 |
| Amargasaurus cazaui Salgado & Bonaparte, 1991 in Cerda, Novas, Carballido & Salgado, 2022. DOI: 10.1111/joa.13659 Illustration by Gabriel Lio |
Abstract
Dicraeosaurid sauropods are iconically characterized by the presence of elongate hemispinous processes in presacral vertebrae. These hemispinous processes can show an extreme degree of elongation, such as in the Argentinean forms Amargasaurus cazaui, Pilmatueia faundezi and Bajadasaurus pronuspinax. These hyperelongated hemispinous processes have been variably interpreted as a support structure for a padded crest/sail as a display, a bison-like hump or as the internal osseous cores of cervical horns. With the purpose to test these hypotheses, here we analyze, for the first time, the external morphology, internal microanatomy and bone microstructure of the hemispinous processes from the holotype of Amargasaurus, in addition to a second dicraeosaurid indet. (also from the La Amarga Formatin; Lower Cretaceous, Argentina). Transverse thin-sections sampled from the proximal, mid and distal portions of both cervical and dorsal hemispinous processes reveal that the cortical bone is formed by highly vascularized fibrolamellar bone interrupted with cyclical growth marks. Obliquely oriented Sharpey's fibres are mostly located in the medial and lateral portions of the cortex. Secondary remodelling is evidenced by the presence of abundant secondary osteons irregularly distributed within the cortex. Both anatomical and histological evidence does not support the presence of a keratinized sheath (i.e. horn) covering the hyperelongated hemispinous processes of Amargasaurus, and either, using a parsimonious criterium, in other dicraeosaurids with similar vertebral morphology. The spatial distribution and relative orientation of the Sharpey's fibres suggest the presence of an important system of interspinous ligaments that possibly connect successive hemispinous processes in Amargasaurus. These ligaments were distributed along the entirety of the hemispinous processes. The differential distribution of secondary osteons indicates that the cervical hemispinous processes of Amargasaurus were subjected to mechanical forces that generated higher compression strain on the anterior side of the elements. Current data support the hypothesis for the presence of a ‘cervical sail’ in Amargasaurus and other dicraeosaurids.
Keywords: bone histology, dicraeosauridae, neural arch morphology, interspinous ligaments, sauropoda
 |
| Life restoration of Amargasaurus cazaui. Illustration made by Gabriel Lio |
Amargasaurus cazaui Salgado & Bonaparte, 1991
Concluding remarks and future perspectives
• Both anatomical and histological evidence does not support the presence of a keratinized sheath covering the hyperelongated hemispinous processes of Amargasaurus and Dicraeosauridae indet. MOZ-Pv 6126-1. It is possible that the same occur in other dicraeosaurids with similar structures (i.e. Bajadasaurus, Pilmatueia). Future studies are required to test such a possibility.
• Unambiguous evidence for the presence of a supravertebral pneumatic diverticula contacting the medial side of the proximal third of the hemispinous processes of Amargasaurus is not supported by histological evidence.
• The spatial distribution and relative orientation of the Sharpey's fibres suggest the presence of a complex system of interspinous ligaments that possibly connect successive hemispinous processes in Amargasaurus. These ligaments were distributed along the entire length of the hemispinous processes.
• Histological evidence indicates that the cervical hemispinous processes of Amargasaurus were subjected to mechanical forces that generate higher compression strain in the anterior side of the elements.
• Although the current data support the hypothesis for the presence of a ‘cervical sail’ in Amargasaurus and other dicraeosaurids, the same must be re-evaluated using more morphological and histological data based on living forms. Furthermore, the functional significance of these bizarre structures must be analysed using different approaches (e.g. finite elements analyses).
Ignacio A. Cerda, Fernando E. Novas, José Luis Carballido and Leonardo Salgado. 2022. Osteohistology of the Hyperelongate Hemispinous Processes of Amargasaurus cazaui (Dinosauria: Sauropoda): Implications for Soft Tissue Reconstruction and Functional Significance. Journal of Anatomy. DOI: 10.1111/joa.13659
