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teeth vs. bone in the fossil record
Someone raised a question a couple of days ago about why teeth should be
found in a particular sediment and no bones. This is purely a function
of the relative survivability of bone vs. tooth and the process by which
bone weathers. I have been studying the process for some time in mammalian
bones. Although they are not entirely analogous, many of the comments below
apply equally well to dinosaur bone.
When an animal dies, the first thing to happen, is that the scavengers come
to remove the flesh and the bacteria begin to do their work to degrade the
organic matter that they can get reach. This means that, assuming the
carcass is exposed the air (subaerially exposed), the "flesh" will be
stripped off within 1-2 weeks. (The time is different in water and depends
on the type of water in which the carcass lands.) If the bones continue
to be subaerially exposed, they are subjected to a number of different processes
that begin to erode the bone mineral:
1. freeze/thaw: likely not an important process in the Mesozoic except at
high altitude or maybe for short periods (a few days/year?) at high latitudes.
2. drying/wetting: very important in all semi-arid, arid, and temperate
environs, or in humid areas with monsoonal climates.
3. formation of salt crystals during drying (and analogous formation of
ice crystals during freezing).
These three processes cause the bone mineral crystals to separate from one
another. Crystals in bones tend to be long and narrow. Therefore, splinters
of bone tend to form that are needle shaped. Water freezing or salt crystals
forming from water in pore spaces (that exist in all bone) between
the crystals initially causes these pores to widen. As they widen, even more
water is available for forming large crystals. The net effect here is that
within a few weeks to a year in most environments, the bone will begin to show
damage, such as long "green" bone fractures that spiral or "step" along the
long axis of the bone crystals. Smaller fractures will appear where the
bone is very thin, especially at the joint ends (articular surfaces), near
the tooth margins (alveolus), and near any natural foramen or other cracks.
Plaquettes (thin pieces of bone that mimic the outer surface shape) will
flake off the surfaces that are covered by the harder, less porous lamellar
or cortical bone (i.e. on shafts (diaphyses) of long (femur, humerus, etc.)
and short (metatarsals, metacarpals, etc.) bones, on irregular bone (pelves,
pectoral bones, skulls, the mandible, etc.) surfaces).
At the same time, several other processes act upon bone:
4. Plant roots can penetrate to widen cracks, or excrete acids to dissolve
bone mineral. This can include boring algae, fungi, etc., which can actually
colonize the marrow cavities. Blue-green algae (cyanobacteria) are
particularly ubiquitous in the marrow cavities near the surface exposed to
sunlight (some 3-4 cm deep below the upper most surface).
5. Animals can trample or chew the bone causing mechanical damage.
6. Animals can ingest the bone and regurgitate it or excrete it, causing
dissolution of bone mineral and organic matter, as well as mechanical damage.
(Insect larvae may do the same damage.)
7. Acids in the soil can dissolve the bone. This process is particularly
common in forest soils in humid regions. Coniferous forest soils are
among the best at producing acid conditions in the modern spectrum.
8. Organic degradation begins to destroy the organic matrix (mainly
collagen) upon which the mineral crystals are supported.
Meanwhile, several processes work to preserve the bone:
9. Secondary minerals will deposit in porosity elements and between crystals.
This is usually calcite, aragonite, dolomite, any one of several Fe-rich
minerals (including vivianite, hematite), or a few Mg-rich minerals.
Some evidence suggest that bacteria may be involved in this process.
Note that silica is not a common secondary mineral at this stage!
10. Organometallic compounds may precipitate in the bone. These are usually
aided by (or maybe actively precipitated by) bacteria.
The upshot of these different processes is that the destructive forces will
win, unless the bone is buried or covered with water within a year or so.
If covered by water, the process may be slowed considerably, especially in
freshwater, but it will continue. Burial that cuts off light, most water,
and severely limits the infauna (algae, fungi, insects, etc.), really
slows the process ensuring the bones survival.
Ok, now for teeth:
Essentially, the same processes work for teeth. Enamel, however, is extremely
dense with almost no space between crystals and almost no pores. Therefore,
it take about 10-100 times as long to produce the same effects in enamel.
Dentine is more porous and can suffer the same problems as bones, but again
it takes longer, usually 2-10 times longer on average. Cementum tends to
be easily destroyed like bone, and so is rarely preserved on teeth in the
fossil record. The slower times for enamel and dentine destruction means
that the preservation activities tend to win the battle.
Therefore, since the first thing to get destroyed is the anchoring bone for
the teeth, and because the teeth are much more durable in the weathering
environment, you get loose teeth much more commonly in the fossil record.
Bonnie Blackwell, firstname.lastname@example.org
Dept of Geology, (718) 997-3332
Queens College, City University of New York, fax: 997-3349
Flushing, NY 11367-1597