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.
Hard to miss in the field - a Brachiosaurus humerus
2. Where did they live? A rainforest is a terrible place to die if you want to enter the fossil record, in a stagnant lake with a lot of silt is superb. Just living close to a body or water massively increses your preservational potential so marine crocodiles are great, arboreal snakes are bad.
3. When did they live? The longer a fossil has lain in the earth, the longer it has had to be eroded, or destroyed. Recent fossils are far more common than those from the Carboniferous.
4. Were they young or old? Adults have better ossified bones than juveniles and thus preserve better, having more minerals to mineralise during fossilisation.
5. Common or rare? Did the living population number in the millions or dozens.
6. Location in the modern world. No one is going prospecting in Zaire or North Korea anytime soon no matter how good the fossil beds.
7. Any shed body parts? Theropods went through teeth constantly that can enter the fossil record separately to the body of the animal so for every living adult theropod there were probably a few hundred to a few thousand teeth that could be buried and found, but a toothless oviraptorsaur can only leave a sinlge skeleton behind – they might have been present in equal numbers, but you are far more likely to find evidence of one than the other.
8. Any large ossified parts. Large chunks of armour, tail clubs, scutes and plates are all large bony pieces that can enter the fossil record in addition to the main skeleton. This is largely a function of the above point (and number 1) but take an aetosaur and a prosauropod of equal sizes and the former has two or three times the number of bones when you take all the armour pieces into account, and they are nice robust ones too.
9. Any biases from bone destruction? Ankylosaur tail clubs were probably never eaten by theropods, and juveniles are far easier to destroy entirely so they never enter the fossil record.
10. How much has the clade been studied? Theropods get far more research attention than do stegosaurs (prportionally and absolutely), how many have been missed when excavating in the field (deliberately or accidentally) or those that have been dug up misidentified?
11. Is there a shell or skeleton or nothing? Invertebrates are of course typically far more common than vertebrates (think of insects and worms!) but the octopus fossil record is all but non existant.
12. Any special circumstances? Predators can dominate a fossil bed if there is a predator trap, and whole herds of herbivores can be caught in natural disasters and appear to be the only thing there as a result.
Well that’s all I can think of for now, though there are probably soem more (both minor and major) that I have forgotten or just don’t know about. I am very much a zoologist working on dead things than a proper palaeontolgoist or geologist so these things are a little out of my way. Still, hopefully this gives an idea of what to look for. In short a nice big, adult, armoured, marine crocodile that sheds its teeth regularly and lived in the thes of thousands just a few thousand years ago in northern Europe will have an amazing fossil record, but a small, juvenile, scale-less lizard that lived in grasslands 300 million years ago in Colombia will not be described anytime soon, if it was ever even preserved.
These biases are all real and do affect what we find and how we treat them and should be remembered. Just becuase you have a whole bunch of large theropods and little else does not mean it was a predator dominated environment, you might be missing something crucial like a prey item that did not fossilise, or a seasonal one that left when the rains came, that would completley skew the makeup of the ecosystem as it appears to you.
Dave Hone's Archosaur Musings 
Dave,
Your post is on topic, because as a layman, I have been wondering lately about the Jehol Biota – specifically how despite excellent preservation of smaller animals, there isn’t much in the way of larger fossils, except for some fragmentary titanosaur remains and Jinzhousaurus.
Obviously amber, for example, has a preservation bias in favor of small animals, but is it possible to have a lesser preservation bias in favor of smaller animals even with regular sediment?
For example, there is no fossil evidence — or, indeed, physical evidence of any kind — for late-surviving boneless aquatic pterosaurs feeding on blue whales.
And let us not speak of secondarily scansorial plesiosaurids. Like early arborial squamates, if there were any, we know nothing whatever about them, rumors about “treetops nessie” notwithstanding.
Yes Karl there are two obvious possible biases against large animals in the Jehol (though as you note there are a few) that spring to my mind:
1. They might have been genuinely absent. As ever tricky to quantify or even qualify, but there are of course places where few large animals live such as dense forests (yes you still get elephants, but compare the number and diversity of large bodied mammals between the African rainforests and the Massai Mara and it’s obvious where you should go looking.
2. Secondly the actual point of preservation may limit the size (rather like your amber example). The vast majority of fossils are the result of burial in water, but if the Jehol only had small bodies of water (like lots of ponds and streams, but no rivers or lakes) it’s going to be difficult for sauropods to get covered, but relativly easy for something Microraptor sized.
There might even be a taphonomic bias against large size if large chunks of calcium and other bits of bones disrupt the fossilisation process, or tend to crumble when smaller one survive (that’s jsut my speculation, I really don’t know my taphonomy). There could also be a collection bias since most Jehol fossils are collected by local farmers and they may not recognise large bones when a whole fossil bird is quite obvious, or they might even be keeping them hidden and eating them as dragon bones. There really is quite a variety of ways to manipulate the fossil record in this way, and the trick is to spot them and try to work out how they might be influencing what you see.
Rock exposure is important. Fossils in rocks which are currently at the surface in deserts or types of rocks commonly quarried will more likely be found. Most rock is buried under other strata or covered in regolith and the fossils in it will never be found (any time soon).
Please. This is all nonsense.
Wow, what a great argument. Based on evidence, logic and research. Or not of course. I believe the phrase is “That which can be asserted without evidence, can be dismissed without evidence”
Feel free not to post random statements utterly unsupported by anything other than your ignorant conjecture.
Hey Dave,
I’m curious about that big humerus in the photo – is it actually Brachiosaurus or Giraffatitan? It looks a bit slim to be from B. altithorax.