What's interesting with these lists of families with sole surviving members
is that so many of them were present in Cretaceous rocks, more than 60
million years old. They have survived a long, long time. Some of them were
not very prolific then either, like the Pulvinitidae. But there were lots
of species of Trigoniidae. What is their future, I wonder? Will they
dwindle to nothing? Or will they blossom from a single species into many,
like the sea urchins did after the mass extinction at the end of the
Paleozoic?

Andrew K. Rindsberg
Geological Survey of Alabama