Making body fossils

Before burial. Consider what might happen to a dinosaur - or any land-dwelling vertebrate -after it dies (Figure 1.1). Carcasses are commonly disarticulated (dismembered), often by predators and then by scavengers ranging from mammals and birds to beetles. As the nose knows, most of the heavy lifting in the world of decomposition is done by bacteria that feast on rotting flesh. Some bones might be stripped clean of meat and left to bleach in the sun. Others might get carried off and gnawed. Sometimes the disarticulated remains are trampled by herds of animals, breaking and separating them further. So the sum total of all the earthly remains of the animal will end up lying there: a few disarticulated bleached bones in the grass.

Figure 1.1. Bones. A wildebeest carcass, partly submerged in mud and water and on its way to becoming

permanently buried and fossilized. If the bones are not protected from scavengers, air, and sunlight, they decompose rapidly and are gone in high-quality fossils must be buried soon after the death of the animal.

10-15 years. Bones destined to become

If the animal isn't disarticulated right away, it is not uncommon for a carcass to bloat, as feasting bacteria produce gases that inflate it. After a bit, the carcass will likely deflate (sometimes explosively), and then dry out, leaving bones, tissues, ligaments, tendons, and skin hard and inflexible.

Burial. Sooner or later bones are either destroyed or buried. If they aren't digested as somebody's lunch, their destruction can come from weathering, which means that the minerals in the bones break down and the bones disintegrate. But the game gets interesting for paleontologists when weathering is stopped by rapid burial. At this point, they (the bones, not the paleontologists) become fossils. A body fossil is what is produced when a part of an organism is buried. We distinguish these from trace fossils, which are impressions in the substrate left by an organism. Figure 1.2 shows two of the many paths bones might take toward fossilization.

After burial. Bone is made out of calcium-sodium hydroxy apatite, a mineral that weathers easily. This means that, after fossilization, many bones no longer have original calcium-sodium hydroxy apatite present. This is especially likely if the bone comes into contact with fluids rich in dissolved minerals, such as commonly occurs after burial. 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.

Ancient, unaltered bone - and even tissue - do exist, and are crucial for our understanding of the growth of bone tissue (see Chapter 12) and other soft anatomy (for example, the discovery of genuine red blood cells and connective tissues from Tyrannosaurus; see Chapter 9, footnote 3 and Chapter 10).

Most bones are altered to a greater or lesser degree. Since bones are porous, the spaces once occupied by blood vessels, connective tissue, and nerves fill up with minerals. This situation is called permineralization (Figure 1.3). Bones can also be replaced, in which case the

Replacement and/or permineralization

Nearly complete specimen exposed

Quick burial

Replacement and/or permineralization

Nearly complete specimen exposed

Quick burial

Isolated bones buried and mineralized

Isolated bones exposed

Dismemberment before burial scavenging and other natural processes

Isolated bones buried and mineralized

Isolated bones exposed

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, there is exposure. Under these conditions, when the fossil is exhumed, it is largely complete and the bones articulated (connected). 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. The remains 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.

Figure 1.3. Permineralized bone from the Jurassic-aged Morrison Formation, Utah, USA. The fossilized bone is now a solid piece of rock.

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, there is exposure. Under these conditions, when the fossil is exhumed, it is largely complete and the bones articulated (connected). 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. The remains 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.

original bone minerals are replaced with other minerals, retaining the exact original form of the fossil. Most fossil bones undergo a combination of replacement and permineralization. The resultant fossil, therefore, is a magnificent natural forgery: chemically and texturally not bone, but retaining the exact shape and delicate features of the original bone.

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