Reptilian ancestors of birds are :

Options

A. Pterosaurs
B. Pleiosaurs
C. Ichthyosaurs
D. Dinosaurs

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the reptilian ancestors of birds. To answer this, we need to identify which of the given options are indeed reptilian ancestors.

2. **Analyzing Each Option**:
- **Option 1: Pterosaurs**: Pterosaurs were flying reptiles that lived during the Mesozoic era. They are considered to be among the earliest vertebrates to evolve powered flight, making them a significant group in the evolutionary lineage leading to birds. Therefore, this option is correct.

- **Option 2: Plesiosaurus**: Plesiosaurus was a marine reptile that existed during the Mesozoic era but was not a direct ancestor of birds. It was more adapted to life in water and does not share a lineage with birds. Thus, this option is incorrect.

- **Option 3: Ichthyosaurus**: Similar to Plesiosaurus, Ichthyosaurus was also a marine reptile and not a direct ancestor of birds. It lived in the Mesozoic era but is not related to the lineage leading to birds. Therefore, this option is incorrect.

- **Option 4: Dinosaurs**: While birds are considered to be descended from theropod dinosaurs, the term "dinosaur" itself encompasses a wide range of reptiles that do not all contribute directly to the ancestry of birds. Thus, this option is also incorrect in the context of the question.

3. **Conclusion**: The only correct answer among the options provided is **Pterosaurs**, as they are the only group that fits the criteria of being a reptilian ancestor of birds.

**Final Answer**: Pterosaurs

---