While hurridly ploughing through various things over the last week (including submitting my Palaeo Paper Challenge paper) I stuck up a very lame post which consisted entirely of a photo of a pterosaur sacrum and a label to say it was a pterosaur sacrum. Now I have a little more time, I’ll revisit this topic in at least a bit more detail.
The sacrum is a block of vertebrae that are fused together and fused to their respective ribs to form a solid, structurally strong, unit. The pterosaur sacrum here is seen from above (dorsal view) and as is normal for pterodactyloids (this is from an ornithocheiroid pterosaur) tapers strongly towards the posterior end so that the anterior vertebra (on the left) is much bigger than the last one (on the right). It’s actually composed of five vertebrae, but the ribs of the final one have broken off.
The reptilian pelvis typically consists of three bones – the ilium that lies dorsally, below this is the pubis facing forwards and then to the rear is the posteriorly pointing ischium. The three between them border the acetabulum which is the socket that takes the head of the femur. Here you can see these three bones, though actually they have fused together into a solid unit such that the divisions between them are really hard to work out and so the yellow lines I have added to separate them out on the second photo should really only be a guide.
So much so normal, but pterosaurs also have a fourth pelvic bone in the form of the pre-pubis. This pair of bones (or for each side) lie, and no points for guessing this, in front on, and articulate with, the pubes. I won’t dwell on the prepubis more here, partly because this otherwise excellent specimen does not preserve them and also because they are worth more discussion in their own right and what I really wanted to show here was how the bones fit together.
The fossil here is exceptionally well preserved and while there are some missing bits, it’s one of the best pelves going and well worth a good look. As you can see from the other photos I have managed to cobble together by holding the various parts in place, the bones fit together incredibly snugly with each other and the sacrum to form a very solid unit. For all that pterosaurs are lightly built, they do tend to have a very robust pelvis and while I’m not aware of any explicit studies the assumption for this has long been, rather reasonably, that it’s to do with the forces of landing, which can be pretty hard going. Plenty of birds have very robust pelves for the same reason, take a look at your turkey over Christmas for a great example (or indeed this post).
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