Geochronology of a rich early Pleistocene vertebrate fauna, Leisey Shell Pit, Tampa Bay, Florida

Abstract

Leisey Shell Pit, on the southeastern edge of Tampa Bay, has produced the largest early Pleistocene vertebrate fauna in North America, consisting of about 30,000 cataloged specimens. The predominantly terrestrial and freshwater vertebrate fauna was transported into an estuarine environment during a regressive phase in an otherwise marine, bay-bottom sequence of sandy shell beds. The biochronological age of the Leisey Shell Pit local fauna is late Sappan (or late early Irvingtonian) based on the presence of such mammalian species as Mammuthus meridionalis and Smilodon gracilis, the absence of any Blancan holdovers, and early records of Lutra, Nothrotheriops, Palaeolama, and a new small glyptodont. Magnetostratigraphy indicates that the entire Pleistocene section at Leisey is reversed and thus presumably accumulated in the post-Olduvai part of the Matuyama Magnetochron (i.e., 1.66 myr or younger). Strontium isotope ratios ( 87Sr 86Sr ) sampled from aragonitic bivalve shells ( Chione cancellata) trend higher with increasing height in the section, yielding a mean age estimate for the two major bone beds of 1.7 ± 0.35 myr. In combination, these three geochronologic methods provide a high-resolution approach toward age determination and delimit the age of Leisey Shell Pit local fauna as between 1.66 and 1.4 myr.