Biomechanics of the rostrum and the role of facial sutures

The rostrum is a large diameter, thin-walled tubular structure that receives loads from the teeth. The rostrum can be conceptualized both as a rigid structure and as an assemblage of several bones that interface at sutures. Using miniature pigs, we measured in vivo strains in rostral bones and sutures to gain a better understanding of how the rostrum behaves biomechanically. Strains in the premaxillary and nasal bones were low but the adjacent maxillary-premaxillary, internasal, and intermaxillary suture strains were larger by an order of magnitude. While this finding emphasizes the composite nature of the rostrum, we also found evidence in the maxillary and nasal bones for rigid structural behavior. Namely, maxillary strain is consistent with a short beam model under shear deformation from molar loading. Strain in the nasal bones is only partially supported by a long beam model; rather, a complex pattern of dorsal bending of the rostrum from incisor contact and lateral compression is suggested. Torsion of the maxilla is ruled out due to the bilateral occlusion of pigs and the similar working and balancing side strains, although it may be important in mammals with a unilateral bite. Torsional loading does appear important in the premaxillae, which demonstrate working and balancing side changes in strain orientation. These differences are attributed to asymmetrical incisor contact occurring at the end of the power stroke.     

Disparity and geometry of the skull in Archosauria (Reptilia: Diapsida)

A metric comparison of 155 fossil and extant species in lateral view based on the proportions of three homologous units (braincase, orbit and rostrum) reveals the existence of an archosaurian skull geometry. An empirical morphospace depicting skull proportions shows that the most variable unit is the rostrum. Three skull types based on rostral proportion are proposed: meso-, longi- and brevirostral. These types depend, on one hand, on a direct numerical relationship between the braincase and the orbit, with a mean ratio of 1:1; never surpassing a 2:1 or 1:2 ratio limit. On the other hand, skull types show a significant negative correlation between braincase and rostrum proportions. Close relationships have been obtained between orbit and the rostrum, although with lower significance and a geometric meaning specific to each group. Skull types depend mainly on the proportional relationship between the rostrum and the braincase. Mesorostral types account for more natural occurrences within morphospace, implying a plesiomorphic condition in Archosauria. Skulls with highest longirostral values (flying forms) display a more restrictive braincase–orbit ratio relationship. Brevirostrals are limited to the smallest skull lengths, up to approximately 180 mm. 85% of brevirostral modern birds have altricial post-hatchling development. General allometric pattern is very similar for all sampled archosaurs, although giant taxa (i.e. non-avian theropods) display a different type of skull proportional growth, closer to isometry. Results reveal the existence of a constructional skull geometry, highlighting the importance of the deviance of the structural design from adaptive explanations on craniofacial morphology in macroevolution.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 80, 67–88.     

The marine diapsid reptile Endennasaurus from the Upper Triassic of Italy

Abstract:  The marine reptile Endennasaurus from the Upper Triassic Zorzino Limestone of northern Italy is redescribed and reassessed. New details of the skull and postcranial skeleton are revealed, confirming the attribution of this genus to the diapsid reptile clade Thalattosauriformes. Phylogenetic analysis suggests that Endennasaurus was related to the European genus Askeptosaurus and the Chinese Anshunsaurus . Despite a rather conservative postcranial morphology, Endennasaurus clearly occupied a highly specialized dietary niche as it combined a slender tapering premaxillary rostrum with a complete absence of either marginal or palatal teeth.     

An experimental study on the effect of rigid fixation on the developing craniofacial skeleton

Rigid fixation of the craniofacial skeleton has proven of great value in adult orthognathic and traumatic reconstructive surgical procedures. This technique has gained increased acceptance in the surgical treatment of infants and young children with congenital malformations, despite the fact that its effects on subsequent craniofacial growth are unknown. To examine this question, an experimental model using 25 young kittens was developed to compare rigid fixation with conventional wire fixation, with and without osteotomy. Our findings demonstrate a regional restriction of growth in the developing craniofacial skeleton when both wire and plate and screw fixation are utilized in concert with osteotomy. Further, a compensatory growth was observed in individual animals when plate fixation was utilized that was not seen in the wire-treated group. This suggests that there is a dynamic growth interaction between restriction and compensation in this setting.     

Influences of osteoclast deficiency on craniofacial growth in osteopetrotic (op/op) mice

It is well known that the defect in bone resorption in osteopetrotic (op/op) mice brings about deformation of the cranium and failure of tooth eruption. However, the influences on longitudinal growth of the craniofacial skeleton have not been elucidated. This study was thus conducted to examine craniofacial morphology and longitudinal changes in the op/op mice by means of morphometric analysis with lateral cephalograms. Lateral cephalograms, taken every 10 days from 10- to 90-day-old mice, were analyzed on a personal computer for 11 measurement items. For the nasal bone region, the most prominent differences were found between the op/op and normal mice. The anterior cranial base and occipital bone height presented almost equivalent growth changes in both the op/op and normal mice. The size of mandible, meanwhile, was significantly smaller in the op/op mice than in the normal controls. The gonial angle was also significantly larger in the op/op mice than in the normal mice throughout the experimental period. Thus, substantial differences in craniofacial growth were demonstrated in various areas of the craniofacial complex, which are assumed essentially due to the lack of osteoclastic bone resorption during growing period. Since the difference became more prominent in the anatomic regions relevant to the masticatory functions, it would be a reasonable assumption that reduced masticatory function is also a key determinant for the less-developed craniofacial skeleton in the op/op mouse.     

Microstructure and mechanical properties of rostrum in Cyrtotrachelus longimanus (Coleoptera: Curculionidae)

The microstructure, composition and mechanical properties of the rostrum in Cyrtotrachelus longimanus (JHC Fabre) were studied utilizing light, fluorescent, scanning electron microscopy (SEM) and energy-dispersive spectroscopy. SEM images show the morphological characteristics of rostrum’s cross section; it is a typical lightweight multilayer structure – one rigid exocuticle layer and dense endocuticle layers, which construct unevenly overlapping fiber structures. The composition analysis of the rostrum shows that it is mainly composed of C, H, N, O, as well as some metal elements and microelements, such as Mg, Si, Zn, Ca and Na, which contribute to its mechanical performance. The mechanical properties of the rostrum were tested by the electronic universal testing machine, which shows it has high-specific strength and is almost the same as that of the stainless steel. The results may provide a biological template to inspire biomimetic lightweight structure design.     

The capsule: an organic skeletal structure in the Late Cretaceous belemnite Gonioteuthis from north‐west Germany

Abstract: An unusual, bilaterally symmetrical black structure that embraces the protoconch and the phragmocone and is overlain by a rostrum has been studied in the Santonian–early Campanian (Late Cretaceous) belemnite genus Gonioteuthis from Braunschweig, north‐west Germany. The structure is here named the capsule. Energy dispersed spectrometry analyses of the capsule show a co‐occurrence of sulphur with zinc, barium, iron, lead and titanium, suggesting their chemical association. The capsule was originally made of organic material that was diagenetically transformed into sulphur‐containing matter. The material of the capsule differs from the chitin of the connecting rings in the same specimens. The capsule has a complex morphology: (1) ventral and dorsal wing‐like projections that are repeated in a breviconic shape of the alveolus, (2) an aperture with lateral lobes and ventral and dorsal sinuses copied by growth lines and (3) a ventral ridge that fits with the position of the fissure in the rostrum. The alveolus in the most anterior part of the rostrum is crater‐like. It is lined with thin, pyritized, laminated material, which appears to be the outermost portion of the capsule attached to the inner surface of the rostrum. A flare along the periphery of the alveolus marks a region where the rostrum was not yet formed, suggesting that the capsule extended beyond the rostrum. Modification of the skeleton in Gonioteuthis comprises a set of supposedly interrelated changes, such as innovation of the organic capsule, partial elimination of the calcareous rostrum and a diminishing of the pro‐ostracum, resulting in the appearance of a new type of pro‐ostracum that became narrower and shorter and lost the spatula‐like shape and gently curved growth lines of a median field that are typical for the majority of Jurassic and Cretaceous belemnites. The partial replacement of a calcareous rostrum with an organic capsule in belemnitellids may have been an adaptive reaction to an unfavourable environmental condition, perhaps related to difficulties in calcium carbonate secretion during the Late Cretaceous that forced animals to reduce carbonate production and to secret an organic capsule around the protoconch and the phragmocone.     

Evolution of a developmental mechanism: Species-specific regulation of the cell cycle and the timing of events during craniofacial osteogenesis

Neural crest mesenchyme (NCM) controls species-specific pattern in the craniofacial skeleton but how this cell population accomplishes such a complex task remains unclear. To elucidate mechanisms through which NCM directs skeletal development and evolution, we made chimeras from quail and duck embryos, which differ markedly in their craniofacial morphology and maturation rates. We show that quail NCM, when transplanted into duck, maintains its faster timetable for development and autonomously executes molecular and cellular programs for the induction, differentiation, and mineralization of bone, including premature expression of osteogenic genes such as Runx2 and Col1a1. In contrast, the duck host systemic environment appears to be relatively permissive and supports osteogenesis independently by providing circulating minerals and a vascular network. Further experiments reveal that NCM establishes the timing of osteogenesis by regulating cell cycle progression in a stage- and species-specific manner. Altering the time-course of D-type cyclin expression mimics chimeras by accelerating expression of Runx2 and Col1a1. We also discover higher endogenous expression of Runx2 in quail coincident with their smaller craniofacial skeletons, and by prematurely over-expressing Runx2 in chick embryos we reduce the overall size of the craniofacial skeleton. Thus, our work indicates that NCM establishes species-specific size in the craniofacial skeleton by controlling cell cycle, Runx2 expression, and the timing of key events during osteogenesis.     

Acute upregulation of hedgehog signaling in mice causes differential effects on cranial morphology

Hedgehog (HH) signaling, and particularly signaling by sonic hedgehog (SHH), is implicated in several essential activities during morphogenesis, and its misexpression causes a number of developmental disorders in humans. In particular, a reduced mitogenic response of cerebellar granule cell precursors to SHH signaling in a mouse model for Down syndrome (DS), Ts65Dn, is substantially responsible for reduced cerebellar size. A single treatment of newborn trisomic mice with an agonist of the SHH pathway (SAG) normalizes cerebellar morphology and restores some cognitive deficits, suggesting a possible therapeutic application of SAG for treating the cognitive impairments of DS. Although the beneficial effects on the cerebellum are compelling, inappropriate activation of the HH pathway causes anomalies elsewhere in the head, particularly in the formation and patterning of the craniofacial skeleton. To determine whether an acute treatment of SAG has an effect on craniofacial morphology, we quantitatively analyzed the cranial form of adult euploid and Ts65Dn mice that were injected with either SAG or vehicle at birth. We found significant deformation of adult craniofacial shape in some animals that had received SAG at birth. The most pronounced differences between the treated and untreated mice were in the midline structures of the facial skeleton. The SAG-driven craniofacial dysmorphogenesis was dose-dependent and possibly incompletely penetrant at lower concentrations. Our findings illustrate that activation of HH signaling, even with an acute postnatal stimulation, can lead to localized dysmorphology of the skull by generating modular shape changes in the facial skeleton. These observations have important implications for translating HH-agonist-based treatments for DS.KEY WORDS: Hedgehog signaling, Craniofacial shape, Down syndrome, Geometric morphometrics     

A developmental staging series for the African house snake, Boaedon (Lamprophis) fuliginosus

Embryonic staging series are important tools in the study of morphological evolution as they establish a common standard for future studies. In this study, we describe the in ovo embryological development of the African house snake (Boaedon fuliginosus), a non-venomous, egg-laying species within the superfamily Elapoidea. We develop our staging series based on external morphology of the embryo including the head, eye, facial prominences, pharyngeal slits, heart, scales, and endolymphatic ducts. An analysis of embryonic growth in length and mass is presented, as well as preliminary data on craniofacial skeletal development. Our results indicate that B. fuliginosus embryos are well into organogenesis but lack well-defined facial prominences at the time of oviposition. Mandibular and maxillary processes extend rostrally within 8 days (stage 3), corresponding to the first appearance of Meckel's cartilages. Overall, the development of the craniofacial skeleton in B. fuliginosus appears similar to that of other snake species with intramembraneous bones (e.g., dentary and compound bones) ossifying before most of the endochondral bones, the first of which to ossify are the quadrate and the otic capsule. Our staging series is the first to describe the post-ovipositional development of a non-venomous elapoid based on external morphology. This species is an extremely tractable captive that can produce large clutches of eggs every 45 days throughout the year. As such, B. fuliginosus should be a good model for evolutionary developmental biologists focusing on the craniofacial skeleton, loss of limbs, generational teeth, and venom delivery systems.     

Assessing Species-specific Contributions To Craniofacial Development Using Quail-duck Chimeras

The generation of chimeric embryos is a widespread and powerful approach to study cell fates, tissue interactions, and species-specific contributions to the histological and morphological development of vertebrate embryos. In particular, the use of chimeric embryos has established the importance of neural crest in directing the species-specific morphology of the craniofacial complex. The method described herein utilizes two avian species, duck and quail, with remarkably different craniofacial morphology. This method greatly facilitates the investigation of molecular and cellular regulation of species-specific pattern in the craniofacial complex. Experiments in quail and duck chimeric embryos have already revealed neural crest-mediated tissue interactions and cell-autonomous behaviors that regulate species-specific pattern in the craniofacial skeleton, musculature, and integument. The great diversity of neural crest derivatives suggests significant potential for future applications of the quail-duck chimeric system to understanding vertebrate development, disease, and evolution.     

Occlusal morphology in 45,X females

Seventy-two 45,X females (Turner syndrome) were examined for occlusal anomalies. Prevalence comparisons were made with first-degree female relatives and female population controls. The method of Bj?rk et al [1964] was used for determination of malocclusion. The results clearly suggest an increased prevalence of occlusal anomalies in 45,X females. The most common malocclusions are cross bite, large maxillary overjet, distal molar occlusion, and a tendency to open bite. These anomalies reflect an imbalanced growth of the craniofacial skeleton in three dimensions. These findings suggest that genes on the human X chromosome are of importance in the harmonious growth and development of the craniofacial skeleton and ultimately normal occlusal morphology and relations.     

Vertebrate cranial evolution: Contributions and conflict from the fossil record

In modern vertebrates, the craniofacial skeleton is complex, comprising cartilage and bone of the neurocranium, dermatocranium and splanchnocranium (and their derivatives), housing a range of sensory structures such as eyes, nasal and vestibulo-acoustic capsules, with the splanchnocranium including branchial arches, used in respiration and feeding. It is well understood that the skeleton derives from neural crest and mesoderm, while the sensory elements derive from ectodermal thickenings known as placodes. Recent research demonstrates that neural crest and placodes have an evolutionary history outside of vertebrates, while the vertebrate fossil record allows the sequence of the evolution of these various features to be understood. Stem-group vertebrates such as Metaspriggina walcotti (Burgess Shale, Middle Cambrian) possess eyes, paired nasal capsules and well-developed branchial arches, the latter derived from cranial neural crest in extant vertebrates, indicating that placodes and neural crest evolved over 500 million years ago. Since that time the vertebrate craniofacial skeleton has evolved, including different types of bone, of potential neural crest or mesodermal origin. One problematic part of the craniofacial skeleton concerns the evolution of the nasal organs, with evidence for both paired and unpaired nasal sacs being the primitive state for vertebrates.     

The biology of extinct and extant sawfish (Batoidea: Sclerorhynchidae and Pristidae)

Sclerorhynchids (extinct sawfishes, Batoidea), pristids (extant sawfish, Batoidea) and pristiophorids (sawsharks, Squalomorphi) are the three elasmobranch families that possess an elongated rostrum with lateral teeth. Sclerorhynchids are the extinct sawfishes of the Cretaceous period, which reached maximum total lengths of 100 cm. The morphology of their rostral teeth is highly variable. Pristid sawfish occur circumtropically and can reach maximum total lengths of around 700 cm. All pristid species are globally endangered due to their restricted habitat inshore. Pristiophorid sawsharks are small sharks of maximum total lengths below 150 cm, which occur in depths of 70–900 m. Close examination of the morphology of pectoral fin basals and the internal structure of the rostrum reveals that sclerorhynchids and pristids evolved independently from rhinobatids, whereas pristiophorids are squalomorph sharks. The elongation of the rostrum may be an adaptation for feeding, as all marine vertebrate taxa that possess this structure are said to use it in the context of feeding.     

The rostral micro-tooth morphology of blue marlin,Makaira nigricans

Billfish rostra potentially have several functions; however, their role in feeding is unequivocal in some species. Recent work linked morphological variation in rostral micro-teeth to differences in feeding behavior in two billfish species, the striped marlin (Kajikia audax) and the sailfish (Istiophorus platypterus). Here, we present the rostral micro-tooth morphology for a third billfish species, the blue marlin (Makaira nigricans), for which the use of the rostrum in feeding behavior is still undocumented from systematic observations in the wild. We measured the micro-teeth on rostrum tips of blue marlin, striped marlin, and sailfish using a micro–computed tomography approach and compared the tooth morphology among the three species. This was done after an analysis of video-recorded hunting behavior of striped marlin and sailfish revealed that both species strike prey predominantly with the first third of the rostrum, which provided the justification to focus our analysis on the rostrum tips. In blue marlin, intact micro-teeth were longer compared to striped marlin but not to sailfish. Blue marlin had a higher fraction of broken teeth than both striped marlin and sailfish, and broken teeth were distributed more evenly on the rostrum. Micro-tooth regrowth was equally low in both marlin species but higher in sailfish. Based on the differences and similarities in the micro-tooth morphology between the billfish species, we discuss potential feeding-related rostrum use in blue marlin. We put forward the hypothesis that blue marlin might use their rostra in high-speed dashes as observed in striped marlin, rather than in the high-precision rostral strikes described for sailfish, possibly focusing on larger prey organisms.     

Correlations between leaf structural traits and the densities of herbivorous insect guilds

The functional composition of herbivorous insect assemblages was correlated with aspects of new and mature leaf surface features, anatomy and morphology across 18 co‐occurring plant species. Multivariate analyses of insects and leaf traits revealed that the functional composition of the herbivore assemblage was more strongly correlated with leaf structural traits than with leaf constituents. Leaf traits were more strongly correlated with the functional composition of the herbivore assemblage than with its taxonomic composition. Densities of sessile phloem feeders, ­rostrum chewers, and all herbivores were significantly negatively correlated with specific leaf weight, lamina and cuticle thickness, vascular tissue depth and stomate length, and were significantly positively correlated with stomate density. External chewer densities were significantly negatively correlated with percent lignified vein area, and significantly positively correlated with leaf surface area and the distance between lignified tissues. Spine‐like leaves were associated with significantly lower densities of sessile phloem feeders, external chewers and all herbivores compared to kite leaves (kite leaves are comprised of unfortified leaf tissue supported by a framework of vascular tissue). The presence of a thickened leaf hypodermis was associated with significantly lower densities of external chewers and rostrum chewers, while midrib protection was associated with significantly lower densities of external chewers. Leaf structural traits may not be the proximal factors influencing herbivorous insects, as leaf structural traits are correlated with many other plant traits such as photosynthetic rate, relative growth rate and leaf life‐span. Nonetheless, these results indicate that certain leaf structural traits may potentially be used to predict the functional structure of herbivorous insect assemblages. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 77 , 43–65.     

Mechanical factors in the evolution of the mammalian secondary palate: a theoretical analysis

The secondary palate of mammals is a bony shelf that closes the ventral aspect of the rostrum. The rostrum, therefore, approximates to a tapered semicylindrical tube that is theoretically a mechanically efficient structure for resisting the forces of biting, including the more prolonged bouts of mastication typical of mammals. Certain mammal-like reptiles illustrate stages in the development of the palate in which the shelves projecting medially from each premaxilla and maxilla do not meet in the midline. We evaluate several geometric properties of sections through the rostrum of the American opossum (Didelphis virginiana). For loading at the incisors and canines, these properties indicate the structural strength and stiffness in both bending and torsion of the rostrum and of single maxillae. We then repeat the analysis but progressively omit segments of the palatal shelf, a procedure which simulates, in reverse, the evolutionary development of the structure. The results demonstrate that the secondary palate contributes significantly to the torsional strength and stiffness of the rostrum of Didelphis and to the strength of each maxilla in lateromedial bending. The major evolutionary implications of the results are that the rapid increase in rostral strength with small increments of the palatal shelves may have been a significant factor in the development of the complete structure. The results indicate that there was a marked jump in torsional strength and stiffness when the shelves met in the midline, which is likely to have been important in the subsequent development of the diverse masticatory mechanisms of cynodonts and mammals. On the basis of this analysis the mammalian secondary palate may be interpreted as one of a number of methods, seen in the mammal-like reptiles, for strengthening the rostrum.     

Elastic properties of external cortical bone in the craniofacial skeleton of the rhesus monkey

Knowledge of elastic properties and of their variation in the cortical bone of the craniofacial skeleton is indispensable for creating accurate finite-element models to explore the biomechanics and adaptation of the skull in primates. In this study, we measured elastic properties of the external cortex of the rhesus monkey craniofacial skeleton, using an ultrasonic technique. Twenty-eight cylindrical cortical specimens were removed from each of six craniofacial skeletons of adult Macaca mulatta. Thickness, density, and a set of longitudinal and transverse ultrasonic velocities were measured on each specimen to allow calculation of the elastic properties in three dimensions, according to equations derived from Newton's second law and Hooke's law. The axes of maximum stiffness were determined by fitting longitudinal velocities measured along the perimeter of each cortical specimen to a sinusoidal function. Results showed significant differences in elastic properties between different functional areas of the rhesus cranium, and that many sites have a consistent orientation of maximum stiffness among specimens. Overall, the cortical bones of the rhesus monkey skull can be modeled as orthotropic in many regions, and as transversely isotropic in some regions, e.g., the supraorbital region. There are differences from human crania, suggesting that structural differences in skeletal form relate to differences in cortical material properties across species. These differences also suggest that we require more comparative data on elastic properties in primate craniofacial skeletons to explore effectively the functional significance of these differences, especially when these differences are elucidated through modeling approaches, such as finite-element modeling.     

Distraction osteogenesis of the craniofacial skeleton

In summary, distraction osteogenesis is a safe and effective means of achieving bone lengthening. These techniques were originally applied to the long bones of the extremities; over the past 10 years they have been effectively applied to the bones of the craniofacial skeleton. The new bone regenerate that is observed after distraction osteogenesis is stable, and relapse rates after skeletal advancement are believed to be lower than with conventional osteotomy and bone graft techniques. There is considerable variability in distraction protocols employed in clinical practice, including differences in the types of devices used and in the rate, rhythm, latency, and period of consolidation for distraction osteogenesis. The greatest application for distraction osteogenesis in the craniofacial skeleton has been with mandible lengthening, for which there is presently a 10-year clinical experience. Midfacial advancement is a newer application of distraction osteogenesis, for which clinical experience has been accrued over the past 5 years. This latter experience indicates that distraction osteogenesis is a viable treatment option for lengthening of the hypoplastic mandible and midface. These techniques have advantages over conventional means of bone graft and rigid fixation because of the quality of the bone regenerate, the decrease in the long-term relapse rate of the advanced bone segments in both the mandible and the midface, and the simultaneous soft-tissue elongation that accompanies the distraction process. Distraction osteogenesis is particularly applicable to the correction of severe deformities of the mandible and midface in children with developmental hypoplasia and syndromic craniosynostosis. However, growth is an added variable in this patient population. The amount of overcorrection in lengthening of the hypoplastic bone required to compensate for continued growth discrepancy of the adjacent facial bones is difficult to predict. Therefore, the families of these patients should be informed that many children will require repeated operations at a later age as they reach skeletal maturity.     

Covariation of brain and skull shapes as a model to understand the role of crosstalk in development and evolution

Covariation among discrete phenotypes can arise due to selection for shared functions, and/or shared genetic and developmental underpinnings. The consequences of such phenotypic integration are far-reaching and can act to either facilitate or limit morphological variation. The vertebrate brain is known to act as an “organizer” of craniofacial development, secreting morphogens that can affect the shape of the growing neurocranium, consistent with roles for pleiotropy in brain–neurocranium covariation. Here, we test this hypothesis in cichlid fishes by first examining the degree of shape integration between the brain and the neurocranium using three-dimensional geometric morphometrics in an F₅ hybrid population, and then genetically mapping trait covariation using quantitative trait loci (QTL) analysis. We observe shape associations between the brain and the neurocranium, a pattern that holds even when we assess associations between the brain and constituent parts of the neurocranium: the rostrum and braincase. We also recover robust genetic signals for both hard- and soft-tissue traits and identify a genomic region where QTL for the brain and braincase overlap, implicating a role for pleiotropy in patterning trait covariation. Fine mapping of the overlapping genomic region identifies a candidate gene, notch1a, which is known to be involved in patterning skeletal and neural tissues during development. Taken together, these data offer a genetic hypothesis for brain–neurocranium covariation, as well as a potential mechanism by which behavioral shifts may simultaneously drive rapid change in neuroanatomy and craniofacial morphology.