One of the most diverse families in the ocean today, marine bivalve mollusks - called Lucinidae or lucinids - originated more than 400 million years ago in the Silurian period, with adaptations and life habits like those of its modern members. 

About 500 lucinid species exist today, with by far the highest diversity in shallow-sea seagrass meadows. They did it all with a little help from symbiotic friends.

At its origin, the Lucinidae family remained at very low diversity until the rise of mangroves and seagrasses near the end of the Cretaceous. Mangroves and seagrasses created protective habitats in which the bivalve mollusks could thrive, in turn providing benefit through a sort of tri-level symbiosis. 

A schematic cutaway view of the lucinid genus Codakia in a seagrass bed. Credit: S.M. Stanley, Fig 2. Geology.

Writing in Geology, Steven Stanley of the University of Hawaii  tracks the evolutionary expansion of the lucinids through significant symbiotic relationships notes that what was especially important was the lucinids' development of a symbiotic relationship with seagrasses. The lucinids flourished as they took advantage of the oxygen-poor, sulfide-rich sediments below roots and rhizomes.

These habitats provided a rich supply of sulfur-oxidizing bacteria (or endosymbionts), which the bivalves "farmed" on their gills and then consumed. At the same time, the seagrasses benefited from the uptake of (to them) toxic sulfide by the bivalves.

The Cretaceous mass extinction, which killed off not only the dinosaurs but also many forms of marine life, had little impact on the lucinids. Stanley writes that this can be attributed to the fact that the bivalves relied heavily on the endosymbiont bacteria for nutrition at a time when productivity of marine algae collapsed and many suspension-feeding groups of animals died out.

Citation: Steven M. Stanley, 'Evolutionary radiation of shallow-water Lucinidae (Bivalvia with endosymbionts) as a result of the rise of seagrasses and mangroves 'July 25, 2014, Geology, doi: 10.1130/G35942.1.