Why is vertebral pneumaticity in sauropod dinosaurs so variable?

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.