The hyposphene-hypantrum articulation is an accessory joint found in the vertebrae of several fossil reptiles of the group Archosauromorpha.
[1] In most tetrapods including the human, the vertebrae are connected with each other only via the centrum and the zygapophysis joints.
They evolved to make the vertebral column more rigid and stable and probably had supported the gigantism in sauropod dinosaurs.
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).
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.