Hyposphene-hypantrum joint

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The hyposphene-hypantrum joint is an additional joint between the vertebrae that occurs in some fossil reptiles from the Archosauromorpha group . It consists of an extension on the back of the vertebrae, the hyposphene , which fits into a notch on the front of the subsequent vertebra, the hypantrum . Hyposphene-hypantrum joints occur in the dorsal vertebrae and occasionally in the posterior (posterior) cervical vertebrae and the anterior (anterior) caudal vertebrae.

This connecting element existed in addition to the usual connecting elements of the vertebrae, the post- and prezygapophyses . Additional connecting elements in the vertebrae occurred several times within the reptiles; an example are the zygosphene-zygantrum joints in snakes . Hyposphene-hypantrum joints occur in different, not closely related groups of archosauromorphs, especially in large forms, for example in the Rauisuchids , the Silesaurids and - within the dinosaurs - the Saurischia . They stiffened and stabilized the spine and thus probably supported the huge growth of the sauropod dinosaurs, for example.

In early dinosauromorphs (the precursor forms of the dinosaurs ) such as Marasuchus , Lagosuchus and Euparkeria, as well as in ornithic animals , hyposphere-hypantrum joints are missing. From this it follows that this characteristic only emerged anew with the Saurischia - it is therefore considered a jointly derived characteristic ( synapomorphism ) of this group. Within the Saurischia this feature occurs in all basal forms. Within the theropods it is found in numerous genera, including the very bird-like, derived dromaeosaurid Rahonavis . This feature is absent in modern birds. Within the Sauropodomorpha , the feature was present in prosauropods as well as in sauropods, but was lost independently of one another in two groups mainly occurring in the Cretaceous, the Titanosauria and the Rebbachisauridae .

The hyposphen usually consists of a vertical ridge and is located below the postzygapophyses, while the hypantrum sits between the prezygapophyses. In many genera the lower (ventral) end of the hyposphene is expanded slightly transversely. Within the sauropods, the form of this connection is extremely variable.

supporting documents

  1. ↑ Rafalstrok Piechowskia Jerzy Dzik: The axial skeleton of Silesaurus opolensis . In: Journal of Vertebrate Paleontology 30 (4): pp. 1127-1141, July 2010
  2. ↑ a b c Sebastián Apesteguía: Evolution of the Hyposphene-Hypantrum Complex within Sauropoda . In: Thunder-Lizards: The Sauropodomorph Dinosaurs , edited by Virginia Tidwell and Kenneth Carpenter, 2005, Indiana University Press, ISBN 0-253-34542-1
  3. ↑ a b c Oliver Rauhut: The Interrelationships and Evolution of Basal Theropod Dinosaurs (Special Papers in Palaeontology) , Blackwell Publ, 2003
  4. ↑ a b Jacques Gauthier: Saurischian monophyly and the origin of birds . In: Memoirs of the California Academy of Sciences . tape 8 , no. 1 , 1986, pp. 16-17 .
  5. ↑ Max C. Langer: Basal Saurischia . In: The Dinosauria (sec. Edition), edited by Weishampel, Dodson and Osmólska. University of California Press, 2004, p. 32, ISBN 0-520-24209-2