Where Are The Dinosaurs?
Western culture has popularized extinct animals, especially the very large land-based dinosaurs (the larger the better, or the more teeth the better). Of course, the only evidence we have of these creatures' existence are the fossils---fossils which, according to carbon dating techniques are tens to hundreds of millions of years old.
Some sixty-five million years ago though, the fossil profile tells us an interesting story about something major that must have taken place. Essentially, all non-flight-capable dinosaurs disappeared. There are numerous theories about what must have happened, although the dominant theory, namely meteor impact, has seen favor after the discovery of a "smoking gun" crater in Mexico which matches the time period of the extinction.
What dinosaur extinction does teach us is that far-reaching extinction events are possible. As we'll see---these events have taken place more than once on earth and by extension, we can expect future E.L.E's (Extinction Level Events), possibly even in our own lifetimes.
Even the mighty T-Rex was no match for a large rock from space.
450 Million Years Past: "O-S" Extinction Event
Consensus on just how many extinction events have happened in earths past has not been reached. What is generally agreed-to though is that there are at least five fairly well-defined events (some have suggested as many as twenty).
Around about 450 million years ago, the so-called Ordovician-Silurian event occurred; regarded as being the second most calamitous event (if we're considering the limit-of-five events hypothesis). Although the types of life around at that time were generally limited to corals and prehistoric shellfish, a significant number of species died off as a result of what is speculated to have been an ice age.
450 million years ago, the biosphere looked somewhat different to today---effectively populated by corals and molluscs.
370 Million Years Past: Late Devonian Extinction
This one is a bit of a mystery when it comes to cause--multiple theories abound, from severe and prolonged volcanic activity to the usual suspect: an unusually large meteor striking the earth. What we do know is that this extinction consisted of several extinction "pulses", each one possibly having a different cause. Also, this extinction lasted quite a long time (millions of years) and affected only marine life. During this time, life had started to appear on land in the form of plants and insects---land dwellers appeared to weather this event fairly well.
250 Million Years Past: End Permiam Extinction Event
This is the largest of the "big five" and has been aptly dubbed "the great dying". That's because over 90% of marine life and 70% of land-based vertebrates died in this event. It's also the only known extinction event where considerable numbers of insects died.
Unfortunately, scientists are still puzzling over the possible causes of the End Permiam Extinction, and there are numerous candidates including bolide collisions (aka meteors), enormous volcanic activity and methane release causing a runaway greenhouse effect.
200 Million Years Past: End Triassic Extinction Event
This event saw significant numbers of creatures of several varieties dying off, ironically though, dinosaurs appeared to have been spared (only to die another day).
As has tended to be the case with other extinction events, scientists are still not sure what caused this event, but the normal candidates apply: climate change, volcanic eruptions or meteor impact.
Massive volcano activity has been implicated in E.L.E's.
65 Million Years Past: K-T Extinction
After the End Triassic Extinction, dinosaurs were effectively completely dominant due to a disproportionate bias towards other species dying off; it almost feels like they were being set up because this is the extinction event that has become famous as the one that wiped them out.
Towards the end of the last century proof-positive was delivered as to the cause of the K-T event, namely a large meteor hitting the earth. A crater approximately 180km in diameter was discovered under Chicxulub, Mexico which dated positively to the same time.
Of course, the fact that these catastrophic events took place in the past can only mean that our expectation should be that we'll see one sometime in the future. If you look at the time-lapses between the big five E.L.E's, we'd be forgiven for relaxing a little---it's probably a good couple million years (at least) before we're due for the next one.
Never-the-less, the chances of us being hit by a meteor big enough to cause significant worry aside, the fact is that this is the sort of thing over-which we have very little control. As humans we tend to think that we've achieved marvelous things and that we are masters of our destiny, but certain natural events are simply beyond us to prevent or to repel with our present technology---our best option is then to develop a means of adapting to the circumstances after the event.
If you think that stories like those portrayed in Deep Impact and Armageddon are nothing more than fiction, know that it happened and it will happen again.