Why is vertebral pneumaticity in sauropod dinosaurs so variable?

Mike Taylor,Mathew Wedel

doi:10.32388/1g6j3q.3

Abstract

The vertebrae of sauropod dinosaurs have distinctive and complex pneumatic features including fossae and foramina in the sides of their centra. These vary between individuals, serially within individuals, and even between the left and right sides of single vertebrae. This presents a conundrum because bone is usually the least variable material in the vertebrate body. Blood vessels, however, are much more labile, as can be seen in the varied occurrence of vascular foramina in the vertebrae not only of sauropods, but also of birds, crocodilians and mammals. Vascular variation arises in part from the ontogeny of vertebrae, which in their embryonic state are vascularised from within the neural canal: the hand-off from these vessels to others which penetrate from outside is not always completed. In birds, pneumatizing diverticula enter the vertebrae alongside blood vessels, in the vascular foramina that they form, before excavating the surrounding bone into larger pneumatic foramina. We propose that the same was true in sauropods, and that variation of vascularization directly caused variation of pneumatization. In most vertebrae, a single vascular foramen carries both artery and vein, but occasionally these vessels separate and each forms a separate foramen. This explains why in rare cases individual sauropod vertebrae have two pneumatic cavities on a single side: each arises from the vascular foramen formed by either artery or vein.

Highlights

The vertebrae of sauropod dinosaurs are distinctive because of their size and because they have complex pneumatic features
Travan et al (2015) show that in the cervical vertebrae of humans the transverse foramen, which the vertebral artery and vein pass through, is sometimes double, with the two vessels each passing through its own opening rather than the usual shared opening. (In rare cases, a triple transverse foramen occurs, with the sympathetic nerve plexus passing through a third opening rather than sharing the opening used by one or both blood vessels.) A similar phenomenon can be observed in the tail of the Brontosaurus excelsus holotype YPM 1980, in which the right side of the centrum of caudal 7 has the usual single vascular foramen but that of caudal 8 has two (Figure 3)
If our hypothesis that pneumatization follows vascularization is correct, this could explain why there are sometimes two pneumatic fossae on one side of a centrum, for example the left side of caudal 25 of the Giraffatitan brancai tail MB.R.5000 (Figure 5): the two vascular foramina carrying artery and vein were each followed by a pneumatic diverticulum and each developed into a pneumatic fossa

Summary

Introduction

The vertebrae of sauropod dinosaurs are distinctive because of their size and because they have complex pneumatic features These include fossae and foramina, in both the centrum and neural arch; and laminae connecting landmarks such as the zygapophyses, diapophyses and parapophyses (Wedel 2003). The caudal vertebrae of juvenile diplodocids are less pneumatic than those of adults (Melstrom et al 2016, Hanik et al 2017), and in Giraffatitan extensive caudal pneumaticity is present only in large individuals (Wedel and Taylor 2013) These observations suggest that caudal pneumatization in sauropods continued for several years, after the vascular handoff from neural canal arteries to arteries on the external surface of the centrum, possibly explaining the mix of “centrum first” and “arch first” pneumatization observed in sauropod caudal vertebrae. The consistent appearance of vertebral pneumaticity in areas adjacent to the neural canal corroborates the hypothesis that segmental spinal arteries were crucial in “piloting” pneumatic diverticula as they developed

Discussion

Conclusion