TOOTH MORPHOLOGY AND BREAKAGE PATTERNS

        Trace fossils arise by the interaction of a morphological structure on an organism with a substrate. In many cases this substrate is environmental, or abiotic, and is simply an unconsolidated sediment. In other cases (i.e., interactions between species), the substrate is biotic and consists of the tissues of a second organism. In this exercise we will examine both the effects of an organism on the substrate and the reciprocal effect of the substrate on the organism using tooth and substrate models.

        In this exercise we will examine trace fossil formation by looking at the effect of tooth models of three different morphologies on simulated tissues of two different densities. The goal of this exercise is to correlate wear patterns on teeth and substrates with specific tooth movements.

        Damage to a tooth during feeding is referred to as a wear pattern. Several distinctive wear patterns are commonly observed on fossil teeth. Some of these are very faint (= microwear marks) and can only be easily studied with a dissecting microscope. Microwear is restricted to the surface of the enamel and does not penetrate to the dentine. Others are much larger (= macrowear marks) and can be readily observed with the unaided eye. Macrowear marks can be very large and frequently extend into the dentine. There are five basic tooth wear patterns:

In reality, damage to a tooth frequently contains more than one type of wear pattern. For example, many carnivorous animals have teeth with both spall fractures and microfailures.

        Prey tissues also have distinctive trace fossils on them, referred to as bite marks. Soft tissues are rare in the fossil record (except for naturally mummified carcasses), so we will only consider bite marks on hard tissues here. There are four basic types of bite marks on hard prey tissues:

As with wear patterns, bites marks are frequently composites, such as a large gouge or incision that leads to the fracture of the substrate.

Tooth Models

        The fundamental function of vertebrate teeth is to hold and physically degrade food. Ideally, a tooth should be durable, of a shape suited to specific prey tissues and energetically inexpensive to produce. The evolutionary trade-offs between these three criteria lead to the convergent appearance of teeth of similar form in unrelated species feeding on similar foods.

        Teeth are the most durable structures produced by vertebrates and are accordingly the portion of a vertebrate skeleton most likely to survive fossilization. Durability in teeth is produced by using a thin, extremely strong outer covering of enamel over a tough, resilient core of dentine. This composite structure is largely responsible for the durability of teeth, because it combines the best features of each tissue, while avoiding the worst features of each. Enamel, although strong and highly abrasion resistant, is also very brittle. A tooth made entirely of enamel would be prone to shattering on contact with hard prey tissues. Dentine is more elastic than enamel and can absorb greater impacts without shattering. But dentine is not as durable, and wears much more rapidly than enamel. Combining an outer layer of enamel with an inner core of dentine produces teeth that are tough, sturdy and long-wearing.

        Tooth shape is highly correlated with diet. Animals that feed on soft delicate prey have very different teeth than animals that feed on armored prey. Unfortunately, one of the problems of assessing the role of tooth form and function is in mathematically describing tooth shape. Teeth frequently have extremely complex morphologies that are not easily quantified. However, a number of studies over the past decade have shown that relatively simple mathematical models of tooth form can produce surprisingly robust insights into tooth function (the exercise Paleobiology of the Carnivora later in the course will examine this in more detail).

        For a more-or-less conical tooth, tooth shape is governed by a simple proportionality that relates apical stress (= the stress on the tip of the crown) with the radius of curvature for the coronal tip:

From this proportionality it is clear that as the stress on a tooth tip increases, the radius of curvature must increase even more rapidly (as a squared function), to keep the stress at the tooth tip below the point at which the tooth will fracture. Consequently, teeth adapted to deal with tough, durable prey tissues must be short and blunt. Teeth adapted for feeding on soft delicate tissues are not as constrained by this relationship and can have taller, more delicate shapes. Because these teeth have a long, tapered point thay can more easily penetrate soft tissues than blunt, rounded teeth.

        Finally, tooth size, shape and composition are also under strong selective pressure to balance energetic production costs of teeth and other metabolic functions. Clearly, individuals with teeth that are too delicate for the preferred prey risk rapidly fracturing teeth to the point where they are unable to efficiently capture prey. But individuals with massive teeth that are far stronger than necessary are equally disadvantaged, since the extra energy needed to make the teeth can not be used for other functions, such as growth or reproduction. Studies have shown that vertebrate skeletal structures, such as teeth, are slightly stronger than are needed for the typical stresses they must withstand. This "safety margin" prevents fracture under most conditions, including occasional minor accidents. Fractures are only likely in the event of a rare, catastrophic accident. Teeth of intermediate size and shape balance conflicting energetic demands and are at a selective advantage.

        We will model vertebrate teeth as cones on cylindrical bases. We will examine teeth with crowns of three different shapes -- (1) tall and slender, (2) moderate height and slenderness, and (3) broad and domed. To simulate the hardness differences between the pulp cavity, dentine and enamel, the tooth will consist of a core made from a piece of chalk (= "dentine"), and a thin outer covering of an artificial stone that is considerably harder than chalk (= "enamel"). Because the chalk is white and the artificial stone is pink, damage to the tooth models can be easily detected and evaluated.

Procedures (Week 1)

To begin: Before preparing your artificial teeth, take time to examine the fossil specimens in the lab. Inventory and attempt to identify the type of wear, fracture, or bite marks present.

        Each research group will need to make four sets of each of the three tooth shapes (= total of twelve tooth models):

1. Obtain six pieces of blackboard chalk. 

2. CAREFULLY break each piece of chalk into two equal-sized pieces.

3. Take four of the short pieces and shape one end of each into a long, slender point (i.e., the point is about twice as long as the diameter of the chalk). This is the tall tooth model

4. Take another four short pieces of chalk and shape one end of each into a short, slightly rounded point (i.e., the point is as long as the diameter of the chalk). This is the moderate tooth model.

5. On the final four short pieces of chalk, remove any sharp edges from one end, but otherwise leave the end flat. This is the flat tooth model.

6. Mix dry artificial stone powder into water in the ratio of three to four parts powder to 1 part water. When mixed, beat the artificial stone with a chemical spatula until it is smooth and pourable. Add water if necessary so that the mixture is the thickness of heavy cream.

7. Carefully dip the tip of each  tooth model into water for a few seconds and then dip it into the artificial stone mixture so that about half of the model is covered with artificial stone. Set each tooth model aside to dry until next week.


Procedures (Week 2)

        On the testing day we will also need to (1) make simulated substrates (an agar-cotton mixture to simulate soft tissues, such as muscle, and plaster to simulate bone).

1. Obtain six paper bowls and six small paper plates.

2. Mix 1 part agar with 99 parts water in a beaker. Place on a hot plate and stir continuously until the agar is dissolved and the solution is clear. Mix in enough cotton fibers so the mixture is slightly thickened.

3. Pour the mixture into the six paper bowls, so there is an equal depth in each bowl. Allow to cool and solidify.

4. Mix plaster (see above) and fill the six paper plates to an equal depth. Allow to harden.

5.  Beginning with the sharp tooth models, test the mutual effect of their interaction with both hard and soft substrates:

  • Repeat the above procedure using the plaster substrate.

  • Repeat the above analysis for the rounded and blunt teeth.

  • Questions to Ponder

    1. Are particular wear patterns associated with hard substrates?
        With soft substrates?

    2. Are particular wear patterns correlated with tangential tooth impacts?
        With axial tooth impacts?

    3. Are particular bite marks correlated with tangential tooth impacts?
        With axial tooth impacts?

    4. What effect does tooth shape have on damage patterns?
        Is this interpretation consistent with the inverse relationship between apical stress and the radius of curvature of the tooth tip?
     

    Interpreting Damage Patterns

            Reexamine the fossil specimens you observed previously. This time, draw each specimen, labeling pertinent damage marks. Attempt to reconstruct the feeding behavior that produced the damage pattern on each specimen by analogy with what you observed in the artificial teeth.