Old music-hall joke: I say, I say, I say! What's the difference between a centroparapophyseal lamina and a spinoprezygopophyseal lamina?

16th April 2004

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Basic dinosaur anatomy seems straightforward enough: what vetebrae are, the difference between a femur and a tibia, where to find a dentary, etc., But there seems to be a mass of specialised terminology for vertebrae, most of pretty opaque. What do all those terms mean?


Actually, vertebral anatomy is nowhere near as complicated as it looks. The sheer length of the names of the laminae shouldn't put you off: they're that way because they describe exactly what the laminae are.

Here's a description of vertebrae in six easy stages.

Stage 1: The Centrum


An idealised centrum, viewed from slightly behind

The main part of every vertebra is the centrum (plural centra), a mass of bone that is roughly cylindrical (although this can change dramatically in more derived animals). The centra within a spinal column line up end to end, and are generally joined together by ball-and-socket joints. Within the neck and torso in particular, these tend to have the ball part (the condyle) at the front of the centrum, and the socket part (the cotyle) at the back.

Some vertebrae really are this simple - just a a centrum. For example, the vertebrae near the end of the tail in diplodocids are like this. However, most vertebrae need to be more complex so there's somewhere to anchor the muscles that move them.

Stage 2: The Neural Spine

centrum with neural spine

A centrum with a neural spine, viewed from slightly behind.

Most vertebrae have a bony spike that sticks up from the top surface of the centrum. This is the neural spine. It provides a place for muscles and ligaments to attach, to hold the vertebral column in place and move it, and also provides protection for the spinal cord, which passes through a hole from the front to the back of the neural spine called the neural canal. The tip of the neural spine is called the neurapophysis.

Stage 3: The Zygapophyses

vertebra with zygapophyses

A vertebra, showing the zygapophyses.

For most vertebrae, the contacts between the centra alone do not provide enough stability, so there are extra articular surfaces between adjacent vertebrae. These are called the zygapophyses (sometimes just called ``zygs'' for short).

There are two pairs of zygapophyses on each vertebra, all of them located above the centrum. The prezygapophyses are in front on the neural spine (one each on the left and right), and their articular surfaces face forward, upward and inward (or craniodorsomedially, if you like). The postzygapophyses are behind the neural spine, with their articular surfaces facing backwards, downward and outward (or caudoventrolaterally).

Each vertebra's prezygs articulate with the postzygs of the vertebra in front. The zygs can slide across each other to some extent, being enclosed in a synovial joint when the animal is alive, so there is some flexibility to the neck.

Stage 4: The Ribs

vertebra with ribs attached

A vertebra with a neural spine, zygapophyses and ribs attached to the diapophyses and parapophyses.

Ribs, including cervical ribs on neck vertebra, do not articulate directly with the centrum but with two more pairs of bony processes that emerge from the sides of the centrum. The higher pair is called the diapophyses (or sometimes the lateral processes), and the lower the parapophyses. Most ribs are forked at the top, with the upper fork articulating with the diapophysis and the lower with the parapophysis.

In some sauropods, the cervical ribs are very long (e.g. as long as three vertebrae in Sauroposeidon), so they lie along the lower edge of the neck in bundles, stiffening and strengthening the neck.

Stage 5: Excavation

vertebra with lateral excavations

A vertebra like the previous one, but with depressions (shaded) in the sides.

Many vertebrae, particularly in saurischians, have various hollows in the sides, broadly known as excavations. A shallow depression is called a fossa (or pneumatic fossa), a deeper one a foramen. The Beach Boys used a foramen to great effect in Good Vibrations.

These are most common in the centrum, but in some cases occur in the neural spine as well. They are generally interpreted as an adaptation to lighten the spinal column.

Stage 6: The Laminae

vertebra with laminae

A vertebra with spinoprezygapophyseal and spinopostzygapophyseal laminae.

More complex vertebra, particularly in sauropods, have laminae with long, potentially confusing names. Each lamina is a more or less flat sheet of bone connecting two parts of the vertebra, and it is named for the two parts that it connects. For example, the centroparapophyseal laminae connect the centrum with the parapophyses, forming a flattish sheet of bone at the bottom of the vertebra. And the spinoprezygapophyseal laminae connect the neural spine with the prezygapophyses.

And that's all there is to it!

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