The Rise and Fall of Rudist Reefs
Reefs of the dinosaur era were dominated not by corals but by odd mollusks, which died off at the end of the Cretaceous from causes yet to be discovered
From the mountains of Europe to the deserts of Mexico, one finds geologic formations that contain ancient marine fossils so peculiar in shape that local folk once imagined them to be horns shed by sheep or goats. Some farmers built impressive walls and fences with these odd stones. Many decades ago, they realized that their building materials were in fact the shells of extinct marine organisms. But few of those people could have grasped just how apropos it was to be arranging those particular fossils into imposing barriers. After all, without some training in geology, who would know the ancient animal's true notoriety in the annals of the former world: that of being the only mollusk ever to construct massive reefs.
That understanding, of course, took a long time to develop. When the French naturalist Jean-Baptiste Lamarck examined these fossils in 1819, he coined a term for them—rudistae, or "rudists," from the Latin rudis (meaning "rude" or "raw"), or possibly from rudus (denoting rubble or broken stone). Lamarck may have suspected that his rudists once occupied shallow seas, but he would not have guessed that they might have made a tasty dinner. After all, they didn't look at all like their distant cousins, the scallops, mussels and oysters that he probably enjoyed eating.
For years, scientists tried to shoe-horn rudists into various phyla before placing them ultimately in Mollusca. It was longer still before all rudists were moved from Cephalopoda to Bivalvia, a 42,000-species class that evolved more than 500 million years ago.
Today, at least 10,000 bivalve species live in freshwater lakes and streams, in estuaries and bays, and in shallow coastal and deep-ocean environments. Because bivalves have been successful at invading and flourishing in habitats spanning a wide range of latitudes and temperatures, it's not surprising to find that they also live in tropical reef communities, which are made up of an ecologically complex mixture of corals, sponges, algae, snails, fish and many other groups.
Defined simply, reefs are masses of rock lying at or near the surface of the water. As such, they are barriers to navigation; the most spectacular examples lie off the coasts of Australia and Belize. A more complex definition considers the ecology and the physical and chemical attributes of the surrounding water and sediment—the reef as an integrated community of bottom-dwelling organisms that secrete a rigid skeletal framework, forming a wave-resistant structure.
Although people now refer to tropical ecosystems of this sort simply as coral reefs (in deference to the chief skeletal builder today), reefs were not always constructed of corals. Paleontologists have identified reef ecosystems in strata dating back at least to the start of the Paleozoic, perhaps earlier. These ancient reefs were dominated by animals with unfamiliar names such as archaeocyathids and stromatoporoids, and by the more familiar sponges and extinct corals such as rugosans and tabulates. The modern types of corals are new to the scene, having evolved recently, geologically speaking, just before the arrival of the rudists.