Terrestrial vertebrate sites in the Castor Creek Member of the Fleming
Formation, first discovered in 1993 on Fort Polk in western Louisiana, have
been the only source of Miocene terrestrial vertebrates in the state until
discoveries in southeastern Louisiana in the Pascagoula Formation in June of
2005. 6359 cataloged specimens, mainly mammals, have been recovered from the
Fort Polk sites, including the rhinoceros Aphelops, a gomphothere, two
species of merychippine horse, Prosynthetoceras francisi, a large camel, a
small peccary, Miomustela, a hedgehog, bats, shrews, two small beavers,
Copemys, Texomys, heteromyids, and squirrels, as well as lower vertebrates
including turtles, snakes, frogs, toads, salamanders, and fishes. Heavy
vegetative cover in the region and isolation from other sites in Texas and
Florida impede placing the fossiliferous sites in context, both locally and
globally, leading to approaches on the western Louisiana sites that combined
magnetostratigraphy and sequence stratigraphy with biostratigraphy.
Paleomagnetic work on cores at sites in western Louisiana, places the most
prolific as probably between 14 Ma and 13.5 Ma, which is consistent with the
Late Barstovian Land Mammal Age of their faunas. Concentrations of
calcareous pedogenic nodules from the upper Castor Creek Member, Fleming
Formation, produce most of the fossils. The nodule concentrations have been
attributed to drops in base level causing erosion on interfluves. Timing is
congruent with global cooling associated with Antarctic ice development, an
event tied to global lowering of sea level. One of the stratigraphically
lowest Castor Creek sites is marine, bears both marine and terrestrial large
and small vertebrates, and is currently considered to record a storm event.