The hyposphene-hypantrum articulation is an accessory joint found in the vertebrae of several fossil reptiles of the group Archosauromorpha. It consists of a

process A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic. Things called a process include: Business and management *Business process, activities that produce a specific se ...

on the backside of the vertebrae, the hyposphene, that fits in a depression in the front side of the next vertebrae, the hypantrum. Hyposphene-hypantrum articulations occur in the dorsal vertebrae and sometimes also in the posteriormost cervical and anteriormost caudal vertebrae. In most tetrapods including the human, the vertebrae are connected with each other only via the centrum and the zygapophysis joints. Additional joints like the hyposphene-hypantrum articulations, which add rigidity to the vertebral column, are found in several different reptile lineages; a known example are the zygosphene-zygantrum articulations found in snakes. Hyposphene-hypantrum articulations are found in several unrelated groups within the Archosauromorpha. They occur especially in large forms, for example in

rauisuchids Rauisuchidae is a group of large (up to or more) predatory Triassic archosaurs. There is some disagreement over which genera should be included in Rauisuchidae and which should be in the related Prestosuchidae and Poposauridae, and indeed wheth ...

and in silesaurids and – within the Dinosauria – in

saurischia Saurischia ( , meaning "reptile-hipped" from the Greek ' () meaning 'lizard' and ' () meaning 'hip joint') is one of the two basic divisions of dinosaurs (the other being Ornithischia), classified by their hip structure. Saurischia and Ornithis ...

ns. They evolved to make the vertebral column more rigid and stable and probably had supported the gigantism in sauropod dinosaurs. Early Dinosauromorphs (early ancestors of dinosaurs) like ''

Marasuchus ''Marasuchus'' (meaning "Mara crocodile") is a genus of basal dinosauriform archosaur which is possibly synonymous with '' Lagosuchus''. Both genera lived during the Late Triassic in what is now La Rioja Province, Argentina. ''Marasuchus'' conta ...

'', ''

Lagosuchus ''Lagosuchus'' is an extinct genus of avemetatarsalian archosaur from the Late Triassic of Argentina. The type species of ''Lagosuchus'', ''Lagosuchus talampayensis,'' is based on a small partial skeleton recovered from the early Carnian-age Cha ...

'' and ''

Euparkeria ''Euparkeria'' (; meaning "Parker's good animal", named in honor of W.K. Parker) is an extinct genus of archosauriform from the Middle Triassic of South Africa. It was a small reptile that lived between 245-230 million years ago, and was close to ...

'' as well as ornithischian dinosaurs lack hyposphene-hypantrum articulations. Because these articulations are absent in both saurischian ancestors and all non-saurischian dinosaurs, they are considered a

synapomorphy In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to hav ...

(a distinctive feature) of the Saurischia, as proposed by Gauthier (1986). Hyposphene-hypantrum articulations are found in all the basal members of the Saurischia. However, they became lost in several saurischian lineages. They were present in the derived and birdlike

dromaeosaurid Dromaeosauridae () is a family of feathered theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Greek ('), meaning ...

Rahonavis ''Rahonavis'' is a genus of bird-like theropods from the Late Cretaceous (Maastrichtian, about 70 mya) of what is now northwestern Madagascar. It is known from a partial skeleton ( UA 8656) found by Catherine Forster and colleagues in Maevaran ...

'', but are lost in modern day's birds, probably due to their highly modified vertebrae. Within the

Sauropodomorpha Sauropodomorpha ( ; from Greek, meaning "lizard-footed forms") is an extinct clade of long-necked, herbivorous, saurischian dinosaurs that includes the sauropods and their ancestral relatives. Sauropods generally grew to very large sizes, had lon ...

, they were present in

prosauropod Sauropodomorpha ( ; from Greek, meaning "lizard-footed forms") is an extinct clade of long-necked, herbivorous, saurischian dinosaurs that includes the sauropods and their ancestral relatives. Sauropods generally grew to very large sizes, had l ...

s and most sauropods, but became independently lost in two cretaceous sauropod lineages, the

Titanosauria Titanosaurs (or titanosaurians; members of the group Titanosauria) were a diverse group of sauropod dinosaurs, including genera from all seven continents. The titanosaurs were the last surviving group of long-necked sauropods, with taxa still thr ...

and the

Rebbachisauridae Rebbachisauridae is a family of sauropod dinosaurs known from fragmentary fossil remains from the Cretaceous of South America, Africa, North America, Europe and possibly Central Asia. Taxonomy In 1990 sauropod specialist Jack McIntosh included t ...

. The hyposphene usually consists of a vertical ridge and is situated below the postzygapophysis, whereas the hypantrum is situated between the prezygapophysis. In sauropods, this joint is extremely variable.

References

{{Cite journal, volume = 8, issue = 1, last = Gauthier, first = Jacques, title = Saurischian monophyly and the origin of birds, journal = Memoirs of the California Academy of Sciences, year = 1986, pages=16–17 Dinosaur anatomy Reptile anatomy