stegoceras and Pachycephalosaurus are the best examples. Late Cretaceous.
The horned dinosaurs. Early to Late Cretaceous.
Ancestral and most primitive of the ceratopsians; represented by the hornless and bipedal Psittacosaurus. Early Cretaceous.
Primitive quadrupedal ceratopsians, with short frills and very modest horns. Protoceratops and Leptoceratops are the best examples. Late Cretaceous.
Advanced quadrupedal ceratopsians, with prominent horns and frills. Examples are Monoclonius, Torosaurus, and Triceratops. Late Cretaceous.
The plated and armoured dinosaurs. Late Triassic or Early Jurassic to Late Cretaceous.
Primitive stegosaurs, with less well-developed back plates. Scelidosaurus is the most primitive form; Scutellosaurus perhaps the most advanced. Late Triassic or Early Jurassic.
Advanced stegosaurs, usually with well-developed back plates and spines. Stegosaurus and Kentrosaurus are the best known. Middle Jurassic to Early Cretaceous.
Primitive ankylosaurs, usually with less completely developed armour. Nodosaurus, Hylaeosaurus, and Sauropelta are well-known kinds. Early to Late Cretaceous.
Advanced ankylosaurs such as Euoplocephalus and Ankylosaurus. Late Cretaceous.