Our Fossil Friday specimen for this week is a pair of Pleistocene horse vertebrae, still in their field jacket.
These two vertebrae are the sixth and seventh thoracics (see this link for an overview of vertebral regions in mammals). In the image the bottoms of the vertebrae are to the left and the tops are to the right. The sixth thoracic is the upper of the two, and you’re looking at the posterior surface. In contrast, anterior surface of the seventh thoracic is visible. In the articulated skeleton these vertebrae lie roughly adjacent to the shoulder blades, as shown below in this mounted skeleton at the Texas Memorial Museum:
The long projections coming from the top of each vertebra are called “neural spines” or “spinous processes”. They are rather tall in horses, especially in the anterior half of the thoracic region. These tall processes form the horse’s withers, the hump present above the shoulders:
Tall neural spines are present in lots of different animals, and can serve a variety of functions including display and supporting fat bodies. They’re pretty much always present in land mammals like horses that have large, heavy heads. Horses spend the vast majority of their time in a posture like the one in the image above; head to the ground, eating. It’s hard to keep balanced with that heavy head stuck out like that, and ligaments that run along the neural spines transfer much of the load throughout the vertebral column. The neural spines also serve as attachment points for muscles that move and support the head, neck, and shoulders.
These two vertebrae were collected in 1997 near the east dam of Diamond Valley Lake.