Which of the following characteristics first developed in reptiles ?

Options

A. Amniotic egg
B. Scales on skin
C. Four-legged body
D. Skull and vertebral column

Explanation

To determine which characteristic first developed in reptiles, let's analyze the options and the key traits of reptiles:

1. **Understanding Reptiles**:
- Reptiles are characterized by their creeping and crawling habits, which is why they are named "reptiles."
- They have dry scales covering their bodies, which helps in reducing water loss.
- Reptiles typically do not have external ear openings.
- Most reptiles have a three-chambered heart, with the exception of crocodiles, which have a four-chambered heart.
- They are poikilothermic, meaning their body temperature varies with the environment.

2. **Key Characteristic - Amniotic Egg**:
- One of the most significant adaptations in reptiles is the development of the amniotic egg.
- The amniotic egg has a protective shell and membranes that create a safe environment for the embryo to develop.
- This adaptation allows reptiles to reproduce in a terrestrial environment without the need for water for the embryo's development.

3. **Conclusion**:
- Among the characteristics listed, the amniotic egg is the one that first developed in reptiles. This adaptation is crucial as it allows for reproduction away from water, which was a significant evolutionary step.

Therefore, the answer is **the amniotic egg**.