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1. Tooth development and replacement in the Atlantic Cutlassfish, Trichiurus lepturus , with comparisons to other Scombroidei

   https://doi.org/10.1002/jmor.20919  

   Bemis, Katherine E. ; Burke, Samantha M. ; St. John, Carl A. ; Hilton, Eric J. ; Bemis, William E. ( December 2018 , Journal of Morphology)

   Atlantic Cutlassfish,Trichiurus lepturus, have large, barbed, premaxillary and dentary fangs, and sharp dagger‐shaped teeth in their oral jaws. Functional teeth firmly ankylose to the dentigerous bones. We used dry skeletons, histology, SEM, and micro‐CT scanning to study 92 specimens ofT. lepturusfrom the western North Atlantic to describe its dentition and tooth replacement. We identified three modes of intraosseous tooth replacement inT. lepturusdepending on the location of the tooth in the jaw. Mode 1 relates to replacement of premaxillary fangs, in which new tooth germs enter the lingual surface of the premaxilla, develop horizontally, and rotate into position. We suggest that growth of large fangs in the premaxilla is accommodated by this horizontal development. Mode 2 occurs for dentary fangs: new tooth germs enter the labial surface of the dentary, develop vertically, and erupt into position. Mode 3 describes replacement of lateral teeth, in which new tooth germs enter a trench along the crest of the dentigerous bone, develop vertically, and erupt into position. Such distinct modes of tooth replacement in a teleostean species are unknown. We compared modes of replacement inT. lepturusto 20 species of scombroids to explore the phylogenetic distribution of these three replacement modes. Alternate tooth replacement (in which new teeth erupt between two functional teeth), ankylosis, and intraosseous tooth development are plesiomorphic to Bluefish + other Scombroidei. Our study highlights the complexity and variability of intraosseous tooth replacement. Within tooth replacement systems, key variables include sites of formation of tooth germs, points of entry of tooth germs into dentigerous bones, coupling of tooth germ migration and bone erosion, whether teeth develop horizontally or immediately beneath the tooth to be replaced, and how tooth eruption and ankylosis occur. Developmentally different tooth replacement processes can yield remarkably similar dentitions.

    

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2. Tooth and consequences: Heterodonty and dental replacement in piranhas and pacus (Serrasalmidae)

   https://doi.org/10.1111/ede.12306  

   Kolmann, Matthew A. ; Cohen, Karly E. ; Bemis, Katherine E. ; Summers, Adam P. ; Irish, Frances J. ; Hernandez, L. Patricia ( August 2019 , Evolution & Development)

   Tooth replacement in piranhas is unusual: all teeth on one side of the head are lost as a unit, then replaced simultaneously. We used histology and microCT to examine tooth‐replacement modes across carnivorous piranhas and their herbivorous pacu cousins (Serrasalmidae) and then mapped replacement patterns onto a molecular phylogeny. Pacu teeth develop and are replaced in a manner like piranhas. For serrasalmids, unilateral tooth replacement is not an “all or nothing” phenomenon; we demonstrate that both sides of the jaws have developing tooth rows within them, albeit with one side more mineralized than the other. All serrasalmids (except one) share unilateral tooth replacement, so this is not an adaptation for carnivory. All serrasalmids have interlocking teeth; piranhas interdigitate lateral tooth cusps with adjacent teeth, forming a singular saw‐like blade, whereas lateral cusps in pacus clasp together. For serrasalmids to have an interlocking dentition, their teeth need to develop and erupt at the same time. We propose that interlocking mechanisms prevent tooth loss and ensure continued functionality of the feeding apparatus. Serrasalmid dentitions are ubiquitously heterodont, having incisiform and molariform dentitions reminiscent of mammals. Finally, we propose that simultaneous tooth replacement be considered as a synapomorphy for the family.

    

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3. Histology and μCT reveal the unique evolution and development of multiple tooth rows in the synapsid Endothiodon

   https://doi.org/10.1038/s41598-021-95993-6  

   Olroyd, Savannah L. ; LeBlanc, Aaron R. H. ; Araújo, Ricardo ; Angielczyk, Kenneth D. ; Duhamel, Aliénor ; Benoit, Julien ; Amaral, Marisa ( August 2021 , Scientific Reports)

   Several amniote lineages independently evolved multiple rows of marginal teeth in response to the challenge of processing high fiber plant matter. Multiple tooth rows develop via alterations to tooth replacement in captorhinid reptiles and ornithischian dinosaurs, but the specific changes that produce this morphology differ, reflecting differences in their modes of tooth attachment. To further understand the mechanisms by which multiple tooth rows can develop, we examined this feature inEndothiodon bathystoma, a member of the only synapsid clade (Anomodontia) to evolve a multi-rowed marginal dentition. We histologically sampledEndothiodonmandibles with and without multiple tooth rows as well as single-rowed maxillae. We also segmented functional and replacement teeth in µ-CT scanned mandibles and maxillae ofEndothiodonand several other anomodonts with ‘postcanine’ teeth to characterize tooth replacement in the clade. All anomodonts in our sample displayed a space around the tooth roots for a soft tissue attachment between tooth and jaw in life. Trails of alveolar bone indicate varying degrees of labial migration of teeth through ontogeny, often altering the spatial relationships of functional and replacement teeth in the upper and lower jaws. We present a model of multiple tooth row development inE. bathystomain which labial migration of functional teeth was extensive enough to prevent resorption and replacement by newer generations of teeth. This model represents another mechanism by which multiple tooth rows evolved in amniotes. The multiple tooth rows ofE. bathystomamay have provided more extensive contact between the teeth and a triturating surface on the palatine during chewing.

    

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4. Bat Dentitions: A Model System for Studies at the Interface of Development, Biomechanics, and Evolution

   https://doi.org/10.1093/icb/icac042  

   Santana, Sharlene E. ; Grossnickle, David M. ; Sadier, Alexa ; Patterson, Edward ; Sears, Karen E. ( May 2022 , Integrative And Comparative Biology)

   The evolution of complex dentitions in mammals was a major innovation that facilitated the expansion into new dietary niches, which imposed selection for tight form–function relationships. Teeth allow mammals to ingest and process food items by applying forces produced by a third-class lever system composed by the jaw adductors, the cranium, and the mandible. Physical laws determine changes in jaw adductor (biting) forces at different bite point locations along the mandible (outlever), thus, individual teeth are expected to experience different mechanical regimes during feeding. If the mammal dentition exhibits functional adaptations to mandible feeding biomechanics, then teeth are expected to have evolved to develop mechanically advantageous sizes, shapes, and positions. Here, we present bats as a model system to test this hypothesis and, more generally, for integrative studies of mammal dental diversity. We combine a field-collected dataset of bite forces along the tooth row with data on dental and mandible morphology across 30 bat species. We (1) describe, for the first time, bite force trends along the tooth row of bats; (2) use phylogenetic comparative methods to investigate relationships among bite force patterns, tooth, and mandible morphology; and (3) hypothesize how these biting mechanics patterns may relate to the developmental processes controlling tooth formation. We find that bite force variation along the tooth row is consistent with predictions from lever mechanics models, with most species having the greatest bite force at the first lower molar. The cross-sectional shape of the mandible body is strongly associated with the position of maximum bite force along the tooth row, likely reflecting mandibular adaptations to varying stress patterns among species. Further, dental dietary adaptations seem to be related to bite force variation along molariform teeth, with insectivorous species exhibiting greater bite force more anteriorly, narrower teeth and mandibles, and frugivores/omnivores showing greater bite force more posteriorly, wider teeth and mandibles. As these craniodental traits are linked through development, dietary specialization appears to have shaped intrinsic mechanisms controlling traits relevant to feeding performance.

    

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5. Not your father’s homodonty—stress, tooth shape, and the functional homodont

   https://doi.org/10.1111/joa.13248  

   Cohen, Karly E. ; Weller, Hannah I. ; Summers, Adam P. ( July 2020 , Journal of Anatomy)

   Teeth tell the tale of interactions between predator and prey. If a dental battery is made up of teeth that look similar, they are morphologically homodont, but if there is an unspecified amount of regional specialization in size or shape, they are morphologically heterodont. These are vague terms with no useful functional implication because morphological homodonty does not necessarily equal functional homodonty. Teeth that look the same may not function the same. Conical teeth are prevalent in fishes, superficially tasked with the simple job of puncture. There is a great deal of variation in the shape and placement of conical teeth. Anterior teeth may be larger than posterior ones, larger teeth may be surrounded by small ones, and patches of teeth may all have the same size and shape. Such variations suggest that conical dentitions might represent a single morphological solution for different functional problems. We are interested in the concept of homodonty and using the conical tooth as a model to differentiate between tooth shape and performance. We consider the stress that a tooth can exert on prey as stress is what causes damage. To create a statistical measure of functional homodonty, stress was calculated from measurements of surface area, position, and applied force. Functional homodonty is then defined as the degree to which teeth along the jaw all bear/exert similar stresses despite changes in shape. We find that morphologically heterodont teeth are often functionally homodont and that position is a better predictor of performance than shape. Furthermore, the arrangement of teeth affects their function, such that there is a functional advantage to having several smaller teeth surrounding a singular large tooth. We demonstrate that this arrangement of teeth is useful to grab, rather than tear, prey upon puncture, with the smaller teeth dissipating large stress forces around the larger tooth. We show that measurements of how shape affects stress distribution in response to loading give us a clearer picture of the evolution of conically shaped teeth.

    

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