Extinct Dinosaurs

There were other mass extinctions besides the one that killed the dinosaurs.

The Ordovician-Silurian extinction event - 440-450 million years ago, killed about 85% of all species

The Devonian extinction - 360-375 million years ago, about 70% of all species went extinct

The Permian-Triassic extinction - about 251 million years ago, about 96% of all marine species and 70% of land species went extinct

And of course the Cretaceous-Tertiary extinction - about 65 million years ago, about 75% of all species went extinct including the dinosaurs

Among a couple other extinction events: http://www.newworldencyclopedia.org/entry/Mass_extinction

Also here are some other examples of what may cause extinction events.

A comet is not what would have killed all dinosaurs but the effect an impact may have, the evidence for it is in the geologic record.

Impacts can be simulated, on much smaller scales of course, by scientists to get an idea of the effect that they may have.

Not to mention the direct observation we got from Jupiter’s collision with comet fragments (travling at 60 km/s) Jupiter’s gravity caused the comet to break up into fragments up to about 2 km across and even still the collisions left explosions that were nearly the size of Earth (generally speaking)

The scars left by the impacts were easily visible and persisted for many months.