Did sauropod necks have intervertebral discs?
November 8, 2013
One aspect of sauropod neck cartilage that’s been overlooked — and this applies to all non-avian dinosaurs, not just sauropods — is the configuration of the cartilage in their necks. It’s not widely appreciated that birds’ necks differ from those of all other animals in this respect, and we don’t yet know whether sauropods resembled birds or mammals.
Here’s a classic sagittal view of a mammal neck — in this case a human — from The Basics of MRI (Joseph P. Hornak, 1996-2013):
You can see two distinct kinds of structure alternating along the neck: the big, square ones are vertebral centra (slightly hollow at each end), and the narrower lens-shaped ones are the intervertebral discs.
In mammals, and most animals, we find this distinct fibrocartilaginous element, the disc, between the centra of consecutive vertebrae. These discs have a complex structure of their own, consisting of an annulus fibrosus (fibrous ring), made of several layers of fibrocartilage, surrounding a nucleus pulposus (pulpy centre) with the consistency of jelly.
But in birds, uniquely among extant animals, there is no separate cartilaginous element. Instead, the articular surfaces of the bones are covered with layers of hyaline cartilage which articulate directly with one another, and are free to slide across each other. The adjacent articular surfaces are enclosed in synovial capsules similar to those that enclose the zygapophyseal joints. You can see this in the hemisected Rhea neck from last time:
The difference between these two constructions is very apparent in dissection: in birds, adjacent vertebrae come apart easily once the surrounding soft tissue is removed; but in mammals, it is very difficult to separate consecutive vertebrae, as they are firmly attached to the intervening intervertebral disc.To complicate matters further, thin articular discs occur in the necks of some birds — for example, the ostrich (see illustration below), the swan, and the king penguin. But these discs do not occur in all birds — for example, they are absent in the turkey and the rhea. When they are present, these articular discs divide the synovial cavity and prevent the (cartilage-covered) bones on either side from ever articulating directly with each other, just like the articular discs in the human temporomandibular and sternoclavicular joints. These discs are thinner than the true intervertebral discs of mammals and crocodilians; and they are different in composition, lacking the annulus/nucleus structure and consisting of a simple sheet of fibrocartilage.
Crucially, the extant phylogenetic bracket (EPB) does not help us to establish the nature of the intervertebral articulations in sauropods, as the two extant groups most closely related to them have different articulations. As noted, birds have synovial joints; but crocodilians, like mammals, have fibrocartilaginous intervertebral discs. So their most recent common ancestor, the ur-archosaur, could equally have had either condition, and so could its various descendants.
This seems like a mystery well worth solving. For one thing, in the wholly inadequate database that we assembled for the paper, the birds had much thinner cartilage than the other animals. Since they are also the only animals with synovial neck joints, thin cartilage correlates with this kind of joint — at least across that tiny database. Is that correlation reliable? Does it hold out across a bigger sample? Is there a causation? If so, then finding out what kind of intervertebral joints sauropods had would help us to determine how thick their cartilage was, and so what their actual neutral posture was.
But we can’t tell this directly unless we find sensationally well preserved specimens that let us see the structure of the cartilage. We might speculate that since birds have unique saddle-shaped joints and sauropods have ball-and-socket joints like those of mammals and crocs, they’d be more likely to resemble the latter in this respect, too, but that’s rather hand-wavey.
Can we do better?
If we can, it will be through osteological correlates: that is, features of the bones (which are preserved in fossils) that are consistently correlated with features of the soft tissues (which are not). We’d want to find out from analysis of extant animals what correlates might exist, then go looking for them in the bones of extinct animals.
A couple of times now, I’ve pitched this as an abstract for a Masters project, hoping someone at Bristol will work on it with me as co-supervisor, but so far no-one’s bitten. Maybe next year. It would be a very specimen-based project, which I’d think would be a plus in most people’s eyes.
Anyway, the awful truth is that at the moment we know spectacularly little about the cartilage in the necks of sauropods. We don’t know whether they had true intervertebral discs. If not, we don’t know whether they had articular discs like those of ostriches. We don’t know how thick these elements, if present, were. We don’t know how thick the hyaline cartilage on the bones’ articular surfaces was, or how evenly it covered its those surfaces.
And until we know those things, we don’t really know anything about neck posture or range of movement.
There’s lots of work to be done here!