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 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.
Hyposphene-hypantrum articulations were present in the derived and birdlike dromaeosaurid Rahonavis, but are lost [not present] in modern day's birds, probably due to their highly modified vertebrae.
Early Dinosauromorphs (early ancestors of dinosaurs) like Marasuchus, Lagosuchus and Euparkeria 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 (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 Rahonavis, but are lost in modern day's birds, probably due to their highly modified vertebrae. Within the Sauropodomorpha, they were present in prosauropods and most sauropods, but became independently lost in two cretaceous sauropod lineages, the Titanosauria and the Rebbachisauridae.
Avialan characters of scansoriopterygids include: Hyposphene-hypantrum articulations in trunk vertebrae absent (according to Senter).BIRDS
Chiappe (2001) united the Pygostylia in possessing four unambiguous synapomorphies. The trait that gives the group its name is the presence of a pygostyle. Next is the absence of a hyposphene-hypantrum. Next is a retroverted pubis separated from the main axis of the sacrum by an angle of 45 to 65 degrees. Last is a bulbous medial condyle of the tibiotarsus.