EXTINCTIONS

The Late Devonian Extinctions

~372 to 359 million years ago

Phanerozoic · Paleozoic · Devonian

The Late Devonian extinctions began about 372 million years ago, when two long episodes of seawater without oxygen, the Kellwasser events, destroyed the reef ecosystems.

The Late Devonian extinctions are two pulses of extinction thirteen million years apart. The earlier pulse, 372 million years ago, destroyed the reef ecosystems. Seawater became anoxic, that is, lost its oxygen, in two prolonged episodes known as the Kellwasser events, the first of about ninety thousand years and the second of a hundred and ten thousand. Their duration matches the slow swing of the Earth's axial tilt, the angle at which the planet's axis of rotation leans. That swing alters the contrast between seasons and with it the mixing of the ocean. The later pulse, at the boundary with the Carboniferous, the period that follows the Devonian, was the heavier one for vertebrates, the animals with a backbone. More than half of all vertebrate diversity disappeared, in fresh water as completely as in the sea, so that no habitat served as a refuge.

Why it matters

The groups that dominated the Devonian scarcely recovered, first among them the placoderms, armoured fishes. The groups that had been marginal, ray-finned fishes, sharks and four-limbed vertebrates, took their place and have kept it since. Survival in these extinctions, and not any particular aptitude, determined which lineages reached the continents.

Dating & uncertainty

The extinctions were staggered over several million years. Da Silva and colleagues (2020) date the first pulse, at the Frasnian-Famennian boundary between the last two stages of the Devonian, to 371.87 million years ago, to within 0.11 million. The second follows around 359 million years ago. The kill mechanisms remain disputed. Researchers propose oceanic anoxia, volcanic outpourings and nutrient runoff from the new forests.

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