Evolution of the hyposphene-hypantrum complex within Sauropoda

SEBASTIÁN APESTEGUÍA

Thunder-Lizards: the Sauropodomorph Dinosaurs

Indiana University Press

Lugar: Bloomington & Indianapolis; Año: 2005; p. 248 - 267

The Hyposphene-hypantrum accessory articulation complex is present in several archosauromorphs (e.g. rauisuchids). However, because it is absent in early dinosauriomorphs (e.g. Marasuchus) and ornithischians, it can be regarded as a saurischian synapomorphy, and thus plesiomorphic for sauropods. The shape of this complex within Sauropoda is extremely variable and its homologies far from clear. The rhomboidal primitive configuration remains present in Diplodocoidea. In Macronaria, conversely, the hyposphene-hypantrum complex is sharply different, as it is in some basal titanosaurs (e.g. Andesaurus delgadoi and Phuwiangosaurus sirindhornae) that bear hollow rhomboidal hyposphenes. In basal Macronaria, the hyposphene is ventrally widened, and in Brachiosaurus brancai and other basal titanosauriforms there are both ventrally widened and completely bifid hyposphenes. Several Late Cretaceous sauropod lineages are independently convergent in the loss of accessory articulations such as the hyposphene-hypantrum complex in dorsal vertebrae (e. g. rebbachisaurids in Diplodocoidea and titanosaurs in Macronaria). In the titanosaur lineage the loss of the hyposphene-hypantrum is characteristic, but titanosaurs acquired a new kind of accessory structure, developed from the bifid hyposphene plus the medial centropostzygapophyseal laminae. In advanced titanosaurs, no type of hyposphene-hypantrum complex is present what, added to a light camellate bony structure, a skeleton vastly replaced by cartilage, and a wide ilium anterior blades for muscular insertions, allowed them to develop a relatively free and fast movement, even in heavily armored forms. These titanosaur specializations are expectable in context of growing angiosperm importance and the relatively low ornithischian diversity in the South American, and perhaps the Gondwanan terrestrial ecosystems during Late Cretaceous.