Burial

Sooner or later bones become either destroyed or buried. If they aren't turned into somebody's lunch, destruction can come from weathering; eventually, the minerals in the bones break down and the bones disintegrate. But the game becomes interesting for paleontologists when

Figure I. I. 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 10-15 years. Bones destined to become high-quality fossils must be buried soon after the death of the animal. (Photograph courtesy of A. K. Behrensmeyer)

Figure I. I. 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 10-15 years. Bones destined to become high-quality fossils must be buried soon after the death of the animal. (Photograph courtesy of A. K. Behrensmeyer)

the bones are buried. At this point, they become fossils (the bones, not the paleontologists). The word "fossil" comes from the latin word fodere (to bury), and refers to anything that is buried. There is no implication of how much time the remains have been buried; a dog burying a bone is technically producing a fossil. 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 sediment left by an organism.

Burial can take several forms. The simplest type of burial is when a bone or accumulation of bones is covered by sediment. For example, in a desert, burial might occur when a sand dune migrates over another, covering anything that was there before: desert lore resonates with mysteries of shifting sands relentlessly burying all who passed through. Equally inevitably, floodwaters also bury; ask the unhappy homeowner whose basement was silted as the waters receded. A more subtle type of deposition can occur, however, when sediments are reworked, which means that they are actively eroded from wherever they were originally deposited, and redeposited somewhere else.

Rivers are notorious reworkers. They flow, well behaved, within the confines of their channels. But in storms, they can breach their channels, flooding the landscape and eroding the edges of the channel. The eroded material from the channel margin is carried within the channel or spewed out with the floodwaters onto the floodplain. Rivers practice equal-opportunity erosion: if any buried bones are within the eroded part of the channel margin, they too will be swept along wherever the floodwaters see fit.

Here is where a paleontologist can be fooled. Seeing bones in a river channel, he or she might interpret these to be the remains of organisms that died together. But reworking concentrates fossils within the confines of the channel, and a collection of fossils in a channel is not necessarily the remnants of a community that actually lived (and died) together. Instead, it might be a reworked assemblage of bones that includes fossils eroded out of a much older floodplain, mixed with material of the same age as the channel. In the upper Great Plains of North America, 65 million-year-old dinosaur bones have been found jumbled with deposits of 10,000-year-old bison bones: as glaciers melted 10 thousand or so years ago, stream channels of glacial meltwater eroded 65 million-year-old floodplain sediments and mixed the bones of Mesozoic dinosaurs and ice-age mammals.

So paleontological work is not just digging up old bones. We want to know how the bones got the way in which we find them, because that may tell us something of how dinosaurs lived. Different types of concentration of bones can come about through different processes (or can be explained by different scenarios). When the bones are articulated (connected), this suggests that they have not been transported far from where the animal died. The idea here is that none of the destructive forces we described above has had much effect on the fossils that we have found. On the other hand, if we find a collection of disarticulated bones of several types of vertebrates, we can be fairly sure that the deposit has been reworked, and that the bones got there through sedimentary processes sometime after death and initial burial. Then there are the bonebeds: accumulations of bones of many individuals of a very few kinds of organism, sometimes articulated and sometimes not. A bonebed with one or two species may represent a herd subjected to a catastrophic event; alternatively, if we could determine that the bonebed accumulated over a long period of time, then it might simply represent a location where many animals of the same type chose to live (and die). Finally, isolated finds - a thigh bone here or a vertebra there - could represent almost any of the possibilities that we described above. In Figure 1.2, two taphonomic sequences, leading to two different results, are shown.

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