Mesozoic era was dominated by

Options

A. Gymnosperms
B. Mammals
C. Dinosaurs
D. Fishes

Answer

### Step-by-Step Solution

1. **Understanding the Mesozoic Era**: The Mesozoic era is a significant geological period that falls between the Paleozoic and Cenozoic eras. It is often referred to as the "middle era."

2. **Divisions of Geological Time**: Geological time is divided into eons, eras, periods, and epochs. The Mesozoic era is part of the Phanerozoic eon, which is the most recent eon in geological history.

3. **Periods within the Mesozoic Era**: The Mesozoic era is further divided into three periods:
- Triassic
- Jurassic
- Cretaceous

4. **Dominant Life Forms**: The Mesozoic era is famously known as the "Age of Dinosaurs." This indicates that dinosaurs were the dominant terrestrial vertebrates during this time.

5. **Timeline of the Mesozoic Era**: The Mesozoic era lasted approximately 180 million years, from about 250 million years ago to 65 million years ago.

6. **Comparison with Other Groups**:
- Gymnosperms originated in the Paleozoic era and were prevalent during the Mesozoic but were not the dominant life forms.
- Mammals became prominent in the Cenozoic era, following the extinction of dinosaurs.
- Fishes were common in the Paleozoic era and continued to exist but were not the dominant group during the Mesozoic.

7. **Conclusion**: Based on the information, the Mesozoic era was primarily dominated by dinosaurs.

### Final Answer
The Mesozoic era was dominated by **Dinosaurs**.

---