1. Background

2. Preparation
     Specimen selection    Embedding and sectioning    Examination  

3. Dinosaur dental anatomy
     Tissue types    Growth    Tooth replacement pattern    Tooth attachment    Dental batteries  

4. Significance

5. References

Dinosaur dental histology is the study of the dental microanatomy of dinosaur teeth. This involves the creation of histology thin sections from fossils, which are then examined through microscopy.

A variety of anatomical information about teeth can be collected from the microscopy thin sections, including the types of dental tissues present, tooth wear, tooth replacement patterns, how the teeth are attached and the frequency of replacement.

Background

The use of histology in paleontology has traditionally been more focused on examining long bones such as the femur or the humerus.{{fact|date=December 2017}} Previous work on long bone histology revealed differences in the growth patterns of polar dinosaurs,^[1] identified a case of dwarfism in Europasaurus,^[2] reconstructed the life history of Dysalotosaurus by examining multiple specimens of different ontogenetic stages,^[3] and suggested that Psittacosaurus underwent a postural change from a quadruped to biped as it matured.^[4]

By contrast, dental histology has not been looked at in great detail in dinosaurs until more recently and there has been an increase in interest in this particular sub-field.{{fact|date=December 2017}} Histology studies traditionally rely upon the destructive process of creating and examining thin sections under microscopy, often restricting studies to taxa that have plentiful specimens such as isolated teeth or damaged specimens. While non-destructive means of analysis are sometimes possible through the use of scanning electron microscopy (SEM) or micro computed tomography, much anatomical information is difficult to obtain without creating thin sections.^[5][6]

Preparation

Specimen selection

Different types of histological studies are chosen to examine different aspects of dinosaur dental anatomy. 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.^[5] On the other hand isolated teeth would be sufficient if the goal is to examine wear on the tooth surface.

Embedding and sectioning

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.^[5] 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.^[5]

Examination

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.^[6]

Dinosaur dental anatomy

The dental histology of some of the major groups of dinosaurs have been examined through histology, these include the carnivorous theropods and herbivorous groups such as the sauropods, hadrosaurs and ceratopsians.^{[5][6][7][8][9]} Listed below are some of the dental anatomy that has been identified through histology and interpretations on their significance.

Tissue types

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.^[5]

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.^&91;5&93; 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.^&91;10&93; All teeth are not 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.^{&91;11&93; &91;12&93;} Diagenetic alterations modify the structure and composition of both enamel and dentin.^{&91;13&93;&91;14&93;&91;15&93;&91;16&93;}
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.^&91;5&93;
3. Cementum - This tissues covers the root of a teeth 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.^&91;5&93;
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.^&91;5&93;
5. Alveolar bone - This is a type of bone that is typically spongy in appearance and forms the tooth socket itself.^&91;5&93;

Growth

When 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.^[17] 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.^[17]

Tooth replacement pattern

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 and it then migrates outwards, resorbing part of the functional tooth as it grows until it is ready to erupt and replace it.^[5][18]

Tooth attachment

The tooth attachment mode of dinosaurs has been referred to as thecodonty.^[5] 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.^[5][19]

Dental batteries

One of the most complex dentition found in dinosaurs are the dental batteries present in hadrosaurs.^[20] These batteries are formed from hundreds of teeth that are stacked in rows upon rows and form a grinding surface to process plant material.^[20] Histological study of these batteries found that they are not cemented together as previously thought and that each tooth in the battery is separated by ligament.^[7][20]

Significance

The data that has been collected through dinosaur dental histological studies are typically unavailable through other means and has provided us with a more in-depth understanding on their biology. 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.^[21][19] Another finding is that the lines of von Ebner can be counted similar to tree rings to determine the age of a tooth. Furthermore, as dinosaurs continuously replace their teeth, the difference in age between the largest and oldest teeth and the smallest and youngest teeth provides us with the rate of tooth replacement.^[17] Work on the early theropod dinosaur Coelophysis established that the ancestral dental attachment mode of dinosaurs is the same thecodonty as in mammals and crocodilians,^[5] 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.^[5] Histological work on the hadrosaur dental batteries led to the creation of a developmental model that explains how the batteries were formed by the teeth growing faster and maturing earlier, to the point that the pulp cavity of individual teeth are totally filled with dentine before it even erupts.^[20] The lack of pulp in the tooth post-eruption means that the tooth is 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.^[20] While other dinosaurs such as some ceratopsians and sauropods also possesses dental batteries, they all evolved independently and differ in some form or function from those of hadrosaurs.^[20] This shows that some dinosaurs had evolved extremely sophisticated oral processing capabilities independent of the type of mammalian chewing we are familiar with.^[20]

References

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• Starship Catan - 1st Mission: The Space Amoeba
• Starship Catan 1st Mission: The Space Amoeba
• Starship Catan - 2. Mission: The Asteroid
• Starship Catan 2nd Mission: The Asteroid