The most prominent feature of the Wood Turtle is its shell. In fact, the Wood Turtle received its name not because it inhabits forested areas (i.e., the woods), but because of the sculpted appearance of its carapace. A turtle's shell is made up of two main parts. The top portion is called the carapace (Fig. 1) and the bottom chest plate is called the plastron (Fig. 2). The side portions, which join these two parts together, are referred to as the bridges.
Figure 1 Figure 2
The shell is composed of two major materials: keratin and bone. When we look at a turtle, what we see is the outer layer made of a substance called keratin. Keratin is simply the same material your fingernails are made of. Under this keratin layer is the bony defensive structure that protects the turtle from most predators. This bone layer is visible in Fig. 2, as the turtle sustained an injury to her upper plastron which resulted in a fairly large section of the scute layer being removed.
You may be surprised to learn that the bones
which make up the carapace are the vertebral column and fused ribs.
Where us humans have an endoskeleton (endo = inner), turtles essentially
have the same thing but on the outside of their bodies, an exoskeleton
(exo = outer). So, we are not so different after all! ...it's just that
our carapace is a rib cage covered in soft skin. At this
point, it should be clear that a turtle can no more leave its shell behind...than you can
leave your vertebral column and rib cage behind!
Newly hatched Wood Turtles are rather bland in color. Hatchlings begin life with a grey carapace (Fig. 3).
As they age, the carapace begins to take on a tan color at first (Fig. 4), eventually becoming a rich brown.
Thus, the color of the adult carapace is an overall brown.
Although I've seen the adult carapaces of Wood Turtles described as grey, they are not. The seemingly grey color observed in adults (Fig. 5) is actually a fine layer of silt (i.e., dirt).
Depending on the population, the Wood Turtle carapace can have yellow or even pumpkin orange undertones.
Last updated: February 7th, 2010
The outer keratin layer on a turtle is divided into multiple sections called "scutes". A turtle's carapace is made up of four different types of scutes, based on their locations. These are: 1) ertebral scutes; 2) ostal scutes; 3) arginal scutes; and 4) one uchal scute. Their locations are indicated in Figure 6. Clearly, vertebral scutes are those that cover the spinal vertebrae. Costal scutes are so named because of their location as well...they cover the ribs. The Latin word for "rib" is costa. With this pattern in mind, it takes no great leap to understand that marginal scutes are those on the outer margin of the carapace. Nuchal? Yep, they 'went Latin' on us again! The Latin word nucha means nape of the neck. Basically, the nuchal scute is the one at the base of the neck.
So, why does the Wood Turtle have such a rugged looking carapace? It is the particular way in which the scutes grow on Wood Turtles that gives them an almost pyramidal shape. Why doesn't this occur in all turtles? The easy answer is that many turtles shed their scutes as they grow; whereas, the Wood Turtle does not.
Painted Turtles grow slightly larger scutes each year under the old
ones. In late summer, the old scutes detach as the turtle dries out
while basking. As they dry, the scutes curl up at the edges and just
fall off. Snapping Turtles, on the other hand, are continuously shedding
little tiny bits of scute, much like our human skin flakes off in tiny
bits. The Wood Turtle scutes grow in a similar fashion as the Painted
Turtle's scutes, except the older layer remains attached and never falls
off. So, as the years go by, each scute on the Wood Turtle's shell adds
steps, so to speak.
For the first 15-20 years of the Wood Turtle's life, one growth ring per year is produced. You can therefore age a Wood Turtle like you would counting the rings on the cross-section of a tree stump or cut limb. This means that when a Wood Turtle hatches, it isn't very sculptured at all. Also, as Wood Turtles grow older, their scutes start to wear down from being dragged over gravel and sand or wedged under branches. The oldest turtles in a population tend to be rather smooth. It doesn't help that growth slows considerably passed about 20 years, so that you can barely make out any new growth rings at all.