THE GROWTH RATE AND PERIOD OF THE ADULT CANINE IN SMILODON GRACILIS, AND INFERENCES ON DIET USING STABLE ISOTOPES

Trophic structure and interconnectedness have important implications for diversity and stability in ecosystems. It is generally difficult to determine trophic structure and the specific prey of a particular predator in ancient ecosystems. However, variation in the stable carbon isotope ratio in tooth enamel typically reflects differences in diet, and can be used to determine if a carnivore preferred prey that predominantly ate C₃ or C₄ plants. Using this technique, it is possible to determine what taxa, or at least exclude particular taxa, on which a predator fed. This study investigates the rate of growth and growth period of the adult canine in Smilodon gracilis from the Leisey Shell Pit 1A locality in Hillsborough County Florida, and determines whether it shows a preference for preying upon C₃ or C₄-feeders in the hope of determining its specific prey by analyzing stable carbon and oxygen isotope ratios found in tooth enamel. The oxygen isotope results show that this individual of S. gracilis had a mean d¹⁸O value of 0.2‰, and a range from -0.7‰ to 1.3‰. The oxygen isotope pattern indicates that nearly one year was sampled, showing about 60 mm of enamel growth over the year, which results in a growth rate of about 5 mm/month. The total length of enamel in the S. gracilis canine was about 80 mm suggesting a 16-month growth period if enamel growth rate remained constant. The carbon isotope results show that this individual had a mean d¹³C value of -8.8‰, and a range from -9.1‰ to -8.3‰. The carbon isotope values suggest consumption of animals that depended on a C₃ diet. Analysis of the herbivores from the Leisey Shell Pit 1A locality suggest that Hemiauchenia and Platygonus may have been included in the diet of this individual of Smilodon, while Equus and Mammuthus were probably excluded from its diet. Also, because the carbon isotope values of S. gracilis were more negative than many of the individual herbivores at the Leisey Shell Pit 1A locality, the results might be reflecting prey capture in a closed environment with the more negative numbers indicating a canopy effect. This study shows that determining trophic relationships and interconnectedness between organisms within a particular ancient ecosystem is possible.