The recent phylogenetic study reveals that the Unionidae most likely originated in Southeast and East Asia in the Jurassic, with the earliest expansions into North America and Africa (since the mid-Cretaceous) followed by the colonization of Europe and India (since the Paleocene).

The edge of the female's body that protrudes from the valves of the shell develops into an imitation of a small fish complete with markings and false eyes.

Whatever they see, they approach for a closer look and the mussel releases huge numbers of larvae from her gills, dousing the inquisitive fish with her tiny, parasitic young.

These glochidial larvae are drawn into the fish's gills, where they attach and trigger a tissue response that forms a small cyst in which the young mussel resides.

In large enough quantities, unionid shells can have enough of an impact on environmental conditions to affect the ability of organic remains in the local environment to fossilize.

[24] For example, in the Dinosaur Park Formation, fossil hadrosaur eggshell is rare[24] because the breakdown of tannins from local coniferous vegetation would have caused the ancient waters to become acidic.