Introduction The fossils in the creeks around Gainesville are well known to  local collectors, but nothing has been published on their taxonomy, paleoecology or taphonomy.  Tessman's masters thesis (1969) described the relative abundance of the fossil shark taxa, but his data was based on collections at the Florida Museum of Natural History and gravel screened from Gainesville creeks.  As a result, it is not known whether the fossils found in the creeks are derived from one or several different stratigraphic units.  For this project we will collect stratigraphic samples, perform a grain size analysis, pick and sort the fossils, decribe their preservation, and make interpretations about paleoenvironment and depositional processes.

Geologic History

    The Hawthorn Group of Florida consists of a complex, highly variable package of interbedded and intermixed siliciclastic, carbonate, and phosphatic sediments.  These sediments range from Early Miocene to Early Pliocene and are contemporaneous with the Hawthorn Group sediments of Georgia (Scott, 1988).

    During the Paleogene, the Georgia Channel System prohibited siliclastics from invading the Florida Platform and carbonate deposition prevailed.  However, during the Late Oligocene, siliclastics filled the Georgia channel System and began to infringe upon the Florida Platform.  Siliclastic deposition on the Northern Florida Platform started during the Early Miocene and slowly transgressed across the southern Florida peninsula until the Pliocene (Randazzo, 1997).

    The structural features that influenced the deposition of the Hawthorn Group include the Ocala Platform, Sanford High, and the Jacksonville Basin.  In northern Florida, the Hawthorn Group unconformably overlies the Upper Eocene Ocala Limestone and dips gently to the east and northeast, away from the Ocala Platform and towards the Jacksonville Basin.  Hawthorn Group sediments are absent from much of the Ocala Platform and Sanford High, but are present along the margins of these features and thicken toward the Jacksonville Basin to over 500 feet (Scott, 1988).

    Miocene sea level fluctuations strongly influenced the characteristics of these sediments.  Deposition likely occurred during transgressions, while erosion and reworking of the sediments occurred during regressions (Scott, 1988).  Increased global cooling and glaciation, closure of the Tethys, and the formation of steep, latitudinal temperature gradients also occurred contemporaneously with Hawthorn Group deposition, further complicating this sedimentary package (Jones, 1997).

Northern Florida Hawthorn Group Fauna

    Various land vertebrates have been found in Alachua, Suwannee, Marion, and Gilchrist Counties.  These faunas range from Early to Late Miocene and represent deposition in sinkholes, fluvial, and estuarine environments.  They are particularly useful in interpreting relative sea level fluctuations throughout the Miocene (MacFadden and Webb, 1982).

    Jones and Portell (1988) identified over 40 invertebrate taxa from the Lower to Middle Miocene Brooks Sink in Bradford County.  The taxa are mostly preserved as molds and casts, making identifications difficult.  Specimens that could be identified are typical of normal shallow marine conditions.  However, some fossils are indicative of brackish water, high energy conditions, and terrestrial environments.  Williams et al (1977) reported the occurrence of oyster bioherms and brecciated zones throughout the Hawthorn Group of western Alachua County.

Hawthorn Group in Gainesville

    The fossils most frequently encountered in the creeks of Gainesville include: phosphatized internal and external molds of bivalves and gastropods, Callianassid pincers, ray plates, barnacles, shark teeth (Carcharodon megalodon, Hemipristis sp., Negaprion sp., Carcharinus spp., Galeocerdo sp., Rhizoprionodon sp., and Isurus sp.), fish mouth parts, crocodile teeth (Gavilosucas), horse teeth (Cormohipparian spp.), dolphin (Pomatodelphis sp.) and whale teeth and vertebrae, and dugong (Metaxytherium floridanum) ribs, teeth, and vertebrae.  It should be noted that the fossils of the creeks in Gainesville typically occur in zones and abundances differ between creeks.  It is not known whether this truly reflects the distribution and abundance of the fossils in the Hawthorn Group, or is just an artifact of stream hydrology and/or collecting bias'.
 

Shark links

Florida Museum of Natural History
Mote Marine Laboratory
Discovery prehistoric sharks
Bill Heim Fossil Sharks
The Pelagic shark research foundation
The University of California Museum of Paleontology
Miocene shark fossils