Vocal tract dynamics shape the formant structure of conditioned vocalizations in a harbor seal.

Formants, or resonance frequencies of the upper vocal tract, are an essential part of acoustic communication. Articulatory gestures-such as jaw, tongue, lip, and soft palate movements-shape formant structure in human vocalizations, but little is known about how nonhuman mammals use those gestures to modify formant frequencies. Here, we report a case study with an adult male harbor seal trained to produce an arbitrary vocalization composed of multiple repetitions of the sound wa. We analyzed jaw movements frame-by-frame and matched them to the tracked formant modulation in the corresponding vocalizations. We found that the jaw opening angle was strongly correlated with the first (F1) and, to a lesser degree, with the second formant (F2). F2 variation was better explained by the jaw angle opening when the seal was lying on his back rather than on the belly, which might derive from soft tissue displacement due to gravity. These results show that harbor seals share some common articulatory traits with humans, where the F1 depends more on the jaw position than F2. We propose further in vivo investigations of seals to further test the role of the tongue on formant modulation in mammalian sound production.

Chewing shrews: Examining the morphology and function of the masticatory musculature in Soricidae via diffusible iodine-based contrast-enhanced computed tomography.

Essential for sustaining a high metabolic rate is the efficient fragmentation of food, which is determined by molar morphology and the movement of the jaw. The latter is related to the jaw morphology and the arrangement of the masticatory muscles. Soricid jaw apparatuses are unique among mammals, as the articulation facet on the condylar process is separated into a dorsal and a ventral part, which has often been linked to more differentiated jaw motions. Soricidae also possess a remarkably elongated angular process. However, the precise function of the unique morphology of soricid jaw apparatuses has not been fully understood yet. By digitally reconstructing the masticatory musculature via the diffusible iodine-based contrast-enhanced computed tomography technique, we show how the unique jaw morphology is reflected in the spatial organization as well as the inner architecture and respective fascicle orientations of the muscles. From the lines of action of the m. masseter and the m. pterygoideus internus, both muscles inserting on the lateral and medial side of the angular process, respectively, we infer that the angular process is substantial for roll and yaw rotations of the mandible. The m. masseter is subdivided into four and the m. pterygoideus internus into five subunits, each exhibiting a slightly different line of action and torque. This enables Soricidae to adjust and adapt these rotational movements according to the properties of the ingested food, allowing for more efficient fragmentation. Additionally, those guided rotational motions allow for precise occlusion despite tooth wear. The temporalis is the largest of the adductor muscles and is mainly responsible for exerting the bite force. Overall, the unique jaw bone morphology in conjunction with the complex muscle arrangement may contribute towards a more efficient energy gain and the maintenance of a high metabolic rate, which is crucial for small-bodied mammals such as shrews.

Evolution of tooth morphological complexity and its association with the position of tooth eruption in the jaw in non-mammalian synapsids.

Heterodonty and complex molar morphology are important characteristics of mammals acquired during the evolution of early mammals from non-mammalian synapsids. Some non-mammalian synapsids had only simple, unicuspid teeth, whereas others had complex, multicuspid teeth. In this study, we reconstructed the ancestral states of tooth morphological complexity across non-mammalian synapsids to show that morphologically complex teeth evolved independently multiple times within Therapsida and that secondary simplification of tooth morphology occurred in some non-mammalian Cynodontia. In some mammals, secondary evolution of simpler teeth from complex molars has been previously reported to correlate with an anterior shift of tooth eruption position in the jaw, as evaluated by the dentition position relative to the ends of component bones used as reference points in the upper jaw. Our phylogenetic comparative analyses showed a significant correlation between an increase in tooth complexity and a posterior shift in the dentition position relative to only one of the three specific ends of component bones that we used as reference points in the upper jaw of non-mammalian synapsids. The ends of component bones depend on the shape and relative area of each bone, which appear to vary considerably among the synapsid taxa. Quantification of the dentition position along the anteroposterior axis in the overall cranium showed suggestive evidence of a correlation between an increase in tooth complexity and a posterior shift in the dentition position among non-mammalian synapsids. This correlation supports the hypothesis that a posterior shift of tooth eruption position relative to the morphogenetic fields that determine tooth form have contributed to the evolution of morphologically complex teeth in non-mammalian synapsids, if the position in the cranium represents a certain point in the morphogenetic fields.

Study of the influence of macro-structure and micro-structure on the mechanical properties of stag beetle upper jaw.

Macrostructural control of stress distribution and microstructural influence on crack propagation is one of the strategies for obtaining high mechanical properties in stag beetle upper jaws. The maximum bending fracture force of the stag beetle upper jaw is approximately 154, 000 times the weight of the upper jaw. Here, we explore the macro and micro-structural characteristics of two stag beetle upper jaws and reveal the resulting differences in mechanical properties and enhancement mechanisms. At the macroscopic level, the elliptic and triangular cross-sections of the upper jaw of the two species of stag beetles have significant effects on the formation of cracks. The crack generated by the upper jaws with a triangular section grows slowly and deflects easily. At the microscopic level, the upper jaw of the two species is a chitin cross-layered structure, but the difference between the two adjacent fiber layers at 45° and 50° leads to different deflection paths of the cracks on the exoskeleton. The mechanical properties of the upper jaw of the two species of stag beetle were significantly different due to the interaction of macro-structure and micro-structure. In addition, a series of bionic samples with different cross-section geometries and different fiber cross angles were designed, and mechanical tests were carried out according to the macro-structure and micro-structure characteristics of the stag beetle upper jaw. The effects of cross-section geometry and fiber cross angle on the mechanical properties of bionic samples are compared and analyzed. This study provides new ideas for designing and optimizing highly loaded components in engineering. STATEMENT OF SIGNIFICANCE: The upper jaw of the stag beetle is composed of a complex arrangement of chitin and protein fibers, providing both rigidity and flexibility. This structure is designed to withstand various mechanical stresses, including impacts and bending forces, encountered during its burrowing activities and interactions with its environment. The study of the upper jaw of the stag beetle can provide an efficient structural design for engineering components that are subjected to high loads. Understanding the relationship between structure and mechanical properties in the stag beetle upper jaw holds significant implications for biomimetic design and engineering.

Widespread convergence towards functional optimization in the lower jaws of crocodile-line archosaurs.

Extant crocodilian jaws are subject to functional demands induced by feeding and hydrodynamics. However, the morphological and ecological diversity of extinct crocodile-line archosaurs is far greater than that of living crocodilians, featuring repeated convergence towards disparate ecologies including armoured herbivores, terrestrial macropredators and fully marine forms. Crocodile-line archosaurs, therefore, present a fascinating case study for morphological and functional divergence and convergence within a clade across a wide range of ecological scenarios. Here, we build performance landscapes of two-dimensional theoretical jaw shapes to investigate the influence of strength, speed and hydrodynamics in the morphological evolution of crocodile-line archosaur jaws, and test whether ecologically convergent lineages evolved similarly optimal jaw function. Most of the 243 sampled jaw morphologies occupy optimized regions of theoretical morphospace for either rotational efficiency, resistance to Von Mises stress, hydrodynamic efficiency or a trade-off between multiple functions, though some seemingly viable shapes remain unrealized. Jaw speed is optimized only in a narrow region of morphospace whereas many shapes possess optimal jaw strength, which may act as a minimum boundary rather than a strong driver for most taxa. This study highlights the usefulness of theoretical morphology in assessing functional optimality, and for investigating form-function relationships in diverse clades.

A new biomechanical model of the mammal jaw based on load path analysis.

The primary function of the tetrapod jaw is to transmit jaw muscle forces to bite points. The routes of force transfer in the jaw have never been studied but can be quantified using load paths - the shortest, stiffest routes from regions of force application to support constraints. Here, we use load path analysis to map force transfer from muscle attachments to bite point and jaw joint, and to evaluate how different configurations of trabecular and cortical bone affect load paths. We created three models of the mandible of the Virginia opossum, Didelphis virginiana, each with a cortical bone shell, but with different material properties for the internal spaces: (1) a cortical-trabecular model, in which the interior space is modeled with bulk properties of trabecular bone; (2) a cortical-hollow model, in which trabeculae and mandibular canal are modeled as hollow; and (3) a solid-cortical model, in which the interior is modeled as cortical bone. The models were compared with published in vivo bite force and bone strain data, and the load paths calculated for each model. The trabecular model, which is preferred because it most closely approximates the actual morphology, was best validated by in vivo data. In all three models, the load path was confined to cortical bone, although its route within the cortex varied depending on the material properties of the inner model. Our analysis shows that most of the force is transferred through the cortical, rather than trabecular bone, and highlights the potential of load path analysis for understanding form-function relationships in the skeleton.

A multi-peak performance landscape for scale biting in an adaptive radiation of pupfishes.

The physical interactions between organisms and their environment ultimately shape diversification rates, but the contributions of biomechanics to evolutionary divergence are frequently overlooked. Here, we estimated a performance landscape for biting in an adaptive radiation of Cyprinodon pupfishes, including scale-biting and molluscivore specialists, and compared performance peaks with previous estimates of the fitness landscape in this system. We used high-speed video to film feeding strikes on gelatin cubes by scale eater, molluscivore, generalist and hybrid pupfishes and measured bite dimensions. We then measured five kinematic variables from 227 strikes using the SLEAP machine-learning model. We found a complex performance landscape with two distinct peaks best predicted gel-biting performance, corresponding to a significant non-linear interaction between peak gape and peak jaw protrusion. Only scale eaters and their hybrids were able to perform strikes within the highest performance peak, characterized by larger peak gapes and greater jaw protrusion. A performance valley separated this peak from a lower performance peak accessible to all species, characterized by smaller peak gapes and less jaw protrusion. However, most individuals exhibited substantial variation in strike kinematics and species could not be reliably distinguished by their strikes, indicating many-to-many mapping of morphology to performance. The two performance peaks observed in the lab were partially consistent with estimates of a two-peak fitness landscape measured in the wild, with the exception of the new performance peak for scale eaters. We thus reveal a new bimodal non-linear biomechanical model that connects morphology to performance to fitness in a sympatric radiation of trophic niche specialists.

Suction Feeding Turned on Its Head: A Functional Novelty Facilitates Lower Jaw Protrusion.

Functional novelties play important roles in creating new ways for organisms to access resources. In fishes, jaw protrusion has been attributed to the massive diversity of suction-based feeding systems, facilitating the dominant mode of prey capture in this group. Nearly all fishes that feed by suction use upper jaw protrusion, achieved by rotation of the mandible at its base, which then transmits forward motion to independently mobile upper jaw bones. In this study, by contrast, we explore an unusual form of lower jaw protrusion in the freshwater invertivore, Nannocharax fasciatus, enabled by a novel intramandibular joint (IMJ). We combine morphological, kinematic, and biomechanical data to show that the added mobility created by the IMJ influences the pattern of suction-based prey capture movements and contributes to lower jaw protrusion (increasing it by 25%, based on biomechanical modeling). Interestingly, the upper jaw bones are fused in N. fasciatus and rotate about a single fixed joint, like the lower jaws of most other suction feeding fishes. We suggest that this vertical inversion of the jaw protrusion mechanism for ventrally directed suction-feeding on benthic prey is a likely exaptation, as the IMJ is used for biting in related taxa. This work highlights the ability of novelties to facilitate ecological specialization by enabling new functional capabilities.

The effect of jaw suspension on cartilage strength in elasmobranchs.

The jaws and their supporting cartilages are tessellated in elasmobranchs and exhibit an abrupt increase in stiffness under compression. The major jaw-supporting cartilage, the hyomandibula, varies widely by shape and size and the extent of the load-bearing role is hypothesized to be inversely related to the number of craniopalatine articulations. Here, we test this hypothesis by evaluating the strength of the hyomandibular cartilage under compression in 13 species that represent all four jaw suspension systems in elasmobranchs (amphistyly, orbitostyly, hyostyly, and euhyostyly). The strength of the hyomandibular cartilages was measured directly using a material testing machine under compressive load, and indirectly by measuring morphological variables putatively associated with strength. The first measure of strength is force to yield (Fy), which was the peak force (N) exerted on the hyomandibula before plastic deformation. The second measure was compressive yield strength (σy, also called yield stress), which is calculated as peak force (N) before plastic deformation/cross-sectional area (mm2) of the specimen. Our results show that the load-bearing role of the hyomandibular cartilage, as measured by yield strength, is inversely related to the number of craniopalatine articulations, as predicted. Force to yield was lower for euhyostylic jaw suspensions and similar for the others. We also found that mineralization is associated with greater yield strength, while the second moment of area is associated with greater force to yield.

Variations in dietary patterns of living sloths revealed by finite element analysis of jaws.

Although extinct sloths exhibited a wide range of dietary habits, modes of locomotion, and occupied various niches across the Americas, modern sloths are considered quite similar in their habits. The dietary habits of living sloths can be directly observed in the wild, and understanding the mechanical behavior of their jaws during chewing through finite element analysis (FEA) provides a valuable validation tool for comparative analysis with their extinct counterparts. In this study, we used FEA to simulate the mechanical behavior of sloth mandibles under lateral mastication loads, using it as a proxy for oral processing. Our research focused on the six extant sloth species to better understand their diets and validate the use of FEA for studying their extinct relatives. We found that all living sloths have the predominancy of low-stress areas in their mandibles but with significant differences. Choloepus didactylus had larger high-stress areas, which could be linked to a reduced need for processing tougher foods as an opportunistic generalist. Bradypus variegatus and Choloepus hoffmanni are shown to be similar, displaying large low-stress areas, indicating greater oral processing capacity in a seasonal and more competitive environment. Bradypus torquatus, Bradypus pygmaeus, and Bradypus tridactylus exhibited intermediary processing patterns, which can be linked to a stable food supply in more stable environments and a reduced requirement for extensive oral processing capacity. This study sheds light on extant sloths' dietary adaptations and has implications for understanding the ecological roles and evolutionary history of their extinct counterparts.

Tradeoffs between bite force and gape in Eulemur and Varecia.

In 1974, Sue Herring described the relationship between two important performance variables in the feeding system, bite force and gape. These variables are inversely related, such that, without specific muscular adaptations, most animals cannot produce high bite forces at large gapes for a given sized muscle. Despite the importance of these variables for feeding biomechanics and functional ecology, the paucity of in vivo bite force data in primates has led to bite forces largely being estimated through ex vivo methods. Here, we quantify and compare in vivo bite forces and gapes with output from simulated musculoskeletal models in two craniofacially distinct strepsirrhines: Eulemur, which has a shorter jaw and slower chewing cycle durations relative to jaw length and body mass compared to Varecia. Bite forces were collected across a range of linear gapes from 16 adult lemurs (suborder Strepsirrhini) at the Duke Lemur Center in Durham, North Carolina representing three species: Eulemur flavifrons (n = 6; 3F, 3M), Varecia variegata (n = 5; 3F, 2M), and Varecia rubra (n = 5; 5F). Maximum linear and angular gapes were significantly higher for Varecia compared to Eulemur (p = .01) but there were no significant differences in recorded maximum in vivo bite forces (p = .88). Simulated muscle models using architectural data for these taxa suggest this approach is an accurate method of estimating bite force-gape tradeoffs in addition to variables such as fiber length, fiber operating range, and gapes associated with maximum force. Our in vivo and modeling data suggest Varecia has reduced bite force capacities in favor of absolutely wider gapes compared to Eulemur in relation to their longer jaws. Importantly, our comparisons validate the simulated muscle approach for estimating bite force as a function of gape in extant and fossil primates.

Ontogenetic changes in jaw leverage and skull shape in tufted and untufted capuchins.

The ontogeny of feeding is characterized by shifting functional demands concurrent with changes in craniofacial anatomy; relationships between these factors will look different in primates with disparate feeding behaviors during development. This study examines the ontogeny of skull morphology and jaw leverage in tufted (Sapajus) and untufted (Cebus) capuchin monkeys. Unlike Cebus, Sapajus have a mechanically challenging diet and behavioral observations of juvenile Sapajus suggest these foods are exploited early in development. Landmarks were placed on three-dimensional surface models of an ontogenetic series of Sapajus and Cebus skulls (n = 53) and used to generate shape data and jaw-leverage estimates across the tooth row for three jaw-closing muscles (temporalis, masseter, medial pterygoid) as well as a weighted combined estimate. Using geometric morphometric methods, we found that skull shape diverges early and shape is significantly different between Sapajus and Cebus throughout ontogeny. Additionally, jaw leverage varies with age and position on the tooth row and is greater in Sapajus compared to Cebus when calculated at the permanent dentition. We used two-block partial least squares analyses to identify covariance between skull shape and each of our jaw muscle leverage estimates. Sapajus, but not Cebus, has significant covariance between all leverage estimates at the anterior dentition. Our findings show that Sapajus and Cebus exhibit distinct craniofacial morphologies early in ontogeny and strong covariance between leverage estimates and craniofacial shape in Sapajus. These results are consistent with prior behavioral and comparative work suggesting these differences are a function of selection for exploiting mechanically challenging foods in Sapajus, and further emphasize that these differences appear quite early in ontogeny. This research builds on prior work that has highlighted the importance of understanding ontogeny for interpreting adult morphology.

Fossils document evolutionary changes of jaw joint to mammalian middle ear.

The dual jaw joint of Morganucodon1,2 consists of the dentary-squamosal joint laterally and the articular-quadrate one medially. The articular-quadrate joint and its associated post-dentary bones constitute the precursor of the mammalian middle ear. Fossils documenting the transition from such a precursor to the mammalian middle ear are poor, resulting in inconsistent interpretations of this hallmark apparatus in the earliest stage of mammaliaform evolution1-5. Here we report mandibular middle ears from two Jurassic mammaliaforms: a new morganucodontan-like species and a pseudotribosphenic shuotheriid species6. The morganucodontan-like species shows many previously unknown post-dentary bone morphologies1,2 and exhibits features that suggest a loss of load-bearing function in its articular-quadrate joint. The middle ear of the shuotheriid approaches the mammalian condition in that it has features that are suitable for an exclusively auditory function, although the post-dentary bones are still attached to the dentary. With size reduction of the jaw-joint bones, the quadrate shifts medially at different degrees in relation to the articular in the two mammaliaforms. These changes provide evidence of a gradual loss of load-bearing function in the articular-quadrate jaw joint-a prerequisite for the detachment of the post-dentary bones from the dentary7-12 and the eventual breakdown of the Meckel's cartilage13-15 during the evolution of mammaliaforms.

A cross-sectional study of the anatomy of the jaws of a central-European caucasian population using cone beam computer tomography as a prerequisite for designing pre-formed calcium phosphate cement scaffolds.

PURPOSE: This study aims to measure the cortical and cancellous bone thickness in the upper and lower jaws, serving as a data template for developing pre-defined calcium phosphate cement primary implant forms. These measurements are crucial for creating a biphasic scaffold. METHODS: Forty complete jaws were assessed for cortical bone shape and thickness using statistical analysis and specific software tools. Sex and age were considered, and four groups were created. RESULTS: The cumulative thickness of the cortical layer varied from region to region. In both the upper and lower jaws, the cortical layer in the molar region was significantly thicker than in the frontal region. Within the alveolar process, cortical thickness increases with distance from the alveolar crest on both sides. The oral side of the lower jaw is significantly thicker than the vestibular side. For the upper jaw, no significant differences between the oral and vestibular sides were found in this study. Additionally, it is noteworthy that men have a significantly thicker cortical layer than women. Regarding age, no significant overall differences were found. CONCLUSION: Mathematical analysis of anatomical forms using polynomial functions improves understanding of jaw anatomy. This approach facilitates the design of patient-specific scaffold structures, minimizing the need for costly and time-consuming planning and enabling more efficient implementation of optimal therapy.

Synthetic analysis of trophic diversity and evolution in Enantiornithes with new insights from Bohaiornithidae.

Enantiornithines were the dominant birds of the Mesozoic, but understanding of their diet is still tenuous. We introduce new data on the enantiornithine family Bohaiornithidae, famous for their large size and powerfully built teeth and claws. In tandem with previously published data, we comment on the breadth of enantiornithine ecology and potential patterns in which it evolved. Body mass, jaw mechanical advantage, finite element analysis of the jaw, and traditional morphometrics of the claws and skull are compared between bohaiornithids and living birds. We find bohaiornithids to be more ecologically diverse than any other enantiornithine family: Bohaiornis and Parabohaiornis are similar to living plant-eating birds; Longusunguis resembles raptorial carnivores; Zhouornis is similar to both fruit-eating birds and generalist feeders; and Shenqiornis and Sulcavis plausibly ate fish, plants, or a mix of both. We predict the ancestral enantiornithine bird to have been a generalist which ate a wide variety of foods. However, more quantitative data from across the enantiornithine tree is needed to refine this prediction. By the Early Cretaceous, enantiornithine birds had diversified into a variety of ecological niches like crown birds after the K-Pg extinction, adding to the evidence that traits unique to crown birds cannot completely explain their ecological success.

The birds living in the world today are only a small part of the larger bird family tree. Around 120 to 65 million years ago, when dinosaurs and other large reptiles roamed the world, the ancestors of modern-day birds were actually rather rare. Instead, another now extinct group of birds called the Enantiornithes (meaning "opposite birds") were the most common birds. Many researchers believe that Enantiornithes may have filled similar roles in ancient ecosystems as living birds do today. For example, some may have hunted other birds or animals, while some may have eaten only plants. Some may have specialized at eating a few specific foods while others may have been 'generalists' that ate many different foods. However, some of the physical features of Enantiornithes set them apart from modern-day birds. For example, unlike living birds, Enantiornithes had teeth and their wings were also constructed very differently. Previous studies suggest that one group of these extinct birds most likely ate insects and another group most likely ate fish, but it remains unclear what variety of foods opposite birds as a whole may have consumed. Miller et al. compared the jaws, claws and various other physical features of fossils from six additional species of opposite birds with the skeletons of modern birds to infer what the diets of these opposite birds may have been. This approach revealed that Enantiornithes may have had a wide variety of different diets. The researchers found that two species probably ate plants, another species most likely ate meat, and another one likely ate a mixture of both. With a large sample across Enantiornithes, Miller et al. were able to predict the diet of their common ancestor. They found the common ancestor to most likely be a 'generalist' eating variety of foods and that some species subsequently evolved to have more specialist diets. Opposite birds probably played many different roles in ecosystems, like living birds do today. Therefore, a better understanding how Enantiornithes evolved may shed light on the factors that have influenced the evolution of modern-day birds. This may aid future conservation efforts to target birds whose descendants may be able to take up the ecological roles of other species that go extinct.

The three-dimensionally articulated oral apparatus of a Devonian heterostracan sheds light on feeding in Palaeozoic jawless fishes.

Attempts to explain the origin and diversification of vertebrates have commonly invoked the evolution of feeding ecology, contrasting the passive suspension feeding of invertebrate chordates and larval lampreys with active predation in living jawed vertebrates. Of the extinct jawless vertebrates that phylogenetically intercalate these living groups, the feeding apparatus is well-preserved only in the early diverging stem-gnathostome heterostracans. However, its anatomy remains poorly understood. Here, we use X-ray microtomography to characterize the feeding apparatus of the pteraspid heterostracan Rhinopteraspis dunensis (Roemer, 1855). The apparatus is composed of 13 plates arranged approximately bilaterally, most of which articulate from the postoral plate. Our reconstruction shows that the oral plates were capable of rotating around the transverse axis, but likely with limited movement. It also suggests the nasohypophyseal organs opened internally, into the pharynx. The functional morphology of the apparatus in Rhinopteraspis precludes all proposed interpretations of feeding except for suspension/deposit feeding and we interpret the apparatus as having served primarily to moderate the oral gape. This is consistent with evidence that at least some early jawless gnathostomes were suspension feeders and runs contrary to macroecological scenarios that envisage early vertebrate evolution as characterized by a directional trend towards increasingly active food acquisition.

Skull osteology, neuroanatomy, and jaw-related myology of the pig-nosed turtle Carettochelys insculpta (Cryptodira, Trionychia).

The osteology, neuroanatomy, and musculature are known for most primary clades of turtles (i.e., "families"), but knowledge is still lacking for one particular clade, the Carettochelyidae. Carettochelyids are represented by only one living taxon, the pig-nosed turtle Carettochelys insculpta. Here, we use micro-computed tomography of osteological and contrast-enhanced stained specimens to describe the cranial osteology, neuroanatomy, circulatory system, and jaw musculature of Carettochelys insculpta. The jaw-related myology is described in detail for the first time for this taxon, including m. zygomaticomandibularis, a muscular unit only found in trionychians. We also document a unique arterial pattern for the internal carotid artery and its subordinate branches and provide an extensive list of osteological ontogenetic differences. The present work provides new insights into the craniomandibular anatomy of turtles and will allow a better understanding of the evolutionary history of the circulatory system of trionychians and intraspecific variation among turtles.

Deploy the proboscis!: Functional morphology and kinematics of a novel form of extreme jaw protrusion in the hingemouth, Phractolaemus ansorgii (Gonorynchiformes).

Premaxillary protrusion and the performance advantages it confers are implicated in the success of diverse lineages of teleost fishes, such as Cypriniformes and Acanthomorpha. Although premaxillary protrusion has evolved independently at least five times within bony fishes, much of the functional work investigating this kinesis relates to mechanisms found only in these two clades. Few studies have characterized feeding mechanisms in less-diverse premaxilla-protruding lineages and fewer yet have investigated the distinctive anatomy underlying jaw kinesis in these lineages. Here, we integrated dissection, clearing and staining, histology, micro-CT, and high-speed videography to investigate an isolated and independent origin of jaw protrusion in the hingemouth, Phractolaemus ansorgii, which employs a complex arrangement of bones, musculature, and connective tissues to feed on benthic detritus via a deployable proboscis. Our goals were to provide an integrative account of the underlying architecture of P. ansorgii's feeding apparatus and to assess the functional consequences of this drastic deviation from the more typical teleost condition. Phractolaemus ansorgii's cranial anatomy is distinct from all other fishes in that its adducted lower jaw is caudally oriented, and it possesses a mouth at the terminal end of an elongated, tube-like proboscis that is unique in its lack of skeletal support from the oral jaws. Instead, its mouth is supported primarily by hyaline-cell cartilage and other rigid connective tissues, and features highly flexible lips that are covered in rows of keratinous unculi. Concomitant changes to the adductor musculature likely allow for the flexibility to protrude the mouth dorsally and ventrally as observed during different feeding behaviors, while the intrinsic compliance of the lips allows for more effective scraping of irregular surfaces. From our feeding videos, we find that P. ansorgii is capable of modulating the distance of protrusion, with maximum anterior protrusion exceeding 30% of head length. This represents a previously undescribed example of extreme jaw protrusion on par with many acanthomorph species. Protrusion is much slower in P. ansorgii-reaching an average speed of 2.74 cm/s-compared to acanthomorphs feeding on elusive prey or even benthivorous cypriniforms. However, this reorganization of cranial anatomy may reflect a greater need for dexterity to forage more precisely in multiple directions and on a wide variety of surface textures. Although this highly modified mechanism may have limited versatility over evolutionary timescales, it has persisted in solitude within Gonorynchiformes, representing a novel functional solution for benthic feeding in tropical West African rivers.

Assessing the adductor musculature and jaw mechanics of Proterochampsa nodosa (Archosauriformes: Proterochampsidae) through finite element analysis.

Proterochampsids are a group of South American nonarchosaurian archosauromorphs whose general morphology has been historically likened to that of the extant Crocodylia, which purportedly exhibited similar habits by convergence. Taxa from the genus Proterochampsa, for example, show platyrostral skulls with dorsally faced orbits and external nares and elongated snouts that might indicate a feeding habit similar to that of crocodilians. Nonetheless, some aspects of their craniomandibular anatomy are distinct. Proterochampsa has comparatively larger skull temporal fenestrae, and a unique morphology of the mandibular adductor chamber, with a remarkably large surangular shelf and a fainter retroarticular region in the mandible. In light of this, we conducted biomechanical tests on a 3-dimensional model of Proterochampsa nodosa including the first Finite Element Analysis for proterochampsians and compared it with models of the extant crocodylians Tomistoma schlegelii and Alligator mississippiensis. Our analyses suggested that, despite the differences in adductor chamber, Proterochampsa was able to perform bite forces comparable to those modeled for Alligator and significantly higher than Tomistoma. However, the morphology of the surangular shelf and the adductor chamber of Proterochampsa renders it more prone to accumulate stresses resulting from muscle contraction, when compared with both analogs. The elongated lower jaw of Proterochampsa, like that of Tomistoma, is more susceptible to bending, when compared with Alligator. As a result, we suggest that Proterochampsa might employ anteriorly directed bites only when handling small and soft-bodied prey. In addition, Proterochampsa exemplifies the diversity of arrangements that the adductor musculature adopted in different diverging archosauromorph groups.