Thehyposphene-hypantrum articulationis an accessory joint found in the vertebrae of several fossil reptiles of the groupArchosauromorpha.It consists of aprocesson the backside of the vertebrae, thehyposphene,that fits in a depression in the front side of the next vertebrae, thehypantrum.Hyposphene-hypantrum articulations occur in the dorsal vertebrae and sometimes also in the posteriormost cervical and anteriormost caudal vertebrae.[1]
In most tetrapods including the human, the vertebrae are connected with each other only via the centrum and thezygapophysisjoints. 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 thezygosphene-zygantrum articulationsfound in snakes.[1]
Hyposphene-hypantrum articulations are found in several unrelated groups within the Archosauromorpha. They occur especially in large forms, for example inrauisuchidsand insilesaurids[2]and – within theDinosauria– insaurischians.They evolved to make the vertebral column more rigid and stable and probably had supported the gigantism insauropoddinosaurs.[1][3]
EarlyDinosauromorphs(early ancestors of dinosaurs) likeMarasuchus,LagosuchusandEuparkeriaas well asornithischiandinosaurs lack hyposphene-hypantrum articulations. Because these articulations are absent in both saurischian ancestors and all non-saurischian dinosaurs, they are considered asynapomorphy(a distinctive feature) of the Saurischia, as proposed by Gauthier (1986).[4]Hyposphene-hypantrum articulations are found in all the basal members of the Saurischia.[5]However, they became lost in several saurischian lineages. They were present in the derived and birdlikedromaeosauridRahonavis,but are lost in modern day's birds, probably due to their highly modified vertebrae.[4]Within theSauropodomorpha,they were present inprosauropodsand mostsauropods,but became independently lost in two cretaceous sauropod lineages, theTitanosauriaand theRebbachisauridae.[1][3]
The hyposphene usually consists of a vertical ridge and is situated below thepostzygapophysis,whereas the hypantrum is situated between theprezygapophysis.In sauropods, this joint is extremely variable.[1][3]
References
edit- ^abcdeApesteguía, Sebastián (2005). "Evolution of the Hyposphene-Hypantrum Complex within Sauropoda". In Virginia Tidwell; Kenneth Carpenter (eds.).Thunder-Lizards: The Sauropodomorph Dinosaurs.Indiana University Press.ISBN978-0-253-34542-4.
- ^Piechowski, Rafal; Jerzy Dzik (2010). "The axial skeleton ofSilesaurus opolensis".Journal of Vertebrate Paleontology.30(4):1127–1141.doi:10.1080/02724634.2010.483547.S2CID86296113.
- ^abcRauhut, Oliver W.M. (2003).The interrelationships and evolution of basal theropod dinosaurs.Special Papers in Palaeontology. Vol. 69. pp.1–213.ISBN978-0-901702-79-1.
- ^abGauthier, Jacques (1986). "Saurischian monophyly and the origin of birds".Memoirs of the California Academy of Sciences.8(1):16–17.
- ^Langer, Max C. (2004)."Basal Saurischia"(PDF).InWeishampel, David B.;Dodson, Peter; Osmólska, Halszka (eds.).The Dinosauria(2nd ed.). Berkeley: University of California Press. p. 32.ISBN978-0-520-24209-8.Archived fromthe original(PDF)on 2010-12-07.Retrieved2012-12-14.