Built to Bite: Feeding Kinematics, Bite Forces, and Head Shape of a Specialized Durophagous Lizard, Dracaena Guianensis (Teiidae)

Clade-wide variation in bite-force performance is determined primarily by size, not ecology

Performance traits are tightly linked to the fitness of organisms. However, because studies of variation in performance traits generally focus on just one or several closely related species, we are unable to draw broader conclusions about how and why these traits vary across clades. One important performance trait related to many aspects of an animal's life history is bite-force. Here, we use a clade-wide phylogenetic comparative approach to investigate relationships between size, head dimensions and bite-force among lizards and tuatara (lepidosaurs), using the largest bite-force dataset collated to date for any taxonomic group. We test four predictions: that bite-force will be greater in larger species, and for a given body size, bite-force will be greatest in species with acrodont tooth attachment, herbivorous diets, and non-burrowing habits. We show that bite-force is strongly related to body and head size across lepidosaurs and, as predicted, larger species have the greatest bite-forces. Contrary to our other predictions, tooth attachment, diet and habit have little predictive power when accounting for size. Herbivores bite more forcefully simply because they are larger. Our results also highlight priorities for future sampling to further enhance our understanding of broader evolutionary patterns.

Body size miniaturization in a lineage of colubrid snakes: Implications for cranial anatomy

As body size strongly determines the biology of an organism at all levels, it can be expected that miniaturization comes with substantial structural and functional constraints. Dwarf snakes of the genus Eirenis are derived from big, surface-dwelling ancestors, considered to be similar to those of the sister genus Dolichophis. To better understand the structural implications of miniaturization on the feeding apparatus in Eirenis, the morphology of the cranial musculoskeletal system of Dolichophis schmidti was compared with that of the miniature Eirenis punctatolineatus and E. persicus using high-resolution µCT data. The gape index was compared between D. schmidti and 14 Eirenis species. Our results show a relatively increased neurocranium size and decreased maximal jaw muscle force in E. persicus, compared with the D. schmidti, and an intermediate situation in E. punctatolineatus. A significant negative allometry in gape index relative to body size is observed across the transition from the Dolichophis to Pediophis and Eirenis subgenera. However, the gape index relative to head size showed a significant negative allometry only across the transition from the Dolichophis to Pseudocyclophis subgenus. In Dolichophis-Eirenis dwarfing lineages, different structural patterns are observed through miniaturization, indicating that overcoming the challenge of miniaturization has achieved via different adaptations.

Bite force data suggests relationship between acrodont tooth implantation and strong bite force

Extant and extinct reptiles exhibit numerous combinations of tooth implantation and attachment. Tooth implantation ranges from those possessing roots and lying within a socket (thecodonty), to teeth lying against the lingual wall of the jawbone (pleurodonty), to teeth without roots or sockets that are attached to the apex of the marginal jawbones (acrodonty). Attachment may be ligamentous (gomphosis) or via fusion (ankylosis). Generally speaking, adaptative reasonings are proposed as an underlying driver for evolutionary changes in some forms of tooth implantation and attachment. However, a substantiated adaptive hypothesis is lacking for the state of acrodont ankylosis that is seen in several lineages of Lepidosauria, a clade that is plesiomorphically pleurodont. The convergent evolution of acrodont ankylosis in several clades of lepidosaurs suggests a selective pressure shaped the evolution of the trait. We hypothesize that acrodont ankylosis as seen in Acrodonta and Sphenodon punctatus, is an adaptation either resulting from or allowing for a stronger bite force. We analyzed bite force data gathered from the literature to show that those taxa possessing acrodont dentition possess a stronger bite force on average than those taxa with pleurodont dentition. Dietary specialists with pleurodont dentition may also possess relatively high bite forces, though body size may also play a role in their ability to bite hard. Furthermore, our results have implications for the evolution of acrodont ankylosis and potential behaviors related to strong bite force that influenced the evolution of acrodonty within Acrodonta and Rhynchocephalia.

Beyond Description: The Many Facets of Dental Biomechanics

Teeth lie at the interface between an animal and its environment and, with some exceptions, act as a major component of resource procurement through food acquisition and processing. Therefore, the shape of a tooth is closely tied to the type of food being eaten. This tight relationship is of use to biologists describing the natural history of species and given the high instance of tooth preservation in the fossil record, is especially useful for paleontologists. However, correlating gross tooth morphology to diet is only part of the story, and much more can be learned through the study of dental biomechanics. We can explore the mechanics of how teeth work, how different shapes evolved, and the underlying forces that constrain tooth shape. This review aims to provide an overview of the research on dental biomechanics, in both mammalian and non-mammalian teeth, and to synthesize two main approaches to dental biomechanics to develop an integrative framework for classifying and evaluating dental functional morphology. This framework relates food material properties to the dynamics of food processing, in particular how teeth transfer energy to food items, and how these mechanical considerations may have shaped the evolution of tooth morphology. We also review advances in technology and new techniques that have allowed more in-depth studies of tooth form and function.

The Key to Understanding the European Miocene Chalcides (Squamata, Scincidae) Comes from Asia: The Lizards of the East Siberian Tagay Locality (Baikal Lake) in Russia

The early middle Miocene (MN 5) lizards from the East Siberian Tagay locality (Baikal Lake, Russia) in Asia are described here. The lizard fauna consists of two clades, Lacertidae and Scincidae. The skink material is allocated to Chalcides. While this taxon was previously reported from Europe, it has rarely been observed in the Neogene record with only jaw fragments and frontal bones described. Its taxonomy was therefore enigmatic. The Tagay material is almost identical to the European fossils of Chalcides from Austria and Hungary, but it also contains the parietal bone. While the material is also similar to the extant Ch. ocellatus, it exhibits several morphological differences. A new species is therefore erected-Chalcides augei sp. nov. These findings further support the connection of the Baikal Lake area with central Europe during the first half of the Miocene. The comparative anatomy of the frontals, parietals and lower jaws was evaluated by micro-CT in selected skink taxa. This comparison highlights several important differences, for example, paired frontals are present in Broadleysaurus (an outgroup taxon), in Acontias and all studied members of Scincidae herein. The character optimization in Mesquite supports fused frontals as being the condition at the basal node of the Ateuchosauridae + Sphenomorphidae + Eugongylidae + Lygosomidae + Egerniidae + Mabuyidae clade. While the parapineal foramen is restricted to the parietal in most taxa studied herein, it is absent (or vestigial) in Acontias and Feylinia. In contrast to all other skinks, this foramen is located on the frontal in Ateuchosaurus chinensis. Anat Rec, 2019. © 2019 American Association for Anatomy Anat Rec, 303:1901-1934, 2020. © 2019 American Association for Anatomy.

Neutron scanning reveals unexpected complexity in the enamel thickness of an herbivorous Jurassic reptile

Eilenodontines are one of the oldest radiation of herbivorous lepidosaurs (snakes, lizards and tuatara) characterized by batteries of wide teeth with thick enamel that bear mammal-like wear facets. Unlike most reptiles, eilenodontines have limited tooth replacement, making dental longevity particularly important to them. We use both X-ray and neutron computed tomography to examine a fossil tooth from the eilenodontine Eilenodon (Late Jurassic, USA). Of the two approaches, neutron tomography was more successful and facilitated measurements of enamel thickness and distribution. We find the enamel thickness to be regionally variable, thin near the cusp tip (0.10 mm) but thicker around the base (0.15–0.30 mm) and notably greater than that of other rhynchocephalians such as the extant Sphenodon (0.08–0.14 mm). The thick enamel in Eilenodon would permit greater loading, extend tooth lifespan and facilitate the establishment of wear facets that have sharp edges for orally processing plant material such as horsetails (Equisetum). The shape of the enamel dentine junction indicates that tooth development in Eilenodon and Sphenodon involved similar folding of the epithelium but different ameloblast activity.

An Enigmatic Small Neosuchian Crocodyliform from the Woodbine Formation of Texas

New discoveries at the Arlington Archosaur Site (AAS), a Cenomanian (Late Cretaceous) locality in north-central Texas, are filling gaps in our knowledge of mid-Cretaceous Appalachian ecosystems, which remain poorly characterized. The AAS is notable because it preserves a diverse crocodyliform record. As seen in other sites that preserve four or more crocodyliform taxa, the species present at the AAS exhibit different snout shapes and body sizes, indicating that this high diversity of sympatric species was likely sustainable due to niche partitioning. Here we describe Scolomastax sahlsteini gen. et sp. nov., a new species of crocodyliform from the AAS, currently known from a partial right mandibular ramus. This species differs from other crocodyliforms in possessing features associated with durophagy or omnivory, including a shortened mandible, reduced tooth count, heterodonty, a dorsally expanded surangular, and enlarged attachments for jaw adductor muscles. Our phylogenetic analysis places this new taxon within Eusuchia as a member of Paralligatoridae and sister taxon to Paralligator gradilifrons. Scolomastax sahlsteini extends the record of paralligatorids into the Late Cretaceous of North America. This discovery represents the first appearance of this clade on the poorly known landmass of Appalachia, supporting a biogeographic connection between North America and Asia in the Early Cretaceous prior to completion of the Western Interior Seaway. However, relationships among other endemic crocodyliforms and tree instability within Paralligatoridae suggest further analysis is needed to resolve phylogenetic and biogeographic relationships (http://zoobank.org/urn:lsid:zoobank.org:pub:DC114471-6687-4BB5-8FAE-96F7278B1DAF). Anat Rec, 303:801-812, 2020. © 2019 Wiley Periodicals, Inc.

Specialized specialists and the narrow niche fallacy: a tale of scale-feeding fishes

Although rare within the context of 30 000 species of extant fishes, scale-feeding as an ecological strategy has evolved repeatedly across the teleost tree of life. Scale-feeding (lepidophagous) fishes are diverse in terms of their ecology, behaviour, and specialized morphologies for grazing on scales and mucus of sympatric species. Despite this diversity, the underlying ontogenetic changes in functional and biomechanical properties of associated feeding morphologies in lepidophagous fishes are less understood. We examined the ontogeny of feeding mechanics in two evolutionary lineages of scale-feeding fishes: Roeboides, a characin, and Catoprion, a piranha. We compare these two scale-feeding taxa with their nearest, non-lepidophagous taxa to identify traits held in common among scale-feeding fishes. We use a combination of micro-computed tomography scanning and iodine staining to measure biomechanical predictors of feeding behaviour such as tooth shape, jaw lever mechanics and jaw musculature. We recover a stark contrast between the feeding morphology of scale-feeding and non-scale-feeding taxa, with lepidophagous fishes displaying some paedomorphic characters through to adulthood. Few traits are shared between lepidophagous characins and piranhas, except for their highly-modified, stout dentition. Given such variability in development, morphology and behaviour, ecological diversity within lepidophagous fishes has been underestimated.

The biomechanical role of the chondrocranium and sutures in a lizard cranium

The role of soft tissues in skull biomechanics remains poorly understood. Not least, the chondrocranium, the portion of the braincase which persists as cartilage with varying degrees of mineralization. It also remains commonplace to overlook the biomechanical role of sutures despite evidence that they alter strain distribution. Here, we examine the role of both the sutures and the chondrocranium in the South American tegu lizard Salvator merianae We use multi-body dynamics analysis (MDA) to provide realistic loading conditions for anterior and posterior unilateral biting and a detailed finite element model to examine strain magnitude and distribution. We find that strains within the chondrocranium are greatest during anterior biting and are primarily tensile; also that strain within the cranium is not greatly reduced by the presence of the chondrocranium unless it is given the same material properties as bone. This result contradicts previous suggestions that the anterior portion (the nasal septum) acts as a supporting structure. Inclusion of sutures to the cranium model not only increases overall strain magnitudes but also leads to a more complex distribution of tension and compression rather than that of a beam under sagittal bending.

The evolution of bite force in horned lizards: the influence of dietary specialization

Dietary specialization is an important driver of the morphology and performance of the feeding system in many organisms, yet the evolution of phenotypic specialization has only rarely been examined within a species complex. Horned lizards are considered primarily myrmecophagous (ant eating), but variation in diet among the 17 species of horned lizards (Phrynosoma) makes them an ideal group to examine the relationship between dietary specialization and the resultant morphological and functional changes of the feeding system. In this study, we perform a detailed analysis of the jaw adductor musculature and use a biomechanical model validated with in vivo bite force data to examine the evolution of bite force in Phrynosoma. Our model simulations demonstrate that bite force varies predictably with respect to the gape angle and bite position along the tooth row, with maximal bite forces being attained at lower gape angles and at the posterior tooth positions. Maximal bite forces vary considerably among horned lizards, with highly myrmecophagous species exhibiting very low bite forces. In contrast, members of the short-horned lizard clade are able to bite considerably harder than even closely related dietary generalists. This group appears to be built for performing crushing bites and may represent a divergent morphology adapted for eating hard prey items. The evolutionary loss of processing morphology (teeth, jaw and muscle reduction) and bite force in ant specialists may be a response to the lack of prey processing rather than a functional adaptation per se.

On the evolutionary advantage of multi-cusped teeth

A hallmark of mammalian evolution is a progressive complexity in postcanine tooth morphology. However, the driving force for this complexity remains unclear: whether to expand the versatility in diet source, or to bolster tooth structural integrity. In this study, we take a quantitative approach to this question by examining the roles of number, position and height of multiple cusps in determining sustainable bite forces. Our approach is to use an extended finite-element methodology with due provision for step-by-step growth of an embedded crack to determine how fracture progresses with increasing occlusal load. We argue that multi-cusp postcanine teeth are well configured to withstand high bite forces provided that multiple cusps are contacted simultaneously to share the load. However, contact on a single near-wall cusp diminishes the strength. Location of the load points and cusp height, rather than cusp number or radius, are principal governing factors. Given these findings, we conclude that while complex tooth structures can enhance durability, increases in cusp number are more likely to be driven by the demands of food manipulation. Structural integrity of complex teeth is maintained when individual cusps remain sufficiently distant from the side walls and do not become excessively tall relative to tooth width.

Large batoid fishes frequently consume stingrays despite skeletal damage

The shapes of vertebrate teeth are often used as hallmarks of diet. Here, however, we demonstrate evidence of frequent piscivory by cartilaginous fishes with pebble-like teeth that are typically associated with durophagy, the eating of hard-shelled prey. High-resolution micro-computed tomography observation of a jaw specimen from one batoid species and visual investigation of those of two additional species reveal large numbers of embedded stingray spines, arguing that stingray predation of a scale rivalling that of the largest carnivorous sharks may not be uncommon for large, predatory batoids with rounded, non-cutting dentition. Our observations demonstrate that tooth morphology is not always a reliable indicator of diet and that stingray spines are not as potent a deterrent to predation as normally believed. In addition, we show that several spines in close contact with the jaw skeleton of a wedgefish (Rhynchobatus) have become encased in a disorganized mineralized tissue with a distinctive ultrastructure, the first natural and unequivocal evidence of a callus-building response in the tessellated cartilage unique to elasmobranch skeletons. Our findings reveal sampling and analysis biases in vertebrate ecology, especially with regard to the role of large, predatory species, while also illustrating that large body size may provide an escape from anatomical constraints on diet (e.g. gape size, specialist dentition). Our observations inform our concepts of skeletal biology and evolution in showing that tessellated cartilage-an ancient alternative to bone-is incapable of foreign tissue resorption or of restoring damaged skeletal tissue to its original state, and attest to the value of museum and skeletal specimens as records of important aspects of animal life history.

Ontogeny of bite force in a validated biomechanical model of the American alligator

Three-dimensional computational modeling offers tools with which to investigate forces experienced by the skull encountered during feeding and other behaviors. American alligators (Alligator mississippiensis) generate some of the highest measured bite forces among extant tetrapods. A concomitant increase in bite force accompanies ontogenetic increases in body mass, which has been linked with dietary changes as animals increase in size. Because the flattened skull of crocodylians has substantial mediolaterally-oriented muscles, they are an excellent model taxon in which to explore the role of mediolateral force components experienced by the feeding apparatus. Many previous modeling studies of archosaur cranial function focused on planar analysis, ignoring the mediolateral aspects of cranial forces. Here we use three-dimensionally accurate anatomical data to resolve 3D muscle forces. Using dissection, imaging, and computational techniques, we developed lever and finite element models of an ontogenetic series of alligators to test the effects of size and shape on cranial loading and compared estimated bite forces to those previously measured in vivo in Alligator mississippiensis. We found that modeled forces matched in vivo data well for intermediately sized individuals, and somewhat overestimated force in smaller specimens and underestimated force in larger specimens, suggesting that ontogenetically static muscular parameters and bony attachment sites alone cannot account for all the variation in bite force. Adding aponeurotic muscle attachments would likely improve force predictions, but such data are challenging to model and integrate into analyses of extant taxa and are generally unpreserved in fossils. We conclude that anatomically accurate modeling of muscles can be coupled with finite element and lever analyses to produce reliable, reasonably accurate estimate bite forces and thus both skeletal and joint loading, with known sources of error, which can be applied to extinct taxa.

X-ray microtomography provides first data about the feeding behaviour of an endangered lizard, the Montserrat galliwasp (Diploglossus montisserrati)

Reporting the diet of recently extinct or very rare taxa, only known by a few museum specimens, is challenging. This study uses X-ray microtomography, a non-destructive investigation method, to obtain the first data about feeding behaviours in the Montserrat galliwasp (Diploglossus montisserrati) by scanning one of the two specimens known to date. The scans revealed the occurrence of shell fragments of a freshwater snail (Omalonyx matheroni) in the digestive tract of the specimen. This data combined with morphological evidence shows the occurrence of a durophagous feeding habit and a possible tendency of association with freshwater environments. This information could be crucial to save this critically endangered lizard endemic on Montserrat island.

Illustrating ontogenetic change in the dentition of the Nile monitor lizard, Varanus niloticus: a case study in the application of geometric morphometric methods for the quantification of shape-size heterodonty

Many recent attempts have been made to quantify heterodonty in non-mammalian vertebrates, but the majority of these are limited to Euclidian measurements. One taxon frequently investigated is Varanus niloticus, the Nile monitor. Juveniles possess elongate, pointed teeth (caniniform) along the entirety of the dental arcade, whereas adults develop large, bulbous distal teeth (molariform). The purpose of this study was to present a geometric morphometric method to quantify V. niloticus heterodonty through ontogeny that may be applied to other non-mammalian taxa. Data were collected from the entire tooth row of 19 dry skull specimens. A semilandmark analysis was conducted on the outline of the photographed teeth, and size and shape were derived. Width was also measured with calipers. From these measures, sample ranges and allometric functions were created using multivariate statistical analyses for each tooth position separately, as well as overall measures of heterodonty for each specimen based on morphological disparity. The results confirm and expand upon previous studies, showing measurable shape-size heterodonty in the species with significant differences at each tooth position. Tooth size increases with body size at most positions, and the allometric coefficient increases at more distal positions. Width shows a dramatic increase at the distal positions with ontogeny, often displaying pronounced positive allometry. Dental shape varied in two noticeable ways, with the first composing the vast majority of shape variance: (i) caniniformy vs. molariformy and (ii) mesially leaning, 'rounded' apices vs. distally leaning, 'pointed' apices. The latter was twice as influential in the mandible, a consequence of host bone shape. Mesial teeth show no significant shape change with growth, whereas distal teeth change significantly due primarily to an increase in molariformy. Overall, heterodonty increases with body size concerning both tooth size and shape, but shape heterodonty changes in the mandible are much less pronounced. Although it is unclear to what degree V. niloticus specializes in hard prey items (durophagy), previous studies of varanid feeding behavior, along with research on analogous durophagous vertebrates, indicate a division of labor along the tooth row in adults, due to a possible transition to at least a partial durophagous niche. The geometric morphometric method proposed here, although not without its own limitations, may be ideal for use with a number of dental morphotypes in the future.

Feeding performance of king Mackerel, Scomberomorus cavalla

Feeding performance is an organism's ability to capture and handle prey. Although bite force is a commonly used metric of feeding performance, other factors such as bite pressure and strike speed are also likely to affect prey capture. Therefore, this study investigated static bite force, dynamic speeds, and predator and prey forces resulting from ram strikes, as well as bite pressure of the king mackerel, Scomberomorus cavalla, in order to examine their relative contributions to overall feeding performance. Theoretical posterior bite force ranged from 14.0-318.7 N. Ram speed, recorded with a rod and reel incorporated with a line counter and video camera, ranged from 3.3-15.8B L/s. Impact forces on the prey ranged from 0.1-1.9 N. Bite pressure, estimated using theoretical bite forces at three gape angles and tooth cross-sectional areas, ranged from 1.7-56.9 MPa. Mass-specific bite force for king mackerel is relatively low in comparison with other bony fishes and sharks, with relatively little impact force applied to the prey during the strike. This suggests that king mackerel rely on high velocity chases and high bite pressure generated via sharp, laterally compressed teeth to maximize feeding performance.

The three-dimensional morphological effects of captivity

Many captive animals are fed diets that are drastically different in mechanical properties than their wild diet. Most captive pantherines are fed a nutritionally supplemented diet consisting almost entirely of ground meat. While many zoos supplement this diet with bones, the fact remains that large captive felids are fed diets that require substantially less masticatory effort than those of their wild counterparts. The osteological effects of this dietary difference have not been fully evaluated. To this end, we compared linear measurements and 3D geometric morphometric landmarks of captive and wild lions and tigers. Using Principal Component (PC) analysis of the linear measurements, not only were the sexes and species statistically distinct, but so too was the population clearly divisible in terms of captivity status. The 3D analysis supported these findings: although the most influential variable in the sample (PC1, 21.5% of the variation) separates the two species, the second most influential contributor (PC2) to the overall skull shape is driven not by the sex differences in these highly dimorphic species, but rather by their captivity status. In fact, captivity status drives nearly twice as much of the 3D variation as sexual dimorphism (14.8% vs. 8.0% for PC2 vs. PC3). Thus the shape is influenced nearly twice as much by whether the animal was captive or wild than by whether it was male or female. If a causal relationship can be demonstrated between dietary mechanical properties and morphology, people who oversee the diets of captive carnivores should consider modifying these diets to account for not only nutritional but also the mechanical properties of a carcass-based diet as well. In addition to the husbandry implications, our analyses show the ways in which captive specimens are different than their wild counterparts--ndings that have implications for morphologists when considering anatomical samples.