November 5. Oysters and snails

Cretaceous bivalves were not significantly different from today’s forms. Two types of oysters, Exogyra and Gryphea, are so abundant in some Cretaceous strata that they make up a significant portion of the rock. Their irregular shape and rough surface gives them the nickname “devil’s toenails.” Exogyras are sometimes fossilized by having their shells replaced by pyrite, iron sulfide, especially in some of the Cretaceous rocks of Texas, where their twisted form gives them the name “ram’s horns.” 

Inoceramus photo by Mike Beauregard
from Nunavut, Canada, used under
Creative Commons license.  

Inoceramus were clams that had a pretty much world-wide distribution during the Cretaceous. Some varieties grew to 70 inches or more long, the largest bivalves known. The huge size might have been an adaptation to muddy or stagnant waters, where a larger gill would have allowed the animal to extract more oxygen from low-oxygen water.

We mentioned the rudists last month, when they arrived on the scene. They are bivalves in which one shell became extremely elongate, so that the animal formed tubular structures a meter or more high. Cretaceous rudists became so large and prolific that they formed extensive reefs, or at least were substantial contributors to reefs by providing a framework that trapped sediment. They looked a lot like corals, but they’re actually clams. The Cretaceous Atlantic margin of North America, from Mexico to Canada, was marked by near continuous rudist reefs. The widening Gulf of Mexico was fringed by rudist reefs, some of which became reservoirs for oil and natural gas. Rudists became extinct at the end of the Cretaceous. 

Cretaceous gastropods or snails were largely similar to those of today, but with dozens of varieties, many of which are extinct. Cretaceous snails became adept at predatory drilling into other shells to kill and consume the animals inside. Such drilling has waxed and waned in the fossil record, and probably dates back to the first shelly critters in the late Precambrian or Cambrian, when this arms race between predators and prey may have begun. But the technique became common during the Cretaceous and continues to the present day.

—Richard I. Gibson

Rudist reefs 

Drilling predation 

Inoceramus photo by Mike Beauregard from Nunavut, Canada, used under Creative Commons license.