EVOLUTION OF MAMMALIAN MOLAR

 

Two main theories have been proposed to explain the evolution of molars.

1) CONCRESCENCE THEORY OF ROSE

2) TRLTUBERCULAR THEORY OF COPE & OSBORN.

According to the concrescence theory, the molar tooth has evolved by the fusion of several simple cone-like teeth. This theory Is now abandoned in favor of the second with is now accepted with certain modifications. -

Cope & Osborn theory is based on the paleontological evidence. As such the mammalian molar has been derived from a simple reptilian cone by the development of additional cusps. The Mesozoic mammal, the crown of the molar tooth has three cusps arranged more or less in the form of a triangle. This pattern of tooth has been termed tritubercular. It is assumed that the molars of the modem mammals have originated from the primitive tooth by the development of lateral cusps.

In the upper molars the three cusps are known as paracone, protocone, and metacone, The protocone lies on the apex at the inner side, the paracone lies external to it and the metacone to the back of the paracone. In the lower molars the cusps are named protoconid, paraconid and metaconid. The protoconid and metaconid are anterior and posterior to it in position. These cusps are joined by the ridges and later bell-like extensions or talons are produced from the ridges and additional cusps form on them. In this way, the various types of mammalian molars have evolved.

TOOTH DEVELOPMENT IN MAMMALES

The tooth develops partly from the epidermis and partly from the mesenchyme of dermis. Its enamel is derived from the epidermis and the remaining parts from the mesenchyme. Mammalian teeth develop in two sets. The milk or deciduous teeth develops from the buccal epithelium of the gums which are replaced later by the permanent teeth.

In the beginning the buccal epithelitim of the gum sinks down forming the t ridge. Simultaneously, the cells of malpighian layer of epidermis grow and move into the epidermis forming the dental lamina. Along the dental Lamina, the mesenchymal cells multiply and condense into a dental papilla or tooth bud. This grows upwards and pushes the dental kA Thereby an inverted cup form assumed The malpighian cells become markedly columnar and are known as ameloblasts. The mesenchymal ce are differentiated into a layer of odontoblast cells. The tooth bud is now as ‘enamel organ’. The ameloblast cells secrete enamel’ towards their inner side, while the odontobiasts secrete ‘dentine’. The enamel sur rounds the upper part of dentine like a cap. The remaining mesenchyme cells of the dental papilla form the ‘pulp’ and the cavity of the enamel organ modifies into the pulp cavity. Sooner, the development of the tooth is completed,. The jaw bone forms a socket or alveolus around its base. The tooth increases in size by the secretion of more or mote dentine and finally emerges out from the gum. The odontoblasts now secrete cement around the neck and root of the tooth narrowing the pulp cavity.

 

In majority of the mammals, the odontoblasts stop functioning after the development of tooth and the tooth stops growth. In some cases, like incisors; of rodents and tusks of elephants, the odontobiasts are functional throughout the life and the teeth continue to grow.

Teeth Modifications in Mammals

 

There is an interrelation between the form of teeth arid nature of food. These may be omnivorous,, carnivorous, insectivorous herbivorous and rodents or gnawing. In Mammals dentition is complete in the first three types. Often incomplete in herbivores where the canines are absent. The absence of canines forms a gap between incisors and premolar. This is called diastema Rabbit). The canines are small or absent and the cheek teeth have broad grinding surfaces with transverse ridges - lophodont molars. In omnivorous mammals the incisors are suited for cutting the canines for tearing and cheek teeth with prominent crowns raised into rounded tubercles are adapted for crushing bunodont molars. In the carnivorous mammals, the incisors are either poorly developed or reduced and few in number and the canines are long and sharp suited for tearing and piercing the flesh But the molars with knife-like edges are modified for scratching the flesh from the bones. In the insectivores, the incisors of the upper and lower jaws become closely united to secure small active prey and the molars have sharp pointed crowns.

Depending upon the shape, size and number of cusps, the teeth are differentiated into -

I Monocuspici teeth -Teeth with one cusp or tubercie

ii) Bicuspid teeth - Teeth with two cusps.

iii Tricuspid teeth - Teeth with three cusps and all the cusps are arranged in one plane by forming a triangle.

iv) Bunodont teeth - Teeth with a number of distinct blunt pointed cusps - Man and Monkey.

v) Lophodont teeth - The cusps are fused in the form of ridges- Horse & Elephant.

vi)Selenodont teeth - Here the cusps are wear away, exposing the under lying dentineythich-getnuii by crescent -Shaped bands raf -

vii)Secodont teeth - The cusps are having sharp cuffing surfac es - carnivorous mammals. Carnassial teeth are present.

viii) Hypsodont teeth - The teeth are having long crown and deeply placed neck. - Horse

ix) Brachyodont teeth Teeth with short crow:, and neck at the surface gum. - Dog.

In sirenia (sea - Cow and Manatee), the teeth are devoid of enamel

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DENTAL FORMULA:

DENTAL FORMULA IN MAMMALES

Mostly the number of teeth is fixed in each mammalian species. Mammalian heterodont dentition is expressed by a ‘dental formula’. The number and arrangement of teeth in each half of the upper and lower jaws is constant and identical. Hence the teeth can be expressed by using the initials - I, C, Pm and P4. The number of teeth differs in the various orders of mammals and is closely related to their ‘feeding habits. Typical number of teeth in the mammals is 44. This can be observed in Horse and pig and their dental formula is

 

The numerator indicates the number of teeth on one side of upper jaw. The denominator indicates the number of teeth on one side of the lower jaw. As the two halves of each jaw have same number and kind of teeth, the number of teeth on one side alone is usually ex pressed in the formula.

The dental formulae of some common mammals are given below

Mammalian dentition characters:

 

The mammalian teeth are placed in the muscular sockets in the jaw bones. This type of dentition is called Thecodont. The teeth are ‘diphyodont’. Two sets of teeth develop during life lime. The first set of teeth develop in the young and known as ‘milk or deciduous teeth’, after certain age these are replaced by the second set of permanent teeth. But in bats ‘and, guinea pigs the milk teeth are shed before birth. In platypus, toothed whales, sloths and sirenians the monophyodont dentition’ is present. Milk dentition is reduced by the replacement of the third premolar in marsupials.

All the teeth are not alike in all mammals. Mostly mammals exhibit ‘heterodont dention’. The teeth are modified in the form and function. But in Dolphins and porpoises it is homodont (all teeth are alike). There are four types of mammalian teeth.

1) Incisors : These are present on the premaxillae bones of the upper jaw and dentary bones of the lower jaw. They have only one ro and sharp cutting edges. So this type of teeth is used in sizing and cuffing.

ii) Canine. : These are present between the incisors and premolar. There is a single such troth in each half of each Jaw. These are long, conical teeth with a single root and simple, sharp-pointed crown.

iii) Premolars : These follow the canines. They have double root and compressed crown with one or two cusps. These possess grinding and crushing surfaces. Premolars are replaced once in life time.

IV:Molars : These are next to the premolars. They have more than two roots and cusps. These do not have predessors and always develop in the permanent set. Molars are used for crushing and mastica lion.

Both prermolars and molars are collectively knows, as cheek teeth and are borne on maxillae and dentaries. These have broad crushing and grinding surfaces or cusps.

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DENTITION IN MAMMALS

 

Teeth are the dermal derivatives of integument They are developed as a result of calcification in the mucous membrane of the l cavity. Along with the ridge of the two jaws, the teeth are arranged in a row. The teeth are present in almost all the mammals except in a few mammals In whale, the teeth are fused into plates and lost in the adult stage of Ant eaters. But in Echidna (spiny ant eater) the teeth are absent even in the embryo.

Structure of tooth: Each typical mammalian tooth is placed in the socket over the jaw bone. It is distinguished into three main pads.

1) Root - It is the basal part embedded in the bony socket.

2) Neck - it is the part above the root enclosed by the gum

3) Crown - It is the upper part beyond the surface of the gum.

The toot is separated from the socket by a vascular pridontal membrane. The vertical section tooth consists of the following parts.

I) Pulp cavity: The entire tooth encloses a central pulp cavity surrounded by a layer of odontoblast cells filled with soft pulp. It is made up of connective tissue, blood vessels and nerve fibers.

II) Dentin: A substance chemically similar to bone- dentine forms the major part of the tooth. But the dentine is permeated by numerous thin canaliculi.

III) Enamel: It is present over the de in the crown and neck regions of the tooth. It is hardest and contains only traces of living matter.

IV) Cement: It surrounds the denting of the root portion of the tooth. It is bony in nature,

The next topic is Mammalian dentition characters