The end-Permian mass extinction (~252 Ma ago) drastically altered ecosystems worldwide, not just in the oceans, but also on land. The effects were so profound that it was not until 4-5 million years into the Triassic that the ecosystem of what was then the eastern part of equatorial Pangea, showed signs of recovery. The flora responsible for this recovery appears to have invaded European habitats after their Early Triassic origination elsewhere. Hitherto, no direct evidence exists of where the refugional source areas of the immigrants could have been located. Here, we suggest that North America could be a potential source area for European Triassic conifers. This body of work aims to advance the plant fossil record across the Permo-Triassic boundary in this geographical region.

The first part of chapter one focusses on our current understanding of the potential causes of the end-Permian biotic crisis. While there may be a singular cause that set off a cascade of events, the intricate nature of the Earth’s system makes it difficult to identify the specific drivers behind the ecosystem collapse. It appears that many factors have contributed to some degree to the biotic crisis. The second part of this chapter is focused on the effects the biotic crisis had on life on land, particularly on plants and the data that we have available to study these extinction patterns. Analysis of the data that is currently available through various online databases, suggests that the information is insufficient to adequately study extinction patterns.

The second chapter is a literature review of the role of silica in plants. Silica is absorbed by plant roots in the form of monosilicic acid, which originates from the dissolution of crystalline silicate and weathering of biogenic silicates in the soil. Silica absorption, once thought to be a passive process, has been shown to be driven by active transport mechanisms. Depending on where the silica is deposited in the plant, the crystalized shape—the phytolith—may adopt a shape unique to that plant cell or tissue. However, before it crystalizes, the silica plays an important role in the plant’s defense mechanisms. Once crystalized, the opaline silica continues to beneficial; it not only reduces biotic and abiotic stress, it also appears to provide structural benefits.

Chapter three examines the Permian and Triassic phytolith record and its potential to support (bio)stratigraphic investigations. In Caprock Canyons State Park and Palo Duro State Park in Texas, the Permian-Triassic boundary is well-preserved and the stratigraphy has been resolved in detail. The phytolith assemblages recovered from the Permian and Triassic strata appear to have limited potential for regional biostratigraphy. However, these phytoliths do form the first evidence of plant life at the western portion of the supercontinent Pangea, a region previously believed to be entirely barren.

The Mesozoic era, spanning ~252 to 66 million years ago (Ma), was an important time period in the early evolution of the major plant lineages that dominate modern ecosystems. The origins of the seven living conifer families (e.g., cypresses, pines) can be traced back to at least the beginning of the Jurassic (>200 Ma). They were a diverse and prominent part of global vegetation throughout most of the Mesozoic, and the living members of these families still thrive in many areas today. The flowering plants appeared in the Early Cretaceous (~135 Ma) and rapidly diversified and rose to dominate most lowland warm ecosystems by the time the dinosaurs went extinct. The rise of flowering plants transformed plant communities into their more modern form, as forests once dominated by conifers and other groups became dominated by flowering plants. This body of work addresses three different aspects of the evolution of plants and ecosystems since the Mesozoic.

Chapter one examines the evolution of conifer dispersal strategies from the perspective of the structural adaptations and functional morphology of conifer diaspores (the seed plus other dispersing structures), Seed dispersal is an important process that shapes the distribution of plants and the biotic interactions within ecosystems. This chapter examines macroevolutionary patterns in conifer dispersal ecology by mapping diaspore characteristics across the conifer phylogeny and evaluating the frequency and direction of transitions in dispersal strategies. These analyses reveal multiple examples of convergence and divergence in dispersal strategies, particularly for seed dispersal by wind and animals. The inferred ancestral dispersal characteristics are evaluated in the context of the Mesozoic fossil record, which demonstrates that dispersal strategies were varied and complex even during the early evolution of conifers.

Chapter two investigates the early evolution of the Cupressaceae (cypress family), a diverse and ecologically important conifer family with pan-hemisphere distribution. The early diverging lineages of the family were once diverse and important components of Mesozoic ecosystems, but are now represented by relatively few species with disjunct distributions (e.g., redwoods). The fossil history of the Cupressaceae is investigated through the description of an Early Jurassic (~179 Ma) species from Patagonia, Argentina based on foliage, pollen cones, and seed cones. This species is one of the oldest records for the family based on multiple organs, and provides context for evaluating the early structural evolution and biogeographical history of the family.

Chapter three focuses on the evolution of plant community structure during the ecological expansion of flowering plants. After their origination in the equatorial region, flowering plants increased in diversity as they spread towards higher latitudes. The timing of their ecological takeover of communities by abundance, or biomass, and the resulting changes in community structure have been less well understood than their diversity increases. This critical period in plant evolution is investigated by reconstructing a 74.6 Ma fossil forest from south-central New Mexico, which existed in a moist-wet megathermal (= "tropical") climate. The fossil flora is one of the most diverse known from leaf macrofossils, and is the oldest flora for which the ecologically dominance of flowering plants has been demonstrated across a floodplain. In contrast to modern forests in similar climates, conifers still formed a major component of the canopy in the fossil flora. This non-analog flora provides novel insights into geographic and temporal patterns in the ecological turnover of plant communities, and the early development of angiosperm-dominated forests in warm-wet climates.

The end-Permian biotic crisis, occurring ~252 million years ago (Ma), was the largest extinction event in Earth history and serves as an important case study for understanding how biodiversity crises unfold. During this event, palynological sections across the planet feature abundant pollen and spore abnormalities coinciding with decline of arborescent seed plants and insurgencies of spore-bearing lycopsid-dominated floras. These fossil trends are hypothesized to signal deteriorating atmospheric conditions—specifically elevated solar ultraviolet B [UV-B] radiation exposure resulting from volcanogenic deterioration of the stratospheric ozone layer. This body of work integrates three experimental and observational studies of modern plants to determine whether historic ozone weakening events could have induced end-Permian palynomorph abnormalities and vegetation turnovers.

Chapter one investigates whether pollen malformations and forest recessions in the end-Permian fossil record could result from past ozone weakening events. Many of the seed plant lineages affected by the crisis produced the same ‘winged’ (saccate) pollen type as some modern conifer groups, such as pines. However, the effects of elevated UV-B exposure on pollen development, survival and reproductive capacity in modern conifers are not understood. This chapter experimentally evaluates whether end-Permian modeled UV-B regimes induce pollen malformations, heightened mortality and/or reduced fitness in the modern pine, Pinus mugo ‘Columnaris.’ The results of this study demonstrate that elevated UV-B exposure not only induces the same types of pollen malformations seen in the end-Permian fossil record, but also could have played a role in causing forest recessions by causing premature death of seed cones—rather than killing—ancient seed plants.

Chapter two examines whether the lycopsid lineage [isoetaleans] that dominated end-Permian and early Triassic crisis assemblages may have had a competitive advantage over seed plants under an ozone weakening scenario. Modern quillworts, Isoëtes, represent the closest living relatives and functional counterparts to Late Paleozoic isoetaleans, however the effects of elevated UV-B exposure on their survival and growth are unknown. This chapter experimentally tests whether Isoëtes howellii could survive, grow, and produce spores under end-Permian modeled UV-B regimes. Although irradiated I. howellii survived and were stunted, they produced mature spores that were potentially viable. Despite experiencing more adverse impacts to their vegetative growth, modern isoetaleans may therefore have a fitness advantage over conifers under ozone weakening scenarios.

Chapter three evaluates how frequently saccate pollen malformations are produced by modern conifers under ‘non-stressed’ growth conditions to better interpret malformations in palynological assemblages. Saccate pollen types are produced by members of two living conifer families—Pinaceae and Podocarpaceae. Although prior studies suggest that malformation frequencies of >3% in fossil yields indicate environmental stress, the variability in their expression amongst ‘non-stressed’ extant conifer lineages is not well understood. This chapter serves as a baseline comparison of the frequency and variability in saccate malformation expression in pollen yields of fourteen conifer genera in cultivation spanning Pinaceae and Podocarpaceae. Malformed grains represent <3% of pollen yields in twelve of the fourteen genera studied and no discernable phylogenetic pattern in malformation expression was detected. These results demonstrate that pollen malformations are rarely produced in ‘unstressed’ conifers, specifically lineages with bisaccate grains. Therefore, malformation frequencies exceeding 3% of bisaccate yields in palynomorph assemblages likely indicate a historical environmental stress