Phytoliths, microscopic plant silica bodies, inform us about the types of plants that remain in the fossil record, allowing for reconstruction of past flora, climate, and ecosystems. The existing method for reconstructing vegetative structure and habitat type (open vs. closed) using phytoliths depends on the presence of presumed open-habitat grass types. This method may not be able to reliably reconstruct vegetation type when open-habitat grass phytoliths are absent, and depends on the assumption that open-habitat grasses have always been restricted to open environments. Therefore, a taxon-free, morphological approach may be more informative for inferring habitat openness using fossil phytolith assemblages. This study looks at the effects of light variation on epidermal cell shapes in modern grasses as a first step in creating a proxy for determining habitat type from the fossil record. Based on patterns observed in dicotyledonous plants, we hypothesize that epidermal grass cells will be more undulated when grown in reduced lighting condition. To test this, five species of grasses were grown in varying light treatments (20%, 60%, 100% (control), and 120% relative to the control). We took morphometric measurements of long and short cell types from epidermal peels and calculated their degree of undulation (UI). Results show three species where long cells had higher UI in deep shade (20% light) than in enhanced light (120% light), though the relationship is non-linear. With short cells only one species displayed a continuous increase in UI as light levels decreased. Thus, data from long cells show the greatest promise for use in the fossil record. To further test this method for paleoecological work, the UI of epidermal cells in soil phytolith assemblages from known modern habitat types will also need to be examined to see whether this UI and light correlation persists under natural growth conditions.

The Middle-Miocene Climatic Optimum (MMCO) was a period of rapid warming that occurred 15 million years ago. The climate change likely affected plant community composition as grasses spread across North America, but because of the scarcity of plant macrofossils from the MMCO in North America, the response of vegetation in the area to this climate change is poorly understood. Many modern crops are C3 and C4 grasses (distinguished by their photosynthetic pathway), and thus, gained understanding of the response of forest vs. C3 and C4 grasslands in deep time may help us predict the possible response of modern plants to our changing climate. Plant silica microfossils (phytoliths) are abundant in fossil soil layers however, and can be used to reconstruct the biotic response to the warming event. Polled Herefords is a site in southwest Montana that has easily accessed fossil soil layers from the MMCO that are phytolith rich. Phytolith assemblages from previously collected samples are currently being analyzed. Preliminary results show on average 2/3 – 3/4 of the phytolith assemblage in a given sample is grass silica short cells, diagnostic of grasses. The remainder of the sample is made up phytoliths indicating palms, monocotyledons, dicotyledons, conifers, and other non-grass types of plant. This suggests a fairly open habitat, and a high abundance of C3 grasses within the grass indicators indicate a climate that was warm and wet. More samples will be collected to increase the spatial and temporal resolution of the study, and to help determine if changes in vegetation reflect spatial or temporal variability, or both.

The Middle-Late Miocene (~11 million years ago) was a period of cooling following the Middle Miocene Climatic Optimum, the warmest period in the last 35 million years. However, because of the scarcity of fossil floras of that age, our understanding of vegetation response to this climate change is poor. The Middle-Late Miocene fossil flora from Vasa Park therefore adds invaluable information that allow us to better reconstruct how the vegetation and climate has changed over time in the Pacific Northwest. Previous work on the Middle-Late Miocene in this region has focused on macrofossils and pollen/spores; this study will add information about phytolith assemblages. Phytoliths form when silica from the ground is deposited within and around various types of plant cells. When the plant dies, the phytoliths from the plant gets preserved in sediment, retaining the shapes they had in the living plant. Our understanding of phytolith production in modern day plants and their climate preferences allows us to infer, from fossil phytoliths not just what kinds of plants they represent, but also something about the climate at that site. Specifically, Vasa Park will help us understand the flora of the Middle-Late Miocene in the Pacific Northwest and how it transitioned out of a very warm time into cooler current day temperatures. Based on our knowledge of the general climate during the Middle-Late Miocene, the Vasa Park assemblage should contain evidence of a more warm-adapted flora than what we see currently in the region. Indeed, preliminary evidence from phytoliths, macrofossils (leaves and flowers), and fossil palynomorphs (plant spores and pollen) indicate that the area was a relatively warm subtropical forest. The inclusion of the Vasa Park phytolith assemblage greatly adds to our understanding of Earth’s most recent warming period and helps us infer how vegetation will change with current warming trends.

Understanding modern climate changes hinges upon our ability to assess past climates. While relatively few analogs to present climatic shifts are represented in recent geological history, the Paleozoic Era (542-251 Ma) may offer more. Investigating such transitions in the more distant past though will require expansion of existing techniques. Several methods use fossil leaf morphology to infer past climatic conditions. One such approach, DiLP (Digital Leaf Physiognomy Approach) uses leaf margin serration (toothiness) and leaf shape to infer past mean annual temperature. This technique has proven successful in using modern plant communities to estimate regional climates but can only assess past temperatures as far back as 120 Ma since it currently only utilizes leaves of recently-evolved flowering plants. However, other, more ancient groups of vascular plants have also evolved leaves with toothed margins. With toothed leaves and a fossil record exceeding 350Ma, lycopsids (clubmosses and allies) and ferns have potential to extend paleotemperature assessments back to the Late Paleozoic. To determine whether climate (in particular, temperature) influences leaf shape in living representatives of these lineages, three species of lycopsid (Lycopodium clavatum, Lycopodiella alopecuroides, and L. appressa) and two species of fern (Polystichum munitum and Blechnum brasiliense) were cultivated in a growth chamber under two temperature regimes: 15°C and 25°C, each trial lasting five months. Several leaves were selected from each specimen, laminated on overhead transparencies, and scanned for digital measures using a modified DiLP measuring protocol. While quantitative leaf measurements remain ongoing, preliminary qualitative assessment has shown that lycopsids radically change their growth habit when grown under different temperature regimes. If leaf margin toothiness or shape changes significantly in the leaves of these plants in response to temperature, fossil representatives of these ancient lineages could potentially be implemented to better assess climatic changes long-preceding the age of flowering plants.

Many plants, as part of their growth process, accumulate groundwater-derived silica that is deposited into inter- and intracellular spaces, forming what are known as phytoliths. Due to a combination of preservational and diagnostic characteristics, phytoliths found in the fossil record serve as a powerful paleontological tool for the reconstruction of past environments and climates. For this study, rock samples containing phytoliths were collected from a site called Everson Creek in southwest Montana, once part of the Great Plains region of the United States, which spans in age from 25.3 to 24.5 million years and corresponds to an interval of warming worldwide. The samples have been processed for phytoliths using heavy liquid flotation, whereby silica bodies are separated from other material by means of their unique density. Currently, the phytoliths are in the process of being counted and divided into various diagnostic groups according to morphology. The results of these counts, when viewed in temporal order throughout the site, will shed light on the floral and climatic history of the region based on the prominent plant groups and their growth preferences. This research, which covers a time interval previously unstudied for phytoliths aims to clarify the timing of the rise to dominance of open-habitat grasses in the region, a subject that is currently disputed in the literature. The spread of grasslands is an important event in plant history because it represents the development of open habitats and is intimately linked to the expansion and evolution of various other biota. Since grasses are large producers of highly diagnostic phytoliths, any change in their ecological role will easily be seen in this study.