They are believed to be descendants of the once diverse marine Trigoniacea, today represented only by Neotrigonia . They have been here at least since the Triassic, and no place on earth has as many species as North America. Living in rivers is very different from living in the ocean, and not just for physiological reasons. The world of a unionid is a linear one, with a constant current. Droughts and flood are the norm.
Temperature is much more variable than in the ocean.
In general, it's a rough life. Environmental rigors notwithstanding, one of the greatest problems to unionids is how to distribute their offspring, within and between rivers. The next river may be only a mile away, but the intervening dry land is insurmountable for a unionid. Unionids, like all bivalves, do not copulate. But because rivers have currents, and sperm cannot possibly swim upstream against them, that female must be downstream.