56-3 3-D Morphology Of Fossil Phytoliths Shows That The Earliest Grasslands Of North America And Western Eurasia Were Dominated By Cold-Season Grasses

Session: Phylogenetic and Computational Approaches in Paleobiology and Paleoecology, Part II

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Abstract:

With ~4,000 species, Pooideae is the largest subfamily of grasses (Poaceae). It is thought to have radiated in the northern hemisphere from the Paleocene onward, and its members dominate northern temperate- and high-altitude grasslands thanks to the unique tolerance of many pooids to prolonged periods of freezing. Published fossil phytolith assemblages recording Earth's earliest grassy habitats (Oligocene-early Miocene) are interpreted as consisting of a counterintuitive mix of these cold-adapted grasses and taxa (e.g., palms, spiral gingers) typical of milder climates than most Pooideae occupy today.

To better understand the ecology of Earth’s first grass-dominated habitats, we seek to more precisely constrain the taxonomic and ecological affinities of Eocene-Miocene pooids. To do so, we combine quantitative analysis of the 3-D morphology of modern and fossil grass silica short cell phytoliths (GSSCPs) with phylogenetic comparative analysis of Poaceae climate niches. For this analysis, we created 2,183 3-D GSSCP surface models, from 43 pooid- and 50 other grass species, using confocal microscopy. Models were analyzed through 3-D geometric morphometrics, and we used supervised classification algorithms to develop a framework in which we taxonomically placed ~400 fossil GSSCP in Eocene-Miocene samples from the US Great Plains and Turkey. The climate niches of taxonomic groups used to classify these phytoliths were then reconstructed using ancestral state reconstruction (ASR) to determine the likely climatic affinities of pooids before and during the spread of grassy habitats in these regions.

Our results indicate that pooid grasses were likely present since the late Eocene and confirmed that they were dominant in the earliest grasslands. However, 3-D morphology could not reliably resolve taxonomic affinities below the Pooideae subfamily level. Further, ASR shows that cold tolerance is ancestral to Pooideae, suggesting that the observed, Eocene-Miocene pooids were similarly cold-adapted. The non-analog coexistence of palms, gingers, and cold-adapted grasses during the Cenozoic therefore points to environmental conditions unlike those present today. Alternatively, results suggest that living relatives may be a poor indicator of ecological niches of the grasses which dominated Earth's earliest grassy habitats.

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

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