Hyposphene-hypantrum articulation
Hyposphene-hypantrum articulation is an extra joint found in the vertebrae of several fossil reptiles in Archosauromorpha. It consists of a hyposphene, a projection on the back of one vertebra, that fits into a depression on the front of the next vertebra, the hypantrum. This joint mainly occurs in the mid-back (dorsal) vertebrae and sometimes in the last neck vertebrae or the first tail vertebrae.
Usually, vertebrae are connected mainly by the centrum and the zygapophyses. The hyposphene-hypantrum joint adds another connection, making the spine stiffer and more stable. A similar extra joint is seen in snakes (zygosphene-zygantrum).
These articulations appear in several unrelated groups within Archosauromorpha and are common in large forms, such as rauisuchids and certain early dinosaur relatives called silesaurids. In dinosaurs (the saurischians in particular), they likely helped the spine stay rigid as animals grew larger, possibly supporting sauropod gigantism.
Some early dinosauromorphs (like Marasuchus, Lagosuchus, Euparkeria) and all ornithischian dinosaurs lack the joint. Because it is absent in saurischian ancestors and in all non-saurischian dinosaurs, some researchers view it as a defining feature of Saurischia.
Within Saurischia, the hyposphene-hypantrum joint is found in basal members but was lost in several lineages. It is present in Rahonavis (an early bird-like dromaeosaurid) but is absent in modern birds, probably due to vertebral changes over time.
In Sauropodomorpha, the joint is seen in prosauropods and most sauropods, but was independently lost in two Cretaceous groups, Titanosauria and Rebbachisauridae.
Anatomically, the hyposphene is usually a vertical ridge below the postzygapophysis, and the hypantrum lies between the prezygapophyses. In sauropods, this joint is highly variable.
This page was last edited on 2 February 2026, at 08:16 (CET).