Tweaking the hinge and caps: testing a model of the organization of jaws

Historically, examinations of gnathostome skulls have indicated that for essentially the entirety of their existence, jaws have been characterized by a high degree of fidelity to an initial basic structural design that will then go on to manifest an amazing array of end-point phenotypes. These two traits-bauplan fidelity and elaboration of design-are inter-connected and striking, and beg a number of questions, including: Are all jaws made in the same manner and if not how not? To begin to tackle such questions, we herein operationally define jaws as two appositional, hinged cranial units for which polarity and potential modularity are characteristics, and then address what is necessary for them to form, including delineating both the sources of cells and tissues that will formally yield the jaws as well as what informs their ontogeny (e.g., sources of positional information and factors directing the interpretation of developmental cues). Following on this, we briefly describe a predictive, testable model of jaw development (the "Hinge and Caps" model) and present evidence that the Satb2+cell population in the developing jaw primordia of mice defines a developmentally and evolutionarily significant jaw module such as would be predicted by the model.     

Fishing for jaws in early vertebrate evolution: a new hypothesis of mandibular confinement

The evolutionary origin of the vertebrate jaw persists as a deeply puzzling mystery. More than 99% of living vertebrates have jaws, but the evolutionary sequence that ultimately gave rise to this highly successful innovation remains controversial. A synthesis of recent fossil and embryological findings offers a novel solution to this enduring puzzle. The Mandibular Confinement Hypothesis proposes that the jaw evolved via spatial confinement of the mandibular arch (the most anterior pharyngeal arch within which the jaw arose). Fossil and anatomical evidence reveals: (i) the mandibular region was initially extensive and distinct among the pharyngeal arches; and (ii) with spatial confinement, the mandibular arch acquired a common pharyngeal pattern only at the origin of the jaw. The confinement occurred via a shift of a domain boundary that restricted the space the mesenchymal cells of the mandibular arch could occupy. As the surrounding domains replaced mandibular structures at the periphery, this shift allowed neural crest cells and mesodermal mesenchyme of the mandibular arch to acquire patterning programs that operate in the more posterior arches. The mesenchymal population within the mandibular arch was therefore no longer required to differentiate into specialized feeding and ventilation structures, and was remodelled into a jaw. Embryological evidence corroborates that the mandibular arch must be spatially confined for a jaw to develop. This new interpretation suggests neural crest as a key facilitator in correlating elements of the classically recognized vertebrate head ‘segmentation’.     

Vertebrate head development: Segmentation,novelties, and homology

Vertebrate head development is a classical topic lately invigorated by methodological as well as conceptual advances. In contrast to the classical segmentalist views going back to idealistic morphology, the head is now seen not as simply an extension of the trunk, but as a structure patterned by different mechanisms and tissues. Whereas the trunk paraxial mesoderm imposes its segmental pattern on adjacent tissues such as the neural crest derivatives, in the head the neural crest cells carry pattern information needed for proper morphogenesis of mesodermal derivatives, such as the cranial muscles. Neural crest cells make connective tissue components which attach the muscle fiber to the skeletal elements. These crest cells take their origin from the same visceral arch as the muscle cells, even when the skeletal elements to which the muscle attaches are from another arch. The neural crest itself receives important patterning influences from the pharyngeal endoderm. The origin of jaws can be seen as an exaptation in which a heterotopic shift of the expression domains of regulatory genes was a necessary step that enabled this key innovation. The jaws are patterned by Dlx genes expressed in a nested pattern along the proximo-distal axis, analogous to the anterior–posterior specification governed by Hox genes. Knocking out Dlx 5 and 6 transforms the lower jaw homeotically into an upper jaw. New data indicate that both upper and lower jaw cartilages are derived from one, common anlage traditionally labelled the “mandibular” condensation, and that the “maxillary” condensation gives rise to other structures such as the trabecula. We propose that the main contribution from evolutionary developmental biology to solving homology questions lies in deepening our biological understanding of characters and character states.     

An Ancient Gene Network Is Co-opted for Teeth on Old and New Jaws

Vertebrate dentitions originated in the posterior pharynx of jawless fishes more than half a billion years ago. As gnathostomes (jawed vertebrates) evolved, teeth developed on oral jaws and helped to establish the dominance of this lineage on land and in the sea. The advent of oral jaws was facilitated, in part, by absence of hox gene expression in the first, most anterior, pharyngeal arch. Much later in evolutionary time, teleost fishes evolved a novel toothed jaw in the pharynx, the location of the first vertebrate teeth. To examine the evolutionary modularity of dentitions, we asked whether oral and pharyngeal teeth develop using common or independent gene regulatory pathways. First, we showed that tooth number is correlated on oral and pharyngeal jaws across species of cichlid fishes from Lake Malawi (East Africa), suggestive of common regulatory mechanisms for tooth initiation. Surprisingly, we found that cichlid pharyngeal dentitions develop in a region of dense hox gene expression. Thus, regulation of tooth number is conserved, despite distinct developmental environments of oral and pharyngeal jaws; pharyngeal jaws occupy hox-positive, endodermal sites, and oral jaws develop in hox-negative regions with ectodermal cell contributions. Next, we studied the expression of a dental gene network for tooth initiation, most genes of which are similarly deployed across the two disparate jaw sites. This collection of genes includes members of the ectodysplasin pathway, eda and edar, expressed identically during the patterning of oral and pharyngeal teeth. Taken together, these data suggest that pharyngeal teeth of jawless vertebrates utilized an ancient gene network before the origin of oral jaws, oral teeth, and ectodermal appendages. The first vertebrate dentition likely appeared in a hox-positive, endodermal environment and expressed a genetic program including ectodysplasin pathway genes. This ancient regulatory circuit was co-opted and modified for teeth in oral jaws of the first jawed vertebrate, and subsequently deployed as jaws enveloped teeth on novel pharyngeal jaws. Our data highlight an amazing modularity of jaws and teeth as they coevolved during the history of vertebrates. We exploit this diversity to infer a core dental gene network, common to the first tooth and all of its descendants.     

Jaw morphology and ontogeny in five species of Ophryotrocha

Detailed scanning electron microscopy of jaws within the genus Ophryotrocha (Dorvilleidae, Annelida) was performed on 871 jaw parts. The investigations resulted in new understandings of the ontogeny and jaw morphology and have systematic implications for the family. Five species in the genus (Ophryotrocha alborana, O. diadema, O. gracilis, O. hartmanni, and O. labronica pacifica) were kept in culture, and the development of the jaws was studied by sampling throughout their life history. Ophryotrocha species have mandibular plates that remain the same throughout ontogeny, whereas the posterior shafts elongate. Both mandibular plate morphology and shaft ontogeny have species‐specific distinctions. In Ophryotrocha, the maxillae can be assigned to three to four distinct types, which are replaced by moulting. The maxillary morphology and developmental stages at which moults occur are species specific, although with broad intervals. A redefinition is given for some of the basic jaw elements, and new homologies are proposed for structures that are also present across other dorvilleid taxa. J. Morphol. 2010. © 2009 Wiley‐Liss, Inc.     

Micro- and macroevolutionary decoupling of cichlid jaws: a test of Liem's key innovation hypothesis

The extent to which elements of functional systems can change independently (modularity) likely influences the diversification of lineages. Major innovations in organismal design, like the pharyngeal jaw in cichlid fishes, may be key to a group's success when they relax constraints on diversification by increasing phenotypic modularity. In cichlid fishes, pharyngeal jaw modifications that enhanced the ability to breakdown prey may have freed their oral jaws from serving their ancestral dual role as a site of both prey capture and prey processing. This functional decoupling that allowed the oral jaws to become devoted solely to prey capture has been hypothesized to have permitted the two sets of cichlid jaws to evolve independently. We tested the hypothesis that oral and pharyngeal jaw mechanics are evolutionarily decoupled both within and among Neotropical Heroine cichlids. In the trophically polymorphic species Herichthys minckleyi, molariforms that exhibit enlarged molarlike pharyngeal jaw teeth were found to have approximately 400% greater lower jaw mass compared to H. minckleyi with the alternative papilliform pharyngeal morphology. However, oral jaw gape, lower jaw velocity ratios, anterior jaw linkage mechanics, and jaw protrusion did not differ between the morphotypes. In 40 other Heroine species, there was a weak correlation between oral jaw mechanics and pharyngeal jaw mass when phylogenetic history was ignored. Yet, after expansion of the cytochrome b phylogeny for Heroines, change in oral jaw mechanics was found to be independent of evolutionary change in pharyngeal jaw mass based on independent contrasts. Evolutionary decoupling of oral and pharyngeal jaw mechanics has likely played a critical role in the unparalleled trophic diversification of cichlid fishes.     

Early Middle Ordovician scolecodonts from north‐western Argentina and the emergence of labidognath polychaete jaw apparatuses

Scolecodonts provide fossil evidence of the evolution and diversification of jaw‐bearing polychaetes from the latest Cambrian onwards. However, their record before the Darriwilian (Middle Ordovician) is scarce worldwide, which limits our understanding of key evolutionary events. One such event is the emergence of taxa possessing the asymmetrical labidognath‐type jaw apparatus architecture, which became common in the Middle Ordovician and is often dominant throughout the Palaeozoic. Here, we document a small collection of Dapingian scolecodonts from the Capillas section, Sierras Subandinas, north‐western Argentina. The isolated elements recovered allowed us to reconstruct the distinctive jaw apparatus, and to introduce a new taxon, Andiprion paxtonae gen. et sp. nov. The maxillary apparatus of Andiprion is intermediate between the symmetrognath type of the Early Ordovician Kadriorgaspis and the labidognath type that is present in polychaetaspids and related taxa. The apparatus architecture of Andiprion corresponds best to the labidognath type, but the morphology of the individual jaws suggests that it may be the most primitive representative of this lineage currently known. We propose that Andiprion‐like forms were ancestral to polychaetaspids, polychaeturids and ramphoprionids. The Capillas collection provides supporting evidence for the evolutionary homology of the ‘basal plate’ and the left first maxilla. Thus the labidognath‐type asymmetry, with an unpaired left maxilla III, developed as a result of gradual reduction in size of the first right jaw (‘basal plate’) in front of the carriers, instead of loss or fusion of anterior maxillae.     

Satb2 haploinsufficiency phenocopies 2q32-q33 deletions, whereas loss suggests a fundamental role in the coordination of jaw development

The recent identification of SATB2 as a candidate gene responsible for the craniofacial dysmorphologies associated with deletions and translocations at 2q32-q33, one of only three regions of the genome for which haploinsufficiency has been significantly associated with isolated cleft palate, led us to investigate the in vivo functions of murine Satb2. We find that, similar to the way in which SATB2 is perceived to act in humans, craniofacial defects due to haploinsufficiency of Satb2, including cleft palate (in ~25% of cases), phenocopy those seen with 2q32-q33 deletions and translocations in humans. Full functional loss of Satb2 results in amplification of these defects and leads both to increased apoptosis in the craniofacial mesenchyme where Satb2 is usually expressed and to changes in the pattern of expression of three genes implicated in the regulation of craniofacial development in humans and mice: Pax9, Alx4, and Msx1. The Satb2-dosage sensitivity in craniofacial development is conspicuous—along with its control of cell survival, pattern of expression, and reversible functional modification by SUMOylation, it suggests that Satb2/SATB2 function in craniofacial development may prove to be more profound than has been anticipated previously. Because jaw development is Satb2-dosage sensitive, the regulators of Satb2 expression and posttranslational modification become of critical importance both ontogenetically and evolutionarily, especially since such regulators plausibly play undetected roles in jaw and palate development and in the etiology of craniofacial malformations.     

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.     

Development of the muscles associated with the mandibular and hyoid arches in the Siberian sturgeon,Acipenser baerii (Acipenseriformes: Acipenseridae)

The skeleton of the jaws and neurocranium of sturgeons (Acipenseridae) are connected only through the hyoid arch. This arrangement allows considerable protrusion and retraction of the jaws and is highly specialized among ray‐finned fishes (Actinopterygii). To better understand the unique morphology and the evolution of the jaw apparatus in Acipenseridae, we investigated the development of the muscles of the mandibular and hyoid arches of the Siberian sturgeon, Acipenser baerii. We used a combination of antibody staining and formalin‐induced fluorescence of tissues imaged with confocal microscopy and subsequent three‐dimensional reconstruction. These data were analyzed to address the identity of previously controversial and newly discovered muscle portions. Our results indicate that the anlagen of the muscles in A. baerii develop similarly to those of other actinopterygians, although they differ by not differentiating into distinct muscles. This is exemplified by the subpartitioning of the m. adductor mandibulae as well as the massive m. protractor hyomandibulae, for which we found a previously undescribed portion in each. The importance of paedomorphosis for the evolution of Acipenseriformes has been discussed before and our results indicate that the muscles of the mandibular and the hyoid may be another example for heterochronic evolution.     

Drosophila wing modularity revisited through a quantitative genetic approach

To predict the response of complex morphological structures to selection it is necessary to know how the covariation among its different parts is organized. Two key features of covariation are modularity and integration. The Drosophila wing is currently considered a fully integrated structure. Here, we study the patterns of integration of the Drosophila wing and test the hypothesis of the wing being divided into two modules along the proximo‐distal axis, as suggested by developmental, biomechanical, and evolutionary evidence. To achieve these goals we perform a multilevel analysis of covariation combining the techniques of geometric morphometrics and quantitative genetics. Our results indicate that the Drosophila wing is indeed organized into two main modules, the wing base and the wing blade. The patterns of integration and modularity were highly concordant at the phenotypic, genetic, environmental, and developmental levels. Besides, we found that modularity at the developmental level was considerably higher than modularity at other levels, suggesting that in the Drosophila wing direct developmental interactions are major contributors to total phenotypic shape variation. We propose that the precise time at which covariance‐generating developmental processes occur and/or the magnitude of variation that they produce favor proximo‐distal, rather than anterior‐posterior, modularity in the Drosophila wing.     

The oldest Gondwanan cephalopod mandibles (Hangenberg Black Shale,Late Devonian) and the mid‐Palaeozoic rise of jaws

It is widely accepted that the effects of global sea‐level changes at the transition from the Devonian to the Carboniferous are recorded in deposits on the shelf of northern Gondwana. These latest Devonian strata had been thought to be poor in fossils due to the Hangenberg mass extinction. In the Ma'der (eastern Anti‐Atlas), however, the Hangenberg Black Shale claystones (latest Famennian) are rich in exceptionally preserved fossils displaying the remains of non‐mineralized structures. The diversity in animal species of these strata is, however, low. Remarkably, the organic‐rich claystones have yielded abundant remains of Ammonoidea preserved with their jaws, both in situ and isolated. This is important because previously, the jaws of only one of the main Devonian ammonoid clades had been found (Frasnian Gephuroceratina). Here, we describe four types of jaws of which two could be assigned confidently to the Order Clymeniida and to the Suborder Tornoceratina. These findings imply that chitinous normal‐type jaws were likely to have already been present at the origin of the whole clade Ammonoidea, i.e. in the early Emsian (or earlier). Vertebrate jaws evolved prior to the Early Devonian origin of ammonoids. The temporal succession of evolutionary events suggests that it could have been the indirect positive selection pressure towards strong (and thus preservable) jaws since defensive structures of potential prey animals would otherwise have made them inaccessible to jawless predators in the course of the mid‐Palaeozoic marine revolution. In this respect, our findings reflect the macroecological changes that occurred in the Devonian. [Correction added on 28 July 2016 after first online publication: In the Abstract, the sentence “Vertebrate jaws probably … in the Early Devonian” was amended]     

Ontogeny of the jaw apparatus and suspensorium of the Tetraodontiformes

Konstantinidis, P. and Johnson, G. David 2012. Ontogeny of the jaw apparatus and suspensorium of the Tetraodontiformes. —Acta Zoologica (Stockholm) 93 : 351–366. The jaw apparatus and suspensorium of adult Tetraodontiformes are well adapted to a durophagous feeding habit. Anatomical indicators are the short, stout jaws and a suspensorium in which the quadrate lies in the same vertical plane as the autopalatine. In contrast, the palatoquadrate of larval Tetraodontiformes generally resembles that of larval percomorphs – a more posteriorly positioned quadrate and a slender and long Meckelian cartilage. Among Tetraodontiformes, the Triacanthodidae retain a protrusible upper jaw and a versatile suspensorium. The jaws of the Balistoidei have greater mobility achieved by a reduced autopalatine that has lost its bony contact with the suspensorium. In contrast to the Balistoidei, the beak‐like jaws of the Tetraodontoidei lack individual teeth in the biting part of the jaws. The autopalatine is enlarged, which results in immobilization of the ethmopalatine articulation. The Ostraciidae are exceptional in having the distal part of the autopalatine reduced, while the proximal part remains attached to the suspensorium.     

On the relative frequencies of hominid maxillary and mandibular teeth and jaws as taphonomic indicators

In southern African samples of early hominid remains, maxillary and mandibular teeth (deciduous-plus-permanent) have a virtually equal chance of accumulating in the dolomitic limestone cave deposits, of being preserved therein and recovered therefrom. Thus, of 1066 fossil teeth ofAustralopithecus spp. plusHomo habilis, 51.9 per cent are maxillary and 48.1 per cent mandibular. On the other hand, the East African sample of 847 early hominid, deciduous-plus-permanent teeth, departs more strikingly from a 1∶1 ratio: it comprises 41.0 per cent maxillary and 59.0 per cent mandibular teeth. It is inferred that mandibular teeth have a somewhat better chance of accumulating and being preserved in, and being recovered from, the open, fluvial, lacustrine and deltaic sedimentary environments of the East African sites. The dental proportions are approximately matched by the proportions of jaws. For example, the maxilla: mandible proportions at Koobi fora in northern Kenya are 33.0∶67.0 for teeth and 21.6∶78.4 for jaws. In other words, the preponderance in favour of mandibular remains is somewhat more marked in the case of jaws than of teeth, this distinction doubtless reflecting the more fragile bony structure of the maxilla and the sturdier construction of the mandible. This first study known to the author of the differential distribution of maxillary and mandibular teeth of the Plio-Pleistocene hominids leads the author to hypothesize that, where environmental conditions at the place and time of the death of the hominids have been non-destructive, non-dispersive, relatively mild and protective, maxillae and mandibles may be expected to have been conserved and recovered in approximately equal proportions—and likewise of maxillary and mandibular teeth. On the other hand, the more brutal and destructive the sedimentary environment and other taphonomic influences have been, at the place and time when the hominid individuals died, the more likely it is that the maxillary and mandibular remains of jaws and teeth will deviate from equality of proportions, generally at the expense of the maxillae and upper teeth. Hence, it is proposed that the upper jaw/low jaw ratio (Mx/Mn jaw ratio) and the maxillary teeth/mandibular teeth ratio (Mx/Mn dental ratio) may serve as two useful new gauges of the rigour of palaeo-ecological and taphonomic conditions.     

Histological study of odontogenesis in the pharyngeal jaws of Trachinotus teraia (Cuvier et valenciennes, 1832) (Osteichthyes,Teleostei, Carangidae)

A histomorphological study of the development of the pharyngeal jaws in the Carangid fish Trachinotus teraia shows that they transform progressively from tiny organs with sharp superficial teeth, to thick ones with rounded teeth embedded in bony tissue. The morphological transformations take place simultaneously with a shift to a diet based on molluscs. Though odontogenesis takes place deep in the pharyngeal jaws, at all developmental stages, pharyngeal epithelium participates to the formation of teeth. Long epithelial strands penetrate in the depth of the bony jaw and here induce differentiation of “bell organs.” As the young teeth migrate passively toward the occlusal surface, while the jaw grows, the pharyngeal jaws of Trachinotus teraia almost behave like the “coalesced” teeth of the Tetraodontidae with respect to the morphogenetic processes of their growth. The developmental phenotypic plasticity of the pharyngeal jaws of Trachinotus teraia then may be compared to that of various mollusicivore cichlids. © 1994 Wiley-Liss, Inc.     

Morphology and evolution of the ectethmoid-mandibular articulation in the meliphagidae (Aves)

The ectethmoid-mandibular articulation in Melithreptus and Manorina (Meliphagidae: Aves) consists of the dorsal mandibular process fitting into and abutting against the ventral ectethmoid fossa; it forms a brace for the mandible. This articulation in Melithreptus is a typical diarthrosis with long folded capsular walls. The mandible, thus, has two separate articulations, each with a different axis of rotation. No other genus of Meliphagidae (except Ptiloprora) or any other avian family possesses a similar feature. The jaw and tongue musculature of Melithreptus are described. The two muscles opening the jaws are well developed, while those closing the jaws are small. The tongue muscles show no special developments. A large maxillary gland, presumably muscus secreting, covers the ventral surface of the jaw muscles. Its duct opens into the oral cavity just behind the tip of the upper jaw. The frilled tip of the tongue rests against the duct opening. The ectethmoid-mandibular articulation braces the adducted mandible against dorsoposteriorly directed forces. The mandible can be held closed without a compression force exerted by the mandible on the quadrate, permitting the bird to raise its upper jaw with greater ease and less loss of force. The tongue can be protruded through the slight gap between the jaws, moving against the duct opening and thus be coated with mucus. Presumably, these birds capture insects with their sticky tongue. Hence, the ectethmoid-mandibular articulation is an adaptation for this feeding method; it evolved independently in three genera of the Meliphagidae. The ectethmoid-mandibular articulation demonstrates that a bone can have two articulations with different axes of rotation, that the two articular halves can separate widely, and that articular cartilages can be flat and remain in contact over a large area. Its function suggests that the basitemporal articulation of the mandible found in many other birds has a similar function. And it demonstrates that in the evolution of the mammalian dentary-squamosal articulation, the new hinge did not have to lie on the same rotational axis as the existing quadrate-articular hinge.     

Mandibular morphology and diet in the genusCebus

The influence of hard-object feeding on the size and shape of the mandibular corpus was investigated through a comparative biomechanical analysis of the jaws of adult femaleCebus apella andCebus capucinus. Computed tomography (CT) was used to discern the amount and distribution of cortical bone at M₂ and symphyseal cross sections. From these data, the biomechanical properties of the mandibular corpus were determined to assess the structural rigidity of the jaw with respect to the bending, torsional, and shear stresses that occur during mastication and incision. The mandibles ofC. apella are demonstrably more robust than those ofC. capucinus in terms of biomechanical rigidity; differences in corporeal size rather than shape largely account for the enhanced robusticity in the sample ofC. apella. The differences that separate the two taxa probably represent a structural response to the mechanical demands of durophagy inC. apella. These observations suggest that specialization on a diet of hard objects may be expected to result in an overall hypertrophy of bony contours throughout the mandibular corpus.     

Feeding,Diet, and Jaw Form in West African Colobus and Procolobus

The functional link between mandibular morphology and masticatory stress has been documented by both experimental and comparative investigation. Somewhat more tenuous is the purported connection between dietary variation and the form of the jaws in primates. Several factors complicate the inference of such a connection, including anecdotal or incomplete dietary data from field studies and allometric effects on skeletal form that may have little to do with diet per se. We compared the jaws of sympatric colobines from West Africa to test the effect of diet on mandibular form. Procolobus badius and Colobus polykomos occupy the same habitat yet differ in diet primarily due to the exploitation of hard seeds by C. polykomos. The fact that the two taxa are comparable in body size also obviates the need for allometric qualifications. Colobus polykomos is expected to possess more robust mandibular corpora than Procolobus badius. In fact, the jaws of Colobus polykomos do not differ consistently from those of Procolobus badius in terms of biomechanical function. This apparent failure of mandibular morphology to reflect differences in diet and feeding behavior may be due to a variety of factors. We suspect that functional demands related to canine tooth support are contributing to obliteration of the expected biomechanical signal. Successful prediction of dietary effects on mandibular form requires consideration of competing structural and functional demands. The influence of diet on mandibular corporeal morphology is not equivalent across different primate species.