* Evolutionary Novelty: Fur (or hair) is a unique feature that evolved specifically within the mammal lineage. It wasn't present in their reptilian ancestors.
* Shared Ancestry: All mammals share the trait of fur, even though it might have evolved in different forms across different groups. This indicates a common ancestor that possessed this characteristic.
* Distinguishing Feature: Fur sets mammals apart from other vertebrates, making it a useful marker for distinguishing them from other groups like reptiles, amphibians, and birds.
In contrast, a primitive character is a trait that was present in an ancestral group and has been inherited by their descendants. For example, the presence of a backbone (vertebral column) is a primitive character shared by all vertebrates, including mammals.
Let me know if you'd like to explore other examples of derived characters in mammals!