October 26. Rise of the Rudists

Today is episode 300 of the History of the Earth Calendar. The blog has reached about 23,000 total page views and as near as I can tell, the podcast is getting about 900 downloads a day. I have no idea whether that’s a little or a lot in the grand scheme of things, but I’m very happy that more than two or three people find this stuff interesting. I really do appreciate the support and the feedback you’ve given. Thanks!

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Today’s Jurassic topic is the Rise of the Rudists. As much as I’d like that to be an episode of Dr. Who, it’s a little more mundane. 

During the early part of the late Jurassic, a new form of marine invertebrate appeared. The rudists were mollusks, part of the class of bivalves, and related to clams and oysters. But their shell forms were distinctive, and quite dramatically NOT like clams and oysters. 

One of the shells became highly elongate – a tall tube, ranging from a few centimeters to more than a meter high. The other shell, or valve, sat like a hinged lid on top of that tube, in which the animal lived. These tall tubes, growing near each other, trapped sediment between them, and by the Cretaceous, rudists were really important reef-builders. The porosity in those reefs makes Cretaceous rudist reefs excellent reservoirs for oil. 

During the Jurassic they were proliferating but had generally not become so large and abundant yet that they formed reefs. There’s a wide diversity among rudists, which are grouped into at least 11 different families and more than 1000 species. Some Jurassic forms were coiled with snake-like shells while others were more boxy in shape. All of this variety in morphology led Lamarck to give them their name, a reference to their unusual or “rude” appearance.

Rudists lived mostly in shallow, tropical waters bordering the Tethys Ocean and its remnants, including the Gulf of Mexico.

They only existed from late Jurassic to the end of the Cretaceous. They were wiped out by the extinction event at the end of the Cretaceous. Nonetheless, they had a run of close to 100 million years. 

—Richard I. Gibson

Rudists
Rudists at UCBerkeley
Cretaceous Rudists