443 – 416 MILLION: SILURIAN

Ordovician–Silurian extinction event 60% of marine genera wiped out;
O₂ 70 % of modern level;
mean surface temperature 3 °C above modern level;
sea level 180m above present day with short-term negative excursions (glacial cause);
Euramerica forms as Gondwana drifts north
Wide shallow seas as Ordovician glaciers melt
Warm, greenhouse climate
Most land is barren, lifeless desert
Life begins to creep onto land – primitive plants, ferns
fungi and myriapoda lead the way, the age of Arthropods.

Roughly 430 million years ago, during the Earth’s Silurian Period, melting polar ice sheets meant sea levels were steadily rising, and ocean oxygen was falling fast around the world.

The Ireviken extinction event devastated scores of ancient species. Eighty percent of conodonts, which resembled small eels, were wiped out, along with half of all trilobites (their distant, modern-day relative the horseshoe crab).

The period’s sea level rise and ocean oxygen depletion led to the widespread decimation of marine species.

Geochemical evidence for expansion of marine euxinia during an early Silurian (Llandovery–Wenlock boundary) mass extinction

Name	Diameter (km)	Age (megayears)	Dating method	Morphological type	Notes
Slate Islands, Lake Superior	32	436 Ma ± 3	Ar⁴⁰-Ar³⁹ melt rock	CONFIRMED Complex	Pseudotachylite dating
High Rock Lake, Manitoba	~5	435 ± 10	Geological dating	PROBABLE Complex
Lac Couture, Quebec	8	425 ± 25	Ar⁴⁰-Ar³⁹ melt rock	CONFIRMED Complex	Submerged central peak
Rock Elm, Wisconsin	6	420–440	Geological dating	CONFIRMED Complex	Youngest exposed rocks