189-22 Evidence for a C3 PACMAD-dominated Grassy Ecosystem in Late Miocene Central Australia

Session: Paleontology, Paleoecology/Taphonomy (Posters)

Poster Booth No.: 107

Presenting Author:

Benjamin Lloyd

Authors:

Lloyd, Benjamin A.¹, Strömberg, Caroline A.E.², McInerney, Francesca A.³, Arman, Samuel D.⁴, Couzens, Aidan M.C.⁵, Prideaux, Gavin J.⁶

(1) Earth & Space Sciences, University of Washington, Seattle, WA, USA, (2) Biology, University of Washington, Seattle, WA, USA, (3) CSIRO, Adelaide, South Australia, Australia, (4) Museum and Art Gallery of the Northern Territory, Alice Springs, Northern Territory, Australia, (5) Biological Sciences, Flinders University, Adelaide, South Australia, Australia, (6) Biological Sciences, Flinders University, Adelaide, South Australia, Australia,

Abstract:

Australia’s C₄ grassland, shrubland, and savanna ecosystems cover the vast majority of its land area, providing crucial habitats for endemic plants and animals, underpinning indigenous connection to the natural world, and anchoring Australia’s agricultural sector. To best understand the trajectories of these ecosystems, it is integral to understand the climatic, evolutionary, and ecological forces that drove their assembly. Floral, faunal, and geochemical records capturing the development of central Australian open habitats are rare, but marine sediments (ODP site 763A) show an increase in grass (Poaceae) pollen at 5.3 Ma, followed by a leaf wax n-alkane δ¹³C signal of increasing C₄ biomass beginning at 3.5 Ma. We present a preliminary phytolith record from the ~8 Ma Alcoota bone bed, a well-studied central Australian vertebrate locality in the Northern Territory. This record is the first paleobotanical data in direct association with vertebrate fossils of this age in Australia, and provides new insight into a time of significant ecosystem change. The assemblage is overwhelmingly dominated by grass phytoliths, with only a small fraction of woody dicot phytoliths, indicating a locally open, grassy habitat. Grass phytoliths are primarily of the PACMAD clade—which includes all modern C₄ grass lineages and many C₃ grass lineages. These preliminary data show that grasslands appeared in central Australia earlier than observed in marine sediment palynological data, and before the widespread expansion of C₄ photosynthesis, suggesting regional heterogeneity in C₃ grassland expansion. This pattern follows that seen on other continents, where C₃ open habitats developed before the expansion of C₄ biomes, and indicates that C₃ grass-dominated biomes were present in Australia for at least 4.5 million years before C₄ grasses reached dominance. Beyond constraining the timing of Australian grassland expansion, this phytolith assemblage offers a unique opportunity to link floral data to a marsupial herbivore fauna. Marsupials adapted to arid ecosystems in ways that are both similar and distinct from placentals, but the timing of these changes relative to floral change are unclear. Our new paleobotanical record helps constrain the ecological pressures shaping the evolution of Australia’s unique fauna.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11236

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