Dinosaur classification

thumb|right|450px|Classification of dinosaurs

Dinosaur classification began in 1842 when Sir Richard Owen placed Iguanodon, Megalosaurus, and Hylaeosaurus in "a distinct tribe or suborder of Saurian Reptiles, for which I would propose the name of Dinosauria." In 1887 and 1888 Harry Seeley divided dinosaurs into the two orders Saurischia and Ornithischia, based on their hip structure. These divisions have proved remarkably enduring, even through several seismic changes in the taxonomy of dinosaurs.

The largest change was prompted by entomologist Willi Hennig's work in the 1950s, which evolved into modern cladistics. For specimens known only from fossils, the rigorous analysis of characters to determine evolutionary relationships between different groups of animals (clades) proved incredibly useful. As computer-based cladistics matured in the 1990s, paleontologists were among the first zoologists to broadly adopt the system. Progressive scrutiny and work upon dinosaurian interrelationships, with the aid of new discoveries that have shed light on previously uncertain relationships between taxa, have begun to yield a stabilizing classification since the mid-2000s. While cladistics is the predominant classificatory system among paleontology professionals, the Linnean system is still in use, especially in works intended for popular distribution.

Evolution

Evolutionary relationships

Up until 1985, dinosaurs were usually believed to be polyphyletic, with as many as six independent ancestors giving rise to the various groups popularly called "dinosaurs". However, beginning in 1974 and 1976 independent authors concluded that dinosaurs were a natural group containing both Ornithischia and Saurischia, which became generally accepted with cladistic studies in the 1980s, and is now considered a well-diagnosed clade, and significant among terrestrial vertebrates for the inclusion of birds. Birds and crocodiles are united amongst reptiles by multiple features of the skeleton, soft tissue, and molecular sequencing that separates them from other living vertebrates, with this group now named Archosauria. Dinosaurs form part of the bird-line of archosaurs along with several early Triassic forms, and pterosaurs, while the crocodile-line of archosaurs includes phytosaurs, prestosuchids, rauisuchids, poposaurids, and even ornithosuchids that had once been thought to be ancestral to birds. Originally, Archosauria also included a wide array of extinct reptiles such as rhynchocephalians and anomodonts, and then was modified to include crocodiles, birds, pterosaurs, dinosaurs and the Triassic groups Proterosuchidae and Erythrosuchidae with the earliest member being Archosaurus from the Permian. While some authors retain Archosauria for this more inclusive group, and use the name Avesuchia for the crown group uniting birds and crocodiles, others restrict Archosauria to the crown group and use Archosauriformes for the larger group including the extinct Triassic and Permian members. The placement of turtles relative to other living reptile groups is uncertain, they may form a close relationship with archosaurs, or a close relationship with lizards. Regardless, archosaurs diverged from lizards during the Permian, and each formed their own major clade of reptiles.

Triassic archosaurs, excluding crocodylomorphs, dinosaurs, and pterosaurs, have historically been referred to as thecodontians, though this group has been abandoned as excluding descendents. Early studies of the interrelationships of thecodonts suggested multiple varying arrangements such as the groups Proterosuchia, Parasuchia, and Pseudosuchia, but also Aetosauria, Sphenosuchia, Ornithosuchia and Rauisuchia. Large emphasis was placed on the anatomy of the ankle among these various taxa, identifying a primitive mesotarsal, crocodile-normal, or crocodile-reversed arrangement depending on the location of the hinging joint. Early archosaurs showed a primitive mesotarsal ankle, while crocodylomorphs, phytosaurs, aetosaurs and rauisuchians shared a crocodile-normal ankle, and ornithosuchids were unique in having a crocodile-reversed ankle. Pterosaurs and dinosaurs showed an advanced mesotarsal ankle. On this foundation, ornithosuchids were believed to be the earliest group along the bird-line of archosaurs, and the name Ornithosuchia was used. Restudy of the ankle anatomy and more in the 1990s removed ornithosuchids as one of the earliest crocodile-line archosaurs, leaving only Pterosauria, Dinosauria, and a handful of small Triassic taxa as bird-line archosaurs. Marasuchus and Lagerpeton, from the Triassic of South America, were believed to fall between pterosaurs and dinosaurs within the group Dinosauromorpha, and the Triassic taxon Scleromochlus could be found as either closer to pterosaurs or outside the group of pterosaurs and dinosaurs (Ornithodira) as an early member of Avemetatarsalia. There has also been the suggestion that pterosaurs were only believed to be close to dinosaurs because of convergence, and instead were outside Archosauria. The analysis below shows the evolution of dinosaurs as recovered by British palaeontologist Michael Benton in 2004, using his preferred clade names.

Despite the relative stability of the relationships of the avian stem, there have been few fossils found that display the transitional morphology of the early members, as most taxa known are either highly derived or poorly known. Thus in 2017 a new group of early avemetatarsalians was recognized, Aphanosauria, based primarily upon the anatomy of Teleocrater, the oldest member of the bird-line yet known. From this discovery, it was shown that the bauplan of early bird-line archosaurs was transitional between earlier archosaurs and dinosaurs, with a long neck, quadrupedality, and carnivory more similar to crocodile-line archosaurs and earlier archosauriformes than other avemetatarsalians. Teleocrater also displays a crocodile-normal ankle, showing it to be the ancestral morphology for all of Archosauria. Contrary to earlier studies, the crocodile-normal ankle can also be found on other bird-line archosaurs such as members of Silesauridae, Marasuchus, and even early dinosaurs. Aphanosaurs were widespread across Pangaea like early crocodile-line archosaurs during the Middle Triassic, and their presence shows that the anatomy of small, bipedal taxa like Marasuchus and Lagerpeton were not similar to the ancestral bird-line morphology. a respected undergraduate textbook. While it is structured so as to reflect evolutionary relationships (similar to a cladogram), it also retains the traditional ranks used in Linnaean taxonomy. The classification has been updated from the second edition in 2000 to reflect new research, but remains fundamentally conservative.

Michael Benton classifies all dinosaurs within the Series Amniota, Class Sauropsida, Subclass Diapsida, Infraclass Archosauromorpha, Division Archosauria, Subdivision Avemetatarsalia, Infradivision Ornithodira, and Superorder Dinosauria. Dinosauria is then divided into the two traditional orders, Saurischia and Ornithischia. The dagger (†) is used to indicate taxa with no living members.

Order Saurischia

Suborder Theropoda
†Infraorder Herrerasauria
†Infraorder Coelophysoidea
†Infraorder Ceratosauria+
†Parvorder Neoceratosauria+
†Superfamily Abelisauroidea
†Family Abelisauridae
†Family Noasauridae
†Family Ceratosauridae
Infraorder Tetanurae
†Superfamily Megalosauroidea
†Family Megalosauridae
†Family Spinosauridae
†Parvorder Carnosauria
†Superfamily Allosauroidea
†Family Allosauridae
†Family Carcharodontosauridae
†Family Neovenatoridae
†Family Metriacanthosauridae
Parvorder Coelurosauria
†Family Coeluridae
Division Maniraptoriformes
†Family Tyrannosauridae
†Family Ornithomimidae
Subdivision Maniraptora
†Family Alvarezsauridae
†Family Therizinosauridae
†Infradivision Deinonychosauria
†Family Troodontidae
†Family Dromaeosauridae
Class Aves
†Suborder Sauropodomorpha
†Genus Thecodontosaurus
†Family Plateosauridae
†Genus Riojasaurus
†Family Massospondylidae
†Infraorder Sauropoda
†Family Vulcanodontidae
†Family Omeisauridae
†Parvorder Neosauropoda
†Family Cetiosauridae
†Family Diplodocidae
†Division Macronaria
†Family Camarasauridae
†Subdivision Titanosauriformes
†Family Brachiosauridae
†Infradivision Somphospondyli
†Family Euhelopodidae
†Family Titanosauridae

†Order Ornithischia frameless|40x40px

†Family Pisanosauridae
†Family Fabrosauridae
†Suborder Thyreophora
†Family Scelidosauridae
†Infraorder Stegosauria
†Family Huayangosauridae
†Family Stegosauridae
†Infraorder Ankylosauria
†Family Nodosauridae
†Family Ankylosauridae
†Suborder Cerapoda
†Infraorder Pachycephalosauria
†Infraorder Ceratopsia
†Family Psittacosauridae
†Family Protoceratopsidae
†Family Ceratopsidae
†Infraorder Ornithopoda
†Family Heterodontosauridae
†Family Hypsilophodontidae
†Family Iguanodontidae *
†Family Hadrosauridae

Weishampel / Dodson / Osmólska classification

The following is based on the second edition of The Dinosauria, a compilation of articles by experts in the field that provided the most comprehensive coverage of Dinosauria available when it was first published in 1990. The second edition updates and revises that work.

The cladogram and phylogenetic definitions below reflect the current understanding of evolutionary relationships. The taxa and symbols in parentheses after a given taxa define these relationships. The plus symbol ("+") between taxa indicates the given taxa is a node-based clade, defined as comprising all descendants of the last common ancestor of the "added" taxa. The greater-than symbol (">") indicates the given taxon is a stem-based taxon, comprising all organisms sharing a common ancestor that is not also an ancestor of the "lesser" taxon.

Saurischia frameless|40x40px

(Triceratops/Stegosaurus)

Herrerasauria (Herrerasaurus > Liliensternus, Plateosaurus)
Herrerasauridae (Herrerasaurus + Staurikosaurus)
? Eoraptor lunensis
Sauropodomorpha (Saltasaurus > Theropoda)
? Saturnalia tupiniquim
? Thecodontosauridae
Prosauropoda (Plateosaurus > Sauropoda)
? Thecodontosauridae
? Anchisauria (Anchisaurus + Melanorosaurus)
? Anchisauridae (Anchisaurus > Melanorosaurus)
? Melanorosauridae (Melanorosaurus > Anchisaurus)
Plateosauria (Jingshanosaurus + Plateosaurus)
Massospondylidae
Yunnanosauridae
Plateosauridae (Plateosaurus > Yunnanosaurus, Massospondylus)
Sauropoda (Saltasaurus > Plateosaurus)
? Anchisauridae
? Melanorosauridae
Blikanasauridae
Vulcanodontidae
Eusauropoda (Shunosaurus + Saltasaurus)
? Euhelopodidae
Mamenchisauridae
Cetiosauridae (Cetiosaurus > Saltasaurus)
Neosauropoda (Diplodocus + Saltasaurus)
Diplodocoidea (Diplodocus > Saltasaurus)
Rebbachisauridae (Rebbachisaurus > Diplodocus)
Flagellicaudata
Dicraeosauridae (Dicraeosaurus > Diplodocus)
Diplodocidae (Diplodocus > Dicraeosaurus, Apatosaurus)
Macronaria (Saltasaurus > Diplodocus)
? Jobaria tiguidensis
Camarasauromorpha (Camarasaurus + Saltasaurus)
Camarasauridae
Titanosauriformes (Brachiosaurus + Saltasaurus)
Brachiosauridae (Brachiosaurus > Saltasaurus)
Titanosauria (Saltasaurus > Brachiosaurus)
Andesauridae
Lithostrotia (Malawisaurus + Saltasaurus)
Isisaurus colberti
Paralititan stromeri
Nemegtosauridae
Saltasauridae (Opisthocoelicaudia + Saltasaurus)
Theropoda (Passer domesticus > Cetiosaurus oxoniensis)
? Eoraptor lunensis
? Herrerasauridae
Ceratosauria (Ceratosaurus nasicornis > Aves)
? Coelophysoidea (Coelophysis > Ceratosaurus)
? Dilophosaurus wetherilli
Coelophysidae (Coelophysis + Megapnosaurus)
? Neoceratosauria (Ceratosaurus > Coelophysis)
Ceratosauridae
Abelisauroidea (Carnotaurus sastrei > C. nasicornis)
Abelisauria (Noasaurus + Carnotaurus)
Noasauridae
Abelisauridae (Abelisaurus comahuensis + C. sastrei)
Carnotaurinae (Carnotaurus > Abelisaurus)
Abelisaurinae (Abelisaurus > Carnotaurus)
Tetanurae (P. domesticus > C. nasicornis)
? Spinosauroidea (Spinosaurus aegyptiacus > P. domesticus)
Megalosauridae (Megalosaurus bucklandii > P. domesticus, S. aegyptiacus, Allosaurus fragilis)
Megalosaurinae (M. bucklandii > Eustreptospondylus oxoniensis)
Eustreptospondylinae (E. oxoniensis > M. bucklandii)
Spinosauridae (S. aegyptiacus > P. domesticus, M. bucklandii, A. fragilis)
Baryonychinae (Baryonyx walkeri > S. aegyptiacus)
Spinosaurinae (S. aegyptiacus > B. walkeri)
Avetheropoda (A. fragilis + P. domesticus)
Carnosauria (A. fragilis > Aves)
? Spinosauroidea
Monolophosaurus jiangi
Allosauroidea (A. fragilis + Sinraptor dongi)
Allosauridae (A. fragilis > S. dongi, Carcharodontosaurus saharicus)
Sinraptoridae (S. dongi > A. fragilis, C. saharicus)
Carcharodontosauridae (C. saharicus > A. fragilis, S. dongi)
Coelurosauria (P. domesticus > A. fragilis)
Compsognathidae (Compsognathus longipes > P. domesticus)
Proceratosaurus bradleyi
Ornitholestes hermanni
Tyrannoraptora (Tyrannosaurus rex + P. domesticus)
Coelurus fragilis
Tyrannosauroidea (T. rex > Ornithomimus velox, Deinonychus antirrhopus, A. fragilis)
Dryptosauridae
Tyrannosauridae (T. rex + Tarbosaurus bataar + Daspletosaurus torosus + Albertosaurus sarcophagus + Gorgosaurus libratus)
Tyrannosaurinae (T. rex > A. sarcophagus)
Albertosaurinae (A. sarcophagus > T. rex)
Maniraptoriformes (O. velox + P. domesticus)
Ornithomimosauria (Gallimimus bullatus + Ornithomimus edmontonicus + Pelecanimimus polyodon)
Harpymimidae
Garudimimidae
Ornithomimidae
Maniraptora (P. domesticus > O. velox)
Oviraptorosauria (Oviraptor philoceratops > P. domesticus)
Caenagnathoidea (O. philoceratops + Caenagnathus collinsi)
Caenagnathidae (C. collinsi > O. philoceratops)
Oviraptoridae (O. philoceratops > C. collinsi)
Oviraptorinae (O. philoceratops + Citipati osmolskae)
Therizinosauroidea (Therizinosaurus + Beipiaosaurus)
Alxasauridae
Therizinosauridae
Paraves (P. domesticus > O. philoceratops)
Eumaniraptora (P. domesticus + D. antirrhopus)
Deinonychosauria (D. antirrhopus > P. domesticus or Dromaeosaurus albertensis + Troodon formosus)
Troodontidae (T. formosus > Velociraptor mongoliensis)
Dromaeosauridae (Microraptor zhaoianus + Sinornithosaurus millenii + V. mongoliensis)
Avialae (Archaeopteryx + Neornithes)

Ornithischia frameless|40x40px

(Iguanodon/Triceratops > Cetiosaurus/Tyrannosaurus)

? Lesothosaurus diagnosticus
? Heterodontosauridae
Genasauria (Ankylosaurus + Triceratops)
Thyreophora (Ankylosaurus > Triceratops)
Scelidosauridae
Eurypoda (Ankylosaurus + Stegosaurus)
Stegosauria (Stegosaurus > Ankylosaurus)
Huayangosauridae (Huayangosaurus > Stegosaurus)
Stegosauridae (Stegosaurus > Huayangosaurus)
Dacentrurus armatus
Stegosaurinae (Stegosaurus > Dacentrurus)
Ankylosauria (Ankylosaurus > Stegosaurus)
Ankylosauridae (Ankylosaurus > Panoplosaurus)
Gastonia burgei
Shamosaurus scutatus
Ankylosaurinae (Ankylosaurus > Shamosaurus)
Nodosauridae (Panoplosaurus > Ankylosaurus)
Cerapoda (Triceratops > Ankylosaurus)
Ornithopoda (Edmontosaurus > Triceratops)
? Lesothosaurus diagnosticus
? Heterodontosauridae
Euornithopoda
Hypsilophodon foxii
Thescelosaurus neglectus
Iguanodontia (Edmontosaurus > Thescelosaurus)
Tenontosaurus tilletti
Rhabdodontidae
Dryomorpha
Dryosauridae
Ankylopollexia
Camptosauridae
Styracosterna
Lurdusaurus arenatus
Iguanodontoidea (=Hadrosauriformes)
Iguanodontidae
Hadrosauridae (Telmatosaurus + Parasaurolophus + Corythosaurus)
Telmatosaurus transsylvanicus
Euhadrosauria
Lambeosaurinae
Saurolophinae (=Hadrosaurinae)
Marginocephalia
Pachycephalosauria (Pachycephalosaurus wyomingensis > Triceratops horridus)
Goyocephala (Goyocephale + Pachycephalosaurus)
Homalocephaloidea (Homalocephale + Pachycephalosaurus)
Homalocephalidae
Pachycephalosauridae
Ceratopsia (Triceratops > Pachycephalosaurus)
Psittacosauridae
Neoceratopsia
Coronosauria
Protoceratopsidae
Bagaceratopidae
Ceratopsoidea
Leptoceratopsidae
Ceratopsomorpha
Ceratopsidae (Triceratops + Styracosaurus)
Centrosaurinae
Chasmosaurinae

Baron / Norman / Barrett classification

In 2017 Matthew G. Baron and his colleagues published a new analysis proposing to put Theropoda (except Herrerasauridae) and Ornithischia within a group called Ornithoscelida (a name originally coined by Thomas Henry Huxley in 1870), redefining Saurischia to cover Sauropodomorpha and Herrerasauridae. Amongst other things this would require hypercarnivory to have evolved independently for Theropoda and Herrerasauridae. This scheme is currently debated among palaeontologists, with recent studies finding little difference between the traditional and newly proposed models.

Dinosauria
Ornithoscelida
Ornithischia
Theropoda
Saurischia
Sauropodomorpha
Herrerasauridae

Cau 2018

In his paper about the stepwise evolution of the avian bauplan, Cau (2018) found in the parsimony analysis a polytomy between herrerasaur-grade taxa, Sauropodomorpha and the controversial Ornithoscelida. The Bayesian analysis, however, found weak support for the sister grouping of Dinosauria and Herrerasauria, but strong support for the dichotomy between Sauropodomorpha and Ornithoscelida, as shown below: