Mass extinction

Assembled from 13 blocks · 1 evidence · 13 related

1. Story
2. A mass extinction is conventionally defined as a loss of a large fraction of species in a geologically short interval, across many unrelated groups and across a wide geographic range. Each clause is doing work. Extinction is continuous — the background rate never stops — so what distinguishes a mass extinction is rate and breadth rather than the fact of loss. "Many unrelated groups" excludes the collapse of a single clade, and "wide geographic range" excludes a regional catastrophe. The measurement is harder than the definition. Percentages quoted for species loss are estimates reverse-engineered from counts of genera or families in a fossil record that is spatially uneven, taxonomically inconsistent and biased toward shelly marine animals — which is why the figure for the end-Permian has been revised from around 96 per cent of marine species on the classic 1979 analysis to around 81 per cent on a 2016 reanalysis without any new fossils being required. The "Big Five" grouping itself comes from a statistical treatment of the marine record and is a useful convention rather than a natural category; some workers argue the Late Devonian belongs in a different class, being a failure of speciation more than a spike of extinction.
3. Knowledge
4. Mass extinction
5. Connections
6. Radiometric dating
7. The end-Permian mass extinction
8. The Cretaceous–Palaeogene mass extinction
9. The end-Ordovician mass extinction
10. The Late Devonian crisis
11. The end-Triassic mass extinction
12. Evidence
13. Supports the general Precambrian narrative used here: stromatolite biogenicity reasoning, banded iron formation requirements, endosymbiotic origin of organelles and the framing of mass extinction as a measurement problem. Published in 2003, so it predates the 2016, 2018 and 2024 results also cited in this pack. V55 verification basis: this session had no network access to any source — WebFetch was egress-blocked and the WebSearch budget was exhausted — so the cited work was not retrieved and its pagination was not re-checked.