How Complete Is The Fossil Record?

For some species, there is a near perfect fossil record: almost every animal left a fossil. In other cases, we might find a clutch of huge fossilized eggs, but not find fossils of whatever laid them. Still other species left skewed records. A short summary just doesn't do justice to this complicated situation.

At any given time, some portion of the earth is being eroded away. Mostly, high regions erode, and the resulting bits and pieces get washed downhill. That all has to fetch up somewhere, so, some low-lying regions of the earth will be undergoing deposition. Any creature which lives in an erosion area is extremely unlikely to get buried, so we don't find fossils of mountain goats. Any creature which lives in a deposition area can easily be buried, and it may leave a fossil, particularly if the area is anoxic. Unless, of course, it is a jellyfish, with no hard parts. Casts of soft creatures are truly rare. On the whole planet, there are only a dozen-odd important deposits showing the shapes of soft life forms. We only find insect fossils in these deposits, and in amber.

Next, there's the issue of scavengers and rot. Forests have both, plus forests tend to have acidic soil. So, most of our fossils of forest dwellers are because they foolishly tried to ford a raging river. You may have seen nature films on TV of herds of caribou or wildebeeste doing just that. But only animals big enough to migrate try to ford rivers. Foxes and squirrels don't migrate. So, the fossil record of forest dwellers is absolutely terrible, and highly biased towards big herd animals.

You may have heard that volcanic ash can bury creatures. That does happen, near the eruption site, but acid rain from the eruption dissolves bones over an even wider area. The meteorite that killed the dinosaurs caused a worldwide acid rain, so there is a worldwide foot-thick rock layer with no fossils.

I could go on. We found conodont teeth for 127 years before finally finding a fossilized conodont animal. You know about T. Rex, so you probably think we have a lot of them. No, we coined the word Dinosaur 60 years before we found the first T. Rex. By now we have 22 fairly complete skeletons, but only one single footprint. We only have one complete stegosaur and no complete Triceratops. It was estimated in 1998 that half of all known dinosaur species were found in the preceding 20 years. If our luck holds, the species count will eventually double again. Three quarters of the known Cretaceous bird species were discovered in the 1990's. Basically, this was possible because the situation was terrible, and it still is. Apparently 99% of extinct dinosaur and bird species left no fossils whatsoever.

So, if you have questions about the history of (say) small land animals, then the fossil record is a source of deep frustration. The same is usually true if you have questions about any one specific species. But it helps if you are willing to study groups of species. It helps if you don't mind studying plankton or fish or whales. It helps if you want to learn whatever the record teaches, rather than learn a completely detailed history. And in that case, then the fossil record can be superb. A huge broad picture is clear, and hundreds of specific transitions are there to see.

Last modified: 2 July 2006

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