SPECTACULAR RADIATION OF THE LUCINDAE (BIVALVIA) AFTER 350 MILLION M.Y. OF LOW DIVERSITY: A RESPONSE TO THE INITIAL RADIATIONS OF MARINE ANGIOSPERMS THAT FACILITATE LUCINID BACTERIAL CHEMOSYMBIOSIS

The Lucinidae are an unusual group of marine bivalves, which possess only one (posterior) siphon; take in water via an anteriorly positioned mucous-lined tube; and, in addition to suspension feeding, farm bacterial endosymbionts that oxidize sulfides. Most modern lucinid species are largely restricted to seagrass habitats, where their endosymbionts have access to abundant sulfides. A smaller number are associated with mangrove forests, for the same reason. A few others (not considered here because of their poor fossil record) colonize deep-sea vents and seeps. As of 2006, 88 lucinid genera were recognized in modern seas.

Only two lucinid genera are recognized from the Silurian, when the family arose; these persisted into the Devonian, to be joined by one other known species. No lucinids are known from the Mississippian-Triassic interval, although a few must have existed, and only 4 Early Cretaceous genera are recognized. During the Late Cretaceous, however, lucinids began to radiate spectacularly: 13 genera are known from the Maastrichtian, 12 of which survived into the Paleocene; 28 genera are known from the Paleocene and 50 from the Early Eocene. This rate of diversification greatly exceeded rates for other bivalve taxa that radiated following the terminal Cretaceous mass extinction. Furthermore, the curious history of diversity for the lucinids appears to be without counterpart among other marine invertebrates.

The lucinids' spectacular radiation after persisting at very low diversity for about 350 m.y. must have resulted from the origin and initial expansion of seagrasses and, to a lesser extent, mangroves: plants with which the vast majority of lucinid species are associated today. Molecular clock studies have concluded that seagrasses arose at about 87 Ma (the Coniacean), and the oldest known seagrass fossils are of Early Campanian age (about 80 m.y. old); at least two of the four modern families of seagrasses existed by the end of the Cretaceous. Mangroves are a polyphyletic group that arose in the Maastrichtian. Both plant groups underwent rapid geographic expansion after their origins.

Survival of 12 of the 13 Maastrichtian lucinid genera into the Paleocene probably resulted from the ability to survive without suspension feeding in the "Strangelove" ocean that followed terminal Cretaceous crisis.