TOOTH DETECTIVES: DETERMINING THE DIETS OF EXTINCT ANIMALS

James W. Westgate
Department of Geology
Lamar University
Beaumont, TX 77710

Level: Grades 4 - 6

Estimated Time Required

Two 1-hour sessions. This exercise should be divided into two sessions. The first 1-hour session will deal with teeth of living mammals. The second session will apply skills learned in the first session to the interpretation of the diet of extinct animals.

Anticipated Learning Outcomes

• Students will recognize that teeth display a variety of shapes.
• Students will learn that tooth shapes are correlated with specific ways of obtaining and processing food items.
• Students will be able to recognize the dietary habits of living animal species through the examination of their teeth.
• Students will be able to hypothesize what the diets of extinct animals were through the examination of fossil teeth.

Background

Teeth are the mineral medium through which an animal obtains and begins to process its food. Teeth are designed to resist the extensive abuse that their owner subjects them to throughout its lifetime. They are composed of a wear-resistant and chemical-resistant mineral called apatite (calcium fluorophosphate) and usually display a highly resistant coat of enamel. Teeth are commonly preserved as fossils because of their chemical and structural resistance to destruction. Teeth continue to resist destruction after the death of the individual that made them. Commonly, teeth will survive scavenging and stream transport that totally destroys other skeletal elements. By comparing the teeth of extinct fossil species with the teeth of living species, we can deduce the types of foods that the extinct species ate. In turn, we may determine that species' position in the food web of the ancient community in which it lived. We can even reconstruct an ancient community when nothing but fossil teeth are preserved in a geologic deposit.

Teeth display variable shapes for different functions. There are just a few basic types of tooth shapes, although many variations on those basic types exist. Mammals, such as dogs and humans, commonly display four basic tooth shapes in one jaw. Most mammals are either carnivores (flesh-eaters), herbivores (plant-eaters), or omnivores (flesh and plant-eaters).

The four basic types of mammal teeth are incisors, canines, premolars and molars. Incisors are located at the front of the mouth and are flattened front-to-back. They are used by most mammals to nip food, although rodents have long, ever-growing incisors, designed for gnawing. Some herbivores, such as deer and cattle, have lost the upper incisors. Canines are positioned behind the incisors. One canine is present on each side of the upper and lower jaws. These teeth are long and conical and are used by carnivorous mammals to stab prey, or may be used in self defense. The dog family earned its common name because of its well developed canine teeth. Many omnivorous and herbivorous mammals have small canines or have lost them through evolutionary tooth reduction. Humans, as an omnivorous species, have canines that barely protrude above the level of the incisors.

Premolars are positioned behind the canine teeth. In carnivores, these are usually four blade-like teeth, flattened from side-to-side, which serve to slice through the meat and tendons of their prey. Herbivores have premolars whose crowns tend to wear to flat grinding surfaces and resemble the more posterior molar teeth. Omnivorous mammals display premolars having a shape in between that of carnivores and herbivores. Humans have only two premolars on each side of the upper and lower jaws. The two blade-like cusps in the human upper premolars give these teeth the common name of "bicuspid."

The molars are the posterior-most teeth in the jaw and are the last ones to erupt. In carnivores, these teeth are much flatter and broader crowned than the premolars. Although placental mammals (all living, native North American mammals, except opossums, fall in this category) may have up to three molars, some carnivores possess only one or two in each jaw. Domestic dogs have three lower, but only two upper molars. Molars are designed for crushing or grinding. Herbivores and omnivores generally display three flat-wearing molars having a somewhat square (upper jaw) or rectangular (lower jaw) outline. The difficulty many humans have in successfully erupting their third molars ("wisdom teeth") without help from a dental surgeon, may be fore-shadowing an evolutionary loss of that tooth position in our species. The table below summarizes the common premolar and molar condition in carnivorous and herbivorous placental mammals.

Mammal	Premolars	Upper molars	Lower molars
Carnivores	Up to 4 blade-like	Usually 1 or 2 flat	First one blade-like, 1 or 2 flat
Herbivores	Up to 3 flat-wearing	Usually 3 flat-wearing, square	Usually 3 flat-wearing, rectangular

Other groups of animals display a wide range in tooth shapes. Most non-mammals have only one, or occasionally, two types of teeth. Fish-eating and marine mammal-eating sharks have sharp, flesh-piercing teeth which are frequently shed to prevent undue wear. The bottom dwelling relatives of sharks, the rays, are mollusc-eaters with flat teeth that enable them to crush the external skeletons of their prey. Most bony fishes have cylindrical teeth which allow them to grasp their food long enough to swallow it. Some animals, such as birds and turtles, evolutionarily lost the ability to generate teeth millions of years ago.

Materials

Session 1:

• Skulls and lower jaws of living mammal species. Include at least one carnivore (such as a dog or cat), herbivore (such as a deer or cow), and omnivore (such as a raccoon or pig).
• Hand-held mirrors.
• Student worksheets

Note: Prepared modern skulls are available from the Ward's Biology volume 2 catalog (1-800-962-2660). A classroom collection of skulls may also be collected locally. Farms and hunters are potential sources of skulls in rural areas. In urban areas, skulls may be collected from road kills (the courageous take a hatchet) and mouse traps. Large fresh specimens should be wrapped in window screen with one end open to allow scavenger entrance, weighted down, and left in a secluded area. The specimen should be clean after about 2-3 weeks under summer conditions. A 15-minute bath in a solution of 50% Chlorox and 50% water will remove oils from the bone.

Session 2:

• Fossil skulls, jaws or teeth. Casts are available from the Ward's Earth Science catalog. Close-up pictures of fossil skulls (sabre-tooth cats, Tyrannosaurus rex, etc.) may be substituted for actual specimens, or this part of the activity may be done during a field trip to a natural history museum.
• Student worksheets

Procedures

Session 1. Recognition of tooth types in modern mammals using the discovery method

1. Students should be divided into small groups and allowed to discuss their opinions throughout the exercise. Where class size is large and limited specimens are available, this activity may be conducted at the Science Center in the classroom.
   
2. Give the students 10 minutes for a preliminary examination of the skulls.
   
3. Ask students to focus on the types of teeth they see. Ask how their shapes vary and what the uses of each type might be. Then assign the names incisor, canine, premolar and molar to each shape type.
   
4. Students will then examine their own teeth and their partners' teeth with the aid of a hand mirror and record their results on Table 1. Only count teeth on one side (right or left). (See sample key, top of next page.)
   
5. Students will examine teeth in the skulls and jaws of non-identified mammals (unknowns) and record their observations on Table 1. Again, only count one side.

Specimen	Incisors U/L	Canines U/L	Premolars U/L	Molars U/L	Diet	Name
You	2 / 2	1 / 1	2 / 2	3* / 3*	Omnivore	Human
Unknown	3 / 3	1 / 1	4 / 4	2 / 3**	Carnivore	Dog
Unknown	0 / 3	0 / 0	3 / 3	3 / 3	Herbivore	Deer
Unknown	3 / 3	1 / 1	4 / 4	3 / 3	Omnivore	Pig
Example Key: U=Upper, L=Lower Note: * this varies with age: 6-10 years = 1, 10-15 years = 2, 15-?= 3 ** first molar looks like a premolar

Session 2. Tooth analysis in fossil specimens

1. Show students casts or pictures of the skulls and jaws of fossil species; or give them a list of species to observe, if this exercise is conducted at a natural history museum with mounted specimens. Various types of fossil species might include sabre tooth cats, giant ground sloths, mammoths, oreodonts, and of course dinosaurs such as Tyrannosaurus rex, Apatosaurus ("Brontosaurus"), Deinonychus, or other available specimens.
   
2. Using the same procedures as in Session 1, students should examine fossil specimens and record their observations on Table 2.
   
3. Based on their knowledge of the teeth and diets of living mammals, students should hypothesize what the diet of each extinct species they observed was.
   

Results and Discussion

• After each session, groups should be given time to present their interpretations to each other and defend their identifications and conclusions.
  
• Emphasis should be put on the free development of ideas by the groups rather than whether interpretations are right or wrong. This is the method employed by paleontologists and we do not all agree with each other in our conclusions. Early interpretations of the diet of proboscidians (mastodons and mammoths) suggested that the giant tusks were canine teeth and that these elephant-like species had been carnivores. Remember, no one really knows with 100% certainty what the diet of Apatosaurus or T. rex really was!
  

Additional Activities

Any kind of vertebrate skulls or jaws may be added to this exercise. Shark jaws or skulls of snakes, alligators, birds, turtles, and bony fish, are readily available and are very interesting to elementary students. I encourage teachers to build their own osteology collection. Turkey and chicken skeletons cost only the time of boiling for a few hours until all the scrap meat falls off. This is a great post-Thanksgiving activity for students and leads to an analysis of other skeletal parts and their functional morphology. If session 2 was conducted in class, a good follow up activity would be a field trip to a natural history museum.

Selected References

BURT, W. H. and GROSSENHEIDER, R. P.,1964. A Field Guide to the Mammals. Houghton Mifflin Co., Boston, 284 p. (good skull photos and a table of dental formulas).

NORMAN, D., 1985. The Illustrated Encyclopedia of Dinosaurs. Crescent Books, New York, 208 p. (great illustrations of dinosaur and other reptile skulls and their teeth).

NAME: ___________________


TABLE 1. MODERN MAMMAL SPECIMENS

Specimen	Incisors U/L	Canines U/L	Premolars U/L	Molars U/L	Diet	Name
You	/	/	/	/		Human
Unknown-1	/	/	/	/
Unknown-2	/	/	/	/
Unknown-3	/	/	/	/
Unknown-4	/	/	/	/
Unknown-5	/	/	/	/
Unknown-6	/	/	/	/

TABLE 2. FOSSIL SPECIMENS

Specimen	Incisors U/L	Canines U/L	Premolars U/L	Molars U/L	Diet	Name
Unknown-1	/	/	/	/
Unknown-2	/	/	/	/
Unknown-3	/	/	/	/
Unknown-4	/	/	/	/
Unknown-5	/	/	/	/

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