Here's why:
* Silver dollars belong to the family Characidae, a large and diverse group of fish primarily found in South America. They are known for their peaceful nature and their flat, disk-shaped bodies.
* Piranhas, on the other hand, are also members of the Characidae family, but they are part of a distinct subfamily known as Serrasalminae. This subfamily is characterized by their sharp teeth and predatory behavior.
While they share a common ancestor within the Characidae family, silver dollars and piranhas have evolved significantly differently, with their distinct characteristics reflecting their separate ecological niches.