MIOCENE AMPHIBIANS AND REPTILES FROM FORT POLK IN WESTERN LOUISIANA: EVALUATING THEIR USEFULNESS IN PALEOENVIRONMENTAL RECONSTRUCTION AND BIOSTRATIGRAPHY

The late Barstovian (Middle Miocene) herpetofauna from Fort Polk, Louisiana represents the most taxonomically diverse fossil herpetofauna on the central Gulf Coast and the only fossil herpetofauna from Louisiana. The fauna consists of 15 genera from 10 families with diagnostic elements from all amphibian and reptilian orders except Gymnophionans (caecilians). All sites at Fort Polk fall in the early late Barstovian Land Mammal Age during a time of cooling global temperatures and lowering sea levels shortly after the Middle Miocene Climatic Optimum.

The six herpetofaunal-bearing sites at Fort Polk were previously determined to be paleoenvironmentally distinct from one another, ranging from upland to transitional marine. Recovery rates of herpetological elements per kilogram of matrix processed and colubrine:natricine paleoenvironmental indices were calculated. Anuran data were plotted on ternary diagrams to discriminate between upland versus lowland sites at Fort Polk. The plotted frog data and numbers of herpetological elements per kilogram of matrix processed were in agreement with other environmental proxies, placing the Stonehenge site the most aquatic, followed by the TVOR site.

To determine the extent that climate played in the colubrid radiation and the simultaneous diminution of boid snakes during the Neogene of North America, the total numbers of colubrid snakes from contemporaneous North American sites from the Late Oligocene through the Miocene were plotted and expressed as a percentage of the total number of colubrids and boids in a fauna. The plot was then compared to the δ¹⁸O graph, which is considered a useful proxy for demonstrating global climatic changes through time. In addition, individual taxon stratigraphic ranges were plotted through the Neogene to produce a composite section and evaluate the utility of snakes in biostratigraphic studies. These plots demonstrate the affect of the colubrid radiation on Neogene snake faunas and reveal that fossil snakes can be used to coarsely subdivide the Miocene.