Sacral co‐ossification in dinosaurs: The oldest record of fused sacral vertebrae in Dinosauria and the diversity of sacral co‐ossification patterns in the group

Abstract

The fusion of the sacrum occurs in the major dinosaur lineages, i.e. ornithischians, theropods, and sauropodomorphs, but it is unclear if this trait is a common ancestral condition, or if it evolved independently in each lineage, or even how or if it is related to ontogeny. In addition, the order in which the different structures of the sacrum are fused, as well as the causes that lead to this co‐ossification, are poorly understood. Herein, we described the oldest record of fused sacral vertebrae within dinosaurs, based on two primordial sacral vertebrae from the Late Triassic of Candelária Sequence, southern Brazil. We used computed microtomography (micro‐CT) to analyze the extent of vertebral fusion, which revealed that it occurred only between the centra. We also assessed the occurrence of sacral fusion in Dinosauria and close relatives. The degree of fusion observed in representatives of the major dinosaur lineages suggested that there may be a sequential pattern of fusion of the elements of the sacrum, more clearly observed in Sauropodomorpha. Our analyses suggest that primordial sacral vertebrae fuse earlier in the lineage (as seen in Norian sauropodomorphs). Intervertebral fusion is observed to encompass progressively more vertebral units as sauropodomorphs evolve, reaching up to five or more fully fused sacrals in Neosauropoda. Furthermore, the new specimen described here indicates that the fusion of sacral elements occurred early in the evolution of dinosaurs. Factors such as ontogeny and the increase in body size, combined with the incorporation of vertebrae to the sacrum may have a significant role in the process and in the variation of sacral fusion observed.

An unusually robust specimen attributed to Buriolestes schultzi (Dinosauria: Sauropodomorpha) from the Late Triassic of southern Brazil

“…In ventral view, the attachment of the sacral rib of S2 with its respective vertebra is craniocaudally wider than in S1, extending from the cranial margin of the centrum to beyond its central portion. This pattern is also observed in the sacrum of B. schultzi (Moro et al, 2021), P. barberenai (Langer et al, 2019), and B. agudoensis (Pretto et al, 2019), and apparently is a widespread condition in early sauropodomorphs.…”

A European giant: a large spinosaurid (Dinosauria: Theropoda) from the Vectis Formation (Wealden Group, Early Cretaceous), UK

“…The relative position of the sacral vertebrae is difficult to determine given their incompleteness, and it is perhaps unusual that elements of this size are not more extensively fused to other sacral elements. The plesiomorphic dinosaurian (and archosaurian) sacrum consisted of two “primordial vertebrae” ( Langer & Benton, 2006 ; Moro et al, 2021 ). This count increased to five in tetanurans via the addition of dorso- and caudosacrals ( Holtz, Molnar & Currie, 2004 ).…”

“…The primordial sacral vertebrae are thought to fuse prior to the evolutionarily ‘younger’ elements ( O’Connor, 2007 ), suggesting that IWCMS 2018.30.2 may represent this pair in the absence of a completely fused series. However, recognition of sacral fusion patterns in theropods remain complicated ( Moro et al, 2021 ) and the identification of primordial sacrals is largely based on their sacral ribs and associated attachment points on the ilium ( Nesbitt, 2011 ), neither of which can be assessed here.…”

An integrative insight into the synsacral canal of fossil and extant Antarctic penguins

“…Actually, this would be by no means unique. For example, dinosaurs from all major lineages increased, in the course of their evolution, the number of the sacral vertebrae by incorporation of thoracic and/or caudal ones (the original count was 2; Moro et al 2021). Three sacral vertebrae are also typical of present-day ostriches (e.g.…”