Turtle shells have evolved greatly over the last 210 million years, creating modern turtle shells to be extremely diverse, depending on the environment they live in. Their shells begin to form when the turtle is just an embryo and continue to change as it gets older. The ancestors of the turtle had different shells, which slowly evolved over time.

Why Watch This Video?

  1. Have you ever wondered what a turtle shell is made out of?
  2. Would you like to know how a turtle’s shell evolved from a single hard shell on their belly to two shells that develop separately?
  3. Have you ever been confused by the wide range of environments turtles are able to survive in?

Key Terms

Carapace– The upper part of the turtle’s shell, which is secured by a ring of bone.

Eunotosaurus africanus– The oldest known ancestor of a turtle that had broad flat ribs and dates back to 260 million years ago.

Sutures: An area in a turtle shell where the bones meet and connect.

Loose Ends

How do bone-forming cells develop?

The video could’ve clarified how turtles secrete the signaling protein and how it converts the surrounding cells into bone forming cells.

The conversion of mesenchymal tissue into bone is called intramembranous ossification. This process creates collagen-proteoglycan matrix, which is able to combine with calcium salt, creating the pre-bone matrix to become calcified.

Is a rigid shell really a disadvantage?

The video stated that a turtle’s rigid shell can be a disadvantage due to the fact that the sutures between the bones of the carapace and plastron spread. However, it didn’t say what type of disadvantage this causes.

The rigid shell doesn’t cause much of a disadvantage. Unlike most animals they lack a flexible ribcage making it impossible to breath like other animals. Instead, they have a muscle sling attached to the shell that allows ventilation.

Evolution of different shell types

The different types of shells turtles have could’ve been expanded on, explaining how evolution changes organisms to better fit their environment.

Turtles have adapted to their environment over time, but without actually trying to. Small differences in shells from one individual to the next sometimes provided an advantage, allowing those types of individuals to reproduce and make the population more like themselves. Over time, accumulated changes fine-tuned by a variety of very different environments have added up to big differences in shell types between species.

Self-Test Questions

Scroll down for the answers.

1. A turtle’s shell is:

  1. A single large bone that sits around its body
  2. A large piece of cartilage surrounding its body
  3. An extension of its skeleton
  4. A mass of keratin (like a toenail)

2. How do turtles breath?

  1. They breath air into their gills
  2. They rely on a flexible ribcage to breath
  3. They use their abdominal muscles to breath, one to breath in and one to breath out
  4. They take in water through their mouth, collecting oxygen from it then let the water run back out.

3.What does the outer layer of skin cells produce on a turtle’s shell?

  1. Spots
  2. Scales
  3. Freckles
  4. Hairs

4. What is one of the major advantages turtles have because of their shell?

  1. Protection from predators
  2. A cool spot out of the sun so they don’t get too hot
  3. Helps them swim faster
  4. Helps them sleep during the day

5. How many bones is a turtle shell made out of?

  1. 25
  2. 100
  3. Almost 60
  4. 78


1.C is the correct answer. The turtles shell is an extension of its skeleton. Both the top and bottom shell are connected to the skeleton inside the turtle.

2. C is the correct answer because their shell prevents them from having a flexible rib cage, making it necessary for them to use their abdominal muscles.

3. B is the correct answer. The outer layer on a turtle is its scales, which are created by the skin cells on their outer layer.

4. A is the correct answer. Their hard shell protects them from being eaten by a predator and helps them stay hidden.

5. C is the correct answer because the carapus and plastron are made out of almost 60 bones.

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Item Credit: Judy Cebra Thomas, TED-Ed

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