Slashdot linked up news of the "world's deepest" dinosaur:
While most nations excavate their skeletons using a toothbrush, the Norwegians found one using a drill. The somewhat rough uncovering of Norway's first dinosaur happened in the North Sea, at an entire 2256 metres below the seabed. It had been there for nearly 200 million years, ever since the time the North Sea wasn't a sea at all, but an enormous alluvial plane.
Which brings up a rather interesting point about paleontology. We don't necessarily find, say, dinos mostly in the North American west and hominids in east-central Africa because that's where they lived. We sometimes find them there because that's where the geologic deposits laid down when they lived are exposed to the surface.
In other words, critter A may be rare in the fossil record because critter A was rare. But it may also be that critter A is rare in the fossil record because, while you couldn't swing a dead trilobite without hitting one in swampy marsh A, the only place you can actually dig around in is the remains of swampy marsh C, where they were relatively rare.
In some cases this is not a big hairy deal. Rocks from, say, the Jurassic are eroding out in several different places all over the planet, so it's possible to get a decent sense of populations densities for the various dinos we've discovered so far.
Not so, unfortunately, for other time periods. The one I most remember from college was the Miocene, important to us anthropology undergrads because that's when apes evolved. Miocene apes are quite rare in the fossil record. This might be because there just weren't all that many of them. However, the only deposits accessible which have ever had any Miocene apes in them are eroding out of the Himalayas, an area which isn't particularly close to the Euro-African forests in which they are presumed to have evolved.
So, were they genuinely rare, or are we just not able to look in the right place? Could there be thousands of undiscovered fossils, perfectly preserved, thousands of feet underground, all over the planet?