In contrast, cold-adapted animals living in Arctic waters fared quite well. Much
of Gondwana was in the Antarctic during the Devonian, and seas flooded broad
areas of the continent. The Gondwanan fauna, which lacked reef builders and
other warm-water species, survived the extinction with few losses.
The oldest species living in the world’s oceans today thrive in cold waters.
Many Arctic species, including certain brachiopods, starfish, and bivalves, belong
to biological orders whose origins extend hundreds of millions of years back to
the Paleozoic. In contrast, tropical faunas such as reef communities, battered by
periodic mass extinctions, have come and gone quite rapidly on the geologic
time scale. However, not all animals that shared the same environments such as
corals and mollusks were identically affected by the extinctions.
A possible cause for the end-Devonian extinction was the bombardment
of Earth by one or two large asteroids or comets.The meteorite impact theory
is supported by the discovery of deposits containing glassy beads called tektites
in the Hunan province of China and in Belgium. Tektites form when a large
meteorite strikes Earth and hurls droplets of molten rock into the air that
quickly cool into bits of glass (Fig. 99). The deposits also include an unusually
high iridium content, which strongly indicates an extraterrestrial source. The
Siljan crater in Sweden, about the same age as the tektites, might be the source
Figure 99A Nor th
American tektite found in
Texas in November
1985, showing surface
erosional and corrosional
features.
(Photo by E. C.T. Chao,
courtesy USGS)DEVONIAN FISH