Lecture 14: Extinction, Recovery and Cause - The End-Permian Crisis

Aliases and Identification: - The Great Dying - The Mother of all mass extinctions - The Permian-Triassic (P-Tr) Biotic Crisis
Date and Chronology: - Multiple incidents occurring between $252$ and $250\text{ Million years ago}$ . - Significant peak specifically around $251.902 \pm 0.024\,\text{Ma}$ .
Impact Metrics: - Taxonomic Impact: Extreme; greater than $80\%$ of multicellular genera became extinct. - Phylogenetic Impact: Very high, involving many extinctions at the order and class levels. - Functional Impact: Very high. - Geographic Extent: Full global extent, affecting both marine and terrestrial realms.
Victims by Realm: - Marine: Widespread extinction among metazoans, particularly in shallow water invertebrate records. The fate of other eukaryotes and prokaryotes remains unknown. - Terrestrial: The extent is notoriously difficult to determine due to an incomplete record. However, many lineages forming the trophic base of forest biomes disappeared.
Disappearance of Major Forest Biomes: - Gondwana: Loss of the Glossopterids. - Euramerica: Loss of Walchian conifers. - Sub-Angara: Loss of Cordaites. - Cathaysia: Loss of Arborescent lycopods.

Macroflora Analysis: - Represented by fossilized plant parts that disarticulate and disperse. - Coverage is typically local with poor spatial and temporal range. - Good for determining botanical affinity. - Primarily represents environments like wetlands, riverine, and coastal regions. - Limitations: Not optimal for detailed analysis of extinction and recovery rates.
Microflora (Pollen and Spores) Analysis: - Better spatial and temporal coverage as they represent regional vegetation. - Most producers of pollen and spores are known, aiding in reconstruction. - Synchronicity: Plant community collapse is found to be synchronous with the first pulse of marine extinction at $251.9\,\text{Mya}$ .

Northern Hemisphere observations (Norway, Pakistan, Tibet, Greenland): - General sequence involves initial collapse, a slight recovery, a second collapse, followed by gradual recovery. - The "Seedfern-interregnum" and the dominance of lycopod spores are key markers.
The Case of Pleuromeia sternbergii: - An isoetalean lycophyte standing approximately $120\,\text{cm}$ tall. - Found globally after the crisis. - Characterized as an opportunistic colonizer of empty, nutrient-poor landscapes following massive soil erosion. - Described as "bad competitors" that thrive when other vegetation is absent.
Abnormal Spore and Pollen Development: - Findings of malformed Klausipollenites and Alisporites pollen in China and Russia. - Foster and Afonin (2005) Hypothesis: Malformations were caused by atmospheric pollution, increased levels of UV-B radiation, or a combination of both.
Gondwana Transitions: - Glossopteris was the dominant woody tree in southern Gondwana (tongue-shaped leaves with net-like venation, reproductive structures on modified leaves) but went extinct. - Post-extinction, Dicroidium and Voltzian conifers appeared. - Dicroidium migrated from Euramerica into Gondwana. - All conifers except the Voltzians went extinct; Triassic conifers evolved in Euramerica and spread worldwide.

Recovery Timeline: It took between $5$ and $14\,\text{Ma}$ to re-establish plant communities equivalent to pre-extinction ecosystems.
Successional Stages: - Move from Glossopteris-dominated or Conifer-dominated floras to Pleuromeia-/lycopsid-dominated crisis floras. - Eventual return to diversified floras (Conifer-fern-lycopsid dominated).

Extraterrestrial Hypotheses: Bolide (asteroid/comet) impacts.
Earth-Bound Hypotheses: - Climate change. - Sea level changes. - Ocean anoxia. - Flood basalt volcanism. - Acid rain.
The "Murder on the Orient Express" Scenario: The hypothesis that there may not be a single killer, but rather one ultimate driver (volcanism) responsible for a cascade of different global ecological disruptions.

Description: Prolonged, massive volcanic and subvolcanic activity caused by a mantle plume (upwelling of abnormally hot rock) under the Siberian Craton.
Scale of the Siberian Traps: - Covers approximately $5,000,000\,\text{km}^2$ . - Emplacement of up to $4,000,000\,\text{km}^3$ of basalts (equivalent to burying California under a layer $9,400\,\text{m}$ thick).
Timing of Volcanism: - Main activity pulses: $252.4$ to $251.3\,\text{Ma}$ . - Final emplacement: $250.6 \pm 0.22\,\text{Ma}$ .
Phases: Initially mostly extrusive (lava flows), followed by mostly intrusive (sills) phases.
Atmospheric Emissions and Consequences: - Primary Emissions: $CO_{2}$ , $SO_{2}$ , heavy metals, and halogens. - Direct Effects: Long-term rise in global temperature (interspersed with short volcanic winters), ocean acidification, ocean warming, and acid rain. - Secondary Effects: Soil erosion leading to nutrient runoff and subsequent ocean anoxia; metal poisoning. - Sill Intrusion Impact: Sills burning through coal, organic shales, and salt layers released massive amounts of greenhouse gases and halogens. - Ozone Destruction: Halogen and $SO_{2}$ injections repeatedly destroyed the ozone layer, leading to high UV-B radiation levels.

UV-B Stress on Plants: - Experiments with extant Pinus mugo and Isoëtes howellii show that high UV-B levels cause malformed pollen/spores similar to those seen in the End-Permian record. - UV-B stress causes plants to visibly suffer, utilize sunscreen compounds, and exhaust energy reserves. - While spores/pollen may remain viable, the seed cones often die before pollination, leading to reproductive failure.
Tree Ring Evidence: - Stress is visible in latest Permian conifer wood found within the earliest flows of the Siberian Traps.
Isoëtes howellii findings: - Plant structures consist entirely of sporophylls (no strobilus at the top). - Sequence: First megasporophylls form, then microsporophylls.

Environmental changes are directly linked to volcanic emissions from the Siberian Traps.
The extinction was stepwise in both terrestrial and marine realms.
Critical perturbations included global warming, oceanic anoxia, acidification, acid rain, and metal poisoning.
The destruction of the ozone layer and resulting UV-B radiation is identified as the primary global mechanism that would affect plant life worldwide, leading to reproductive failure and the collapse of forest biomes.