Mass extinction Study Guide

📖 Core Concepts Mass extinction – A rapid, geologically brief interval (< 5 Myr) in which ≥ 50 % of species disappear. Background extinction rate – The “normal” loss of taxa between crises; 2–5 marine families · Myr⁻¹. Press‑pulse model – Long‑term ecological stress (“press”) plus a sudden catastrophic event (“pulse”) together drive a mass extinction. Adaptive radiation – Rapid diversification of surviving lineages into vacant niches after a crisis. Dead clades walking – Survivors that persist but never regain their former diversity. Origination & extinction rates – $p$ = new taxa · Myr⁻¹; $q$ = disappearing taxa · Myr⁻¹. Flood basalt province (LIP) – Vast, rapid eruptions of basaltic lava covering millions of km², releasing CO₂, SO₂, ash, and aerosols. 📌 Must Remember Big Five: End‑Ordovician, Late Devonian, Permian‑Triassic, Triassic‑Jurassic, Cretaceous‑Paleogene. Permian‑Triassic “Great Dying”: 53 % marine families, 84 % marine genera, 81 % marine species lost; 70 % terrestrial vertebrates lost. Cretaceous‑Paleogene (K‑Pg): Chicxulub impact + Deccan Traps; extinction of non‑avian dinosaurs, ammonites, marine reptiles. Background marine family loss: 2–5 families per Myr (baseline for “excess” calculations). Sixth (anthropogenic) extinction: Extinction rates > 1 000 × background since 1900; 1 M of 8 M known species threatened. Key drivers: Flood basalts, bolide impacts, oceanic anoxia/euxinia, rapid sea‑level fall, greenhouse‑gas bursts, climate extremes, human activities. Recovery time: Major events require 15–30 Myr for full biodiversity rebound; vertebrate land recovery after Permian‑Triassic 30 Myr. 🔄 Key Processes Assessing extinction severity Count families/genera before and after the interval. Subtract the expected background loss (2–5 families · Myr⁻¹) to get “excess” loss. Correcting fossil bias Signor‑Lipps effect: Last fossil ≠ true extinction → apparent gradualness. Shareholder quorum subsampling: Sample until a set taxonomic “share” is reached, limiting sample‑size bias. Three‑timer/Two‑timer methods: Track taxa present before, during, after a bin to estimate true rates. Press‑pulse extinction mechanism Press (e.g., long‑term CO₂ rise, sea‑level fall) weakens ecosystems. Pulse (e.g., impact, massive volcanism) delivers the final lethal shock. 🔍 Key Comparisons Impact vs. Flood basalt Impact: Instantaneous dust/ash, shock wave, megatsunamis; e.g., Chicxulub (K‑Pg). Flood basalt: Prolonged CO₂/SO₂ release, climate oscillations; e.g., Siberian Traps (Permian‑Triassic). Global cooling vs. Global warming Cooling: Ice‑sheet expansion, sea‑level drop; major role in End‑Ordovician, Late Devonian, Permian‑Triassic. Warming: Expanded tropical zones, intensified hydrologic cycle, promotes anoxia; key in Triassic‑Jurassic, end‑Cretaceous. Background extinction vs. Mass extinction Background: 2–5 families · Myr⁻¹, steady‑state turnover. Mass: ≥ 50 % species loss in < 5 Myr, rates > 100× background. ⚠️ Common Misunderstandings “All mass extinctions are caused by impacts.” – Only the K‑Pg has strong impact evidence; others involve volcanism, anoxia, sea‑level change, or combos. “Recovery is quick after any extinction.” – Full ecosystem complexity can take tens of millions of years, especially after the Permian‑Triassic. “Sea‑level fall alone kills species.” – It is usually a symptom of broader drivers (global cooling, tectonics) that together reduce habitat area. “The ‘Big Five’ are discrete categories.” – They sit on a continuum of extinction intensity; earlier Cambrian events can be equally severe proportionally. 🧠 Mental Models / Intuition Press‑pulse “two‑hit” model – Picture a stressed athlete (press) who then slips on a banana peel (pulse); the combination leads to collapse. Ecological “vacuum” – After a mass extinction, think of a cleared stage; any surviving “weedy” species can quickly take the spotlight (adaptive radiation). Bias “fog” – Fossil record is like a foggy mirror; methods (Signor‑Lipps, quorum subsampling) clear the fog to see true extinction spikes. 🚩 Exceptions & Edge Cases Pre‑Phanerozoic extinctions – Poorly recorded; the Oxygen Catastrophe (2.45 Ga) is the only well‑documented early event, but its comparability to later mass extinctions is uncertain. Cambrian/early Ordovician spikes – Can match or exceed “Big Five” percentages, but low baseline biodiversity makes absolute species loss smaller. Clathrate gun events – Proposed for end‑Permian and PETM; carbon‑13 negative excursions are the geochemical signature, but direct evidence remains debated. 📍 When to Use Which Choosing a severity metric – Use family‑loss percentage for broad marine trends; use genus‑loss when finer resolution is needed (e.g., Late Ordovician). Bias correction method – Apply shareholder quorum subsampling when sample size varies widely across intervals; use three‑timer when you have well‑resolved stratigraphic ranges. Attributing cause – If a sharp iridium layer + shocked quartz = impact (K‑Pg). If massive LIP coincides with CO₂ rise, sulfur isotopes, and anoxia = flood‑basalt–driven. If rapid sea‑level fall + glaciation markers = cooling‑driven. 👀 Patterns to Recognize Dual‑pulse extinction – Many events (Late Ordovician, Late Devonian) show two closely spaced loss spikes. Anoxia + warming – Nutrient runoff → algal bloom → decay → O₂ depletion; look for δ¹³C negative shift + trace‑metal (Se) anomalies. “Dead clade walking” – Presence of a lineage in the fossil record after the event but with dramatically reduced diversity. Rapid biodiversity dip followed by “disaster taxa” – E.g., Lystrosaurus dominance after Permian‑Triassic. 🗂️ Exam Traps Mistaking “background” for “mass” rates – Remember the background is only a few families · Myr⁻¹; any answer implying millions of species lost per Myr is a distractor. Confusing sea‑level fall with sea‑level rise – Most mass extinctions are linked to falls (habitat loss), not rises. Assuming every LIP caused a mass extinction – Only five of the “Big Five” coincide with LIPs; not every flood basalt leads to a crisis. Over‑generalizing the “impact hypothesis” – Only the K‑Pg event has strong impact evidence; other events rely on multiple lines of evidence. Mixing up “global cooling” vs. “global warming” drivers – Cooling is tied to glaciation & sea‑level drop; warming drives anoxia, clathrate release, and expanded tropics. --- Use this guide for rapid recall before the exam – focus on the bold facts, the cause‑effect pairings, and the signature patterns that most often appear in multiple‑choice stems.