When I was an undergraduate, I think one of the most startling things I learned was how few prehistoric animals we actually know about.
Like many kids, I grew up with books on dinosaurs and other prehistoric animals, and I was captivated by the panoramic artistic recreations of the Cretaceous landscape, with lumbering triceratops and T. rexes, and pterodactyloids gliding overhead (and always, for some reason, with a smoldering volcano in the background). It was my evolutionary biology professor who blew all this away.
Fossilization, he said, is ridiculously rare. It takes a significant series of very unlikely events to result in a fossil at all, much less one that could last 66-plus million years. The deceased organism has to land in, or be covered by, sediments; it can't be eaten up or otherwise destroyed by animals. The sediments it's encased in have to be undisturbed long enough to harden into rock, then that rock has to avoid erosion and the other geological processes that eventually degrade most of the rocks the Earth produces.
Then, that surviving fossil-bearing rock has to be found by scientists.
So we're basing our picture of prehistoric landscapes upon a random sampling of a very small number of species. It is, my professor said, like someone tried to put together a picture of the modern landscape using only the remains of a mouse, a maple tree, a deer, a sparrow, a bullfrog, and a great white shark.
The situation may not be quite that bleak, but it's not far off. For every one pre-Cretaceous-extinction organism we know about, there are likely to be ninety-nine we have no record of. Which is why even after a couple of hundred years of serious fossil-chasing, we still have surprises awaiting.
Take, for example, the discovery of a fossil in Chile that was so weird that for a while, paleontologists had reconstructed it as an entirely different animal. It was a tail that had sharp plates on either side -- clearly some kind of defensive weapon. The plates put the researchers in mind of the stegosaurus:
The spiky tail of the stegosaurus is called the thagomizer -- which came, I kid you not, from Gary Larson's iconic The Far Side, specifically the one with some cave men looking at a diagram of a stegosaurus. One of them is pointing to the tail, and says, "And this is called the thagomizer, after the late Thag Simmons." The name stuck, and the thagomizer it's been ever since.
Well, when the paleontologists looked at the new fossil, they realized that the thagomizer on this puppy was in a class by itself. This thing could have chopped a T. rex off at the knees. But further analysis of the rest of the skeleton showed that it wasn't a stegosaurus relative at all; it was a type of ankylosaur, a group of tank-like dinosaurs, most of which had tails ending in clubs (formidable enough weapons in and of themselves)."It's a really unusual weapon," said Alex Vargas, of the University of Chile, who co-authored the paper on the find this week in Nature. "Books on prehistoric animals for kids need to update and put this weird tail in there. ... It just looks crazy."