Monday, May 28, 2012

Florida fossil shells

A fossilized shark's tooth from Venice Beach. It's about 1/2 inch in height - possibly from a Lemon or a Dusky Shark.

A land at the bottom of the sea
Did you know that much of Florida used to lie at the bottom of a shallow sea?  Sea levels were very high during the Oligocene through Cretaceous times (23 to 145 million years ago), with water covering the entire Florida peninsula.  Marine life flourished: coral reefs and all sorts of invertebrates called these shallow seas home.

Then water levels began to fluctuate, with Florida and parts of the bottom of the Gulf of Mexico exposed in the Pleistocene time.  As the Great Ice Age blossomed around the world, glaciers ate up huge amounts of water, storing them in ice.  Water levels receded, and Florida became a huge landmass.  Over time, sea levels rose once again to rim and outline the coastline boundaries we know today.

Much of Florida lies on layers of skeletons and shells of sea creatures, collected and compressed over time to form limestone bedrock.  Limestone is a sedimentary rock, and makes up most of the exposed rock you might see as you travel through Florida. 

Interestingly enough, the sand on our coastal beaches is mainly composed of eroded and weathered quartz fragments from the Appalachian Mountains.  Bits of the mountains erode over time, and washed into rivers leading to the sea, later to be swept up by tidal currents and deposited onto the shoreline.  Different beaches have different compositions, mixing quartz sand with coral and shell fragments, fossils, and organic matter.
Different species of fossilized shells and coral from a quarry near Sarasota.

Fossil shells found throughout Florida
Florida contains fossiliferous rocks, which simply means “rocks containing fossils.”  A chunk of limestone found in Florida is very likely to contain fossil shells and corals.  You can see my watercolor study and blog post on one of these chunks here.  These rocks can be found in the ground, used as rip-rap in sea walls and groynes, or as garden borders. 

A fossil is created when minerals replace the original structure of the shell or skeleton.  Sometimes the fossil is a complete recreation of the organism, sometimes it’s a cast or mold, where the minerals fill in the void left by the dissolved or decayed parts.

I find fossils of any kind fascinating, and am attracted to the mundane as well as the spectacular.  A past trip to Venice Beach yielded a collection of small prehistoric shark’s teeth, which wash up right onto the beach and become buried in the sand.  Although my finds are an inch long or less, enthusiastic and persistent (or perhaps just lucky) collectors have discovered shark’s teeth as large as the palm of my hand!  Mixed in with the sand and the shark’s teeth, one might find other treasures: petrified wood fragments, small bones, and ray teeth. 

Small fossil shells are often found along gravel roads, as the gravel is quarried and crushed from large pits of limestone throughout the state.  Small quarries have been known to open their doors (pits?) on designated days to fossil enthusiasts for a small fee and a signed waiver.  Some of my larger shells were collected from a past trip to a quarry near Sarasota, which has some nice large invertebrate specimens that can be found intact.

Other fossils found in Florida
Although ocean-dwelling creatures are abundant throughout our state, Florida fossils also include plants and wood, and remnants of mammals such as manatees, horses, mammoths, mastodons, peccaries, sloths, and camels.   Please note that a state permit is required to collect certain types of fossils from state parks; exemptions include shark's teeth and shells.

I’m only touching on the tip of the iceberg in this post, but if you’re interested further, the Florida Museum of Natural History offers an online list of Florida fossil clubs here.   Or look for this book, “Roadside Geology of Florida,” by authors Bryan, Scott, and Means, three geologists who will take you on an historical trip through Florida’s past, and introduce you to specific locations you can visit now. 

Click on images to view larger.
Upper shark's tooth drawn with gray and black Albrecht Durer watercolor pencils on Pentalic Nature Sketch paper.
Lower image is also on Pentalic Nature Sketch paper, with a background of Golden Raw Umber acrylic paint, with drawings on top in white and black Prismacolor colored pencil.