Osteohistology of the hyperelongate hemispinous processes of Amargasaurus cazaui (Dinosauria: Sauropoda): Implications for soft tissue reconstruction and functional significance

CERDA, IGNACIO A.; NOVAS, FERNANDO E.; CARBALLIDO, JOSÉ LUIS; SALGADO, LEONARDO

JOURNAL OF ANATOMY

WILEY-BLACKWELL PUBLISHING, INC

Año: 2022

0021-8782

Dicraeosaurid sauropods are iconically characterized by the presence of elongatehemispinous processes in presacral vertebrae. These hemispinous processes canshow an extreme degree of elongation, such as in the Argentinean forms Amargasauruscazaui, Pilmatueia faundezi and Bajadasaurus pronuspinax. These hyperelongatedhemispinous processes have been variably interpreted as a support structure for apadded crest/sail as a display, a bison-likehump or as the internal osseous cores ofcervical horns. With the purpose to test these hypotheses, here we analyze, for thefirst time, the external morphology, internal microanatomy and bone microstructureof the hemispinous processes from the holotype of Amargasaurus, in addition to asecond dicraeosaurid indet. (also from the La Amarga Formatin; Lower Cretaceous,Argentina). Transverse thin-sectionssampled from the proximal, mid and distal portionsof both cervical and dorsal hemispinous processes reveal that the cortical boneis formed by highly vascularized fibrolamellar bone interrupted with cyclical growthmarks. Obliquely oriented Sharpey's fibres are mostly located in the medial and lateralportions of the cortex. Secondary remodelling is evidenced by the presence ofabundant secondary osteons irregularly distributed within the cortex. Both anatomicaland histological evidence does not support the presence of a keratinized sheath(i.e. horn) covering the hyperelongated hemispinous processes of Amargasaurus, andeither, using a parsimonious criterium, in other dicraeosaurids with similar vertebralmorphology. The spatial distribution and relative orientation of the Sharpey's fibressuggest the presence of an important system of interspinous ligaments that possiblyconnect successive hemispinous processes in Amargasaurus. These ligaments weredistributed along the entirety of the hemispinous processes. The differential distributionof secondary osteons indicates that the cervical hemispinous processes ofAmargasaurus were subjected to mechanical forces that generated higher compressionstrain on the anterior side of the elements. Current data support the hypothesisfor the presence of a ?cervical sail? in Amargasaurus and other dicraeosaurids.