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Crescent-shaped rudists on the floor af a Cretaceous Sea

When we think of living reefs we are likely to think of coral reefs, since the biotic reefs of today are most often composed of cnidarian corals (and coralline algae).  Such has not always been the case –convergent evolution means that other animals have sometimes jumped in and taken over the central reef building role occupied today by corals (indeed there are still oyster reefs in some parts of the ocean although human hunger for oysters has greatly reduced their size).  One of the more interesting and successful of these coral analogs was actually a modified colonial mollusk—the rudist.  Rudists were bivalve mollusks very similar to the clams you enjoy on your linguini.  Like clams, rudists had two shells (or valves) joined at a hinge. However the rudists possessed very different shapes from modern clams.  Some had horn-shaped shells which lay flat on the bottom of the ocean shore (the horns prevented currents from flipping the mollusks or washing them away).  The other major group had cone-shapes with little hinged lids on top –like a cross between a lidded beer stein and an ice-cream cone).  This latter group formed together in huge super colonies.

Rudist Types (with a modern bivalve in the top left for comparison)

Rudists evolved in the Jurassic Era and burgeoned throughout the Mesozoic, but their greatest success came during the Cretaceous when they pushed out corals and sponges to become the major reef-building organisms in the Tethys Ocean and various other warm tropical shelves around the world.  It is believed that rudists were so successful because the ocean’s temperature was so much higher during the Cretaceous (as was the salt content of the water).  It must have been amazing to see a rudist tropical reef thronged with strange colorful belemnites, ammonites, and unknown teleosts.  Huge prehistoric diving birds, mosasaurs, and super sharks would have lurked in the depths beside the reef.

A Fossilized Rudist Reef from the Cretaceous Era

Like the dinosaurs and the ammonites, the rudists were wiped out by the Chicxulub impact. Sometimes I think about the rudists as I fret about coral die-offs.  Coral quickly evolved back into the warm shallow tropical niche left open by the extinction of the rudists.  Is there some little clam with a big destiny waiting for the corals to falter in the ever-warmer, ever-more-acidic oceans of the present?

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