Tutorial 2: Basic vertebral anatomy
October 4, 2007
At the risk of turning this blog into Brachiosaurus brancai 8th Cervical Picture of the Day, here’s a quick tutorial on your basic sauropod vertebral anatomy, using everyone’s favourite cervical vertebra.
This picture shows the same vertebra as was photographed in the very first SV-POW! entry. I’ve composited the figures of this element from Janensch (1950), an exhaustive and lavishly illustrated monograph on the vertebrae and ribs of B. brancai. Parts of this vertebra’s right-hand side are missing, as is apparent in the anterior and posterior views, but most of it is excellently preserved.
First things first: directions. In standard anatomical descriptions of dinosaurs, the direction towards the front is called anterior, and towards the back is posterior — so an anterior view (such as the top left part of this picture) is looking at the front of the vertebra. Upwards is dorsal, downwards is ventral. Sticking out sideways is lateral, and towards the midline is medial. These directions can be combined into single words that describe oblique directions such as anteroventral, posterolateral and anteroventromedial. (Some poor misguided souls use “cranial” and “caudal” in place of “anterior” and “posterior”, but we’ll have none of that here.)
The main body of the vertebra, the roughly cylindrical part, is called the centrum. At the front of the centrum in sauropod cervicals and most dorsals is a ball which fits into a corresponding socket at the back of the preceding one. The ball is called a condyle, the socket is called a cotyle.
Dorsal to the centrum is the neural arch, which is surmounted by the neural spine. A hollow passageway runs through the neural arch from front to back: this houses the spinal cord, and is called the neural canal. (You can’t really see it in the pictures above; you can make it out much more clearly in the BMNH R2523 photos, anterior and posterior.)
Two processes (which just means pointy bits) project laterally from each side of the vertebra: in cervicals they mostly hang downwards a bit, i.e. they project ventrolaterally. (It is a bit of a mystery to me why we say “project ventrolaterally” in scientific writing instead of “hang down”, but there it is.) The dorsal pair of processes are the diapophyses (singular diapophysis, pronounced dye-a-POFF-a-siss). The ventral pair are the parapophyses. They are the articulation points for the ribs. In cervical vertebrae, the ribs are often fused to the processes that support them; in dorsal vertebrae they are free, attached only by soft tissue. (In the pictured specimen, the ribs are broken off very close to their point of origin. In life, they would have projected backwards as thin cylinders a couple of meters long.)
As you see, the diapophysis, parapophysis and fused cervical rib form a loop lateral to the centrum. This loop doesn’t really have a name — it’s called the ansa costro-transversaria in birds, but very few palaeontologists use avian anatomical nomenclature for sauropods. So we three just call it the “cervical rib loop”.
Finally (for now), we have the zygapophyses. There are two pairs of these: prezygapophyses at the front, and postzygapophyses at a location that I will not insult your intelligence by stating. When vertebrae are strung together in a line, their zygs articulate, but they can slide past each other to a limited degree, allowing some flexibility. How much flexibility is a matter of some debate which we may revisit in another post. The facets of the prezygs always face anteromedially (that’s upwards and inwards, remember?) and the postzygs ventrolaterally, although their precise orientation varies along the spinal column.
That it for now. Coming soon in subsequent tutorials: fossae and foramina; laminae; variation along the column; and much, much more. Remember to tell all your friends that this is where the party is.