Saurischia (/sɔːˈrɪskiə/saw-RIS-kee-ə, meaning "reptile-hipped" from theGreeksauros (σαῦρος) meaning 'lizard' andischion (ἴσχιον) meaning 'hip joint')[3] is one of the two basic divisions ofdinosaurs (the other beingOrnithischia), classified by their hip structure. Saurischia and Ornithischia were originally calledorders byHarry Seeley in 1888,[4] though today most paleontologists classify Saurischia as an unrankedclade rather than an order.[5]
Saurischian dinosaurs are traditionally distinguished fromornithischian dinosaurs by their three-pronged pelvic structure, with thepubis pointed forward. The ornithischians'pelvis is arranged with the pubis rotated backward,parallel with theischium, often also with a forward-pointing process, giving a four-pronged structure. The saurischian hip structure led Seeley to name them "lizard-hipped" dinosaurs, because they retained the ancestral hipanatomy also found in modern lizards and other reptiles. He named ornithischians "bird-hipped" dinosaurs because their hip arrangement was superficially similar to that of birds, though he did not propose any specific relationship between ornithischians and birds. However, in the view which has long been held, this "bird-hipped" arrangement evolved several times independently in dinosaurs, first in the ornithischians, then in the lineage of saurischians including birds (Avialae), and lastly in thetherizinosaurians. This would then be an example ofconvergent evolution: avialans, therizinosaurians, and ornithischian dinosaurs all developed a similar hip anatomy independently of each other, possibly as an adaptation to their herbivorous oromnivorous diets.[7]
Edmontosaurus pelvis (showing ornithischian structure – left side)
In his paper naming the two groups, Seeley reviewed previous classification schemes put forth by other paleontologists to divide up the traditional order Dinosauria. He preferred one that had been put forward byOthniel Charles Marsh in 1878, which divided dinosaurs into four orders:Sauropoda,Theropoda,Ornithopoda, andStegosauria (these names are still used today in much the same way to refer to suborders orclades within Saurischia and Ornithischia).[4]
Seeley, however, wanted to formulate a classification that would take into account a single primary difference between major dinosaurian groups based on a characteristic that also differentiated them from other reptiles. He found this in the configuration of the hip bones, and found that all four of Marsh's orders could be divided neatly into two major groups based on this feature. He placed the Stegosauria and Ornithopoda in the Ornithischia, and the Theropoda and Sauropoda in the Saurischia. Furthermore, Seeley used this major difference in the hip bones, along with many other noted differences between the two groups, to argue that "dinosaurs" were not a natural grouping at all, but rather two distinct orders that had arisen independently from more primitivearchosaurs.[4] This concept that "dinosaur" was an outdated term for two distinct orders lasted many decades in the scientific and popular literature, and it was not until the 1960s that scientists began to again consider the possibility that saurischians and ornithischians were more closely related to each other than they were to other archosaurs.
Although his concept of apolyphyletic Dinosauria is no longer accepted by most paleontologists, Seeley's basic division of the two dinosaurian groups has stood the test of time, and has been supported by moderncladistic analysis of relationships among dinosaurs.[8] A node-base clade,Eusaurischia, was named for the least inclusive group containing sauropodomorphs (represented byCetiosaurus) and theropods (represented byNeornithes). Any saurischian that diverged before the theropod-sauropodomorph split is therefore outside clade Eusaurischia.[9]
One alternative hypothesis challenging Seeley's classification was proposed byRobert T. Bakker in his 1986 bookThe Dinosaur Heresies. Bakker's classification separated the theropods into their own group and placed the two groups of herbivorous dinosaurs (the sauropodomorphs and ornithischians) together in a separate group he named thePhytodinosauria ("plant dinosaurs").[10] The Phytodinosauria hypothesis was based partly on the supposed link between ornithischians andprosauropods, and the idea that the former had evolved directly from the latter, possibly by way of an enigmatic family that seemed to possess characters of both groups, thesegnosaurs.[11] However, it was later found thatsegnosaurs were an unusual type of herbivorous theropod saurischianclosely related to birds, and the Phytodinosauria hypothesis fell out of favor.
A 2017 study by Matthew Grant Baron, David B. Norman and Paul M. Barrett did not find support for a monophyletic Saurischia, according to its traditional definition. Instead, the group was found to beparaphyletic. As a solution, Theropoda was removed from the group and placed as the sister group to the Ornithischia in the newly defined cladeOrnithoscelida. As another result, the authors redefined Saurischia as "the most inclusive clade that containsD[iplodocus] carnegii, but notT[riceratops] horridus", resulting in a clade containing only the Sauropodomorpha andHerrerasauridae.[12][13]Thomas Holtz (2017) recommended using the name Sauropodomorpha to refer to a possible clade that includes traditional sauropodomorphs and herrerasaurids; alternatively, he proposed redefining the long-disused taxonPachypodosauria to include Sauropodomorpha and Herrerasauridae as subclades.[14] Cau (2018) also supported Ornithoscelida but placed herrerasaurids,Tawa andDaemonosaurus in a clade (Herrerasauria) outside Dinosauria.[15] Other recent studies support a view closer to the traditional Saurischia hypothesis, with theropods closer to sauropodomorphs than to ornithischians. Novas et al. (2021) support Cau's herrerasaur phylogeny but place this clade in Saurischia.[16]
Phylogenetic position of saurischians in different topologies
^abCau, A. (2018). "The assembly of the avian body plan: a 160-million-year long process".Bollettino della Società Paleontologica Italiana (1):1–25.doi:10.4435/BSPI.2018.01 (inactive 12 July 2025).S2CID44078918.{{cite journal}}: CS1 maint: DOI inactive as of July 2025 (link)
^abNovas, F.E.; Agnolin, F.L.; Ezcurra, M.D.; Müller, R.T.; Martinelli, A.; Langer, M. (2021). "Review of the fossil record of early dinosaurs from South America, and its phylogenetic implications".Journal of South American Earth Sciences.10 103341.Bibcode:2021JSAES.11003341N.doi:10.1016/j.jsames.2021.103341.
.svg)
