Why Teeth Fossilize Better Than Bone

Bonnie Blackwell

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.