Why do scientists think that mussels and snails have been living the same lifestyles for hundreds of millions of years?

Painting showing several snails and mussels living in shallow water.

The behaviors and habitat preferences of modern mussels and snails influenced the placement of the mussels and snails in this painting. Click to zoom.

Snails and fresh-water mussels alive today have very conservative life styles, meaning that there is little variety among the thousands of species in the ways they feed and live. This makes scientists think that Cretaceous species probably behaved in the same way as their modern descendants. If modern snails and mussels had evolved a variety of different lifestyles, it would be hard to make a convincing argument about how their ancient ancestors lived.