Tag Archives: Fossil

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  Its gold color is due to the original organic material having been replaced by pyrite – also known as fool’s gold.

Geo-pic of the week: Graptolites

 

Graptolites

This is a picture of shale, collected from the Womble Formation, near Lake Ouachita State Park, Arkansas.  The photo shows examples of the, now extinct, Graptolites: fossilized colonies of tiny marine animals.

There were many types of Graptolites.  Some were attached to the sea floor, like corals, while others floated in the water, like plankton.  The feather-shaped fossils pictured here are actually the nests in which the animals lived.  Each tooth-like tube, on the edges of the nests, housed a tiny animal.  Several of these nests would be linked together into a larger colony.

At one time the oceans were full of Graptolites, but by about 300 million years ago they died out for unknown reasons.  Because they were abundant, widespread, and continually evolving, Graptolites are important fossils for dating ancient marine rocks. 

To download a copy of our self-guided tour of Lake Ouachita geology, click here http://www.geology.ar.gov/pdf/Lake%20Ouachita%20Geologic%20Float.pdf

Geo-pic of the week: Fossiliferous mudstone

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Above is a picture of loosely-consolidated, fossiliferous mudstone from the Midway Group: a group of near-shore marine rocks exposed in southwestern Arkansas.  The Midway Group was deposited in a variety of near-shore marine environments that were common in what is now Arkansas around 60 million years ago. The majority of the fossils pictured here are casts of a variety of sea snails (gastropods), but there are also oysters and tiny bryozoans (marine filter feeders).

 

To see more views of fossiliferous mudstone click here

Geo-pic of the week: Amber

Amber_croppedandcolored

Pictured above are several polished samples of the fossil amber; the sample in the top left is unpolished. 

Amber is the fossilized resin of a tree.  As you can see, some of the wood of the trees got preserved with the resin, which gives this amber a wood-grain appearance .  These pieces were collected near Malvern, Arkansas, where amber is found in beds of lignite: a soft, low-grade coal.

People have prized amber since pre-historic times for its beauty and its scent.  It continues to be popular today in the production of jewelry and perfume.