The dinosaurs descended from ______ ancestors.

Options

A. Therapsids
B. Pelycosaurs
C. Synapsids
D. Thecodonts

Answer

**Step-by-Step Solution:**

1. **Understanding Dinosaurs**: Dinosaurs are classified as reptiles and were the dominant species during the Mesozoic era. This gives us a context for their evolutionary lineage.

2. **Identifying the Ancestors**: To fill in the blank, we need to identify which of the given options are related to reptiles. The options provided are:
- Theropsids
- Pelicosaurus
- Synapsids
- Thecodons

3. **Analyzing the Options**:
- **Theropsids**: These are a group of reptiles that are more closely related to mammals.
- **Pelicosaurus**: This is an early group of reptiles, but not directly related to dinosaurs.
- **Synapsids**: These are also more closely related to mammals and not reptiles.
- **Thecodons**: This term refers to socket-toothed reptiles, which are indeed ancestors of modern reptiles, including dinosaurs.

4. **Conclusion**: Since dinosaurs are reptiles and thecodons are socket-toothed reptiles that are related to the lineage of dinosaurs, the correct answer to fill in the blank is "thecodons".

**Final Answer**: The dinosaurs descended from **thecodon** ancestors.

---