Hyposphene-hypantrum articulation

The Hyposphen Hypantrum - joint is an additional joint between the vertebrae that some reptiles fossil occurs from the group of Archosauromorpha. It consists of an extension at the back of the vertebrae, the Hyposphen that fits into a notch on the front of the trailing vortex, the Hypantrum. Hyposphen - Hypantrum - joints are in the vertebrae and occasionally in addition to the back ( posterior ) cervical vertebrae and the front ( anterior ) and caudal vertebrae.

This connecting element existed in addition to the usual connection elements of the vertebrae, and the post- prezygapophyses. Additional fasteners in the vertebrae occurred within the reptiles on several occasions; an example is the Zygosphen - Zygantrum joints in snakes. Hyposphen - Hypantrum joints come in different, not more closely related to each other groups of Archosauromorphen especially for large forms, for example, in the Rauisuchiden, the Silesauriden and - within the dinosaurs - at the Saurischia. She stiffened and stabilized the spine and thus supported, for example, probably the gigantism of sauropod dinosaurs.

In early Dinosauromorphen ( the precursor forms of the dinosaurs ), such as Marasuchus, Lagosuchus and Euparkeria and in Ornithischiern missing Hyposphen - Hypantrum joints. It follows that this new feature took shape only in the Saurischia - therefore it is considered common derived character ( synapomorphy ) of this group. Within the Saurischia occurs this feature found in all basal forms. Within the theropods it is found in many species, including the very bird-like, derived Dromaeosauriden Rahonavis. In modern birds, this feature is missing. Within the Sauropodomorpha the feature at prosauropods as well as in sauropods was present, but was lost in two occurring mainly in the Cretaceous groups, the Titanosauria and Rebbachisauridae independently.

The Hyposphen usually consists of a vertical ridge and is located below the postzygapophyses while the Hypantrum between prezygapophyses sitting. In many classes, the lower ( ventral ) is expanded end of the Hyposphens slightly transversely. Within the sauropods, the form of this compound is extremely variable.

Documents

• Anatomy of Reptiles
• Joint