Fossils: What They Tell Us About Florida’s Natural History
Fossils found in the phosphate mining pits in Florida have helped uncover a lot of what we know about what Florida was like millions of years ago.
Fossils can be found everywhere in Florida. They are found in limerock, beach sand, riverbeds and phosphate mines. Florida is known for its rich fossil record and is acknowledged as unique in North America for the number and variety of vertebrate fossils that have been found.
Many areas of North America have no fossils. Fossils that are found in other areas are mostly of invertebrate specimens such as mollusks, corals or crinoids. Any vertebrate fossils that are found are usually encased in rock and hard to reach and dig out. This makes Florida’s land vertebrate fossils from the Oligocene, Miocene and Pliocene periods extremely important because they are so rare elsewhere in eastern North America. It is Florida’s fossil record that helps complete the understanding of Cenozoic terrestrial life on this continent. Florida’s Fossils, Robin C. Brown, Pinapple Press, 1996.
“Fossils from Bone Valley, the phosphate mining district in Central Florida, are rare and highly-priced specimens. Due to the unique geological characteristics of the phosphate-rich region, most of the fossils are beautifully preserved with amazing detail and color. Unlike the majority of southeastern U.S. fossils retrieved from rivers and streams, Bone Valley specimens are found in dry earth and are not stained with the typical cruddy black and brown muck from rivers.” www.FossilMuseum.net, 2003
In general, Florida fossils are in relatively good condition and lie close to the surface where they can be easily discovered. They are mostly found lying loose in beach sand, river gravel or uncovered as earth is moved. The vertebrate fossils that help unravel the mysteries of ancient Florida and North America date back to the Eocene Period, 56.5 million years ago. Florida’s Fossils, Robin C. Brown, Pinapple Press, 1996.
This 50 million-year record covers the age of mammals and shows the variety of creatures that roamed the land and the sea because Florida during much of this time was underwater. Throughout this period, glacial activity caused the oceans to rise and fall many times, as recently as 10,000 years ago. At times, the state was twice its current width. At other times, it was less than 100 miles long and 10 miles wide.
Since the sea rose and receded from Florida many times and sediments were reworked each time, the remains of ancient land and sea animals are often mixed together, making it difficult to be sure where animals lived and at what time. Typically, however, the oldest land animals are found in higher elevations in the northern third of the state and along the central ridge, but the creatures of Florida’s last two million years – the Pleistocene animals – are found throughout Florida’s strata.
Phosphate mining uncovers ancient shorelines millions of years old that at different times were home to both land and sea creatures. From the middle Miocene, 16 million years ago, to the earliest Pliocene, about 4.5 million years ago, no other region in North America can claim a more varied and richer wealth of important vertebrate fossil finds than from the famous Bone Valley region in the phosphate mining district of Central Florida.
During this time, thick forests and grassy plains covered a short, wide, peninsula that only went as far south as what is now Polk County. If you were to visit this area at that time, you would find six-foot tortoises, shovel-tusked mastodons, hornless rhinos, humpless camels, iguanas, gila monsters, and 30-foot crocodiles. The warm waters surrounding the area were filled with a rich variety of life as well, including long-beaked dolphins, bony fish, rays, sea cows and sharks including the notorious and now extinct giant killer shark, megalodon. www.FossilMuseum.net, 2003
Fossils such as the teeth of giant sharks and the bones of huge whales can be seen at the Mulberry Phosphate Museum in central Florida.
Fossils found in Bone Valley include many species that came to tropical and sub-tropical frost-free Florida forests to escape the advancing glaciers of the great "Ice Ages." Some of these animals migrated to North America from other parts of the world. For example, some of these animals came across the Bering Strait land bridge from Asia when sea level was lower. Others traveled around the rim of the Gulf of Mexico, when areas that are now submerged were exposed.
You will not, however, find dinosaur fossils associated with the Florida phosphate formation. The extinction of dinosaurs occurred about 65 million years ago, well before the lands now known as Florida began to emerge from the sea.
How Fossils Form
There are a number of different ways fossils are formed. Sometimes the actual remains of plants and animals are preserved. For instance, the frozen remains of a baby Woolly Mammoth were found at Cripple Creek, Alaska. In Florida, shells are often found when digging canals near the coast. Appearing as they do at the beach, these buried shells are actually fossils from a time when the sea covered the area where they were found.
Petrified wood represents another kind of fossil. In the formation of petrified wood, dissolved mineral matter fills in the pores of the wood. Over many years, the wood is replaced, leaving only the mineral matter behind. A similar process occurs in the case of bones and teeth. With bones and teeth, mineral matter fills in the pore spaces, but the bone matter or dental enamel does not fully dissolve. Instead, the mineral matter makes the bone or tooth more durable, and it is thus able to survive for millions of years. In rare cases, whole skeletons are preserved in this way, but usually the bones are found disarticulated, meaning separated from one another.
Another kind of fossil is called a mold. Molds are formed when plants or animals are encased in mud or some other sediment. The sediment hardens and eventually becomes rock. Meanwhile, the organism decays away, leaving behind a mold of itself inside the rock. If some substance fills this mold and hardens inside, a cast can be created. Casts look like the original organism externally, but lack any internal detail.
Similar in appearance to the inside of a mold are imprints and impressions. Imprints are made when an animal or plant is pressed into a clay or mud layer. Often this is how tracks of animals are preserved. Paleontologists can then tell how much an animal weighed by the track depth and how the animal moved by reconstructing its steps.
A number of conditions are necessary for fossils to form. Since scavengers, the elements, bacteria, and fungi quickly destroy or decay the remains of living things; the decay process must be slowed for fossils to form. Hard parts like bones, teeth, and shells decay more slowly than wood, flesh, or other soft tissue so most fossils come from organisms that had hard parts. They are most commonly found in layers of sedimentary rock formed from mud and silt at the bottom of lakes and shallow seas because plants and animals that are buried quickly decay more slowly.