Dinosaur tooth

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A tooth from a Tyrannosaurus Rex

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

Bone histology of the non-avian dinosaur Shuvuuia and the early bird Confuciusornis

The use of histology in paleontology has traditionally been more focused on examining long bones such as the femur or the humerus.[citation needed] 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.[citation needed] 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][7][8][9][6] Listed below are some of the dental anatomy that has been identified through histology and interpretations on their significance.

Tissue types

Diagram of a cross section of a typical theropod dinosaur tooth in side view. All dinosaur teeth possess the same tissue types but can differ in their appearance.

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.[5] 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.[10]
  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.[5]
  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.[5]
  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.[5]
  5. Alveolar bone - This is a type of bone that is typically spongy in appearance and forms the tooth socket itself.[5]

Growth

Tooth replacement in Coelophysis. Top down view of a replacement tooth (in middle) that has partially made its way into the pulp cavity of the previous tooth.

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.[11] 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.[11]

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][12]

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][13]

Dental batteries

One of the most complex dentition found in dinosaurs are the dental batteries present in hadrosaurs.[14] These batteries are formed from hundreds of teeth that are stacked in rows upon rows and form a grinding surface to process plant material.[14] 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.[8][14]

Close up of the dental battery of Edmontosaurus regalis
Dental batteries from an adult and juvenile hadrosaur. Both Batteries are part of the lower jaw.
The early theropod dinosaur Coelophysis

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.[15][13] 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.[11] 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.[16][17] 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.[14] 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.[14] 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.[14] This shows that some dinosaurs had evolved extremely sophisticated oral processing capabilities independent of the type of mammalian chewing we are familiar with.[14]

References

  1. ^ Chinsamy, Anusuya; Rich, T.; Vickers-Rich, P. (1998-06-15). "Polar dinosaur bone histology". Journal of Vertebrate Paleontology. 18 (2): 385–390. doi:10.1080/02724634.1998.10011066. ISSN 0272-4634.
  2. ^ Martin Sander, P.; Mateus, Octávio; Laven, Thomas; Knötschke, Nils. "Bone histology indicates insular dwarfism in a new Late Jurassic sauropod dinosaur". Nature. 441 (7094): 739–741. doi:10.1038/nature04633.
  3. ^ Hübner, Tom R. (2012-01-06). "Bone Histology in Dysalotosaurus lettowvorbecki (Ornithischia: Iguanodontia) – Variation, Growth, and Implications". PLOS ONE. 7 (1): e29958. doi:10.1371/journal.pone.0029958. ISSN 1932-6203.
  4. ^ Zhao, Qi; Benton, Michael J.; Sullivan, Corwin; Sander, P. Martin; Xu, Xing (2013-06-28). "Histology and postural change during the growth of the ceratopsian dinosaur Psittacosaurus lujiatunensis". Nature Communications. 4: ncomms3079. doi:10.1038/ncomms3079.
  5. ^ a b c d e f g h i j k l m n o p Fong, R.K.M.; LeBlanc, A.R.; Berman, D.S.; Reisz, R.R. (2016). "Dental histology of Coelophysis bauri and the evolution of tooth attachment tissues in early dinosaurs". Journal of Morphology. 277: 914–924.
  6. ^ a b c Brink, K. S.; Reisz, R. R.; LeBlanc, A. R. H.; Chang, R. S.; Lee, Y. C.; Chiang, C. C.; Huang, T.; Evans, D. C. (2015-07-28). "Developmental and evolutionary novelty in the serrated teeth of theropod dinosaurs". Scientific Reports. 5 (1). doi:10.1038/srep12338. ISSN 2045-2322.
  7. ^ Erickson, Gregory M.; Sidebottom, Mark A.; Kay, David I.; Turner, Kevin T.; Ip, Nathan; Norell, Mark A.; Sawyer, W. Gregory; Krick, Brandon A. (2015-06-01). "Wear biomechanics in the slicing dentition of the giant horned dinosaur Triceratops". Science Advances. 1 (5): e1500055. doi:10.1126/sciadv.1500055. ISSN 2375-2548.
  8. ^ a b Erickson, Gregory M.; Krick, Brandon A.; Hamilton, Matthew; Bourne, Gerald R.; Norell, Mark A.; Lilleodden, Erica; Sawyer, W. Gregory (2012-10-05). "Complex Dental Structure and Wear Biomechanics in Hadrosaurid Dinosaurs". Science. 338 (6103): 98–101. doi:10.1126/science.1224495. ISSN 0036-8075. PMID 23042891.
  9. ^ Sereno, Paul C.; Wilson, Jeffrey A.; Witmer, Lawrence M.; Whitlock, John A.; Maga, Abdoulaye; Ide, Oumarou; Rowe, Timothy A. (2007-11-21). "Structural Extremes in a Cretaceous Dinosaur". PLOS ONE. 2 (11): e1230. doi:10.1371/journal.pone.0001230. ISSN 1932-6203.
  10. ^ Hwang, S.H. (2005). "Phylogenetic Patterns of Enamel Microstructure in Dinosaur Teeth". Journal of Morphology. 266: 208–240.
  11. ^ a b c Erickson, G. M. (1996-12-10). "Incremental lines of von Ebner in dinosaurs and the assessment of tooth replacement rates using growth line counts". Proceedings of the National Academy of Sciences. 93 (25): 14623–14627. doi:10.1073/pnas.93.25.14623. ISSN 0027-8424. PMC 26184. PMID 8962103.
  12. ^ Richman, Joy M.; Handrigan, Gregory R. (2011-04-01). "Reptilian tooth development". genesis. 49 (4): 247–260. doi:10.1002/dvg.20721. ISSN 1526-968X.
  13. ^ a b LeBlanc, Aaron R. H.; Brink, Kirstin S.; Cullen, Thomas M.; Reisz, Robert R. (2017-09-29). "Evolutionary implications of tooth attachment versus tooth implantation: A case study using dinosaur, crocodilian, and mammal teeth". Journal of Vertebrate Paleontology. 0 (0): e1354006. doi:10.1080/02724634.2017.1354006. ISSN 0272-4634.
  14. ^ a b c d e f g LeBlanc, Aaron R. H.; Reisz, Robert R.; Evans, David C.; Bailleul, Alida M. (2016-07-28). "Ontogeny reveals function and evolution of the hadrosaurid dinosaur dental battery". BMC Evolutionary Biology. 16: 152. doi:10.1186/s12862-016-0721-1. ISSN 1471-2148.
  15. ^ LeBlanc, Aaron R. H.; Reisz, Robert R. (2013-09-04). "Periodontal Ligament, Cementum, and Alveolar Bone in the Oldest Herbivorous Tetrapods, and Their Evolutionary Significance". PLOS ONE. 8 (9): e74697. doi:10.1371/journal.pone.0074697. ISSN 1932-6203.
  16. ^ Wood, Sarah A.; Strait, David S.; Dumont, Elizabeth R.; Ross, Callum F.; Grosse, Ian R. "The effects of modeling simplifications on craniofacial finite element models: The alveoli (tooth sockets) and periodontal ligaments". Journal of Biomechanics. 44 (10): 1831–1838. doi:10.1016/j.jbiomech.2011.03.022.
  17. ^ Porro, Laura B.; Holliday, Casey M.; Anapol, Fred; Ontiveros, Lupita C.; Ontiveros, Lolita T.; Ross, Callum F. (2011-08-01). "Free body analysis, beam mechanics, and finite element modeling of the mandible of Alligator mississippiensis". Journal of Morphology. 272 (8): 910–937. doi:10.1002/jmor.10957. ISSN 1097-4687.
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