Cartilage accounts for nearly a fifth of the length of the neck of a baby giraffe
March 26, 2011
In a comment on the previous post, Dean asked: “What was the difference in length between the neck with its cartilage and the bones flush together?”
I’m glad you asked me that. You’ll recall from last time that the fully fleshed neck — intact apart from the removal of the skin and maybe some superficial muscle — was 51 cm in length from the front of the atlas to the back of the centrum of the seventh cervical vertebra. When I pose the cleaned and cartilage-free bones together, the total length of the series is only 41 cm — 10 cm shorter, coming in at just over 80% of the live length. Don’t believe me? Here are the photos!
I’m sure I need hardly say, but the top image is the neck as we got it, the second is the cleaned bones posed in more or less the arrangement they must have been in life (both of these taken from the previous post) and the bottom image the bones fully abutting.
So! The neck of Wallace the baby giraffe was very nearly a quarter as long again as the bones alone suggest. Does this mean that the neck of Giraffatitan was really 10.6 m long instead of 8.5 m?
It’s an exciting prospect, but I’m afraid the answer is no. As I hinted last time, while it’s perfectly acceptable, indeed obligatory, to recognise the important role of cartilage in sauropod necks qualitatively, we can’t blindly apply the numbers from Wallace the baby giraffe to adult sauropods for two reasons: 1, Wallace is a baby; and 2, Wallace is a giraffe.
The first of these reasons is part of why I am keen to do this all over again with an adult giraffe when I get the opportunity; but there’s not much we can do about the second. One might think that a more closely related extant animal such as an ostrich might have a neck that is more homologous with those of sauropods; and that’s true, but my feeling is that the giraffe is more analogous. That is, although the birds share more recent common ancestry with sauropods, giraffes’ more similar size seem to have encouraged them to evolve cervicals that are in some ways more similar to those of sauropods, most notably in the possession of ball-and-socket intervertebral joints rather than the saddle-shaped joints that are ubiquitous in birds.
How big a deal is Wallace’s juvenile status? Well, take a look at his fifth cervical vertebra in posterior view:
If this bone were found in 150 million years by competent palaeontologists, in a world where there were no extant artiodactyls to compare with, what would they make of it? Most of the articular area of the centrum is very obviously damaged, exposing the internal spongy texture of cancellous bone — presumably the bone surface was attached more firmly to the cartilaginous posterior end of the element than to the inner part of the bone, so it came away with the cartilage during simmering. So it would be obvious to our future palaeontologists that the articular surface was missing, and that the complete vertebra would have been somewhat longer — but it would be hard to judge by how much.
But the state of this bone is particularly interesting because the middle part of the centrum does have a preserved bone surface. It would be easy to extrapolate that out across the whole area of the posterior end of the centrum, and assume that this was the maximum posterior extent of the element’s functional length in life — an assumption that we know, having taking the neck apart ourselves, would be completely wrong.
Are we making similar incorrect assumptions with our sauropod vertebrae?
An even more interesting case is the postzygapophyses. The posterodorsal surface of the left postzyg is slightly damaged, but the bone of the right postzyg has a nice, perfectly preserved surface. But I can tell you that the functional articular surface of the postzyg was totally different from this: different size, different shape, different position, different orientation. If we tried to calculate range of movement from these zygapophyseal facets, the results we got would be literally meaningless.
The good news is, there’s a clue that would prevent us from making this mistake — a really nice, obvious one. The texture of the bone on the postzyg is irregularly crenellated in a way that strongly indicates a cartilaginous extension: it’s the same texture you see on the ends of the long-bones of (even mature) birds if you peel off the cartilage caps. (It’s also what you see, at a much bigger scale, on the ends of the sauropod long-bones.)
But while the presence of this texture indicates the presence of cartilage, I don’t know whether the converse is true. In the absence of such a texture, can we assume the absence of cartilage? I just don’t know. Anyone?