Developmental Toxicity Studies with Pregabalin in Rats: Significance of Alterations in Skull Bone Morphology

Pregabalin was administered to pregnant Wistar rats during organogenesis to evaluate potential developmental toxicity. In an embryo‐fetal development study, compared with controls, fetuses from pregabalin‐treated rats exhibited increased incidence of jugal fused to maxilla (pregabalin 1250 and 2500 mg/kg) and fusion of the nasal sutures (pregabalin 2500 mg/kg). The alterations in skull development occurred in the presence of maternal toxicity (reduced body weight gain) and developmental toxicity (reduced fetal body weight and increased skeletal variations), and were initially classified as malformations. Subsequent investigative studies in pregnant rats treated with pregabalin during organogenesis confirmed the advanced jugal fused to maxilla, and fusion of the nasal sutures at cesarean section (gestation day/postmating day [PMD] 21) in pregabalin‐treated groups. In a study designed to evaluate progression of skull development, advanced jugal fused to maxilla and fusion of the nasal sutures was observed on PMD 20–25 and PMD 21–23, respectively (birth occurs approximately on PMD 22). On postnatal day (PND) 21, complete jugal fused to maxilla was observed in the majority of control and 2500 mg/kg offspring. No treatment‐related differences in the incidence of skull bone fusions occurred on PND 21, indicating no permanent adverse outcome. Based on the results of the investigative studies, and a review of historical data and scientific literature, the advanced skull bone fusions were reclassified as anatomic variations. Pregabalin was not teratogenic in rats under the conditions of these studies     

中国和欧洲早期智人的比较研究

中国与欧洲的早期智人头骨在颧骨额蝶突前外侧面的朝向、颧颌角、上颌骨颧突、鼻区、上面部高度、额鼻额颌缝形状、眉间区、矢状脊、印加骨和铲形门齿诸特征的形态或出现率等方面有明显差异。那时此两大地区存在相对独立的人类进化线,其间还有一定程度的基因交流。当时此两地区的人类居群分属于不同的人种。这一假说还可从古文化和古环境的资料得到支持。     

Evolution of a complex phenotype with biphasic ontogeny: Contribution of development versus function and climatic variation to skull modularity in toads

The theory of morphological integration and modularity predicts that if functional correlations among traits are relevant to mean population fitness, the genetic basis of development will be molded by stabilizing selection to match functional patterns. Yet, how much functional interactions actually shape the fitness landscape is still an open question. We used the anuran skull as a model of a complex phenotype for which we can separate developmental and functional modularity. We hypothesized that functional modularity associated to functional demands of the adult skull would overcome developmental modularity associated to bone origin at the larval phase because metamorphosis would erase the developmental signal. We tested this hypothesis in toad species of the Rhinella granulosa complex using species phenotypic correlation pattern (P‐matrices). Given that the toad species are distributed in very distinct habitats and the skull has important functions related to climatic conditions, we also hypothesized that differences in skull trait covariance pattern are associated to differences in climatic variables among species. Functional and hormonal‐regulated modules are more conspicuous than developmental modules only when size variation is retained on species P‐matrices. Without size variation, there is a clear modularity signal of developmental units, but most species have the functional model as the best supported by empirical data without allometric size variation. Closely related toad species have more similar climatic niches and P‐matrices than distantly related species, suggesting phylogenetic niche conservatism. We infer that the modularity signal due to embryonic origin of bones, which happens early in ontogeny, is blurred by the process of growth that occurs later in ontogeny. We suggest that the species differing in the preferred modularity model have different demands on the orbital functional unit and that species contrasting in climate are subjected to divergent patterns of natural selection associated to neurocranial allometry and T3 hormone regulation.     

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.     

Aspects of comparative cranial mechanics in the theropod dinosaurs Coelophysis, Allosaurus and Tyrannosaurus

The engineering analysis technique finite element analysis (FEA) is used here to investigate cranial stress and strain during biting and feeding in three phylogenetically disparate theropod taxa: Coelophysis bauri , Allosaurus fragilis and Tyrannosaurus rex . Stress patterns are generally similar in all taxa with the ventral region of the skull tensed whilst the dorsal aspect is compressed, although the skull is not purely behaving as a cantilever beam as there is no discernible neutral region of bending. Despite similarities, stress patterns are not wholly comparable: there are key differences in how certain regions of the skull contain stress, and it is possible to link such differences to cranial morphology. In particular, nasal morphology can be explained by the stress patterns revealed here. Tyrannosaurus models shear and compress mainly in the nasal region, in keeping with the indistinguishably fused and expanded morphology of the nasal bones. Conversely Allosaurus and Coelophysis models experience peak shear and compression in the fronto-parietal region (which is tightly interdigitated and thickened in the case of Allosaurus ) yet in contrast the nasal region is lightly stressed, corresponding to relatively gracile nasals and a frequently patent internasal suture evident in Allosaurus . Such differences represent alternate mechanical specializations between taxa that may be controlled by functional, phylogenetic or mechanical constraints. Creation of finite element models placed in a phylogenetic context permits the investigation of the role of such mechanical character complexes in the cranium of nonavian theropods and the lineage leading towards modern birds.  © 2005 The Linnean Society of London, Zoological Journal of the Linnean Society , 2005, 144 , 309–316.     

Morphological integration and modularity in the hyperkinetic feeding system of aquatic‐foraging snakes

The kinetic skull is a key innovation that allowed snakes to capture, manipulate, and swallow prey exclusively using their heads using the coordinated movement of eight bones. Despite these unique feeding behaviors, patterns of evolutionary integration and modularity within the feeding bones of snakes in a phylogenetic framework have yet to be addressed. Here, we use a dataset of 60 μCT‐scanned skulls and high‐density geometric morphometric methods to address the origin and patterns of variation and integration in the feeding bones of aquatic‐foraging snakes. By comparing alternate superimposition protocols allowing us to analyze the entire kinetic feeding system simultaneously, we find that the feeding bones are highly integrated, driven predominantly by functional selective pressures. The most supported pattern of modularity contains four modules, each associated with distinct functional roles: the mandible, the palatopterygoid arch, the maxilla, and the suspensorium. Further, the morphological disparity of each bone is not linked to its magnitude of integration, indicating that integration within the feeding system does not strongly constrain morphological evolution, and that adequate biomechanical solutions to a wide range of feeding ecologies and behaviors are readily evolvable within the constraint due to integration in the snake feeding system.     

中国穿山甲和马来穿山甲头骨量度的测定及比较

全球共有7种穿山甲。本文报道的是中国穿山甲和马来穿山甲的头骨形态学资料,涉及头骨特征描述和一些形态学指标测定。被测定的指标主要有颅全长、基底长、腭长、眶间宽、后头宽、鼻骨长、鼻骨宽、脑颅高、听泡长和下颌长,并且在这两个物种之间进行了比较。结果表明,(1)这两个物种头骨形态十分相似,但马来穿山甲头骨细长,听泡长小于10mm(n=12),鼻骨狭长,前后部宽窄相似,两鼻骨外侧缘内凹或平行,鼻骨宽长之比值小于0 3000(0 2025～0 2811,n=12);而中国穿山甲头骨较粗短,听泡长大于11mm(n=46),鼻骨宽短,后部较前端宽,外侧缘外凸,鼻骨宽长之比值大于0 3000(0 3154～0 5325,n=33)。这些异同之处,可以作为区分这两个物种的依据。(2)中国穿山甲华南亚种与海南亚种头骨各变量之间的差异不显著(P>0 05),用颅全长是否大于83mm来区分这两个亚种不能成立,但海南亚种头骨上颌骨的颧突和鳞骨的颧突通常越过眶颞窝完全相接触,如果不是这样,则在它们之间有一个小的骨质棒状结构将它们连接起来。     

南京1号直立人头骨与肯尼亚KNM-ER 3733人类头骨化石的形态比较

KNM-ER 3733人类头骨化石的年代为距今1.78百万年,1975年发现于肯尼亚。Walker和Leakey注意到这具头骨与周口店直立人的在脑颅形态上很相近,但二者在年代上相差大约1百万年,故认为直立人形态在这1百万年期间是稳定的。长期来此观点缺乏更多的人类化石证据来支持。1993年在中国发现了南京1号人类头骨化石。该头骨与KNM-ER 3733头骨一样兼具脑颅和面颅,且都属于成年女性个体,但南京1号人类头骨化石的年代比KNM-ER 3733人类头骨化石的要晚大约1百万年。因此,南京1号人类头骨是目前所知的可用来验证直立人头骨形态是否在1百万年期间保持稳定的唯一合适的人类头骨化石材料。形态比较表明,这两个人类头骨化石的脑颅虽然在眶上圆枕上沟的发育程度、眶后收缩的程度、额骨横向隆起的程度、角圆枕和乳后突的发育与否、顶骨形状以及骨壁厚度的表现上有所差异,但有更多的形态性状显示出相近。这些相近表现在脑颅的长、宽、高值上;颅容量上;脑颅的低矮性上;脑颅最大宽之位置上;额骨、顶骨、枕骨之矢弧值的比例上;眶上圆枕的纤细上;顶骨的大小和矢向扁平性上;颞线位置和颞鳞顶缘的形状上;枕鳞的低宽形状上;上枕鳞与下枕鳞之间的转折形状和比例上;枕骨圆枕和枕骨圆枕上沟的发育程度上等。这两具头骨的面颅虽然有同属突颌型的面角、皆发育有鼻骨间嵴、两鼻骨组成的上部宽度与下部宽度皆差别很大,但有更多的形态性状显示出差别。这些差别表现在面型上、颜面上部扁平度上、眶形和眶型上、上颌额突外侧面的朝向上、鼻骨横向隆起程度上、鼻梁外突程度上、鼻型上、颧骨下缘外展程度上、颊高上、颧上颌下缘的形状上、上颌颧突基部的位置上以及颧结节的位置上等。因此,南京1号头骨与KNM-ER 3733头骨之间在脑颅上显示出较多的相近性状,在面颅上则显示出较多的相异性状。脑颅方面的相近性状大多具有分类上的鉴别价值。这两个头骨脑颅形态的相近支持把KNM-ER 3733头骨鉴定为"直立人"的观点;也提示了南京1号头骨的脑颅似乎保持着1百多万年前的"祖先"形态。如果直立人的某些成员在至少1百万年期间保持着形态稳定的话,则这种形态上的稳定主要是表现在脑颅形态上。这两具头骨的面颅形态上较大差异的意义,目前尚不清楚。     

Functional structure of the skull in hominoidea

Finite elements stress analysis (FESA) was used to investigate the flow of compressive forces which occur if a homogenous, three-dimensional body representing the skull is loaded by simulated bite forces against the tooth row. Model 1 represents the snout alone. Bite forces are applied simultaneously, but increase rearward. Stresses in the model concentrate along the anterior contour and the lower surface of the model, leaving unstressed a nasal opening and a wide naso-oral connection. Model 2 represents the facial region, as far as the temporomandibular joint. The orbits and the nasal cavity are assumed to be present a priori. Model 3 applies reactions to the bite forces in the temporal fossa, corresponding to the origins of the masticatory muscles. Regions of the model under compressive stress correspond closely to the arrangement of bony material in a hominoid skull. If only the stress-bearing finite elements on each section are combined, and the stress-free parts neglected, the resulting three-dimensional shape is surprisingly similar to a hominoid skull. If bite forces are applied to parts of the tooth row only, the stress patterns are lower, asymmetrical and do not spread into all regions that are stress-bearing in simultaneous biting on all teeth. In model 2, the highest stresses occur at the tooth roots and along the forehead on top of the nasal roof. There are no marked stress concentrations on top of the orbits. The resulting shape resembles that of an orang-utan. In model 3, the highest stresses also occur at the tooth roots, but the circles of force mostly close below the brain case, so that the stress concentration in the forehead region remains much less marked. In this model, however, the stress concentrations are very similar to hollow brow ridges. The entire resulting shape resembles that of gorilla or chimpanzee skulls. A typical gracile australopithecine skull (STS-5) also shows clear similarities to the patterns of stress flow in our models. Compared to our earlier study of the modern human skull, differences relate to: the relative length and width of the dental arcade, the relative size of the brain case and the position of the arcade relative to the brain case. It seems that these traits are the points of attack of selective pressures, while all other morphological details are simply consequences of stress flow.     

Homology of the palpebral and origin of supraorbital ossifications in ornithischian dinosaurs

Maidment, S.C.R. & Porro, L.B. 2010: Homology of the palpebral and origin of supraorbital ossifications in ornithischian dinosaurs. Lethaia , Vol. 43, pp. 95–111.
The palpebral is a small ossification that projects across the orbit in some ornithischian dinosaurs and its presence is considered a synapomorphy of the clade. By contrast, other ornithischians lack the palpebral but possess accessory ossifications, commonly termed supraorbitals, which form the dorsal margin of the orbit. The homology of the ornithischian palpebral to one or more of the supraorbitals is widely accepted in the literature, but this homology has never been explicitly tested and no hypotheses have been proposed regarding the function of the palpebral or why it was incorporated into the orbital margin. As homology is synonymous with synapomorphy, incorrect homology statements can lead to incorrect hypotheses of relationships being obtained during cladistic analysis. The primary and secondary homologies of the ornithischian palpebral and the anterior supraorbital of more derived members of the major ornithischian clades are tested and we demonstrate that these homology hypotheses can be accepted. Osteological correlates indicate that the palpebral supported a layer of connective tissue that roofed the orbit; ossification of this connective tissue resulted in the incorporation of the palpebral into the skull roof and gave rise to additional supraorbital elements, which are neomorphic ossifications. Large-scale structural changes in the ornithischian skull, including dermal ossifications associated with display or defence and the development of complex feeding mechanisms, may have led to the incorporation of the palpebral into the skull roof. □ Functional morphology , homology , Ornithischia , palpebral , supraorbital.     

Regional variations in the outer retina of atherinomorpha (Beloniformes,Atheriniformes, Cyprinodontiformes: Teleostei): photoreceptors,cone patterns,and cone densities

The outer retinae of adults of 13 atherinomorph species, representing nine different families, were examined by both light and electron microscopy. The retinae were investigated with respect to photoreceptor types, cone densities, and cone patterns. All data were composed to eye maps. This procedure allows an interspecific comparison of the regional differences within the outer retina among these shallow-water fish. Furthermore, for a more detailed pattern analysis nitro-blue tetrazolium chloride- (NBT)-stainings in the retina of Melanotaenia maccullochi are presented. Apart from rods, eight morphologically different cone types could be identified: short, intermediate, and long single cones, double cones (equal and unequal), triple cones (triangular and linear), and in Ameca splendens one quadruple cone. Dimensions and occurrence of photoreceptors vary among the respective species and within the retinal regions. In the light-adapted state, the cones are arranged in highly ordered mosaics. Five different cone tessellation types were found: row patterns, twisted row patterns, square patterns, pentagonal patterns, and, exclusively in Belone belone, a hexagonal pattern. In Melanotaenia maccullochi the different spectral photoreceptor classes correspond well with the distribution of morphological photoreceptor classes within the mosaic. Double cone density maxima together with a highly ordered cone arrangement usually occur in the nasal and/or ventral to ventrotemporal retina. In most of the species that were examined these high-density regions are presumed to process visual stimuli from the assumed main directions of vision, which mainly depend on feeding behavior and predator pressure. Our findings are discussed with respect to the variable behavioral and visual ecology and phylogeny of the respective species.     

Development of the supraorbital and mandibular lateral line canals in the cichlid,Archocentrus nigrofasciatus

The development of two of the cranial lateral line canals is described in the cichlid, Archocentrus nigrofasciatus. Four stages of canal morphogenesis are defined based on histological analysis of the supraorbital and mandibular canals. "Canal enclosure" and "canal ossification" are defined as two discrete stages in lateral line canal development, which differ in duration, an observation that has interesting implications for the ontogeny of lateral line function. Canal diameter in the vicinity of individual neuromasts begins to increase before ossification of the canal roof in each canal segment; this increase in canal diameter is accompanied by an increase in canal neuromast size. The mandibular canal generally develops later than the supraorbital canal in this species, but in both of these canals development of the different canal segments contained within a single dermal bone is asynchronous. These observations suggest that a dynamic process requiring integration and interaction among different tissues, in both space and time, underlies the development of the cranial lateral line canal system. The supraorbital and mandibular canals appear to demonstrate a "one-component" pattern of development in Archocentrus nigrofasciatus, where the walls of each canal segment grow up from the underlying dermal bone and then fuse to form the bony canal roof. This is contrary to numerous published reports that describe a "two-component" pattern of development in teleosts where the bony canal ossifies separately and then fuses with an underlying dermal bone. A survey of the literature in which lateral line canal development is described using histological analysis suggests that the occurrence of two different patterns of canal morphogenesis ("one-component" and "two-component") may be due to phylogenetic variation in the pattern of the development of the lateral line canals.     

Elastic properties of human supraorbital and mandibular bone

Elastic constants, including the elastic modulus, the shear modulus, and Poisson's ratio, were measured on human craniofacial bone specimens obtained from the supraorbital region and the buccal surfaces of the mandibles of unembalmed cadavers. Constants were determined using an ultrasonic wave technique in three directions relative to the surface of each sample: 1) normal, 2) tangential, and 3) longitudinal. Statistical analysis of these elastic constants indicated that significant differences in the relative proportions of elastic properties existed between the regions. Bone from the mandible along its longitudinal axis was stiffer than bone from the supraorbital region. Directional differences in both locations demonstrated that cranial bone was not elastically isotropic. It is suggested that differences in elastic properties correspond to regional differences in function. © 1993 Wiley-Liss, Inc.     

Relationship of strain magnitude to morphological variation in the primate skull

In a comparative study of variation in primate skulls, Wood and Lieberman ([ 2001 ] Am. J. Phys. Anthropol. 116:13–25) proposed that a predictable relationship exists between in vivo bone‐strain magnitudes and the extent of morphological variation in skeletal structures. They hypothesized that regions subject to high strains are prone to enhanced levels of variation. Three questions are posed with respect to the plausibility of this hypothesis. First, does the proposed relationship hold at different levels of analysis (e.g., for more restricted anatomical regions in which large strain gradients are present)? Second, is the biomechanical rationale for the hypothesis sound, given the current understanding of bone biology? Third, is the hypothesis obviated by consideration of the functional matrix concept of skull development, in which osseous growth is posited to be governed by surrounding soft tissues (e.g., muscle and tendon) and developing spaces (e.g., the nasal capsule)? The different perspectives explored by these questions suggest that the validity of the hypothesis, despite having a defensible theoretical rationale, is likely to be context‐specific. A direct role for strain magnitude in conditioning morphological variation is difficult to demonstrate either comparatively or theoretically, and it is unlikely that a single strain threshold or interval can be directly associated with elevated variation in the skeleton. The conceptual framework of the functional matrix (which allows for independent growth among different regions of the skull) conceivably contravenes the premise of a uniform relationship of strain magnitude to morphological variability. Am J Phys Anthropol, 2003. © 2003 Wiley‐Liss, Inc.     

Skull growth and the presence of auxiliary fontanels in rhinolophoid bats (Microchiroptera)

 Cephalometry was used to detect patterns of cranial growth in fetal bats that have been differentially stained for bone and cartilage. Rhinolophoid bats exhibit elaborate nasal cavities with coincidental distortions of the maxilla. The expansion of these cavities creates paired auxiliary fontanels among the nasal, maxillary, and frontal bones. This distortion of the rostrum is also associated with the loss of the lacrimal bones and the modification of the infraorbital foramen into a shallow canal. The use of the head as an acoustical horn is discussed with reference to the ontogeny of echolocation. Accepted: 12 October 1996     

Incorporation of titanium mesh in orbital and midface reconstruction

Several authors have demonstrated the safety and effectiveness of titanium in orbital reconstruction. One question posed by clinicians is what happens to large pieces of titanium in communication with the paranasal sinuses or nasal-oral-pharyngeal area. This question becomes increasingly relevant as titanium is used to reconstruct extensive defects for which the destruction of bony architecture requires the placement of mesh in proximity to these areas. The objective of this study was to examine the gross and histologic soft-tissue response to large segments of titanium mesh in the setting of orbital and midface reconstruction, particularly when exposed to the nasal-oral-pharyngeal area and paranasal sinuses. In this study, large segments of titanium mesh were used in eight patients to reconstruct orbital and midface defects, with direct communication between the mesh and nasal-oral-pharyngeal area and paranasal sinuses. Four patients had suffered self-inflicted gunshot wounds; as a result, much of their midface was missing, including the inferior and medial orbital floor, maxilla, nose, naso-orbital-ethmoid complex, and hard palate. Extensive sheets of titanium mesh were used to reconstruct their medial and inferior orbital walls, nasal bridge, and maxilla. In the fifth patient, titanium mesh was used to reconstruct the maxilla after resection of a squamous cell carcinoma of the nasolacrimal duct. In the sixth and seventh patients, mesh was used to reconstruct the nasal bridge after severely comminuted nasal fractures resulted in the loss of bone and mucosa. Finally, the eighth patient had titanium mesh used to replace cocaine-induced bone loss involving the left medial orbital floor and wall and part of the maxilla. On gross examination by either endoscopy or direct inspection, all eight patients had rapid soft-tissue incorporation of the titanium mesh. Initial examination typically revealed budding of soft tissue through mesh interstices, followed by progressive incorporation. One patient's mesh was covered in only 15 days. Two patients underwent biopsies of this newly formed soft tissue. One had biopsies performed at 3, 15, and 31 months after the original operation. Biopsy examination at 3 months revealed incorporation of the titanium with fibrous soft tissue covered by ciliated respiratory epithelium, goblet cells, and squamous epithelium with metaplasia. In addition, the dense, acute inflammation present at 3 months evolved into mild, chronic inflammation at 31 months. The second patient had a single biopsy 4 months after secondary orbital reconstruction for delayed enophthalmos. Biopsy examination revealed a fibrous soft-tissue sheath lined by squamous epithelium with metaplasia. Again, mild chronic inflammation was present within the soft tissue. This study provides evidence of titanium's compatibility with soft tissue. The mesh underwent progressive incorporation with soft tissue that was then resurfaced by indigenous cells, including respiratory epithelia and goblet cells. This phenomenon occurred despite communication with the nasal-oral-pharyngeal area and paranasal sinuses.     

Patterns of morphospace occupation and mechanical performance in extant crocodilian skulls: A combined geometric morphometric and finite element modeling approach

Extant and fossil crocodilians have long been divided into taxonomic and/or ecological groups based on broad patterns of skull shape, particularly the relative length and width of the snout. However, these patterns have not been quantitatively analyzed in detail, and their biomechanical and functional implications are similarly understudied. Here, we use geometric morphometrics and finite element analysis to explore the patterns of variation in crocodilian skull morphology and the functional implications of those patterns. Our results indicate that skull shape variation in extant crocodiles is much more complex than previously recognized. Differences in snout length and width are the main components of shape variation, but these differences are correlated with changes in other regions of the skull. Additionally, there is considerable disparity within general classes such as longirostrine and brevirostrine forms. For example, Gavialis and Tomistoma occupy different parts of morphospace implying a significant difference in skull shape, despite the fact that both are traditionally considered longirostrine. Skull length and width also strongly influence the mechanical performance of the skull; long and narrow morphotypes (e.g., Tomistoma) experience the highest amount of stress during biting, whereas short and broad morphotypes (e.g., Caiman latirostris) experience the least amount of stress. Biomechanical stress and the hydrodynamic properties of the skull show a strong relationship with the distribution of crocodilians in skull morphospace, whereas phylogeny and biogeography show weak or no correlation. Therefore, ecological specializations related to feeding and foraging likely have the greatest influence on crocodilian skull shape. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Creating morphological diversity in reptilian temporal skull region: A review of potential developmental mechanisms

Reptilian skull morphology is highly diverse and broadly categorized into three categories based on the number and position of the temporal fenestrations: anapsid, synapsid, and diapsid. According to recent phylogenetic analysis, temporal fenestrations evolved twice independently in amniotes, once in Synapsida and once in Diapsida. Although functional aspects underlying the evolution of tetrapod temporal fenestrations have been well investigated, few studies have investigated the developmental mechanisms responsible for differences in the pattern of temporal skull region. To determine what these mechanisms might be, we first examined how the five temporal bones develop by comparing embryonic cranial osteogenesis between representative extant reptilian species. The pattern of temporal skull region may depend on differences in temporal bone growth rate and growth direction during ontogeny. Next, we compared the histogenesis patterns and the expression of two key osteogenic genes, Runx2 and Msx2, in the temporal region of the representative reptilian embryos. Our comparative analyses suggest that the embryonic histological condition of the domain where temporal fenestrations would form predicts temporal skull morphology in adults and regulatory modifications of Runx2 and Msx2 expression in osteogenic mesenchymal precursor cells are likely involved in generating morphological diversity in the temporal skull region of reptiles.