Dinosaur tooth

Different specimens will be suitable for looking at particular anatomical features. For example, specimens with teeth intact within the jaws are necessary to study tooth attachment as this information is lost on isolated teeth.[6] On the other hand, isolated teeth would be sufficient if the goal is to examine wear on the tooth surface.

Thin sections are prepared by first embedding a suitable specimen in epoxy resin. The embedded specimen can then be mounted and cut with a precision saw.[6] The resulting slice is attached to a slide and ground down, then polished, until it is thin enough, with a suitable surface to be examined with a microscope.[6]

Thin sections are typically examined with a petrographic microscope using plain light and cross-polarized light. Some structures are more easily visible and distinct using one type of light over the other due to differences in their mineral properties. Some specimens can also be examined with a SEM.[7]

There are generally 5 tissue types present in dinosaurs, and these have been found to be identical to those of their closest living non-avian relatives, the crocodilians.[6] One of the most significant findings is that despite differences in their appearance, dinosaur teeth are essentially composed of the same dental tissues found in modern mammals, crocodilians and other amniotes, suggesting that these tissues first evolved in a common ancestor and has been retained ever since.[11][12]

1. Enamel - This is the hard coating on the outside of the teeth and typically appears as a clear, thin featureless band on the tooth surface when viewed in cross section.[6] SEM analysis of the surface of dinosaur teeth revealed that their enamel form in prisms similar to mammals and that there is sufficient difference in the enamel microstructure to help pinpoint what group a tooth belonged to, sometimes to the genus level, when only isolated teeth are found.[13] Not all teeth are covered by a prismatic enamel, and in most taxa, prisms are perpendicular to the outer surface of the tooth. Complex arrangements such as visible in mammals are rare.[14][15] Diagenetic alterations modify the structure and composition of both enamel and dentin.[16][17][18][19]
2. Dentine - This tissue makes up the bulk of the tooth and is characterized by long thin parallel tubules running throughout the body of the tooth.[6]
3. Cementum - This tissue covers the root of a tooth and is an attachment tissue that forms part of the periodontium. It is typically infilled with Sharpey's fibers that help anchor the tooth in place in the socket.[6]
4. Periodontal ligament - This is a soft tissue layer between the cementum and the tooth socket. While this is not preserved in fossils, there is always a mineral filled gap that is present in all dinosaur teeth between the cementum and the tooth socket, which infers the presence of soft tissue in life.[6]
5. Alveolar bone - This is a type of bone that is typically spongy in appearance and forms the tooth socket itself.[6]

Top down view of a replacement tooth in Coelophysis growing up through the middle of pulp cavity in the older tooth.

In some examples viewed in cross section, growth lines can be observed in the dentine of dinosaur teeth. These are known as lines of von Ebner and represent daily deposition of dentine.[20] Counting these lines provides the age of a tooth and comparing the age of the mature tooth to the replacement tooth in a socket provides an estimate of the tooth replacement rate.[20]

The difference in age between the oldest teeth and the youngest teeth is used to determine the rate of tooth replacement.[20]

Many dinosaur teeth have been found to have a replacement pattern similar to other reptiles where a replacement tooth grows in the dental lamina on the inside of the jaw before migrating outwards, resorbing part of the growing functional tooth, until ready to erupt and replace it.[6][21]

The tooth attachment mode of some dinosaurs has been referred to as thecodonty.[6] This is a condition where the tooth is deeply implanted into the tooth socket with periodontal ligament present, as is the case in crocodilians and mammals.[6][12] In mammals, thecodonty is associated with dental occlusion while in crocodilians it has been proposed as a means to reduce stresses from bite forces.[22][23] Coelophysis possessed neither dental occlusion nor a strong bite, raising questions as to why it possesses thecodonty.[6]

Close up of hadrosauridae dental batteries in various Hadrosaurid dinosaurs

Diagram showing the dental battery in the rebbachisaurid genus Nigersaurus

One of the most complex dentition found in dinosaurs are the dental batteries present in hadrosaurs (whose members were dominant species across the planet), Neoceratopsia (for example, Triceratops), and Rebbachisauridae.[24]

These batteries were formed from hundreds of teeth which were stacked in rows upon rows and formed a grinding surface to process plant foods.[24] Histological study of these batteries found that they were not cemented together as previously thought, but that each tooth in the battery was separately moving and supported by ligamenture such that the whole structure was flexible.[9][24] Comparable to shark teeth, dental batteries exhibited polyphyodonty, growing new teeth on the inside which migrated over time to replace the outer teeth. Unlike sharks however, who lose all of their old teeth, teeth in the rapidly growing dental battery would wear completely down and be reabsorbed by the renewing structure around it.[25]

The batteries were formed by the teeth growing fast and maturing early, to the point that the pulp cavity of individual teeth—usually filled with cells and connective tissue—were totally filled with dentine before it even erupted. The lack of pulp in the tooth post-eruption means that the tooth was essentially dead and able to be completely worn away through use, and replaced without the risk of exposing the normally sensitive dental pulp to infection and pain. While other dinosaurs, such as some ceratopsians and sauropods, also possessed dental batteries, they all evolved independently and differ in some form or function from those of hadrosaurs. This shows that some dinosaurs had evolved extremely sophisticated chewing capabilities.[24][25]