________were present but they all fell to form coal deposits slowly.

Answer

**Step-by-Step Solution:**

1. **Understanding the Context**: The question refers to a time in evolutionary history when certain plants were present that eventually contributed to coal deposits.

2. **Identifying the Era**: The Mesozoic Era is known as the Age of Reptiles, during which dinosaurs and other reptiles dominated the land and air.

3. **Recognizing the Flora**: During this era, giant ferns were prominent. These ferns are classified as pteridophytes, which are non-flowering vascular plants.

4. **Extinction and Fossilization**: As the dinosaurs became extinct, the giant ferns also fell to the ground. Over time, these plants were fossilized, leading to the formation of coal deposits.

5. **Final Answer**: Therefore, the correct term to fill in the blank is "giant ferns". They were present but eventually fell to form coal deposits slowly.

**Complete Sentence**: "Giant ferns were present but they all fell to form coal deposits slowly."

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