The rise and fall of Rudists
Rudists are an extinct group of bivalves that lived attached to the floor of tropical shallow seas during the Cretaceous period. They were so successful that became the biggest reef builders during the Mesozoic.
Due to the fact that rudists evolved in a variety of sizes and shapes, they were able to fill a wide range of ecological niches during almost 100 million years. Rudists are not that different from other bivalved molluscs, but they are usually larger and more robust. Their valves are commonly asymmetric; the lower valve is attached to the substrate and the upper valve is smaller and served as a lid for the animal.
Early rudists (such as diceratids and requienids) had relatively symmetrical valves; asymmetrical valves developed later (in caprinids, for example). Most primitive rudists generally presented tightly coiled valves and as they evolved, their valves became more uncoiled.
Just like other bivalves, rudists had a double layered calcium carbonate wall formed by an internal aragonitic layer and an external calcitic layer. Primitive rudists had a larger proportion of the aragonitic layer, but as they developed, their shells became more calcitic. Considering that calcitic-shelled rudists are more likely to be preserved than aragonitc-shelled rudists (due to chemical properties), calcitic-shelled rudists could be overrepresented in the fossil record. This should be taken in consideration when studying the fossil record of these animals.
Rudists built reefs in equatorial latitudes where the waters were warm and shallow. These conditions were perfect for rudists to proliferate, and as a result, they created extensive carbonate platforms around what are now Meso-America, the Mediterranean, North of Africa, the Middle East, and Southeast Asia.
As it happens with most bivalves, rudists were filter-feeding animals that obtained their nutrients from sea currents. This, along with the fact that they were sessile (unable to move), means that their requirements to live were quite simple: anchor themselves firmly to the substrate and filter water efficiently to obtain nutrients.
At the end of the Cenomanian (100 million years ago), an extinction event wiped out the capronids and other families of bivales nearly disappeared. However, other rudists (such as the hippuritids and radiolitids) proliferated, taking advantage of the niches left by the extinct capronitid rudists.
All rudists were extinguished at the end of the Maastrichtian (aka late Cretaceous or roughly 65 million years ago), disappearing in the same event that killed the non-avian dinosaurs. Since rudists dwelled in shallow waters, it's possible that the habitat that once allowed them to flourish also made them vulnerable to the drastic climatic changes that occurred 65 million years ago.
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