UC Berkeley

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