About 66 million years ago, the
Cretaceous period, and the Mesozoic era with it, ended. Like the great Permian
extinction that began the Mesozoic, it too wiped out a large portion of the
life on earth (around 75%). But most notable among these extinctions was the
demise of the non-avian dinosaurs, leaving only the birds to continue into the
Cenozoic era that we live in today.
For well over a century, scientists
have debated about what wiped out these creatures and why none of them
continued on to today. It has only been in the last 20-30 years that we have
begun to cement a firm theory as to what happened (though many alternative
theories do still exist). By looking at the layer of rock known as the K-Pg (or sometimes K-T) boundary, we can look at the moment of extinction in the fossil
record. Below it, are the older periods of time, such as the Cretaceous and the
Triassic before that. Above this line, is the Paleogene, first period of the
Cenozoic era. Within that line, however, exists some rather interesting things,
namely the concentration of iridium a metallic element similar to platinum. It
exists in this layer in much higher concentrations than in the rock above or
below it.
In 1980, a physicist named LuisAlvarez and his son, noticed that this band of odd rock could be found all over
the world. Iridium being an incredibly rare element on earth, it lead
scientists to the idea that the earth was hit by some foreign body, causing
some global catastrophe. This is now known as the Alvarez Impact Hypothesis and
is the most widely held theory as to the cause of the Cretaceous-Paleogene Extinction.
Thirty-five years of science since then, have lead to further evidence to
support this theory. The discovery of the Chicxulub Crater off the coast of
Mexico seemed to be the clincher. All evidence pointed to a large astronomical
object (probably 10 km wide) smashing into the earth with the force of over 2
million nuclear bombs.
But it wasn't just the impact itself
that would have wiped out the majority of life across the globe. While anything
near the site would just be incinerated, the rest of the planet suffered
through years of horrendous side effects. The cloud of dust and debris from the
impact would have blocked out the rays of the sun, killing off plants and
phytoplankton that could no longer create food. This would have worked its way
up the food chain, killing off herbivores who no longer had plants to eat, and
then carnivores who lost things to hunt.
Because dinosaurs made up the greater
bulk of the food chain, they would have been hit the hardest. They were large
animals who needed an enormous amount of food to survive. On top of that, they
would have been more vulnerable to the toxic gases, acid rain, firestorms and
shifting weather brought on by the cataclysm . It would be the smallest of
things, lizards and early mammals, insects of various kinds, and some birds
that would survive by seeking shelter underground, requiring less food, or
being able to make long trips to find suitable shelter and nourishment.
This has been the standard model for
many years. Often focus shifts on the aftermath as to the leading cause of the
long term die off around the world. Sometimes it is a massive global winter,
while other times it is something else. But other theories do still exist that
play down the importance that even the Chicxulub impact had on the event.
The Deccan Traps Hypothesis points
the finger at a massive group of volcanic eruptions leading to a slow die off
as toxic gases and dust filled the air. There is a lot of evidence to support
this theory as well, which has lead many scientists to add it into the larger
Alvarez Hypothesis. It is unlikely any one thing caused the global devastation,
but more likely an incalculable chain of events that coalesced into the event
that nearly wiped out all life on earth once again. It is often these combination
theories that make the most sense for such a wide spread phenomena. Theories of
a severe drop in sea levels, multiple impacts, volcanic eruptions, climate changes and radiation are prevalent. But all could easily work together to
create a really bad 'wrong place at the wrong time' kind of event.
Regardless of the how, scientists do
agree on one thing. The Cretaceous-Paleogene Extinction event saw the end of
the non-avian dinosaurs. We find no fossil evidence of them above the K-Pg
Boundary. It is unlikely that even the smaller non-avian dinosaurs lived beyond
a few hundred thousand years of the event. Evidence found, just outside my
door, in the Red Deer River also points to the fact that these dinosaurs were on the decline in the last 10 million years leading up to the event. As their
numbers paled, a massive ecology altering event would easily wipe the remaining out.
But every story has to have an
ending. Though the event took out all the non-avian dinosaurs, we were left
with the avian ones who survive to this day. Sure we have fossils and other
traces of dinosaurs that grace the exhibits of a thousand museums, but there is
just something awesome about knowing that there are still dinosaurs just
outside your window.
Next Time:
Further Reading
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