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1. The Evolutionary Continuum of Functional Homodonty to Heterodonty in the Dentition of Halichoeres Wrasses

   https://doi.org/10.1093/icb/icaa137  

   Cohen, Karly E ; Weller, Hannah I ; Westneat, Mark W ; Summers, Adam P ( September 2020 , Integrative and Comparative Biology)

   null (Ed.)

   Synopsis Vertebrate dentitions are often collapsed into a few discrete categories, obscuring both potentially important functional differences between them and insight into their evolution. The terms homodonty and heterodonty typically conflate tooth morphology with tooth function, and require context-dependent subcategories to take on any specific meaning. Qualifiers like incipient, transient, or phylogenetic homodonty attempt to provide a more rigorous definition but instead highlight the difficulties in categorizing dentitions. To address these issues, we recently proposed a method for quantifying the function of dental batteries based on the estimated stress of each tooth (inferred using surface area) standardized for jaw out-lever (inferred using tooth position). This method reveals a homodonty–heterodonty functional continuum where small and large teeth work together to transmit forces to a prey item. Morphological homodonty or heterodonty refers to morphology, whereas functional homodonty or heterodonty refers to transmission of stress. In this study, we use Halichoeres wrasses to explore how a functional continuum can be used in phylogenetic analyses by generating two continuous metrics from the functional homodonty–heterodonty continuum. Here we show that functionally heterodont teeth have evolved at least 3 times in Halichoeres wrasses. There are more functionally heterodont teeth on upper jaws than on lower jaws, but functionally heterodont teeth on the lower jaws bear significantly more stress. These nuances, which have functional consequences, would be missed by binning entire dentitions into discrete categories. This analysis points out areas worth taking a closer look at from a mechanical and developmental point of view with respect to the distribution and type of heterodonty seen in different jaws and different areas of jaws. These data, on a small group of wrasses, suggest continuous dental variables can be a rich source of insight into the evolution of fish feeding mechanisms across a wider variety of species. 

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2. Dental Dynamics : A Fast New Tool for Quantifying Tooth and Jaw Biomechanics in 3D Slicer

   https://doi.org/10.1093/iob/obae015  

   Cohen, K_E ; Fitzpatrick, A_R ; Huie, J_M ( May 2024 , Integrative Organismal Biology)

   Teeth reveal how organisms interact with their environment. Biologists have long looked at the diverse form and function of teeth to study the evolution of feeding, fighting, and development. The exponential rise in the quantity and accessibility of computed tomography (CT) data has enabled morphologists to study teeth at finer resolutions and larger macroevolutionary scales. Measuring tooth function is no easy task, in fact, much of our mechanical understanding is derived from dental shape. Categorical descriptors of tooth shape such as morphological homodonty and heterodonty, overlook nuances in function by reducing tooth diversity for comparative analysis. The functional homodonty method quantitatively assesses the functional diversity of whole dentitions from tooth shape. This method uses tooth surface area and position to calculate the transmission of stress and estimates a threshold for functionally homodont teeth through bootstrapping and clustering techniques. However, some vertebrates have hundreds or thousands of teeth and measuring the shape and function of every individual tooth can be a painstaking task. Here, we present Dental Dynamics, a module for 3D Slicer that allows for the fast and precise quantification of dentitions and jaws. The tool automates the calculation of several tooth traits classically used to describe form and function (i.e., aspect ratio, mechanical advantage, force, etc.). To demonstrate the usefulness of our module we used Dental Dynamics to quantify 780 teeth across 20 salamanders that exhibit diverse ecologies. We coupled these data with the functional homodonty method to investigate the hypothesis that arboreal Aneides salamanders have novel tooth functions. Dental Dynamics provides a new and fast way to measure teeth and increases the accessibility of the functional homodonty method. We hope Dental Dynamics will encourage further theoretical and methodological development for quantifying and studying teeth.

    

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3. Puncture performance tests reveal distinct feeding modes in pinniped teeth

   https://doi.org/10.1242/jeb.244296  

   Peredo, Carlos Mauricio ; Ingle, Danielle N. ; Marshall, Christopher D. ( June 2022 , Journal of Experimental Biology)

   Marine mammals have undergone a dramatic series of morphological transformations throughout their evolutionary history that facilitated their ecological transition to life in the water. Pinnipeds are a diverse clade of marine mammals that evolved from terrestrial carnivorans in the Oligocene (∼27 million years ago). However, pinnipeds have secondarily lost the dental innovations emblematic of mammalian and carnivoran feeding, such as a talonid basin or shearing carnassials. Modern pinnipeds do not masticate their prey, but can reduce prey size through chopping behavior. Typically, small prey are swallowed whole. Nevertheless, pinnipeds display a wide breadth of morphology of the post-canine teeth. We investigated the relationship between dental morphology and pinniped feeding by measuring the puncture performance of the cheek-teeth of seven extant pinniped genera. Puncture performance was measured as the maximum force and the maximum energy required to puncture a standardized prey item (Loligo sp.). We report significant differences in the puncture performance values across the seven genera, and identify three distinct categories based on cheek-teeth morphology and puncture performance: effective, ineffective and moderate puncturers. In addition, we measured the overall complexity of the tooth row using two different metrics, orientation patch count rotated (OPCR) and relief index (RFI). Neither metric of complexity predicted puncture performance. Finally, we discuss these results in the broader context of known pinniped feeding strategies and lay the groundwork for subsequent efforts to explore the ecological variation of specific dental morphologies.

    

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4. 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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5. Lip service: Histological phenotypes correlate with diet and feeding ecology in herbivorous pacus

   https://doi.org/10.1002/ar.25075  

   Cohen, Karly E. ; Lucanus, Oliver ; Summers, Adam P. ; Kolmann, Matthew A. ( October 2022 , The Anatomical Record)

   Complex prey processing requires the repositioning of food between the teeth, as modulated by a soft tissue appendage like a tongue or lips. In this study, we trace the evolution of lips and ligaments, which are used during prey capture and prey processing in an herbivorous group of fishes. Pacus (Serrasalmidae) are Neotropical freshwater fishes that feed on leaves, fruits, and seeds. These prey are hard or tough, require high forces to fracture, contain abrasive or caustic elements, or deform considerably before failure. Pacus are gape‐limited and do not have the pharyngeal jaws many bony fishes use to dismantle and/or transport prey. Despite their gape limitation, pacus feed on prey larger than their mouths, relying on robust teeth and a hypertrophied lower lip for manipulation and breakdown of food. We used histology to compare the lip morphology across 14 species of pacus and piranhas to better understand this soft tissue. We found that frugivorous pacus have larger, more complex lips which are innervated and folded at their surface, while grazing species have callused, mucus‐covered lips. Unlike mammalian lips or tongues, pacu lips lack any intrinsic skeletal or smooth muscle. This implies that pacu lips lack dexterity; however, we found a novel connection to the primordial ligament which suggests that the lips are actuated by the jaw adductors. We propose that pacus combine hydraulic repositioning of prey inside the buccal cavity with direct oral manipulation, the latter using a combination of a morphologically heterodont dentition and compliant lips for reorienting food.

    

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