Synapsids were

Options

A. Mammal-like reptiles
B. Bird-like reptiles
C. Early primates
D. Extinct amphibians.

Answer

**Step-by-Step Solution:**

1. **Understanding Synapsids**: Synapsids are a group of animals that are characterized by their unique skull structure. They have a single temporal fenestra (an opening in the skull behind the eyes), which distinguishes them from other groups like diapsids (which have two).

2. **Relation to Mammals**: Synapsids are more closely related to mammals than to reptiles or birds. This means that all mammals, including humans, are descendants of synapsid ancestors.

3. **Classification of Synapsids**: Within the synapsid group, there are various species that exhibit traits resembling both reptiles and mammals. This is why they are often referred to as "mammal-like reptiles".

4. **Exclusion of Other Options**: The options provided include bird-like reptiles, early primates, and extinct amphibians. However, synapsids do not fit these categories:
- They are not bird-like reptiles as they do not share the same evolutionary lineage as birds.
- They are not early primates since primates are a distinct group that evolved much later.
- They are also not extinct amphibians, as amphibians belong to a different evolutionary branch.

5. **Conclusion**: Based on the characteristics and evolutionary relationships, the correct classification for synapsids is "mammal-like reptiles".

**Final Answer**: Synapsids were mammal-like reptiles.

---