SIZE-FREQUENCY DISTRIBUTION OF LEOPARD FROGS (RANA PIPIENS COMPLEX) FROM THE LATE TERTIARY PIPE CREEK SINKHOLE, GRANT COUNTY, INDIANA

The Pipe Creek Sinkhole (PCS) preserves a continental vertebrate fauna of late Tertiary (latest Hemphillian Land Mammal Age, c. 5 million years BP) age. The fauna is dominated by aquatic species, particularly leopard frogs. Unconsolidated sediments from the deposit were wet-sieved, yielding a large collection of leopard frog ilia (hip bones--the most diagnostic skeletal element for frogs). We digitally scanned 202 left ilia from the fossil assemblage, as well as ilia from modern leopard frogs of known size (snout-vent length [SVL]). From the digitized images we measured the vertical diameter (AD) across the acetabulum (hip socket). AD is positively correlated with SVL, indicating that AD is a suitable proxy for frog size. The size-frequency distribution of AD for the PCS leopard frogs suggests three possible modes. If the PCS leopard frogs represent a single species, these modes probably reflect a mixture of age classes and sexual dimorphism (females larger than males). The effects of sex and age might be disentangled by a skeletochronological study of the PCS frog ilia.