The plastron is the flat, bottom portion of a turtle or tortoise’s shell. It serves as the ventral, or underside, connecting to the upper part known as the carapace. This entire shell is part of the animal’s skeletal system, with the plastron shielding the lower body. It is considered by paleontologists to be the oldest part of the turtle’s shell to have evolved.
The Structure of a Plastron
The plastron’s strength comes from its two-layer construction. The inner layer consists of nine fused bony plates. These bones develop directly within tissue rather than from cartilage first. This bony foundation includes elements derived from the clavicles (collar bones) and parts of the ribs.
Covering these bones is an outer layer of keratinous plates called scutes. These scutes, made of a material similar to human fingernails, do not align perfectly with the bony plates beneath them. This offsetting of the seams creates a stronger, more resilient structure. The arrangement of these scutes is symmetrical, with named pairs including:
- Gular
- Humeral
- Pectoral
- Abdominal
- Femoral
- Anal
What the Plastron Does
The primary function of the plastron is to shield the turtle’s soft internal organs. It protects the underside from injury from ground hazards and offers a defense against predators attempting to attack from below.
Some species have developed a specialized, functional adaptation in their plastron. Certain turtles, most notably box turtles, possess a hinged plastron. This hinge, a flexible area across the plastron, allows the turtle to pull the front and back sections of its lower shell upwards, tightly sealing against the carapace. This action effectively encloses the animal’s head and limbs completely within the shell, providing a formidable defense against predators.
How to Identify Turtles by Their Plastron
The plastron is frequently used to determine the sex of a turtle, a characteristic known as sexual dimorphism. Male turtles often have a concave, or inwardly curved, plastron. This shape helps the male mount the female’s domed carapace during mating.
In contrast, female turtles typically possess a flat or slightly convex plastron. A flatter bottom shell provides greater internal volume, allowing more space for the development and passage of eggs. Beyond sex identification, the distinct colors and markings on the plastral scutes can also serve as a useful tool for identifying different turtle species.