How did dinosaurs arrive on our planet?
The usual assumption is that they evolved here from other (smaller)
species of reptiles over a very long time, some tens of millions of years.
By "evolved" I mean that slowly, each generation of some small reptile
became bigger and bigger on the average, and after millions of years they
were quite huge. If this seems strange to you, rest assured it seemed
strange to most people when it was first proposed. But the idea is
strongly supported by the fossils people have dug up. If you examine these
closely, you can see a reasonably smooth development over a very long time
of the big dinosaurs from earlier, smaller reptiles.
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Update: June 2012