Australia: The Land Where Time Began

A biography of the Australian continent 

Anatomy based relationships - teeth

Cells derived from the base of a deep groove of surface epithelium and from underlying jaw tissues coalesce to form the teeth of mammals. The tooth bud initially develops as a core covered by epithelium that dentine and enamel is later deposited. During tooth development, the tooth bud rises to the jaw surface, where it pierces the groove and erupts as a fully formed tooth, to be followed by a succession of tooth buds developing into teeth back along the jaw. The structure of the front teeth is simple, with a single cusp and a single root. The back teeth develop as do the front teeth, but with the addition of subsidiary buds, the position of which, in relation to the primary cusp, is characteristic of the particular tooth and the particular species.

It has been found that the expression of developmental genes in the tooth bud determines whether the tooth will develop into a simple incisor or a more complex molar. Other genes in the bud determining the position of subsidiary buds in relation to the primary bud, the expression of these genes occurring 1-2 days before the morphological changes to cells of the jaw. This indicates that the pattern is determined before the beginning of bud formation, so not resulting from interaction between adjacent tooth buds. Future cusp pattern is therefore predicted by gene expression, affecting the very earliest stages of tooth development. The possibility exists of profound effects on the earliest stages of tooth morphology resulting from minor changes in gene expression, that could have a significant effect on the evolution of the teeth.

A comparison has been made of specific gene expression between the formation of molars in the mouse,  Mus musculus, and a vole, Microtis rossiameridionalis, both of which shared a common ancestor about 20 Ma in the Early Miocene (Jervall et al., 2000). The large differences in molar pattern between the 2 species resulted from small changes of gene expression that determines the position of the secondary cusps relative to the primary cusp, parallel in the mouse and diagonal in the vole.

Marsupial teeth develop in the same way as placental teeth, indicating that for more then 20 Ma they shared a common ancestor. From this it could be assumed that the marsupial pattern of molar cusps is controlled by the same or similar genes.

All ancestral placentals and marsupials, among the earliest mammals, were small, insectivorous animals. In small carnivores, the dentition is more similar to ancestral dentition of mammals, whereas the several types of herbivorous dentition have evolved from these ancestral dentitions. The molars of mammals provides the greatest amount of information about the diet of an animal, interspecies relationships, as well as about the main descent lines from ancestors of the Cretaceous.

The earliest known mammals had triangular, 3-cusped molars in the upper and lower jaws. Seen in surface view, the apex of each protocone (upper molar) pointed inwards, the apex of each protoconid (lower molar) pointed outwards. The molars of the upper and lower jaws fitted closely between each other in a zigzag shearing surface when the jaws closed. Small carnivorous marsupials and placentals still have this pattern of dentition. The simple arrangement of 3 cusps was extended, prior to the separation of the marsupials and placentals, at an early stage of mammalian evolution. On the posterior face of the molars in the lower jaw, 2 additional cusps developed, the entoconid and the hypoconid, that provided a depression into which the protocone, or apex, of the upper molar fitted, adding a grinding surface to the shearing surface of the molars. Additional cusps developed in the upper molars outside the primary 3 cusps, as well as between them. The stylar shelf, the 5 small cusps of the outer series, don't completely mesh with complementary parts of the lower molars, their function being unknown. The earliest placentals had 2 stylar cusps, but they were lost in later placentals. The full complement of 5 cusps are present in all early marsupials, the dasyurids and American opossums retaining most of them to the present. Fossil placentals are differentiated from fossil marsupials by the stylar shelf.

Phylogenetic relationships between extinct and extant mammals can be determined by the patterns of ridges that can form between several cusps and the relative sizes of the cusps. Further information on relationships and food type can be provided by the fine surface structure and surface scratches on the teeth. Long snouts with a battery of simple, sharp-pointed teeth, are found on carnivorous Australian species and all American marsupials. There are 4 molars and 3 premolars, 1 prominent canine, and 4-5 incisors in the upper jaw and 3 in the lower jaw. They are grouped together as the Polyprotodontia (many front teeth). The bandicoots are more omnivorous than the dasyurids, but are included in this group, based on this criterion. The bandicoots have molars that have an extra cusp at the back, the metaconule, giving them a more squared up appearance.

There are fewer premolars in the Australian herbivorous species, and their canines are small or completely absent. In the upper jaw, there are 1-3 incisors and a single large pair of incisors in the lower jaw. Having fewer teeth, there is often a diastema, a gap between the front teeth and the cheek teeth, allowing the tongue to move the food to the grinding molars.

This diverse group, the Diprotodontia (2 front teeth),  is defined, by the single pair of incisors in both the upper and lower jaw. There is a superficial link to the caenolestidae, a group of marsupials from South America, that have large, procumbent first incisors, but the other incisors are also present. In the development of teeth in the Protodontia embryo the large incisor is the second, not the first incisor, indicating that it is a case of convergence rather than close relationship.

The Diprotodontidia have the metaconules on the upper molars, as occurs in bandicoots, but the stylar shelf has been lost, and the loss of the paraconid at the front of their lower molars has resulted in them also being squared up. The grinding surface of the molars is more rounded than the sharp-pointed cusps of the Polyprotodontia, being variously shaped into crescents or ridges. 2 groups of grass eaters achieve the greatest development of the grinding battery of molars, wombats and large kangaroos. The incisors in the upper jaw of wombats has been reduced to 1 pair, and all the teeth grow continuously throughout life, as occurs in rabbits and rodents. The molar teeth are high crowned in large kangaroos, as they are in sheep, the whole battery moving forward in the jaw, as the molars at the front are worn down and shed, those from the back replace them, as occurs in elephants.

Sources & Further reading
  1. Chris Johnson, Australia's Mammal Extinctions, a 50,000 year history, Cambridge University Press, 2006
  2. M.Archer, S.J. Hand & H. Godthelp in Hill, Robert S., (ed.), 1994, History of the Australian Vegetation, Cambridge University Press.
  3. Tyndale-Biscoe, Hugh, 2005, Life of Marsupials, CSIRO Publishing.

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated  30/09/2011
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading