Rudists

Lore & Background

The Late Jurassic forms of rudists were elongated, with both valves similarly shaped, often pipe- or stake-shaped. In contrast, the reef-building forms of the Cretaceous had one valve that became a flat lid, with the other valve becoming an inverted spike-like cone. The size of these conical forms ranged widely from just a few centimeters to well over a meter in length. Their classic morphology consisted of a lower, roughly conical valve attached to the seafloor or to neighboring rudists, and a smaller upper valve that served as a lid, which could take forms including a simple flat lid, a low cone, a spiral, or even a star-shaped form.

Reader's Guide

Rudists were among the most important constituents of reefs during the Cretaceous Period. Their classification as true reef-builders is controversial because they would catch and trap much sediment between their lower conical valves, meaning they were not completely composed of biogenic carbonates as a coral would be. However, rudist reefs were so successful that they may have driven scleractinian corals out of many tropical environments, including shelves that are today the Caribbean and the Mediterranean. Their success as reef builders was likely due to the extreme environment of the Cretaceous, when tropical waters were between 6°C and 14°C warmer than today and also more highly saline—conditions less hospitable to corals. These rudist reefs were sometimes hundreds of meters tall and often ran for hundreds of kilometers on continental shelves, at one point fringing the North American coast from the Gulf of Mexico to the present-day Maritime Provinces. Because of their high porosity, rudist reefs are highly favored oil traps. The rudists became extinct at the end of the Cretaceous, apparently as a result of the Cretaceous–Paleogene extinction event, though it had been thought that this group began a decline about 2.5 million years earlier which culminated in complete extinction half a million years before the end of the Cretaceous.

Did You Know?

• The oldest true rudists are found in late Jurassic rocks in France.
• Rudist reefs were sometimes hundreds of meters tall and often ran for hundreds of kilometers on continental shelves.
• The small upper valve of rudists could take a star-shaped form.
• Rudist reefs are highly favored oil traps due to their high porosity.