A living fossil is

Options

A. Coelacanth
B. Limulus
C. Sphenodon
D. All the above

Answer

**Step-by-Step Solution:**

1. **Understanding the Concept of Living Fossils:**
- A living fossil is a term used to describe a living organism that closely resembles species known only from fossil records. These organisms typically have no close living relatives, as their relatives have gone extinct.

2. **Analyzing the Options:**
- We need to evaluate each of the options provided to determine if they qualify as living fossils.

3. **Option A: Coelacanth (Latimeria chalumnae)**
- The coelacanth is a fish that was believed to have been extinct for around 66 million years until it was discovered alive in 1938. It is considered a living fossil because it closely resembles ancient species known only from fossils, and it has no close relatives.

4. **Option B: Limulus (Horseshoe Crab)**
- Limulus, commonly known as the horseshoe crab, is a marine arthropod that has existed for approximately 450 million years. It has retained many primitive features and is considered a living fossil due to its resemblance to ancient species.

5. **Option C: Sphenodon (Tuatara)**
- Sphenodon, or tuatara, is a reptile endemic to New Zealand. It is the only surviving member of its order, Rhynchocephalia, which was once widespread. Its lineage dates back over 200 million years, making it a living fossil.

6. **Conclusion:**
- Since all three options (Coelacanth, Limulus, and Sphenodon) are considered living fossils, the correct answer to the question is option D: all of the above.

**Final Answer: D. All of the above.**

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