3.1 THE TRITUBERCULAR THEORY

I. INTRODUCTION

The history of teeth with two or more cusps has long interested investigators. Teeth are very useful for studying evolutionary history: they are durable in the fossil record, they have clearly discernable anatomic features, and their morphology is under tight genetic control. The similarities of molar teeth in modern humans and our hominid ancestors are readily seen, and they remain one of the most useful guides in sorting fossil remains in the field.

What is the origin of cusps? Several theories have been proposed to account for the origin of cusps and molar patterns. We will mention two. The concrescence theory assumes that mammalian teeth originated by the fusion of originally separate reptilian teeth. The evidence needed to support this theory would be the discovery of transitional forms. None have been found. In the absence of evidence, this theory is pretty much discredited today.

Another theory is the differentiation theory. This hypothesis says that even the most complicated mammalian molar has originated from a simple conical reptilian tooth. The tritubercular theory, the main focus of this paper is a differentiation theory. It sets out to organize the homologies of cusps, explain where the cusps came from, assigns terms to them, and traces their evolutionary history. It is much more successful with the terms than with evolution.

Before outlining the tritubercular theory, we need to clarify a few points about dental evolution covered in section. 2.1.

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II. TRENDS IN DENTAL EVOLUTION FROM REPTILES TO MAMMALS

Summarized in brief, there has been a reduction in the number of teeth and in the generations of teeth. In general, there was a reduction from polyphyodonty (many generations of teeth) to two functional generations (diphyodonty), sometimes one functional generation (monophyodonty), or in the case of toothless whales and anteaters, no functional generations of teeth (anodontia). The term functional is imperative here because in some animals without functional teeth, they form, erupt and are shed in utero before birth.

Early placental (eutherian) mammals had 44 teeth in the permanent dentition. Per quadrant there were three incisors, one canine, four premolars and three molars.

Question: based on our paleontological definition of tooth classes in unit 2.1, how many teeth were in the deciduous dentition?

In general, there have been tooth reductions in many mammals. New World monkeys most often have 36 teeth, while Old World monkeys, apes and man have only 32.

An increase in the number of teeth above the typical mammalian formula is not common, but there are notable examples. The opposum, a marsupial, has fifty. The manatee has forty-four molar teeth instead of twelve. The greatest increase in the number of teeth is in toothed whales, the porpoise, and dolphin. There may be up to two hundred teeth. In those animals, heterodonty has been lost and they have reverted to a simple peg-shaped homodont condition.

In broad perspective, the trend has been in regionalization from homodonty (teeth all similar in form) to teeth in specialized classes (heterodonty). This brings us back to the issue of how molars acquired their cusps.

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III. THE ORIGIN OF CUSPS AND THEIR NOMENCLATURE

The tritubercular theory was first put forth by the American paleontologist Edward Drinker Cope in 1875 and modified by Henry Fairfield Osborn in 1888. In older texts you will find it referred to as the 'Cope-Osborn Theory of Tuberculy'. A short vignette about Cope is at the end of this section.

The theory goes like this: the haplodont (Gr haplo = simple) conical teeth of reptiles evolved to form molars consisting of a series of in-line cusps. In the maxilla, the oldest cusp representing the original reptilian conical tooth is the protocone (Gr proto = first in time). To the mesial of the protocone, the cusp that appeared is the paracone (Gr para = 'at the side of'). Just distal to the protocone is the metacone (Gr = 'in the midst of or after').

In the lower, the process is identical. The naming is similar, except for the addition of the suffix -id. (Gr = connected with). The terms for cusps n the lower are thus protoconid, paraconid, and metaconid.

Figure one Protocone (upper) and Protoconid (lower)

As in reptiles, the upper and lower teeth alternate in the jaws, allowing them to interdigitate on jaw closing. Thus far these simple cone-shaped teeth can be used for piercing and tearing. The jaws function in a simple hinge fashion.

In time, the accessory cusps increased in size and they rotated in relation to the principle cusp to form triangular teeth. They also acquired connecting ridges called lophs (Gr = crest or ridge, terms more understandable to a dentist). In the upper, the protocone displaced to the lingual with the base of the triangle to the buccal.


Figure two

Figure three

In the lower the protoconid remained to the buccal with the base of the triangle to the lingual. The triangles were therefore reversed in relation to each other. This allowed interdigitation between the triangles in function. These trigon/trigonid teeth now could function by puncturing food as before and also by the shearing action of the crests acting against each other.

The next step in evolution was the acquisition of the distal talonid (heel) on the distal of lower molars. This formed a surface for crushing food against the surface of the protocone in the upper. These cuspal patterns form tribosphenic molars (Gr to rub / a wedge) that are ancestral to the cuspal patterns of primate molars, including our own. The fully formed tribosphenic molar probably took an additional 100 million years to develope.

(My note: the term tribosphenic was introduced by Simpson in 1936. This term was intended to replace the older terms "tubersectorial" or "tritubercular." You can best understand them as a sort of letter 'V' with the open part of the letter facing facially in the upper and lingually in the lower. In occlusion, upper tribosphenic molars are normally located to the midline of their lower molar counterparts.

(In large herbivores, this replicates the slicing action of the carnassials of carnivores, but in the horizontal place using the many little ridges of enamel interspersed with the softer valleys of dentin and coronal cementum.)

The upper molar acquired additional cusps: the hypocone on the distal aspect of the molar, completing its appearance as a four-cusped tooth. Small cusp additions to the mesial and distal of the trigon are the protoconule and metaconule. Cusps along the buccal margin are styles: parastyle, mesostyle, and metastyle.

Figure four

Figure five

The lower molar talonid (the heel, remember?) gave rise to three cusps, the hypoconid, the entoconid, and the hypoconulid producing a six-cusped molar.

In humans, the paraconid in first molars is lost, producing a five-cusped tooth. In lower second molars, the hypoconulid is lost, producing a four cusp molar. The five-cusp molar occurs in anthropoid apes and man. The Y-5 pattern named after the Y-shaped pattern of fissures separating the five cusps represents very conservative genetically determined patterns still present in 90% of lower first permanent molars of modern Homo sapiens.

 

Figure six
Figure six

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Edward Drinker Cope was the son of a Quaker family. A precocious child with a brilliant intellect, he became interested in dinosaurs. His career was in a time of competition for finds in the western United States. In 1870, Cope showed a newly reconstructed dinosaur to his competitor, Othniel Charles Marsh who pointed out that Cope had accidentally placed the head at the wrong end of the skeleton! This led to a bitter feud between the men and an intense rivalry that consumed the rest of their lives. Henry Fairfield Osborn who later refined the tritubercular theory was one of America's leading paleontologists in the early 20th century. It was a time of less heated and more careful exploration.

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IV. THE EVIDENCE OF EMBRYOLOGY

Consider the premise: if a cusp appeared earlier in evolution, it should also appear earlier in embryology. Krause and Jordan (1965), dental embryologists have studied in very great detail the developing crown relief of human teeth. The expectation was that the protocone/protoconid being the 'first cusp' in evolution should also be the first to appear in embryology.

As you will recall, in humans the second deciduous molars share the same occlusal anatomy with first permanent molars. Krause and Jordan found that these teeth share a similar pattern of timing and mode of inception of structures.

In lower second deciduous/lower first permanent molars, the sequence of development is protoconid, metaconid and hypoconid. In upper second deciduous/upper first permanent molars, the order is different: paracone, protocone, and metacone. According to embryology, the theory holds for the lower, but not for the upper. The mesial buccal cusp developes first in each case.

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V. CONCLUDING REMARKS

The tritubercular terminology is a very useful system for the description of placental molar teeth. As evolutionary theory, however, the theory is not acceptable. The order of

events is simply not confirmed by fossil evidence. In conclusion, the names--but not the evolutionary theory--remain useful to us today.

..... CJ 98

Sources and further reading

Brown, T. and Kraus, M. "Origin of the Tribosphenic Molar and Metatherian and Eutherian Dental Formula" in Mesozoic Mammals. Lilegraven, J., Kielan-Jaworowska, Z., and Clements, W. eds. Berkeley: University of California Press, 1979.

Crompton, A. and Sita-Lumsen, A. "Functional Significance of the Therian Molar Pattern" Nature. July 11, 1970 pp 197-199.

Gaunt, W. and Miles, A. 'Fundamental Aspects of Tooth Morphogenesis' in Structural and Chemical Organization of Teeth. New York: Academic Press, 1967.

Jordan, R. and Abrams, L. Kraus' Dental Anatomy and Occlusion. St. Louis: Mosby Year Book, 1992.

Kraus, B. and Jordan, R. The Human Dentition Before Birth. Philadelphia: Lea & Febiger, 1965.

Norman. D. The Illustrated Encyclopedia of Dinosaurs. New York: Crescent Books, 1985.

Peyer, B. Comparative Odontology. Chicago: The University of Chicago Press, 1968.

Romer, A. Vertebrate Paleontology. Chicago: The University of Chicago Press, 1967.

Scott, J. and Symons, N. Introduction to Dental Anatomy. Edinburgh: Churchill Livingston, 1974.
Figure 7

Deciduous dentition

Figure 8

Permanent dentition

The above diagrams show the type and number of teeth in primitive placental mammals.