Rudist reef fragment in Camarines Sur

Rudist reef fragment in Camarines Sur
Rudist reef fragment in Camarines Sur | @nmbicol

Rudist reef fragment in Camarines Sur

Rudists, the ancient reef-builders

If you are fascinated with the ocean and the life it supports, for today’s Discover Geology we feature an organism that once dominated the reef-building communities many millions of years ago.

Reefs are marine structures that provide habitats for different types of marine life. The Scleractinia, also known as stony corals or hard corals, are now the primary reef-builders. But did you know that before corals, reefs were made of clams called rudists? About 100 million years ago, during the age of the dinosaurs, rudists inhabited the shallow ocean waters and became the reef- builders at that time. They dominated the marine waters until their extinction about 66 million years ago, along with the dinosaurs.

Rudists were invertebrates with two strange-looking shells that differ from each other. One form resembled a trashcan. One was a large shell attached to the seafloor, while the other smaller shell was found on top like a lid. Other rudist forms look like horns and croissant bread. Their fossils are found in limestone rocks throughout the tropics in the Mediterranean, the Middle East, the Caribbean, and Southeast Asia.

The National Museum of the Philippines found a rudist reef fragment from the Garchitorena formation in Cabacongan, Caramoan, Camarines Sur, estimated to be in Late Cretaceous in age (93.9 – 89.8 million years ago). You may view this fossil at the National Museum PH Bicol as soon as it finally re-opens.

Skene, Jennifer. (September 10, 2009). Rudists. University of California Museum of Paleontology Retrieved from
Johnson, Claudia. (2002). The Rise and Fall of Rudist Reefs. American Scientist – AMER SCI. 90. 10.1511/2002.2.148.
Smithsonian. (n.d). Rudist Clams. Retrieved from
Smithsonian. (n.d). Long Before Coral, Mollusks Built the Ocean’s Reef. Retrieved from
Ross, Rachel (September 25, 2018). What Are Coral Reefs? Retrieved from

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