After burial

Bone is made out of calcium (sodium) hydroxyapatite, a mineral that is not stable at temperatures and pressures at or near the surface of the earth. This means that bones can change with time, which in turn means that most no longer have original bone matter present after fossilization.

Replacement and/or permineralization

Nearly complete specimen exposed

Replacement and/or permineralization

Nearly complete specimen exposed

Permineralisation

Isolated bones buried and mineralized

Figure 1.2. Two endpoint processes of fossilization. In both cases, the first step is the death of the animal. Some decomposition occurs at the surface. In the upper sequence (a), the animal dies, the carcass undergoes quick burial, followed by bacterial decomposition underground, and permineralization and/or replacement. Finally, perhaps millions of years later exposure. Under these conditions, when the fossil is exhumed, it is largely complete and the bones articulated.This kind of preservation yields bones in the best condition. In the lower sequence (b), the carcass is dismembered on the surface by scavengers and perhaps trampled and distributed over the region by these organisms. It may then be carried or washed into a river channel and buried, replaced and/or permineralized, eventually to be finally exposed perhaps millions of years later Under these conditions, when the fossil is exhumed, it is disarticulated, fragmented, and the fossil bones may show water wear and/or the gnaw marks of ancient scavengers, Different conditions of fossil preservation tell us something about what happened to the animals after death.

Isolated bones buried and mineralized

Figure 1.2. Two endpoint processes of fossilization. In both cases, the first step is the death of the animal. Some decomposition occurs at the surface. In the upper sequence (a), the animal dies, the carcass undergoes quick burial, followed by bacterial decomposition underground, and permineralization and/or replacement. Finally, perhaps millions of years later exposure. Under these conditions, when the fossil is exhumed, it is largely complete and the bones articulated.This kind of preservation yields bones in the best condition. In the lower sequence (b), the carcass is dismembered on the surface by scavengers and perhaps trampled and distributed over the region by these organisms. It may then be carried or washed into a river channel and buried, replaced and/or permineralized, eventually to be finally exposed perhaps millions of years later Under these conditions, when the fossil is exhumed, it is disarticulated, fragmented, and the fossil bones may show water wear and/or the gnaw marks of ancient scavengers, Different conditions of fossil preservation tell us something about what happened to the animals after death.

This is especially likely if the bone is bathed in the variety of fluids that is associated with burial in the earth (e.g., ground water). If, however, no fluids are present throughout the history of burial (from the moment that the bone is buried, to when it is exhumed by paleontologists, a time interval that could be measured in millions of years), the bone could remain unaltered, which is to say that original bone mineralogy remains. This situation is not that common, and is progressively rarer in the case of older and older fossils. Unaltered bone, however, is obviously crucial for studies involving aspects of bone mineralogy (see Chapter 15); for example, the discovery of genuine red blood cells from Tyrannosaurus required unaltered bones.

Most bones, as we have suggested, are altered to a greater or lesser degree. Since bones are porous, the spaces in fossil bones fill up with minerals. This situation is called permineralization. More significant than permineralization is recrystallization, where the bone itself (made of hydroxyapatite) is dissolved and reprecipitated, retaining the exact original form of the fossil. Recrystallization can be very obvious - for example, when small crystals are replaced with large ones - but it can also be very subtle, and occur on a microscopic scale. Bones can also be replaced, in which case the original bone minerals are replaced with other minerals.

Nothing is simple, and, in general, fossil bones undergo a combination of replacement, permineralization, and recrystallization. The resultant fossil, therefore, is a magnificent natural forgery: chemically and texturally unlike the original bone matter, although commonly retaining its exact shape and most delicate features. Still, accept no substitutes: fossil bones tend to be much heavier (they're permineralized) than their living counterparts and are more brittle. In virtually every respect, most fossil dinosaur bone is really closer to rock than to the living inorganic and organic mix of materials that we call bone.

In general, the more quickly a bone is buried, the better the chance it has of being preserved. This is because quick burial generally inhibits the weathering processes that would normally break down the bone minerals.3 In fact, it is not uncommon to find evidence of weathering before fossilization. For example, dinosaur bones that appear to have been transported in water show water-wear that is exactly the same as those found in modern bones that have been transported by water: they are rounded and commonly the surface of the bone is partly or completely worn off. In the case of such dinosaur bones, the bone was transported by flowing water shortly after the animal was alive and was buried water-worn. The resultant fossil perfectly preserves the water-wear.

So if the fossils are buried, how is it that we find them? The answer is really luck: if fossil-bearing sedimentary rocks happen to be eroded, and a paleontologist happens to be looking for fossils at the moment that one is sticking out of an actively eroding sedimentary rock, the fossil may be observed and may be collected. Does this mean that great numbers of fossils lie buried within sedimentary rocks that happen not to be eroding at the earth's surface? Undoubtedly. Have fossils been eroding out of rocks since the very first fossil was formed? No question. Are important fossils currently eroding that will never be collected? No doubt.

3 An exception to this is when a bone ends up buried in an active soil; under certain conditions, the bone is then destroyed by biotic and abiotic soil processes.

Paleontology has been pursued in an active and systematic fashion only for the past 150 years; however, as A. Mayor has noted in her fascinating book, The First Fossil Hunters,4 it was not unknown in classical antiquity. This means that, although fossils with hard parts have been produced for 540 or so million years (and trace fossils have been produced for even longer), most of those few that happened to be fossilized in the first place and then happened to be exposed at the earth's surface were never collected: they eroded away with the rest of their host rock.

Dinosaurs first appeared about 228 million years ago, and all but birds went extinct 65 million years ago or earlier. We may be sure that, throughout the 160 million-year existence of dinosaurs on earth, their fossils were constantly eroding out of sediments, being weathered away and lost for eternity.

The odds are stacked against fossilization. And, although many fossils are found, it is clear that most creatures - even those with hard parts - that have lived on earth are not preserved. Some paleontologists estimate that 25% of all the types of dinosaurs that ever lived have been found as fossils. This may be true but then again it may not. Who knows what percentage of all the dinosaurs that ever lived we presently know? The fossilization process is fraught with opportunities for fossils not to be found by that uniquely fossil-crazy group of primates (ourselves) that has been systematically seeldng them for the past 150 or so years.

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Responses

  • cornelia
    When a dinosaur was buried by soil, what eventually replaced the bones to make a fossil?
    9 years ago

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