Jaw mechanism and dental function in the late cretaceous basal eusuchian Iharkutosuchus

Iharkutosuchus makadii is a basal eusuchian crocodylian with multicusped teeth discovered from the Upper Cretaceous of Hungary. Skull and dentition morphology indicates an active food processing for this crocodylian. First among crocodylians, a combination of different analyses, including cranial adductor muscle reconstruction, tooth wear pattern, and enamel microstructure studies, is applied here to support this hypothesis. Data provide unambiguous evidence for significant dental occlusion that was a result of a unique, transverse mandibular movement. Reconstruction of the jaw adductors demonstrates strong muscles responsible for slow but active jaw closure as the motor of transverse jaw movement; nevertheless muscles producing rapid jaw closure were reduced. Macrowear orientations show a dominantly transverse movement of the mandibles completed by a slight anteroposterior component. Along with quadrate morphology, macrowear further indicates that this motion was accomplished by alternate rotation of the mandibles about the quadrate condyles. Dental morphology and wear patterns suggest two types of power stroke: a slicing–crushing stroke associated dominantly with anterior tooth–food–tooth contact (with a low degree of transverse mandibular movement) during in the early stage of mastication, and a grinding stroke with significant posterior tooth–tooth contact and a dynamic transverse movement occurring later. The patterns of microwear show a diverse diet for Iharkutosuchus including both soft and hard items. This is also supported by the microstructure of the thick, wrinkled enamel built up mostly by poorly developed columnar units. Based on wear patterns, ontogenetic variation in feeding habits of Iharkutosuchus is also recognized. J. Morphol., 2009. © 2009 Wiley‐Liss, Inc.     

Feeding mechanisms of the sauropod dinosaurs Brachiosaurus,Camarasaurus, Diplodocus and Dicraeosaurus

Skull morphologies and dental wear patterns have been examined in four sauropod genera to evaluate their probable feeding mechanisms. Wear facets on teeth are generally confined to their apices in Brachiosaurus and Dicraeosaurus and they are sometimes also present on the mesial and distal carinae. Skull morphology and dental wear patterns in Diplodocus and Dicraeosaurus are consistent with a raking motion of the jaws during feeding. Diplodocus became mechanically adapted to feed in this way by evolving anteriorly directed teeth in the premaxilla and mesial parts of the maxilla, and by changing the direction of jaw adduction relative to the long axis of the skull. Similar features are present in the few known skulls of Apatosaurus and they may also have been present in Dicraeosaurus. In Brachiosaurus dental wear patterns also imply a raking motion of the jaws, although the more robust skull and teeth and the more vertically directed action of the jaw adductor muscles have led some to suggest the possibility of isognathous occlusion. Camarasaurus employed a powerful bite in its feeding, possibly with slight propaliny of the lower jaw, and its skull was modified to cope with increased stresses arising from mastication. Archaic sauropods appear largely to have employed isognathic occlusion in chopping off vegetation. The raking motion employed by diplodocids and dicraeosaurids was an advanced mode of cropping and stripping, linked evolutionarily to their highly apomorphic cranial morphology.     

Cranial anatomy of Shunosaurus, a basal sauropod dinosaur from the Middle Jurassic of China

Shunosaurus, from the Middle Jurassic of China, is probably the best‐known basal sauropod and is represented by several complete skeletons. It is unique among sauropods in having a small, bony club at the end of its tail. New skull material provides critical information about its anatomy, brain morphology, tooth replacement pattern, feeding habits and phylogenetic relationships. The skull is akinetic and monimostylic. The brain is relatively small, narrow and primitively designed. The tooth replacement pattern exhibits back to front replacement waves in alternating tooth position. The teeth are spatulate, stout and show well‐developed wear facets indicative of coarser plant food. Upper and lower tooth rows interdigitate and shear past each other. Tooth morphology, skull architecture, and neck posture indicate that Shunosaurus was adapted to ground feeding or low browsing. Shunosaurus exhibits the following cranial autapomorphies: emargination of the ventral margin of the jugal/quadratojugal bar behind the tooth row; postorbital contains a lateral pit; vomers do not participate in the formation of the choanae; pterygoid is extremely slender and small with a dorsal fossa; quadrate ramus of the pterygoid is forked; quadratojugal participates in the jaw articulation; tooth morphology is a combination of cylindrical and spatulate form; basipterygoid process is not wrapped by the caudal process of the pterygoid; trochlear nerve has two exits; occlusal level of the maxillary tooth row is convex downward, whereas that of the dentary is concave upward, acting like a pair of garden shears; dentary tooth count is 25 or more; and the replacing teeth invade the labial side of the functional teeth. Cranial characters among the basal sauropods are reviewed. As Shunosaurus is the earliest sauropod for which cranial remains are known, it occupies an important position phylogenetically, showing the modification of skull morphology from the prosauropod condition. Although the skull synapomorphies of Sauropoda are unknown at present, 27 cranial synapomorphies are known for the clade Eusauropoda. © 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136 , 145?169.     

Variation in Prey-resource Utilization and Oral Jaw Gape Between two Populations of Largemouth Bass, Micropterus Salmoides

Ontogenetic changes in diet and jaw gape were compared between two indigenous populations of largemouth bass, Micropterus salmoides, to test the hypothesis that ecomorphology varies among broadly distributed fish populations. Two hundred seventy-two temperate (southwestern Michigan) M. salmoides and 265 subtropical (east-central Florida) conspecifics were analyzed for food habits and oral jaw gape height and width. Percent volumetric contributions of four functional prey categories (plankton, insect, crustacean, and fish) were compared among fish-size classes to determine if interval-specific differences in prey consumption existed between populations. Subtropical M. salmoides shifted from feeding on plankton and macroinvertebrates to fish by 20mm standard length (SL); and stopped consuming plankton by 29mm SL. Temperate largemouth bass did not become piscivorous until 37mm SL and continued utilizing plankton up to 69mm SL. Following the onset of piscivory, 100–260mm SL subtropical largemouth bass began utilizing more crustaceans than fish. In contrast, temperate M. salmoides consistently fed on fish following the onset of piscivory. Variations in food habits were associated with differences in gape size between temperate and subtropical populations. Temperate largemouth bass had significantly larger gape height (ANCOVA: F = 103.4; df = 1,536; p < 0.001) and width (ANCOVA: F = 47.0; df = 1,536; p < 0.001) than subtropical bass. Although piscivory is a well-known feature of M. salmoides, the ontogeny of piscivory may vary between populations. We hypothesize that interpopulation differences in jaw gape may be related to variations in prey-resource utilization.     

Gape size,its morphological basis,and the validity of gape indices in western diamond‐backed rattlesnakes (crotalus atrox)

Maximum gape is important to the ecology and evolution of many vertebrates, particularly gape‐limited predators, because it can restrict the sizes and shapes of prey that can be eaten. Although many cranial elements probably contribute to gape, it is typically estimated from jaw length or jaw width, or occasionally from a combination of these two measures. We measured maximum gape directly for 18 individuals of the western diamond‐backed rattlesnake, Crotalus atrox. We measured each individual's body length, several external cranial dimensions, several cranial osteological dimensions from cleaned skeletons, and we calculated gape index values from two published gape indices (GI). Cranial bone lengths and gape circumference showed negative allometry with snout–vent length (SVL), indicating that small individuals have relatively larger heads and gapes than their larger conspecifics. We then used Akaike's Information Criterion to determine which external and osteological measurements were the best predictors of gape. Body size (SVL) was the best predictor of maximum gape overall; however, when SVL was excluded from the analysis, quadrate (QL) and mandible lengths (MdLs) were the best predictors of maximum gape using both external and osteological measurements. Quadrate length probably contributes directly to gape; however, the importance of MdL to gape is less clear and may be due largely to its allometric relationships with head length and SVL. The two published GI did not prove to be better indicators of actual gape than the jaw and QLs in this study, and the gape values they produced differed significantly from our empirically determined gapes. For these reasons, we urge caution with the use and interpretation of computed GI in future studies. The extensive variation in quadrate and mandible morphology among lineages suggest that these bones are more important to variation in gape among species and lineages than within a single species. J. Morphol., 2013. © 2012 Wiley Periodicals, Inc.     

The radula of the Late Cretaceous scaphitid ammonite Rhaeboceras halli (Meek and Hayden, 1856)

Abstract: Radular teeth occur between the jaws in two specimens of the Late Cretaceous scaphitid ammonite Rhaeboceras halli (Meek and Hayden, 1856) from the Western Interior of the United States. The detailed morphology of the teeth has been revealed by propagation phase contrast X‐ray synchrotron microtomography. Each row of the radula of R. halli consists of a total of seven teeth (a central rachidian, two pairs of lateral and one pair of marginal teeth), as in other known ammonoid radulae, although the central tooth could not be confirmed in the specimens examined. The lateral teeth are multicuspid and robust, and the marginal teeth are long (4.6 mm) and slender. In overall morphology, the heterodont and ctenoglossan radula of R. halli is similar that of Jurassic and Cretaceous ammonites with the same aptychus‐type lower jaw, that is, the Aptychophora. This discovery reveals the range of variation in radular morphology, which could be related to ecological or phylogenetic factors. It also invalidates the hypothesis that the hook‐like structures in R. halli previously described are radular elements.     

Comparative study on the cranial morphology of Gymnallabes typus (Siluriformes: Clariidae) and their less anguilliform relatives,Clariallabes melas and Clarias gariepinus

We compare the cranial morphology of four fish species with an increasing anguilliformism in the following order: Clarias gariepinus, Clariallabes melas, Gymnallabes typus, and Channallabes apus. The main anatomical‐morphological disparities are the stepwise reduction of the skull roof along with the relative enlargement of the external jaw muscles, which occurred in each of them. Gymnallabes typus and C. apus lack a bony protection to cover the jaw muscles. The neurocranial bones of C. gariepinus, however, form a closed, broad roof, whereas the width of the neurocranium in C. melas is intermediate. Several features of the clariid heads, such as the size of the mouth and the bands of small teeth, may be regarded as adaptations for manipulating large food particles, which are even more pronounced in anguilliform clariids. The jaw musculature of G. typus is hypertrophied and attached on a higher coronoid process of the lower jaw, causing a larger adductive force. The hyomandibula interdigitates more strongly with the neurocranium and its dentition with longer teeth is posteriorly extended, closer to the lower jaw articulation. The anguilliform clariids also have their cranial muscles modified to enable a wider gape. The adductor mandibulae and the levator operculi extend more posteriorly, and the anterior attachment site of the protractor hyoidei dorsalis shifts toward the sagittal plane of the head. A phylogenetic analysis of the Clariidae, which is in progress, could check the validity of Boulenger's hypothesis that predecessors of the primitive fishes, such as Heterobranchus and most Clarias, would have evolved into progressively anguilliform clariids. J. Morphol. 240:169–194, 1999. © 1999 Wiley‐Liss, Inc.     

Using linkage models to explore skull kinematic diversity and functional convergence in arthrodire placoderms

Biomechanical models offer a powerful set of tools for quantifying the diversity of function across fossil taxa. A computer‐based four‐bar linkage model previously developed to describe the potential feeding kinematics of Dunkleosteus terrelli is applied here to several other arthrodire placoderm taxa from different lineages. Arthrodire placoderms are a group of basal gnathostomes showing one of the earliest diversifications of jaw structures. The linkage model allows biomechanical variation to be compared across taxa, identify trends in skull morphology among arthrodires that potentially influence function and explore the role of linkage systems in the early evolution of jaw structures. The linkage model calculates various kinematic metrics including gape angle, effective mechanical advantage, and kinematic transmission coefficients. Results indicate that the arthrodire feeding system may be more diverse and complex than previously thought. A range of potential kinematic profiles among arthrodire taxa illustrate a diversity of feeding function comparable with modern teleost fishes. Previous estimates of bite force in Dunkleosteus are revised based on new morphological data. High levels of kinematic transmission among arthrodires suggest the potential for rapid gape expansion and possible suction feeding. Morphological comparisons indicate that there were several morphological solutions for obtaining these fast kinematics, which allowed different taxa to achieve similar kinematic profiles while varying other aspects of the feeding apparatus. Mapping of key morphological components of the linkage system on a general placoderm phylogeny illustrates the potential importance of four‐bar systems to the early evolution of jaw structures. J. Morphol. 271:990–1005, 2010. © 2010 Wiley‐Liss, Inc.     

Oral food processing in two herbivorous lizards, Iguana iguana (Iguanidae) and Uromastix aegyptius (Agamidae).

The anatomy and function of the feeding apparatus in Iguana iguana and Uromastix aegyptius were studied by dissection, cinematic and cineradiographic techniques. The feeding behavior of these species differs from that of insectivorous lizards in the cropping action involves movement of both the upper jaw around the atlantooccipital joint and the lower jaw around the mandibular joint; and in Uromastix only, streptostylic movement of the quadrate. Often movements of the whole head play a supplementary role in the cropping action. In both species the feeding apparatus has been modified to facilitate cropping. In Iguana the pleurodont dentition is multicusped and laterally compressed. Each tooth forms a shearing blade whose function does not require contact with other teeth. In Uromastix the dentition is acrodont and the cheek teeth are massive and lack cusps. Occlusion is necessary for shearing plant material. The skull system of Uromastix also has a number of modified structures which allow protraction and retraction of the lower jaw to facilitate cropping while maintaining a gape equivalent to that in Iguana. It is suggested that the differences in the feeding apparatus between Iguana and Uromastix are attributable to differeces in the mode of tooth replacement and implantation.     

Morphofunctional Analysis of the Quadrate of Spinosauridae (Dinosauria: Theropoda) and the Presence of Spinosaurus and a Second Spinosaurine Taxon in the Cenomanian of North Africa.

Six quadrate bones, of which two almost certainly come from the Kem Kem beds (Cenomanian, Upper Cretaceous) of south-eastern Morocco, are determined to be from juvenile and adult individuals of Spinosaurinae based on phylogenetic, geometric morphometric, and phylogenetic morphometric analyses. Their morphology indicates two morphotypes evidencing the presence of two spinosaurine taxa ascribed to Spinosaurus aegyptiacus and? Sigilmassasaurus brevicollis in the Cenomanian of North Africa, casting doubt on the accuracy of some recent skeletal reconstructions which may be based on elements from several distinct species. Morphofunctional analysis of the mandibular articulation of the quadrate has shown that the jaw mechanics was peculiar in Spinosauridae. In mature spinosaurids, the posterior parts of the two mandibular rami displaced laterally when the jaw was depressed due to a lateromedially oriented intercondylar sulcus of the quadrate. Such lateral movement of the mandibular ramus was possible due to a movable mandibular symphysis in spinosaurids, allowing the pharynx to be widened. Similar jaw mechanics also occur in some pterosaurs and living pelecanids which are both adapted to capture and swallow large prey items. Spinosauridae, which were engaged, at least partially, in a piscivorous lifestyle, were able to consume large fish and may have occasionally fed on other prey such as pterosaurs and juvenile dinosaurs.     

Functional morphology of durophagy in black carp,Mylopharyngodon piceus

The black carp, Mylopharyngodon piceus (Osteichthyes: Cyprinidae), crushes its snail and other molluscan prey with robust pharyngeal jaws and strong bite forces. Using gross morphology, histological sectioning, and X‐ray reconstruction of moving morphology (XROMM), we investigated structural, behavioral, and mechanical aspects of pharyngeal jaw function in black carp. Strut‐like trabeculae in their pharyngeal jaws support large, molariform teeth. The teeth occlude with a hypertrophied basioccipital process that is also reinforced with stout trabeculae. A keratinous chewing pad is firmly connected to the basioccipital process by a series of small bony projections from the base of the pedestal. The pharyngeal jaws have no bony articulations with the skull, and their position is controlled by five paired muscles and one unpaired median muscle. Black carp can crush large molluscs, so we used XROMM to compare pharyngeal jaw postures as fish crushed ceramic tubes of increasing sizes. We found that black carp increase pharyngeal jaw gape primarily by ventral translation of the jaws, with ventral rotation and lateral flaring of the jaws also increasing the space available to accommodate large prey items. A stout, robust ligament connects left and right jaws together firmly, but allows some rotation of the jaws relative to each other. Contrasting with the pharyngeal jaw mechanism of durophagous perciforms with fused left and right lower pharyngeal jaws, we hypothesize that this ligamentous connection may serve to decouple tensile and compressive forces, with the tensile forces borne by the ligament and the compressive forces transferred to the prey. J. Morphol. 276:1422–1432, 2015. © 2015 Wiley Periodicals, Inc.     

Feeding mechanism of Epibulus insidiator (Labridae; Teleostei): Evolution of a novel functional system

The feeding mechanism of Epibulus insidiator is unique among fishes, exhibiting the highest degree of jaw protrusion ever described (65% of head length). The functional morphology of the jaw mechanism in Epibulus is analyzed as a case study in the evolution of novel functional systems. The feeding mechanism appears to be driven by unspecialized muscle activity patterns and input forces, that combine with drastically changed bone and ligament morphology to produce extreme jaw protrusion. The primary derived osteological features are the form of the quadrate, interopercle, and elongate premaxilla and lower jaw. Epibulus has a unique vomero-interopercular ligament and enlarged interoperculo-mandibular and premaxilla-maxilla ligaments. The structures of the opercle, maxilla, and much of the neurocranium retain a primitive labrid condition. Many cranial muscles in Epibulus also retain a primitive structural condition, including the levator operculi, expaxialis, sternohyoideus, and adductor mandibulae. The generalized perciform suction feeding pattern of simultaneous peak cranial elevation, gape, and jaw protrusion followed by hyoid depression is retained in Epibulus. Electromyography and high-speed cinematography indicate that patterns of muscle activity during feeding and the kinematic movements of opercular rotation and cranial elevation produce a primitive pattern of force and motion input. Extreme jaw protrusion is produced from this primitive input pattern by several derived kinematic patterns of modified bones and ligaments. The interopercle, quadrate, and maxilla rotate through angles of about 100 degrees, pushing the lower jaw into a protruded position. Analysis of primitive and derived characters at multiple levels of structural and functional organization allows conclusions about the level of design at which change has occurred to produce functional novelties.     

Jaw biomechanics in the South American aetosaur <Emphasis Type="Italic">Neoaetosauroides engaeus</Emphasis>

The function of the jaw apparatus and the possible dietary habits of the aetosaur Neoaetosauroides engaeus from the Triassic of South America were analyzed in comparison with Northern Hemisphere aetosaurs Desmatosuchus haplocerus and Stagonolepis robertsoni and the living short-snouted crocodile Alligator mississippiensis. The adductor and depressor jaw musculature of these was reconstructed on the basis of dental and skeletal comparisons with living closest relatives’ extant phylogenetic bracket (EPB), followed by the analysis of the moment arms of these muscles to infer feeding habits. The aetosaurian skull design indicates that the total leverage of the inferred jaw musculature provides force rather than speed. However, within aetosaurs, the high ratios of muscle moment arms to bite moments indicate stronger bites in the northern Hemisphere forms, and faster ones in Neoaetosauroides. These differences indicate more developed crushing, chopping, and slicing capacities, especially at the back of the tooth series for D. haplocerus and S. robertsoni; whereas it opens a window to consider different abilities in which speed is involved for N. engaeus. There are differences among aetosaurs in dental characteristics, position of the supratemporal fenestra, location of the jaw joint relative to the tooth row, and shape of the lower jaw. Neoaetosauroides does not show evidence of dental serrations and wear facets, probably consistent with a relatively soft and non-abrasive diet, for example soft leaves and/or larvae and insects without hard structures. It might be possible that Neoaetosauroides represents a tendency towards insectivorous feeding habits, exploiting a food source that was widespread in continental environments throughout the Triassic.     

A functional analysis of food procurement in two surgeonfish species,Acanthurus nigrofuscus andCtenochaetus striatus (Acanthuridae)

Synopsis The mechanisms of food procurement in the surgeonfishesCtenochaetus striatus andAcanthurus nigrofuscus from the Great Barrier Reef were determined by functional analyses of the jaws and associated structural elements (based on myological and osteological examinations and X-ray photographs) and by video analyses of actions of the mouth and body during feeding.Acanthurus nigrofuscus has relatively robust jaw bones. The movement of the elements during mouth opening is limited with a mean maximum gape angle of 112.8°. Each bite is relatively fast and is characterized by a quick nip at algal filaments, usually followed by a sidewads flick of the head. The jaws bear several broad multidenticulate teeth. It appears that these teeth engage turf algal strands which are either sheared during mouth closure or torn off as the head flicks sideways. InC. striatus, the jaw bones are considerably lighter than those ofA. nigrofuscus. There is much greater movement of the elements during mouth opening, resulting in a mean maximum gape angle of 177.6°. Each bite is slower than inA. nigrofuscus and is characterized by a wide gape as the mouth is applied to the substratum followed by a quick, upward flick of the lower jaw, with no sideways flick of the head. The jaws bear numerous elongate flexible teeth, with expanded incurved denticulate tips; those on the dentary often possessing a pointed blade-like process. It appears that these teeth brush particulate and epiphytic material from the surface of the turf algal strands and other substrata. These observations demonstrate howA. nigrofuscus andC. striatus are able to remove microalgae and detritus, respectively, from the same substratum. The results also demonstrate how relatively small differences in morphology can have a profound influence on the feeding abilities and trophic ecology of fishes.     

The aquatic Lystrosaurus: An alternative lifestyle

Certain distinctive features of the skull and postcranial skeleton of Lystrosaurus are discussed. The mechanics of the skull are re‐evaluated and it is concluded that the skull was modified relative to Permian forms to produce a bigger bite force and more vertical component of the adductor muscles, while preserving a wide gape. A zone of weakness, acting as a shock absorption system, was present in the premaxillary‐nasal region of the skull. Antero‐posterior movement of the lower jaw had been reduced. It is concluded that the skull was used to crush resistant plant matter.

The external nasal opening presents no evidence for a valvular structure, but may have housed a nasal gland.

The flared scapula produced a slightly greater mechanical advantage in the limb protractors and retractors.

The manus was short and broad, suitable for digging, but the claws were flat and rounded.

The wide knee‐joint indicated powerful foot‐moving muscles.

The palaeoenvironment of the Lystrosaurus‐Thrinaxodon Assemblage Zone is examined. It was probably drier than usually described. The fauna contained many terrestrial elements.

Lystrosaurus was probably a fully terrestrial animal which may have excavated burrows for itself, but was not a committed burrower like Cistecephalus.     

The most basal beaked whale Ninoziphius platyrostris Muizon, 1983: clues on the evolutionary history of the family Ziphiidae (Cetacea: Odontoceti)

Ninoziphius platyrostris, from the late Neogene of Peru, is one of the best‐known fossil beaked whales (Odontoceti: Ziphiidae), with a holotype including the skull with ear bones, mandibles, teeth, and postcranial elements. Furthermore, based on several characters, including a complete functional upper and lower dentition, it is usually considered as one of the most archaic ziphiids. However, the poorly preserved dorsal portion of the holotype skull has led to unresolved phylogenetic relationships. With the addition of two newly prepared skulls from the same Peruvian locality we redescribed N. platyrostris. In the light of recent ziphiid discoveries, an emended diagnosis of the species is proposed here. In our cladistic analysis Ninoziphius is the most basal stem ziphiid. Newly observed or reassessed morphological traits allow functional and ecological considerations. The morphology of the oral apparatus suggests that Ninoziphius was less specialized for suction feeding than most extant ziphiids. Tooth wear in the holotype may indicate benthic feeding. Although the vertebral column of Ninoziphius corresponds to less developed locomotor abilities for deep dives, its cranial morphology does not provide definitive arguments for an echolocation system less efficient than in deep diving extant ziphiids. Finally, the phylogenetic tree produced was used to detail the evolutionary history of several major ziphiid features (dental reduction, development of mandibular tusks, and increased body size). © 2013 The Linnean Society of London     

Dietary correlates of temporomandibular joint morphology in the great apes

Behavioral observations of great apes have consistently identified differences in feeding behavior among species, and these differences have been linked to variation in masticatory form. As the point at which the mandible and cranium articulate, the temporomandibular joint (TMJ) is an important component of the masticatory apparatus. Forces are transmitted between the mandible and cranium via the TMJ, and this joint helps govern mandibular range of motion. This study examined the extent to which TMJ form covaries with feeding behavior in the great apes by testing a series of biomechanical hypotheses relating to specific components of joint shape using linear measurements extracted from three‐dimensional coordinate data. Results of these analyses found that taxa differ significantly in TMJ shape, particularly in the mandibular fossa. Chimpanzees have relatively more anteroposteriorly elongated joint surfaces, whereas gorillas tend to have relatively anteroposteriorly compressed joints. Orangutans were most commonly intermediate in form between Pan and Gorilla, perhaps reflecting a trade‐off between jaw gape and load resistance capabilities. Importantly, much of the observed variation among taxa reflects differences in morphologies that facilitate gape over force production. These data therefore continue to emphasize the unclear relationship between mandibular loading and bony morphology, but highlight the need for further data regarding food material properties, jaw gape, and ingestive/food processing behaviors. Am J Phys Anthropol, 2013. © 2012 Wiley Periodicals, Inc.     

Gape and bite force in the rodents Onychomys leucogaster and Peromyscus maniculatus: Does jaw‐muscle anatomy predict performance?

Compared with the deer mouse, Peromyscus maniculatus, the grasshopper mouse, Onychomys leucogaster, exhibits modifications in its jaw‐muscle architecture that promote wide gapes and large bite forces at wide gapes to prey upon large vertebrate prey. In this study, we determine whether jaw‐muscle anatomy predicts gape and biting performance in O. leucogaster, and we also assess the influence of gape on bite force in the two species. Although O. leucogaster has an absolutely longer jaw, which facilitates larger gapes, maximum passive gape is similar in both species, averaging ～12.5 mm. Thus, when scaled to jaw length, O. leucogaster has a smaller maximum passive gape. These results suggest that predatory behaviors of O. leucogaster may not require remarkably large gapes. On the other hand, both absolute and relative bite forces exerted by O. leucogaster are significantly larger than those of P. maniculatus. The largest bite forces in both species occur at 5.0 mm of gape at the incisors, or 40% of maximum gape. Although bite force in both species decreases at larger gapes, O. leucogaster does maintain a larger percentage of maximum bite force at gapes larger than 40% of maximum passive gape. Therefore, although structural modifications in the masticatory apparatus of O. leucogaster may constrain gape, they may help to maintain bite force at large gapes. These results suggest that increases in gape differentially influence the length‐tension properties of the jaw muscles in the two species. Finally, these results highlight the importance of considering the effect of muscle stretch on force production in comparative studies of bite force. As a first approximation, it appears that gapes of 40–50% of maximum gape in rodents optimizes bite force production at the incisors. J. Morphol., 2009. © 2009 Wiley‐Liss, Inc.