Tag Archives: ammonoid

Geo-pic of the week: Pyritized Ammonoid

ammanoid cropped

Pictured above is the internal mold of an ammonoid fossil – a group of invertebrate marine animals abundant in the world’s oceans from 416 – 66 million years ago.  They died during the same mass extinction that killed the dinosaurs.

Ammonoids were not stationary bottom dwellers, but had an interesting way of getting around in the water.  Their shells were partitioned into chambers, which are evident in the picture above.  The squid-like ammonite only occupied the final chamber of the shell.  The rest were empty so that the animal could control its buoyancy, and swim by taking in and expelling water.

Because ammonoids were abundant, widespread, and evolved new species quickly, geologists use their fossils to correlate rock units of similar age worldwide.  This one was collected from the Fayetteville Shale in northwest Arkansas.  Its gold color is due to the original organic material having been replaced by pyrite – also known as fool’s gold.