Climate - Extinctions

The evidence for glacial deposits is doubtful, even though much of the present environment is of Devonian origin. It is also clear that polar ice caps, if they existed, were much smaller than they are today. This suggests that the Earth was warmer in the Devonian than it is today.

Devonian coral growth lines show that the Devonian year was about 400 days long. The lunar cycle was about 301/2 days, one day longer than today.

Hypoxic or anoxic sediments were common in the Devonian oceans. Some of these are known to have been periods of significant extinction, and all are associated with some form of faunal anomaly in the marine strata.

A strong connection between the Devonian extinctions and the environment is the black shale layer characteristic of hypoxic conditions. It is thought to be related to environmental stresses caused by global warming, which has reduced the rate of mixing between the surface and deeper layers of the ocean.

The possibility of extinctions due to abrupt warming and cooling has also been raised. In the Late Devonian, in particular, extinction may be due to the rapid cooling associated with the development of glaciers and a significant drop in sea level.

At present, there is no single reason that can definitively explain the Devonian extinctions.