Overview of the geology and vertebrate biochronology of the Leisey Shell Pit Local Fauna, Hillsborough County, Florida
Authors: Morgan Gary S. , Hulbert Richard C. 
Published: March 2024

The Leisey Shell Pit Local Fauna was collected from two adjacent commercial shell mines located 7 km southwest of Ruskin and less than 1 km inland from Tampa Bay in Hillsborough County, Florida Leisey Shell Pit is one of the most diverse Irvingtonian vertebrate faunas in North America, composed of at least 203 species: 14 sharks, 9 rays, 50 bony fish, 3 amphibians, 26 reptiles. 52 birds, and 49 mammals. Leisey 1A and Leisey 3A are the largest of the four principal sites that constitute the Leisey Shell Pit Local Fauna. Leisey 1A has extensive samples of large mammals, in particular: the camelids Palaeolama mirifica and Hemiauchenia macrocephala, two species of Equus, the primitive mammoth Mammuthus hayi, the ground sloths Paramylodon harlani and Nothrotheriops texanus, the giant tapir Tapirus haysii, the peccary Platygonus vetus, the gracile sabercat Smilodon gracilis, the short-faced bear Arctodus pristinus, and the canid Canis edwardii. The large mammal fauna from Leisey 3A is dominated by associated juvenile skeletons of Hemiauchenia macrocephala, while its diverse microvertebrate assemblage contains important samples of freshwater fish, sirens, aquatic snakes, birds, and small mammals. Sediments in the Leisey Shell Pit are referred to four formations, each of which has produced vertebrate fossils. At the base of the stratigraphic section is an indurated, tan to light gray dolostone referred to the Arcadia Formation. A horse tooth identified as "Merychippus" tertius derived from this unit is indicative of an early Barstovian age (early middle Miocene). Reworked sediments on top of the Arcadia Formation contain several land mammals, including the horses Nannippus aztecus, Neohipparion eurystyle, and Cormohipparion ingenuum, whose association is characteristic of Florida late early Hemphillian (late Miocene) faunas. Phosphatic gravel and spheroidal metaquartzite pebbles in this unit are typical of the upper Bone Valley Formation. The major concentrations of vertebrate fossils in the Leisey Shell Pit occur in thin, irregular layers of organic-rich sediment distributed throughout about 7 m of sandy marine shell beds referred to the Bermont Formation which unconformably overlies the Arcadia Formation. The large assemblage of land mammals from these organic units in the Bermont Formation is early Irvingtonian (early Pleistocene) in age. Shell beds of the Fort Thompson Formation occur in the Leisey section above an erosional unconformity at the top of the Bermont Formation. The Fort Thompson shell beds contain a distinctly younger mammalian fauna, including the bison Bison sp., the cotton rat Sigmodon hispidus, and the tapir Tapirus veroensis, typical of the late Pleistocene (Rancholabrean). Geochronological data derived from paleomagnetic analysis, strontium isotope geochronology, sea levels, and molluscan and vertebrate biochronology have been incorporated in a multidisciplinary effort to determine the age of the Bermont Formation at the Leisey Shell Pit. Molluscan biochronology implies an early or middle Pleistocene age, whereas mammalian biochronology is considerably more precise strongly favoring an early Pleistocene age between 1.6 and 1.0 Ma. All paleomagnetic samples from the Bermont Formation at Leisey have reversed polarity and are referred to the Matuyama Chron, indicating an age greater than 0.78 Ma. The Leisey Bermont section does not correlate to the Jaramillo Subchron of normal polarity between 1.07 and 0.99 Ma. It is also unlikely the Leisey beds were deposited during the youngest portion of the Matuyama between 0.99 and 0.78 Ma. The combination of data from magnetic polarity, mammalian biochronology, and sea level strongly indicates that the Bermont Formation at Leisey is older than 1.07 Ma and younger than 1.55 Ma. Strontium isotope ratios from Chione cancellata (Mollusca) shells from Leisey indicate an age between 1 and 2 Ma. Leisey is younger than Florida's best known earliest Irvingtonian (latest Pliocene) faunas, Inglis 1A and De Solo Shell Pit, based on the presence of five genera unknown in North America before the Pleistocene: Nothrotheriops, Lutra, Castoroides, Palaeolama, and Mammuthus. Four rodents from Leisey, Geomys pinetis, Erethizon dorsatum, Sigmodon libitinus, and Ondatra annectens, differ at the species level from their congeners at Inglis and De Soto. Leisey also lacks various Blancan holdover species found in the two older faunas, including the dwarf Florida form of Megatonyx leptostomus, Chasmaporthetes ossifragus, Trigonictis macrodon, and Capromeryx arizonensis. The occurrence of numerous species at Leisey that are unknown after the early Irvingtonian, including Glyptotherium arizonae, Pachyarmatherium leiseyi, Holmesina floridanus, Nothrotheriops texanus, Sigmodon libitinus, and Canis edwardii, further constrains the age of this fauna, ruling out a middle Irvingtonian or younger age assignment The Leisey mammalian fauna correlates most closely with the late early Irvingtonian, between about 1.6 and 1.0 Ma. Other Florida faunas similar in age to Leisey are Haile 16A, Haile 21A, Crystal River Power Plant, Pool Branch, Payne Creek Mine, Rigby Shell Pit and Punta Gorda. Of these sites Haile 16A is probably somewhat older (between 1.6 and 1.3 Ma) based on the occurrence of several holdovers from Florida late Blancan and earliest Irvingtonian faunas, including Sylvilagus webbi, Geomys propinetis, and Trigonictis. Western early Irvingtonian faunas that are correlatives of Leisey include: Gilliland, Texas; Holloman, Oklahoma; Kentuck, Nash, and Wathena in Kansas; Sappa, Nebraska; and Java, South Dakota. Among these sites, Leisey has the largest number of diagnostic laxa in common with the Gilliland Local Fauna, including Glyptotherium arizonae, a medium-sized Holmesina floridanus, Nothrotheriops texanus, Canis edwardii, Tapirus haysii and primitive Mammuthus. Leisey and other late early Irvingtonian faunas are younger than earliest Irvingtonian faunas (2.0 to 1.6 Ma) such as Curtis Ranch Arizona and Inglis 1A and De Soto Shell Pit of Florida, and are older than middle Irvingtonian faunas (1.0-0.6 Ma), including: the type Irvington fauna from California; Cudahy, Kansas; Conard Fissure, Arkansas; Cumberland Cave, Maryland; Port Kennedy Cave, Pennsylvania; Hamilton Cave, West Virginia; and the Florida equivalent McLeod Limerock Mine. The occurrence of land mammals in estuarine, freshwater, and terrestrial units within the predominantly nearshore marine late Pliocene and Pleistocene shell bed sequence of southern peninsular Florida (Pinecrest Beds, Caloosahatchee Formation, Bermont Formation, and Fort Thompson Formation in ascending stratigraphic order) has allowed a precision in dating these deposits not previously possible. The Pinecrest Beds, the uppermost unit of the Tamiami Formation, contain land mammal faunas of late Blancan age (2.5-2.0 Ma) based on the association of Nannippus and a large suite of Neotropical immigrants, including Dasypus, Holmesina, Glyptotherium, Glossotherium, Eremotherium, and Neochoerus. Florida late Blancan faunas derived from the Pinecrest Beds, or found in association with Pinecrest molluscan faunas, include Macasphalt Shell Pit, Acline Shell Pit, St. Petersburg Times, Kissimmee River, Brighton Canal, and Lehigh Acres. Vertebrate faunas from the overlying Caloosahatchee Formation lack typical Blancan forms, including Borophagus, Nannippus, Equus (Dolichohippus), and Rhynchotherium, and contain taxa typical of earliest Irvingtonian (2.0-1.6 Ma) faunas, such as Inglis 1A. The two richest earliest Irvingtonian faunas from the Caloosahatchee Formation in southern Florida are the De Soto Shell Pit and Forsberg Shell Pit. Vertebrate faunas from the Bermont Formation are late early Irvingtonian (1.6-1.0 Ma) in age, and are typified by the Leisey Shell Pit, as well as Rigby Shell Pit, Crystal River Power Plant, and Punta Gorda. Vertebrate faunas associated with the Fort Thompson Formation typically contain Bison and other tan characteristic of the Rancholabrean Land Mammal Age.