About 1.5 to 1 million years ago (Hulbert and Morgan, 1989; Morgan and Hulbert, 1995)
Basis of Age
Biostratigraphy using macroinvertebrates and vertebrates, magnetostratigraphy, and strontium isotopes (Webb et al., 1989).
Geology
The Leisey Shell Pit consists of massive marine shell beds, overlain by a layer of quartz sand. The vertebrate fossils at Leisey Shell Pit 1A are primarily contained in a 5 to 30-cm-thick lens of dark silt covering an area of approximately 2000 sq. meters (Hulbert and Morgan, 1989; Morgan and Hulbert, 1995) with numerous marine, freshwater, and terrestrial invertebrate fossils. The bone bed lies at an uncomformity between the underlying Bermont Formation and the overlying Fort Thompson Formation.
Figure 1. Initial stages of excavation at Leisey Shell Pit 1A in April, 1984.
Depositional Environment
Leisey Shell Pit 1A was likely located in a coastal mangrove bay in an estuary of a major river, with both marine and terrestrial influences. The presence of silt in the bone bed suggests that they were deposited in a low energy environment. Possible mangrove roots and palm seeds indicate that these plants may have been present during the deposition of the sediment in Pit 1A, or shortly thereafter.
Figure 2. Detail of the fine-grained bone layer at Leisey Shell Pit 1A with rock hammer for scale. (photo by R. Hulbert)
Fossils
(†=extinct species; *=species no longer living in Florida)
Figure 3. Vertical face showing stratigraphy at Leisey Shell Pit 1A. The arrow points to the bone bed layer and a broken mammoth tusk. (photo by R. Hulbert)
Excavation History and Methods
Leisey Shell Pit 1A was discovered in July 1983 by Frank Garcia. He and a group of friends began to excavate the site. Late in 1983, Garcia informed Florida Museum curator David Webb about the discovery, and donated about 1200 specimens to the museum. Others in Frank’s crew also donated some specimens, most notably Ron Shrader. Recognizing the significance of the find, Webb reached an agreement with mine owners and managers and Garcia to begin an extensive excavation starting April 1, 1984, with all recovered fossils to be housed in the Florida Museum collection. This dig lasted until September 1984 and was manned by museum staff, UF paleontology graduate students, and numerous public volunteers from the Tampa Bay area. Unlike the 1983 excavations, the site was gridded in 1984, positional and other taphonomic data were recorded, and extensive samples of sediment were collected for screenwashing (Pratt and Hulbert, 1995).
Discussion
Leisey Shell Pit 1A was an extremely rich concentration of vertebrate fossils (Figs. 4-5). Over 20,000 identifiable specimens were collected in 1984. Several species present at the site are found in greater numbers than anywhere else, including the sabertoothed cat Smilodon gracilis, the tapir Tapirus haysii, and the llama Palaeolama mirifica (Morgan and Hulbert, 1995). Several new species of birds, rodents, and a new genus and species of armadillo-like mammal were found at Leisey Shell Pit 1A (Emslie, 1988; 1995; Morgan and White, 1995; Downing and White, 1995). No skeletons were found in articulation, but scattered associated skeletons were fairly common. Medium- and large-sized terrestrial vertebrates were particularly abundant at Leisey Shell Pit 1A. Screenwashing produced large numbers of small bones of marine fish and sharks, but only a few small-sized terrestrial mammals (Morgan and White, 1995).
Figure 4. UF graduate student Ann Pratt and Florida Museum vertebrate paleontology collections manager Gary Morgan examine fossils exposed at Leisey Shell Pit 1A. (photo by R. Hulbert)
Figure 5. Bones exposed in situ at Leisey Shell Pit 1A. Rain has cleaned the silt and mud from fossils, but they are otherwise as found. Note upper canine of Smilodon gracilis in center of image. (photo by R. Hulbert)
Prior to the discovery of the Leisey Shell Pit 1A locality, the commercial shell pits of South Florida had not been considered a significant source for sizable concentrations of vertebrate fossils. Leisey changed that view, and in the following decade other important sites were found in Florida, such as Leisey Shell Pit 3A, Macasphalt Shell Pit, Richardson Road Shell Pit, and De Soto Shell Pit. Combined, they greatly increased understanding of vertebrate biochronology in Florida during the late Blancan and early Irvingtonian land mammal ages. These sites permit cross-referencing between chronologies generated by vertebrates, marine macroinvertebrates, marine microfossils, and strontium isotope chronology (Webb et al., 1989; Morgan and Hulbert, 1995). The fossils collected at Leisey Shell Pit 1A continue to be studied by the next generation of paleontologists (e.g., DeSantis et al., 2009; Feranec, 2005; Scherer, 2013).
Mining and fossil collecting at the Leisey Shell Pits sites has permanently ceased. The pits are now either reclaimed or filled with water and remain in private ownership.
Sources
Original Author(s): Sahale N. Casebolt
Original Completion Date: October 5, 2012
Editor(s) Name(s): Richard C. Hulbert Jr., Natali Valdes
Emslie, S. D. 1995. An early Irvingtonian avifauna from Leisey Shell Pit, Florida. Bulletin of the Florida Museum of Natural History 37:299-344.
Feranec, R. S. 2005. Growth rate and duration of growth in the adult canine of Smilodon gracilis, and inferences on diet through stable isotope analysis. Bulletin of the Florida Museum of Natural History 45(4):369-377. (Download PDF)
Hulbert Jr., R. C. and G. S. Morgan. 1989. Stratigraphy, Paleoecology, and vertebrate fauna of the Leisey Shell Pit local fauna, early Pleistocene (Irvingtonian) of Southwestern Florida. Papers in Florida Paleontology. No. 2. Florida Paleontological Society, Inc.
MacFadden, B. J. 1995. Magnetic polarity stratigraphy and correlation of the Shell Pit, Tampa Bay, Florida. Bulletin of the Florida Museum of Natural History 37:107-116.
Morgan, G. S., and J. A. White. 1995. Small mammals (Insectivora, Lagomorpha, and Rodentia) from the early Pleistocene (early Irvingtonian) Leisey Shell Pit local fauna, Hillsborough County, Florida. Bulletin of the Florida Museum of Natural History 37:397-462.
Pratt, A. E., and R. C. Hulbert Jr. 1995. Taphonomy of the terrestrial mammals of Leisey Shell Pit 1A, Hillsborough County, Florida. Bulletin of the Florida Museum of Natural History 37:177-250.
This material is based upon work supported by the National Science Foundation under Grant Number CSBR 1203222, Jonathan Bloch, Principal Investigator. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
