Which one of the following in birds, indicated their reptilian ancestry

Options

A. Eggs with a calcareous shell
B. Scales on their hind limbs
C. Four-chambered heart
D. Two special chambers crop and gizzard in their digestive tract.

Answer

To determine which feature in birds indicates their reptilian ancestry, we can analyze the options provided in the question step by step.

### Step-by-Step Solution:

1. **Understanding the Ancestry**: Birds are believed to have evolved from reptiles. To identify features that indicate this ancestry, we need to look for characteristics that are shared between birds and reptiles.

2. **Evaluating Egg Structure**: The first option mentions eggs with a calcareous shell. Both birds and reptiles lay eggs with a calcium carbonate shell. However, this characteristic is not exclusive to birds and does not specifically indicate their reptilian ancestry.

3. **Analyzing Scales on Hindlimbs**: The second option refers to scales on the hindlimbs of birds. Birds do have scales on their legs, which are similar to the scaly skin found in reptiles. This feature suggests a connection to their reptilian ancestors, as reptiles are known for their scaly skin.

4. **Considering the Heart Structure**: The third option discusses the four-chambered heart. While birds possess a four-chambered heart, this feature is also present in crocodiles (a type of reptile). However, it is not a defining characteristic that indicates a direct ancestry, as it is not unique to birds.

5. **Examining Digestive Structures**: The fourth option mentions the crop and gizzard. These structures are specific to birds and do not indicate reptilian ancestry, as they are not found in reptiles.

6. **Conclusion**: After evaluating all options, the presence of scales on the hindlimbs (Option 2) is the most indicative of birds' reptilian ancestry. This feature shows a direct link to their evolutionary background.

### Final Answer:
The correct answer is **Option 2: Scales on their hindlimbs**.

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