Tutorial 1: Regions of the vertebral column
October 2, 2007
Howdy, Matt Wedel here. This is the Sauropod Vertebra Picture of the Week (or SV-POW! as we like to call it), and you just got a post yesterday, so what are we doing back here already? Mike and Darren and I are trying to get SV-POW! up and running as quickly and smoothly as possible, and the best way to do that is to post. And as long as we’re posting, we might as well get a little work done, too.
(Also, we will follow our titular guideline and post at least one sauropod vertebra picture every week; however, we reserve the right to bring you even more doses of awesome than you expected.)
Today we’re covering the very basics of vertebrae. You’ve been toting around a full set for your entire life, but just in case you’ve never gotten acquainted, here’s the skinny.
Vertebrae are the segments of bone or cartilage that make up the spinal column or backbone. In sharks and rays the vertebrae are composed of cartilage, but in almost all of the bony fish and their descendants–including you–the vertebrae are made of bone.
In most fishes and amphibians there is little or no regional specialization of the vertebral column, but in more derived tetrapods (vertebrate animals with four limbs, or whose ancestors had four limbs) the vertebral column can be divided into at least four regions:
Cervical vertebrae are neck vertebrae. They usually have fixed (non-mobile) ribs on either side. You may not know it, but your own cervical vertebrae have these ribs. They’re just very short. In some sauropods the cervical ribs were mind-bogglingly long, up to 4 meters or more in length, but that’s a story for another post. With a couple of weird exceptions, all mammals have only 7 cervical vertebrae, even the giraffe. But other clades are not so limited. For example, birds can have 24 or more. The largest number of cervical vertebrae in any known sauropod is 19.
Dorsal vertebrae are the vertebrae that fall between the neck and the sacrum. In sauropods they always have large, mobile ribs, of the sort that you usually think of when you hear the word ‘rib’. In mammals the dorsal series is divided into thoracic vertebrae, which have ribs, and lumbar vertebrae, which do not, but in sauropods all of the dorsal vertebrae have ribs. Sauropods had as many as 12 or 13 dorsal vertebrae and as few as 9.*
Behind the ribcage is the sacrum, a block of fused sacral vertebrae. The sacrum articulates with the bones of the pelvis; this is the only bony attachment between the appendicular skeleton (limbs and limb girdles) and the axial skeleton (skull, vertebrae, and ribs). There is no bony connection between the shoulder girdles and the vertebral column. Instead, the shoulder girdles and forelimbs are bound to the body by ligaments and muscles. Sauropods had between 4 and 6 sacral vertebrae.*
Caudal vertebrae are tail vertebrae. There is usually more change along the caudal series than in other regions of the vertebral column. The vertebrae closest to the sacrum often closely resemble unfused sacral vertebrae, but down at the end of the tail the vertebrae are usually reduced to simple rods or nubs of bone. Your coccyx or tailbone is made of fused caudal vertebrae, so technically you do have a tail skeleton, just not an external tail (unless you’re lucky).
Well, that’s quite enough for now. We’ll take up the parts of a vertebra in a future post. And it won’t all be anatomical drudgery; we wouldn’t have created SV-POW! if we didn’t have really cool stuff to show off.
The skeleton in the image above is that of Brachiosaurus. The cervical vertebra, BYU 12867, is also from Brachiosaurus, and it is 94 cm long. The dorsal vertebra, OMNH 1382, is from Apatosaurus, and it is 93 cm tall (however, the top of the neural spine is missing so it would have been more than a meter tall in life).
* Some sauropods might have more dorsal vertebrae or fewer sacral vertebrae, depending on the definition of ‘sauropod’ that one uses. Right now it seems that at least some of the animals traditionally called prosauropods are, in fact, primitive sauropods.