The Cambrian Period lasted from 541
to 485 million years ago. At the beginning of this time period, the small
unicellulars that represented most life on Earth became more complex and
multicellular. They also diversified quite rapidly, bringing forth the first
representatives of all modern animal phyla. Indeed, there is strong evidence
that all animals evolved from a single common ancestor.
Life prospered in the oceans, but
the land is thought to have been relatively barren, with nothing more complex
than a microbial soil crust or biofilm. A few molluscs may have emerged to
browse on that biofilm, but the continents were probably dry and rocky. The global
supercontinent Pannotia had just broken up during the early part of the period,
and the new continents were mostly flanked by shallow seas, which were
relatively warm. Polar ice was absent for much of this period.
Most land masses were clustered in
the Southern Hemisphere during this period, but were drifting north. During the
early portion of the Cambrian, the supercontinent of Gondwana went through some
large, high-velocity rotational movements.
Trilobites (I wrote about them in an
earlier post) were rampant during the Cambrian period. Possibly this was
because without any sea ice, the sea level was high, which meant large areas of
the continents were flooded in warm shallow seas, which is ideal for sea live.
But the sea levels did fluctuate somewhat, suggesting there were ‘ice ages’,
possibly meaning pulses of expansion and contraction of a south polar ice cap.
Although the beginning of the period was cold, the average temperature during
the Cambrian was 7° Celsius warmer than today.
Even so, trilobites were not the
dominant species, as was once thought. It seemed they were, because they had
hard external shells that were easy to fossilize, much easier than the thin
chitinous shells of other arthropods, and so trilobite fossils were much easier
to find by today’s paleontologists.
The Cambrian period is often
referred to as ‘the Cambrian Explosion’, indicating a huge increase in the
variety and diversity of life forms. But it seems (to me) that it might be
better to think of it as ‘the Period of Great Changes’. At the start of the
Cambrian, new creatures with new behaviors and lifestyles destroyed the biofilm
that covered the sea floor, so all the creatures (from the previous time
period) who depended on that biofilm died out.
Around 515 million years ago, the
number of species dying out was larger than the number of new species coming
into existence. 500 million years ago, the oceans saw a big drop in the oxygen
content, and at the same time, the level of toxic hydrogen sulfide increased.
Either of these events alone could produce extinctions, so imagine what
happened when they came in together.
Where would hydrogen sulfide have
come from? There are a few ways nature makes it, including anerobic digestion
by certain biofilms in the absence of oxygen. I can’t rule that one out, but
I’m somewhat more inclined to ‘blame’ volcanoes, which also produce it,
probably in larger quantities and certainly can do it in oceans. Also, the heat
given off by the volcano(es) would tend to drive oxygen out of the water. So,
was there a series of huge volcano events 500 million years ago? I don’t know.
It seems possible.
And there we have the Cambrian
period in a nutshell. No fascinating dinosaurs to study, but the thought of a
spinning Gondwana certainly has my attention.
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