Bone ends & cartilage

I started this post quite literally a year ago but never got very far having moved onto other things I was more interested in and leaving it behind. However a recent new paper out of the Witmer lab as led by Casey Halliday has pushed this to the fore of my thoughts and prompted me to finish this off (though rather shorter than I had originally planned).

It is easy to overlook the fact that in palaeontology we generally only have bones to deal with. It’s a bit of a ‘wood for the trees’ issue. You intuitively know that the bones are not the start and end of an organism, but when that’s all you have it’s easy to forget the rest. All longbones of vertebrates have a cap of cartilage on then end. This acts as a sort of catch-all shock absorber and low friction surface, helping the joint move and protecting the bone from damage (imagine walking with the actual end of the femur grinding against the pelvis with every step, not a great idea).

We know they must have been there, and that they were important, even if we almost never see them. They do turn up occasionally, there are some dinosaur bones with preserved cartilage on the end and some pterosaurs have calcite crystals on their joints where the cartilage lay in life. However, in general this gets completely overlooked since it was hard to guess how much difference the cartilage would have made to the shape of the bones or the sizes of the joints. Hard because we didn’t really know how this looked in living archosaurs, so how should this be applied to dinosaurs.

Well Casey and his team have gone and done just that. Sampled a bunch of birds and crocs to determine just how much cartilage is there and what kind of effect does it have on the joints. Between them we can now have a good idea of just that and how it varies in different taxa and with increasing size. I won’t go into the details here as much has been written about this already and the paper is online at PLoS ONE so go and read it there. LINKS However, it’s worth noting that some of the sauropods might well gain a fair few inches in height thanks to some very big cartilage caps.

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