The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on an occasional schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Wednesday, November 5, 2014

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.     

No comments:

Post a Comment