Fossiliferous Florida
Deborah Painter (USA)
Seated on the jet heading south to Florida, I thought of my upcoming field work in the central portion of the state. I also hoped to see some monkeys during my brief stay. I knew they were ‘invasives’ but I still wanted to see them. I had read that, many decades ago, rhesus monkeys were brought in for atmosphere in the Silver Springs area by an entrepreneur named “Colonel” Tooey, who was running a glass bottom tour boat operation named “The Jungle Cruise”. By now, the animals have established a breeding population. I also knew that the Withlacoochee, a river yielding vertebrate fossils from several Cenozoic epochs, was not far from where I would be. Therefore, I had my collecting permit from the state of Florida just in case I had a few hours to spare to look around the riverbanks.
Upon arrival at the Orlando International Airport, I became instantly aware that most of the travelling public was going to, or coming back from, Walt Disney World and the Epcot Center. I was headed to a mostly wooded area in western Sumter County to do an ecological survey and wetland delineation on Federal lands, and was glad to leave the crazed traffic of the airport behind me, as my rental car sped toward the setting sun of a late September day in 2019.
It did not take long for me to enter the agricultural lands where Brussels sprouts, cassava, pears and ornamental house plants were cultivated, and beef cattle were raised. Little did I know the surprises waiting in central Florida’s Sumter County and Lake County. Hilly country in the Howey-In-The Hills area of Lake County: white limerock (fossil coral) unpaved roadway bed material and a dazzling abundance of birds.
Some say that the geology of central Florida is mostly not visible at the surface. I disagree. The unpaved drives of the Brooksville Ridge region of central Florida’s Sumter and Lake Counties are literally white when seen from the air – this colour is due to the white limerock used for unpaved roadbeds, with the rock being quarried from Leon County in the Florida Panhandle to the northwest and Pasco County just south of Hernando County. Fig. 1 depicts piles of limerock ready to be laid as road material. Humans have placed fossils right at the surface for their unpaved road construction.
Central Florida’s surface is shaped by past sea level events that show themselves as the present-day river drainage patterns, scarps and terraces, together with karst topography. Florida itself is more than just an extension of the south-eastern land mass of North America. It is part of a carbonate platform that extends offshore. The sloping flanks drop off rapidly to deep water. Eighteen thousand years ago, during the Last Glacial Maximum of the Wisconsinian Ice Age, the entirety of that platform was exposed and the west coast of Florida would have been 150km further west than today.
The carbonate platform rests on top of Mesozoic and Paleozoic basement rocks that were once part of what we now call South America and northwest Africa. Atop these metamorphic and igneous rocks is a 2km to 6km thick layer of evaporates and carbonates, and atop that, a thin layer of siliciclastic sands of less than 150m in thickness. Cenozoic era fossils are the only ones near the surface.
Central and north Florida has more sinkholes than other areas within the state because there, older carbonate rocks are exposed at the surface or are covered by siliciclastic sand veneers.
An area within the northern Brooksville Ridge region was the prime focus of my work. It lies to the east of the eastern bank of the Withlacoochee. The Brooksville Ridge is about 177km long and divided into two equal parts by the valley of the Withlacoochee at Dunnellon, Florida. The largest portion of the southern zone is approximately 96km long and varies between 16km and 25km wide. The northern portion is approximately 80km long and is between 6km and 9.6km wide.
This region is typically above the piezometric surface, the imaginary surface to which groundwater rises under hydrostatic pressure in wells or springs. The elevations on the Brooksville Ridge vary in short distances from 21m to 60.9m. The hills of Howey-In-The-Hills are within the Brooksville Ridge. Little surface drainage occurs in this region, and the ground surface comprises sand a few meters thick and underlain with red clastic sediments of the Bone Valley and Alachua Formations.
The Brooksville Ridge region boasts an expansive system of lakes and prairies, especially just north of Orlando. The Ocala Limestone of Eocene age is the uppermost rock unit found at my project site, which is underlain by other limestone units to a depth of about 762m. Its surface is weathered and the depth to the top of rock is quite variable. It is found at or just below the surface and is estimated to average about 1.21m below the land surface over the entire property we were studying.
The limestone in the western area of Sumter County is of Eocene age and there are some Pleistocene deposits. The area features sinkholes, rock pinnacles and erosional pinchouts. Along the eastern shore of the Withlacoochee in Sumter, Eocene invertebrate fossil gastropods, echinoids, foraminifers and bryozoans make up the Ocala Limestone. The Ocala limestone also contains vertebrate fossils which I shall mention shortly. .
All of central Florida is a rich area for Cenozoic fossils and collecting of vertebrate material is allowed if a permit is obtained from the state from the Florida Museum of Natural History. It can be obtained online using the following link: https://www.floridamuseum.ufl.edu/vertpaleo/amateur-collector/fossil-permit/permit-application/. The cost is very modest: US$5 as of this writing.
My first morning in Florida, I stepped out of my rental car into the tiny parking lot of the facility where I was to work and my shirt was tinged with sweat within 20 minutes. It felt like the July conditions I had hoped to put behind me by now since it was now late September. But this was Florida. Then I became aware of strange little insects on my clothes. They were not biting me, as I expected any insects attracted to me to do.
I met Ron, the geotechnical crew leader of the company contracted to take GPR reflector readings of the bedrock in the forested areas of the facility to determine the presence and depth of karst features like sinkholes (Fig. 2). The methodology Ron and his crew were planning to use involved running transects across the study area and using a ground penetrating radar signal to characterise near-surface geological conditions and to identify subsurface geological features related to potential karst features. This would help determine potential for soil subsidence and sinkhole formation that would create site constraints for future development.
When I commented on the odd insects, the geologist identified them. “Love Bugs”, Ron called them. “They are really two insects joined together end to end. They mate this time of year and, when they swarm, they gum up automobile grills.”
That same day, I met the two ecologists from another consulting firm who were to assist in the wetland delineation of the large site (Fig. 3).
They were from their office in the Silver Springs area to the north. Faith, the senior ecologist, chatted about her recent trip to Los Angeles, California. Faith’s son is at college there. Her family is from Southern California and she enjoys the heat and humidity of fall in central Florida less and less each time she visits her son in Los Angeles and gets a bit of comparison. Once in the forest, she and the junior ecologist, Brittany, took out their apps on their smart phones for filling in the data sheets for each plot point, their hand augers to be used to dig to a depth of 50.8cm, and their Munsell soil chart.
That is “chart”, singular. I was startled that they only had one page taken from the Munsell book, which is a small binder that will fit in your hand and has many pages, each page devoted to a particular hue. The pages have non-detachable card stock chips of various values and chromas with circular holes at each. Determining hue, value and chroma is done by placing a fresh soil sample behind the circular hole, preferably in good strong sunlight and not using sunglasses.
The chart will match it with the wide array of hues, chromas and values that constitute the soils of the world. Hue is the colour, chroma measures the saturation of the hue and value measures how dark or light a colour is. For example, there is the 7.5YR page. 7.5YR is the hue, and each circular hole on the page further sorts the soil sample into chromas and values using additional letters and numbers.
“All over Sumter County,” Faith told me, “the soils are 10YR from the surface to the necessary depth of sampling. This is because all the soils are from the same sandy deposits.” In addition, in our conversations, Faith mentioned monkeys before I even had a chance. She had seen some on her backyard cam recently. She lives near Silver Springs State Park where, in years past, the boat operator had worked.
I saw piles of white rock on site to be used for supplementing the unpaved roads on site, and obtained two small pieces. Pliocene age quarries in Leon County in Florida’s Panhandle to the northwest bring fossil scleractinian (stony) coral, mostly the extinct Septastrea crassa (Fig. 4) comprising some of the white “gravel” roadbed material I was observing.
The bulk is like the sample in Fig. 5, quarried from Leon and Pasco Counties, comprised of fragments of gastropods (Velates floridanus), foraminifera (Lepidocyclina ocalana, Amphistegina pinarensis, Nummulites [Camerina] vanderstoki, and Nummulites [Operculinoides] ocalana), plus bryozoans and crustaceans.
Occasionally, the fossils of primitive whales are found. These include Basilosaurus cetoides. Some Pleistocene vertebrate fossils have been collected in Sumter County, but fossils of this age are relatively abundant along the western shores of the Withlacoochee, in Citrus and Hernando Counties. Also found in Eocene age strata are fossils of the shark species Galeocerdo latidens. The only surviving species of Galeocerdo is G. cuvier, the “tiger shark” (Fig. 6). Eocene Galeocerdo fossils usually occur as teeth.
We were rather close to the Withlacoochee River floodplain. Its headwaters are central Florida’s Green Swamp. The river flows west, then north, turning northwest and finally west again before it empties into the Gulf of Mexico. The river is approximately 226.9km long and has a drainage basin of 303,000 hectares. The river and stream bottoms mix up both terrestrial and aquatic animal fossils from the Pleistocene deposits, and concentrate them along both west and east banks. They also concentrate in caves.
Florida’s Pleistocene vertebrate fossil assemblages are among the most outstanding in the world and certainly the most diverse in North America. In Hernando County and Citrus County, both on the west bank of the river, the Pleistocene epoch deposits reveal an enviable abundance of species.
Florida’s fauna resembled a mixture of South American and North American assemblages with many species long vanished. Tapirs (Tapirus veroensis), giant capybaras (Neochoerus pickneyi), resembling the smaller modern South American species (Fig. 7), horses (an extinct species of Equus), sabre-toothed cats (Smilodon floridanus) (Fig. 8), guanaco-like small camels (Tanupolama mirifica) (Fig. 9), mammoths (Mammuthus floridanus), mastodons (Mammut americanum) (Fig. 10), American lions (Panthera atrox), which combined features of lions and jaguars, giant ground sloths (Eremotherium and Megalonyx species), giant armadillos (Glyptodon and Glyptotherium species), and an extinct species of bison (Bison antiquus) shared the landscape with animals associated with modern Florida. These familiar animals include cotton rats (Sigmodon hispidus), mallard ducks (Anas platyrhynchos) (Fig. 11), wild turkeys (Meleagris), Myotis bats (genus Myotis) and American alligators of the genus Alligator.
Sifting the mud along the riverbanks is a popular avocation for fossil collectors, and the more ambitious will scuba dive for specimens. My site was a little under a kilometre from the Withlacoochee but my days were full, so I did not have any opportunity to collect fossils at the river banks during this work trip. I did the next best thing, purchasing a partial mastodon molar of a juvenile Mammut americanum from a rock shop. The molar was labelled “Withlacoochee River, Florida” and is depicted in Figs. 12 and 13.
It is 5.09cm long and 4.99cm wide. M. americanum, the American mastodon, is one of the best known and among the last species of Mammut. The magnificent species had a wide range from Alaska to southern California along the North American west coast, and from New England south to Florida along the east coast. The dentition was different from that of our modern elephants or the extinct elephant like mammoths. The name mastodon means “breast tooth” and refers to the shape of the molars. Mastodons and their earlier relatives in Florida were more adapted to browsing in a woodland environment than the mammoths. The amazing array of fossils one can find in central Florida certainly is a draw for scientists and collectors.
About the author
Deborah Painter is an ecologist and general environmental scientist specialising in government and industrial development planning to minimise deleterious environmental impacts. She lives in the United States.
References
Brown, Robin, 2008. Florida’s Fossils: Guide to Location, Identification, and Enjoyment. Pineapple Press. 212 pages.
Florida Department of Natural Resources (FDNR), 1970. The Geomorphology of the Florida Peninsula, Geological Bulletin No, 51. Bureau of Geology, Division of Interior Resources, Florida Department of Natural Resources, Tallahassee, Florida. 172 pages.
The Florida Museum of Natural History Invertebrate Paleontology Image Gallery
https://www.floridamuseum.ufl.edu/invertpaleo/galleries
Green, Richard C. William L. Evans, III and Seth W. Bassett. 2013 . State of Florida Department of Environmental Protection. Text to accompany geologic map of the USGS Daytona Beach 30 x 60 minute quadrangle, northeast Florida (Open-File Map Series 105)
Hine, Albert C. 2009. Geology of Florida. College of Marine Science, University of South Florida. 32 pages.
Mansfield, Wendell C. 1931. Pliocene Fossils from Limestone in Southern Florida. United States Department of the Interior Professional Paper 170-D. United States Government Printing Office, Washington, D. C. 21 pages.
Olsen, Stanley J. 1959. Fossil Mammals of Florida. State of Florida, State Board of Conservation, Florida Geological Survey Special Bulletin Number 6. 81 pages.
Stetson University, 2020. Florida Formations: Shifting Seas and Sediments.
https://www.stetson.edu/other/gillespie-museum
United States Geological Survey Ocala Limestone series.
https://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=FLEOo%3B0