EXTINCTIONS

Late Devonian Extinctions

~372 to 359 million years ago

Phanerozoic · Paleozoic · Devonian

Two long spells of oxygen-free seawater, ninety and a hundred and ten thousand years each, paced by the wobble of the Earth's orbit, destroyed the reef ecosystems at 372 million years ago. The heavier blow for backboned animals came thirteen million years later: at the Devonian boundary more than half of all vertebrate diversity was lost, in fresh water as thoroughly as in the sea, so there was nowhere to shelter.

Why it matters

The groups that had ruled the Devonian, armoured placoderms above all, barely recovered. The groups that had been marginal, ray-finned fishes, sharks and four-limbed vertebrates, radiated into the space and have held it ever since. Which lineages walked onto land was decided here, by who survived rather than by who was best at it.

Dating & uncertainty

A protracted crisis rather than a single event. The Frasnian-Famennian boundary is dated to 371.87 ± 0.11 Ma and a second pulse follows near 359 Ma; the kill mechanisms, from ocean anoxia to nutrient runoff from the new forests, are still contested.

Sources

1. Da Silva et al. (2020), Scientific Reports · Anchoring the Late Devonian mass extinction in absolute time by integrating climatic controls and radio-isotopic dating
2. Sallan & Coates (2010), PNAS · End-Devonian extinction and a bottleneck in the early evolution of modern jawed vertebrates
3. Racki (2020), Global and Planetary Change · A volcanic scenario for the Frasnian-Famennian major biotic crisis and other Late Devonian global changes
4. Zheng et al. (2023), Earth and Planetary Science Letters · Mercury isotope evidence for recurrent photic-zone euxinia triggered by enhanced terrestrial nutrient inputs during the Late Devonian mass extinction

Earth history as one day

22:02:021 h 58 min before midnight

if Earth’s 4.54 billion years were compressed into a single day