When I first started reading the article on the Ordovician Period, I got to the mention of Gondwana, which made me stop and wonder, "Wait, haven't we been through this breakup before?" And the answer is yes, we have, in the blog I wrote about the Paleozoic Era, which the Ordovician Period is part of. I had to remind myself that this is not really déjà vue, that the Paleozoic Era has 6 periods to it, and my blog on the Era would have been like an overview, while the blogs on the periods would have more details. So, some of the big events, like the breakup of Gondwana, will be mentioned in both posts.
The Ordovician Period spans 41.6
million years, from the end on the Cambrian Period some 485.4 million years ago
to 443.6 million years ago.
Life continued to flourish during the
Ordovician Period, although there was an extinction event at the end of the
period. The Ordovician Period is known for its biodiversification event, which
considerably increased the diversity of life. Invertebrates, namely molluscs
and arthropods, dominated the oceans, although fish, the world's first true
vertebrates, continued to evolve, and fish with jaws may have first appeared
late in the period. Life on land had yet to diversify.
However many meteorites strike the
Earth in a year today, there were 100 times that many hitting the Earth per
year during this period.
The southern continents were collected
into Gondwana, which started the period in equatorial latitudes but then
drifted toward the South Pole. Meanwhile, other continents, Laurentia (part of
North American), Siberia, and Baltica (northern Europe), were drifting north,
and Baltica started moving towards Laurentia later in the period. Another small
continent, Avalonia, separated from Gondwana and began moving north towards
Baltica and Laurentia.
Temperatures were mild in the early
and middle Ordovician Period, but from 460-450 million years ago, volcanoes
along one of the oceans spewed massive amounts of carbon dioxide (a greenhouse
gas) into the atmosphere, turning the planet into a hothouse. These volcanic
island arcs eventually collided with proto-North America and formed the
Appalachian Mountains. [At last! I've been waiting for billions of years for
the Appalachians to appear!]
Initially, sea levels were high, but
as Gondwana moved south, ice accumulated into glaciers and the sea levels
dropped. At first, low-lying sea beds increased diversity, but later glaciation
led to mass extinctions as the seas drained and continental shelves became dry
land. By the end of the period, the volcanic emissions had stopped. By then,
Gondwana had neared the South Pole and was largely glaciated.
Reef-forming corals first appeared
early in this period. Land plants probably evolved from green algae, first
appearing in a form resembling liverworts. Fungi was also an early adopter of
living on land, and facilitated the colonization of land by making mineral
nutrients available to plant cells.
This period closed with a series of
extinction events that are generally regarded as one major event, in which 49%
of all fauna died. It is generally agreed that this event or series of events
were caused by an ice age. That ice age had several pulses of
increasing/decreasing glaciation. Each time the glaciation increased, the sea
level dropped, killing many of the fauna that inhabited the shallow seas. When
the sea levels rose during the next decrease of glaciation, there were entire
families of fauna that had not survived to re-establish themselves in the
shallow seas. This may have happened several times, producing a series of
extinction events.
Well, now we're starting to get some
place. Plants (of sorts) on land, starting to make the soil arable. Not sure
about the carbon dioxide level in the atmosphere, but the meteor showers would
be beautiful, as long as they didn't land too close. Hunting probably wouldn't
do much good yet, but, hey, there's fish! Too bad I can't stand fish.
Maybe the next period will be even
more amenable?
https://en.wikipedia.org/wiki/Ordovician
