Despite what people might think, fossils are not evenly distributed on the Earth’s surface. That might make intuitive sense – surely not everywhere can produce dinosaurs, mammals, fish, ammonites and trees, but even where fossils are found the numbers can be strongly skewed towards some kinds and away from others, and not necessarily in the way one might imagine. This is intended as only the sketchiest review of the major factors that affect the frequency with which some animals are fossilised and some are found (the two not necessarily being correlated) but is well worth knowing about.
These biases do exist, but of course they are incredibly hard to quantify – how do you tell how much more of some species you should have, or if there is probably a large predator missing from the fossil record? Even if you have a comparable extant fauna, there may be some aspect you have missed that drastically changes the fauna and makes the comparison far more subjective than you realise. Thus these tend to be only relative, there IS a bias towards fish in the fossil record, but how much of one, nobody knows. None the less these must be kept in mind when examining fossil faunas and looking at palaeoecology and biogoegraphy as it is all too easy to think that the fossil record accurately represents the fauna that it sampled when clearly this is likley to be far from the case.
Since this is the archosaur musings, I’ll try and stick to archosaurian examples and effects, though of course these points are largely applicable to any fossil organism and a few are applicable to the fossil record as a whole rather than individual beds or formations.
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