Stable Isotope Studies of Vegetation Change and Mammal Evolution in Australia

University Of Utah, Salt Lake City UT

Investigators

Abstract

We propose to study the stable carbon and oxygen isotopes preserved in the tooth enamel of Australia's herbivorous mammals to reveal dietary shifts and environmental changes over the past 24 million years, especially in relation to the spread of the tropical grasses which employ the C4 photosynthetic pathway. As an island continent, Australia's physiographic and climatic histories are unique. Intercontinental biotic migration has been severely constrained by vast transoceanic dispersal distances, which isolated the continent to terrestrial faunal migration for most of the Cenozoic. Superimposed upon this is the fact that most of Australia's endemic mammals are marsupials. They diversified in situ and filled a broad spectrum of niches comparable with placental mammals on other continents. Australia thus presents us with an unparalleled experiment in mammal evolution: an isolated radiation of fundamentally different mammals, adapting to landscape and climatic changes in the absence of competing immigrants. Stable isotope analyses of Australian herbivore teeth will allow us to examine the relative strengths of hypotheses previously advanced for the expansion of C4 vegetation and faunal turnover in the over the past 24 million years. Does Australia fit the global model, and if not, why? The proposal seeks to address the following key issues. 1. Timing of the appearance and expansion of C4 vegetation in Australia compared with the rest of the world, as interpreted from d13C values in the tooth enamel of the three terrestrial herbivore lineages: kangaroos, wombats and diprotodontoids. 2. Relationships between evolutionary trends in Australian Neogene terrestrial herbivores and shifts in prevailing climate/vegetation regimes inferred from enamel d13C and d18O values. 3. Development of "seasonality" in Australian climate since the Miocene deduced from isotopic fluctuations down the profile of ever-growing wombat molars, and relationship to occurrence of C4 plants. 4. Paleoecologies of late Cenozoic Australian herbivores and communities. Critical to tackling these issues is the need to establish modern Australian isotope baselines. We will ascertain diet-enamel enrichment factors for extant kangaroos and wombats, and dietary isotopic variation within species across different seasons and environments. There is no basis for assuming that isotopic offsets between diet and animal tissues established for herbivorous placental mammals will hold for marsupials, especially given their overt differences in developmental biology. Indeed, our pilot data do reveal marked ontogenetic variation along the molar row for several different kangaroo species. Our preliminary d13C data of fossil marsupials also indicate a diet containing C4 plants for animals from only the two youngest of 11 northern, central and eastern fossil localities so far sampled. Both are ~3.4 million years old and, if their C4 signatures can be attributed to grasses, are much younger than those sites in which the earliest appearances of C4 grass diets are observed in Asia, Africa and the Americas. We intend to extend this initial work to cover all of the significant Neogene fossil deposits in Australia to better outline the isotope changes recorded in the teeth of fossil marsupials through time.