Thursday 30 December 2021

Pneumatisation in a nanoid Saltasaurid Titanosaur from the Upper Cretaceous of western São Paulo State, Brazil.

The Dinosaurs were (and are) and exceptionally diverse group of Animals, and developed a wide range of novel innovations over their long evolutionary history. One of these inovations was the pneumatisation of the post-cranial skeleton, a trait found in both Sauropods and Theropods (including the living Birds). In these Dinosaurs the axial skeleton is infiltrated by a system of diverticula (pouches) which derive from the lungs. Although the pneumatisation of  the post-cranial skeleton has been known about in non-Avian Dinosaurs for a long time, the phenomenon has not been widely studied, as traditionally the only way to do this was through the observation of macroscopic structures, which could not usually be observed without cutting into the bone, and which were not always preserved anyway. More recently, the advent of computed tomography has enabled the non-destructive study of a wider range of bones, and led to a better understanding of the distinctive bone histologies associated with pneumatisation.

In a paper published in the journal Scientific Reports on 17 December 2021, Tito Aureliano of the Laboratory of Paleontology and Paleohidrogeology at the University of Campinas, the Diversity, Ichnology and Osteohistology Laboratory at the University of Rio Grande do Norte, and the Laboratório de Paleoecologia e Paleoicnologia at the Federal University of São Carlos, Aline Ghilardi, also of the Diversity, Ichnology and Osteohistology Laboratory at the University of Rio Grande do Norte, Bruno Navarro, also of the Laboratório de Paleoecologia e Paleoicnologia at the Federal University of São Carlos, and of the Laboratório de Paleontologia and Museu de Zoologia at the University of Sao Paulo, Marcelo Fernandes, again of the Laboratório de Paleoecologia e Paleoicnologia at the Federal University of São Carlos, Fresia Ricardi‑Branco, also of the Laboratory of Paleontology and Paleohidrogeology at the University of Campinas, and Mathew Wedel of the College of Osteopathic Medicine of the Pacific and College of Podiatric Medicine at the Western University of Health Sciences, present the results of a study of a posterior dorsal vertebra from an adult Saltasaurid Titanosaur from the Upper Cretaceous São José do Rio Preto Formation of western São Paulo State, Brazil.

The specimen studied by Aureliano et al. (LPP-PV-0200) was collected by Marcelo and Luciana Fernandes at the 'Vaca morta' locality in Ibirá in western São Paulo State, and subsequently prepared by Aline Ghilardi. It is part of a skeleton which is currently in the process of being described as a new species of  'nanoid' Saltasaurid Titanosaur, with a total length of only 5.7 m, despite clearly being mature, and indeed apparently of advanced years. This specimen is one of the three members of the new species discovered, with one of the other specimens apparently suffering from pathologies associated with acute osteomyelitis and preserved phosphatized blood parasites inside the vascular canals.

Posterior dorsal vertebra of the Upper Cretaceous nanoid Saltasaurid LPP-PV-0200. Three-dimensional reconstruction from computed tomography scan in left lateral view (A). Circle and rectangle show sampling planes and the respective thin sections are in (B) and (C). Abbreviations: ce, centrum; ns, neural spine; pn, pneumatopore; poz, postzygaphophysis; prz, prezygapophysis. Scale bar in (A) 10 cm; in (B) and (C) 1 cm. Aureliano et al. (2021).

A three-dimensional vertebra was made using computed tomography. The internal structure of the bone proved to be well preserved, allowing assessment of most of the pneumatic structures. The centrum (disk) of the vertebra has an array of elongated parallel cavities extending dorsoventrally in anterior view and anteroposteriorly in lateral view. This creates a honeycomb of cavities within the bone, which is also present within the neural arch. The cavities within the centrum average 3.9 mm in width, those within the neural arch 4.8 mm.

Dorsal vertebra internal structures of LPP-PV-0200. Reconstructed tomography model in distal (A) and right lateral (B) views illustrating subvertical tangential computed tomography scan slices in false colour (1)–(9). Images show that only a few structures had survived diagenesis which restricted the assessment of the internal architecture to limited spaces. Lighter blue and green indicate lower densities (e.g., pneumatic cavities). Purple and darker blue demonstrate denser structures (e.g., camellate bone). Dashed lines indicate internal plates of bone that sustain radial camellae. Abbreviations: ce, centrum; cc, circumferential chambers; cml, camellae; hc-cml, ‘honeycomb’ camellae; ns, neural spine; pf, pneumatic foramen; pn, pneumatopore; pacdf, parapophyseal-centrodiapophyseal fossa; pocdf, postzygapophyseal-centrodiapophyseal fossa; rad, radial camellae. Aureliano et al. (2021).

The vertebra shows a complex arrangement of foramina, fossae, laminae, and camellate internal architecture, which in life would have housed a system of pulmonary diverticula, similar to the air sac system in extant Birds. These camellae are elongate within the central part of the centrum, but radially arranged close to the surface. As a similar arrangement has been seen in cervical vertebrae of the Titanosaurs Austroposeidon and Uberabatitan, Aureliano et al. assume that this pattern relates to the to the structural needs of the vertebral articulation surfaces, rather than the position of the vertebae within the spine or body. Plates of bony tisue separate the camellae from the concave surfaces of the centrum; this has also been seen in vertabrae of the Opisthocoelicaudiine Titanosaur Alamosaurus and the Saltasaurid Titanosaur Saltasaurus. Curiously, both LPP-PV-0200 and Alamosaurus have multiple bony plates, whereas Saltasaurus had one single plates, despite Saltasaurus and LPP-PV-0200 being more closely related to one-another than to Alamosaurus

Circumferential camellae are found around the margins of the centrum, and in particular close to the concave faces. This arrangement has previously been recorded in the Diplodocoid Sauropod Apatosaurus, but not in a specimen with as extensive a (preserved) camellae system as LPP-PV-0200. The basal Titanosaur Giraffatitan has a different structure, with the surface camellae becoming chaotically arranged closer to the concave surfaces, possibly suggesting that the situation seen in LPP-PV-0200 and Apatosaurus evolved convergently in Diplodocoids and Titanosaurs.

Dorsal vertebra centrum near the cotyle. Detail of internal structures of the Saltasaurid Titanosaur LPP-PV-0200 in (A)-(C) and comparison with Apatosaurus (OMNH 01094) in (E). Reconstructed tomography model in lateral view (D) illustrating computed tomography scan slices in false colour (A)–(C). Small circumferentially arranged chambers are present in LPP-PV-0200 near the cotyle. A similar condition has previously been documented in the camerate vertebra of Apatosaurus (E). Abbreviations: cc, circumferential chambers; cmr, radially arranged camerae. Scale bar 10 cm. Aureliano et al. (2021).

The high level of pneumatisation seen in LPP-PV-0200 has been reported in other Titanosaurs, and the suggestion has been made that this is related to the extreme size of some of these Animals. However, the presence of a similar level of pneumatisation in the dwarf LPP-PV-0200 suggests that whatever prompted the evolution of these chambers, they were not exclusively linked to large size.

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