Fossil Friday – mastodon axis vertebra

With the opening of the Valley of the Mastodons exhibit a little over a month away, we’ve started pulling specimens from the collections to work on layouts. The lab and collections workroom are full of mastodons!

Today’s specimen is a mastodon 2nd cervical (neck) vertebrae, more commonly referred to as the axis. It’s shown above in anterior view, and below in posterior view:

Animals with large heads face real biomechanical hurdles. If you’re a quadruped, your head is stuck out on the end of a neck and is otherwise unsupported. This puts tremendous stress on the neck vertebrae, and requires massive muscles to hold up the weight, which affects the entire skeleton. In proboscideans the problem is even worse, because they have a large, heavy brain and, right out in front where it causes the most stress, massively heavy teeth, tusks, and trunk. Proboscideans have addressed this issue in two ways: the skull itself is actually pretty lightly built for its size, and the neck vertebrae are compressed front to back so that the neck is relatively short. But even with a short neck, elephants sometimes need to move their heads. So while the neck vertebrae are short, they have massive attachment areas for muscles and some range of mobility between the vertebrae. The huge, robust neural arch and spine on this mastodon vertebra reflect the large muscles attached to it. Just as a “gee, whiz!” point, the mass of bone that forms the top of the neural canal is roughly the same size as the entire centrum of a bison cervical vertebra, and it’s not as if bison have tiny heads!

In most mammals, nodding the head up and down largely involves movement between the skull and the first cervical vertebra (the atlas). In contrast, if an animal rotates its head along a longitudinal axis, such as the RCA dog below (from Wikipedia), or the elephant in this video, much of the movement is occurring between the atlas and axis vertebrae.

(OK, the pedant in me has to point out that, while nodding and rotating largely involve the atlas and axis respectively, the rest of the cervical vertebrae are also involved in these movements, so injury or fusion at any point in the neck will cause a decrease in mobility. In addition, humans, because our bipedal stance has resulted in precariously-balanced and misshapen heads, work a bit differently. From a vertebral standpoint, our equivalent motion to what the RCA dog is would be looking from left-to-right, but when we cock our head to the side, that’s the vertebral equivalent to looking left-to-right in other mammals. Finally, the necks in non-mammal tetrapods such as reptiles and birds work quite differently from mammals, so much of this doesn’t apply to them.)

The mastodon axis shown here is actually part of an associated skeleton that came from the West Dam of Diamond Valley Lake, which includes several other vertebrae, tusks, and a number of ribs. Like most of our mastodon material, it will be on exhibit starting in August.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s