What Is a Vertebra? The Bone That Identifies an Animal Nobody Else Can

A vertebra has a solid body at its base, called the centrum, which bears the load and articulates with the bones in front and behind. Above that sits an arch enclosing the channel through which the spinal cord runs, and projecting from the arch are several processes: one pointing upward for muscle and ligament attachment, two pairs of facets that interlock with the neighbouring vertebrae and limit how the spine can bend, and lateral projections that carry the ribs where ribs are present. The proportions of all of these vary along the column, so a neck vertebra, a trunk vertebra and a tail vertebra from the same animal look quite different, and identifying where in the column an isolated bone came from is the first step in interpreting it.

Several features combine to make these bones unusually informative:

• •Complexity, since a vertebra carries many separate surfaces and projections while a limb bone has comparatively few
• •Variation between groups, with the arrangement of laminae and hollows differing consistently between lineages
• •Abundance, since an animal has dozens and only four limbs, so vertebrae are simply more likely to be found
• •Durability of the centrum, which is dense and survives where thinner processes break
• •The record of air spaces, since invasion by air sacs leaves openings and chambers that indicate breathing anatomy
• •Articulation surfaces, which record how the spine moved and therefore how the animal held itself

The shape of the articulating surfaces constrains how much the spine could bend and in which directions, which is the basis of reconstructions of posture and movement. Interlocking facets oriented one way permit vertical flexion and prevent twisting, and oriented another permit side-to-side movement, so measuring them across a column establishes what the animal could physically do. That has been applied extensively to the long necks of sauropods, where the question of how high the head could be raised depends entirely on the joints, and where successive studies using different methods have reached different answers, partly because the cartilage between the bones is missing and its thickness changes the result substantially. Similar work on tails has addressed whether they were held clear of the ground, and the trackway evidence, which almost never shows a tail drag, supports the reconstructions rather than the older drooping posture.

The spine is divided into regions with different jobs and the bones reflect that. Neck vertebrae are generally elongated and lightly built, with extensive hollowing in the groups that had air sacs, since reducing mass far from the body matters most there. Trunk vertebrae are shorter and heavier, carry the ribs and bear the weight of the body, and in several groups they fuse or interlock to stiffen the back. The vertebrae over the hips fuse into a single block in many dinosaurs, which anchors the pelvis and transfers the load to the legs, and the number fused varies between groups in a way that is diagnostic. Tail vertebrae shorten and simplify towards the tip, losing their processes progressively, and in some groups the last section is stiffened by elongated projections that lock them together. Reading an isolated bone therefore begins with working out which region it came from.

A great many dinosaur species were named in the nineteenth and early twentieth centuries from a single vertebra or a small series, and the results have been mixed. The bones are diagnostic enough that such a name can be valid, and they are not always diagnostic enough to be distinguishable from later and better material, which leaves a name attached to a fragment that nobody can match to anything. Those names occupy a legal position that blocks later naming while supplying no information, and specialists have petitioned to set several aside. Modern practice discourages naming from insufficient material and requires a clear statement of what distinguishes the specimen, which is difficult to satisfy from one bone. Where a vertebra is genuinely distinctive, as some are, a name from it stands, and several important animals are known from little else.

A load-bearing body, an arch around the spinal cord and several projections for muscles, ribs and interlocking with neighbours, all varying along the column so position must be identified first. Complexity and abundance make these the most diagnostic bones available. Articulation surfaces constrain posture, though missing cartilage changes the answer. Many species were named from one vertebra, with mixed results.