Australia: The Land Where Time Began

A biography of the Australian continent 

Fish Teeth

The fundamental building blocks of vertebrate teeth first appeared in the fish. The agnathans, as the name suggests lack teeth, but some parasitic forms, such as the lampreys have tooth-like structures. These are sharp, horny, uncalcified structures around the mouth. Some regard them as real teeth because they contain enamel-like antigens and proteins, the molecular weight being as high as that of gnathostome tooth enamel. If this is the case, then teeth originated before jaws. Another of the many examples of structures pre-adapting organisms for changes taking place further along their evolutionary path.  Some other fossil agnathans had pointed bony structures around the mouth, possibly functioning in some way similar to teeth, though unlike true teeth, they did not grow and were not replaced.

The earliest known true teeth are found in the Late Silurian in acanthodians and actinopterygians. At this time, placoderms also had tooth-like structures on their jaws, that weren't set into the jaws, and not delineated from the jaw bone. Placoderm teeth are regarded as cusps, not true teeth.

True teeth were developed by osteichthyan fish. These were composed of 3 layers, central pulp cavity surrounded by a layer of dentine, which was surrounded by layers of enameloid. The teeth lacked root systems as in chondrichthyans, and are ankylosed to the jaws. The teeth are homodont in most fish, all the teeth being of roughly similar size and shape. In crossopterygians size variation is common. These have very large stabbing fangs surrounded by small rows of marginal teeth.

Teeth developed by fish that feed by crushing hard-shelled invertebrates can be flat pavement-like reinforced by tubes of pleuromic dentine, as in holocephalans and bradyodont (cochliodont) sharks.

Chondrichthyans developed one of the earliest innovations, "dental lamina", a structure producing teeth throughout the life of the animal. Sharks shed teeth throughout their life and they are continually replaced.

Some lungfish have flat crushing tooth-plates, some with numerous rows of teeth on each tooth plate. These structures, growing out from a dentine base, have outer enameloid over the individual tooth cusps. The central region of each tooth cusp on the tooth plate is strengthened by hypermineralised tissues such as petrodentine.

In some crossopterygians and early tetrapods (amphibians), complex labyrinthine infolding of dentine and enamel is a specialised feature. It strengthens the large teeth, the degree of complexity in cross-sections in some cases characterises groups. E.g., porolepiforms have dendrodont style while others have polyplocodont style. In the strict definition of enamel, it occurs only in crossopterygian fish and higher terrestrial vertebrates. It is the shiny, highly mineralised outer layer of the tooth, with highly-ordered, perpendicular crystalline orientations when viewed in cross section. Some primitive fish have a similar kinds of hard layers on their teeth, but in these cases the enamel lacks the same degree of regular orientation of the crystallites, the substance being called enameloid.

Teeth may also occur inside the mouth cavity on small dental plates located on the gill arch bones, as well as on the jaw bones, in some higher osteichthyans. It is taken one stage further in some specialised teleostean actinopterygians, where it occurs are "pharyngeal mills", pavements of teeth set on hard bony bases in the throat to grind up food.

Sources & Further reading

1. John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995