Primary homologies of the circumorbital bones of snakes

Some snakes have two circumorbital ossifications that in the current literature are usually referred to as the postorbital and supraorbital. We review the arguments that have been proposed to justify this interpretation and provide counter‐arguments that reject those conjectures of primary homology based on the observation of 32 species of lizards and 81 species of snakes (both extant and fossil). We present similarity arguments, both topological and structural, for reinterpretation of the primary homologies of the dorsal and posterior orbital ossifications of snakes. Applying the test of similarity, we conclude that the posterior orbital ossification of snakes is topologically consistent as the homolog of the lacertilian jugal, and that the dorsal orbital ossification present in some snakes (e.g., pythons, Loxocemus, and Calabaria) is the homolog of the lacertilian postfrontal. We therefore propose that the terms postorbital and supraorbital should be abandoned as reference language for the circumorbital bones of snakes, and be replaced with the terms jugal and postfrontal, respectively. The primary homology claim for the snake “postorbital” fails the test of similarity, while the term “supraorbital” is an unnecessary and inaccurate application of the concept of a neomorphic ossification, for an element that passes the test of similarity as a postfrontal. This reinterpretation of the circumorbital bones of snakes is bound to have important repercussions for future phylogenetic analyses and consequently for our understanding of the origin and evolution of snakes. J. Morphol. 274:973–986, 2013. © 2013 Wiley Periodicals, Inc.     

Osteogenesis,homology, and function of the intercostal plates in ornithischian dinosaurs (Tetrapoda,Sauropsida)

Intercostal plates are bony structures positioned lateral to the anterior dorsal ribs in some ornithischian dinosaurs. Some propose these plates are homologous, or functionally analogous, with the uncinate processes of extant avian dinosaurs that assist in breathing, while others suggest they served a defensive function. To elucidate their osteogenesis, homology, and function, a histological survey of intercostal plates from three taxa (Hypsilophodon, Talenkauen, and Thescelosaurus) was undertaken. This study reveals that osteogenesis of intercostal plates closely resembles that of secondary centers of ossification in endochondral bone, typically present in the epiphyses of mammalian long bones. In contrast, ossification of avian uncinate processes begins at a primary ossification center via the development of a bony collar around a cartilaginous model. Based on these data, intercostal plates and avian uncinate processes are likely not evolutionary homologs. Dense packets of obliquely oriented Sharpey’s fibers within the parallel-fibered bone of somatically mature intercostal plates indicate these plates were positioned medial to at least a portion of the hypaxial musculature, which does not support their use as bony armor. Rather, we propose that intercostal plates performed some biomechanical function, either assisting in breathing in a way analogous to avian uncinate processes, or working together with the sternal ribs and sternal plates of these ornithischian taxa to provide increased rigidity to the anterior portion of the ribcage.     

New Developmental Evidence Clarifies the Evolution of Wrist Bones in the Dinosaur–Bird Transition

From early dinosaurs with as many as nine wrist bones, modern birds evolved to develop only four ossifications. Their identity is uncertain, with different labels used in palaeontology and developmental biology. We examined embryos of several species and studied chicken embryos in detail through a new technique allowing whole-mount immunofluorescence of the embryonic cartilaginous skeleton. Beyond previous controversy, we establish that the proximal–anterior ossification develops from a composite radiale+intermedium cartilage, consistent with fusion of radiale and intermedium observed in some theropod dinosaurs. Despite previous claims that the development of the distal–anterior ossification does not support the dinosaur–bird link, we found its embryonic precursor shows two distinct regions of both collagen type II and collagen type IX expression, resembling the composite semilunate bone of bird-like dinosaurs (distal carpal 1+distal carpal 2). The distal–posterior ossification develops from a cartilage referred to as “element x,” but its position corresponds to distal carpal 3. The proximal–posterior ossification is perhaps most controversial: It is labelled as the ulnare in palaeontology, but we confirm the embryonic ulnare is lost during development. Re-examination of the fossil evidence reveals the ulnare was actually absent in bird-like dinosaurs. We confirm the proximal–posterior bone is a pisiform in terms of embryonic position and its development as a sesamoid associated to a tendon. However, the pisiform is absent in bird-like dinosaurs, which are known from several articulated specimens. The combined data provide compelling evidence of a remarkable evolutionary reversal: A large, ossified pisiform re-evolved in the lineage leading to birds, after a period in which it was either absent, nonossified, or very small, consistently escaping fossil preservation. The bird wrist provides a modern example of how developmental and paleontological data illuminate each other. Based on all available data, we introduce a new nomenclature for bird wrist ossifications.     

The anatomy of the basal ornithischian dinosaur Eocursor parvus from the lower Elliot Formation (Late Triassic) of South Africa

Ornithischia is a morphologically and taxonomically diverse clade of dinosaurs that originated during the Late Triassic and were the dominant large‐bodied herbivores in many Cretaceous ecosystems. The early evolution of ornithischian dinosaurs is poorly understood, as a result in part of a paucity of fossil specimens, particularly during the Triassic. The most complete Triassic ornithischian dinosaur yet discovered is Eocursor parvus from the lower Elliot Formation (Late Triassic: Norian–Rhaetian) of Free State, South Africa, represented by a partial skull and relatively complete postcranial skeleton. Here, the anatomy of Eocursor is described in detail for the first time, and detailed comparisons are provided to other basal ornithischian taxa. Eocursor is a small‐bodied taxon (approximately 1 m in length) that possesses a plesiomorphic dentition consisting of unworn leaf‐shaped crowns, a proportionally large manus with similarities to heterodontosaurids, a pelvis that contains an intriguing mix of plesiomorphic and derived character states, and elongate distal hindlimbs suggesting well‐developed cursorial ability. The ontogenetic status of the holotype material is uncertain. Eocursor may represent the sister taxon to Genasauria, the clade that includes most of ornithischian diversity, although this phylogenetic position is partially dependent upon the uncertain phylogenetic position of the enigmatic and controversial clade Heterodontosauridae. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 160 , 648–684.     

Paedomorphosis and skull structure in Malagasy chamaeleons (Reptilia: Chamaeleoninae)

The phylogenetic analysis of skull characteristics, along with data derived from lung and hemipenial morphology, does not support monophyly of the predominantly terrestrial chamaeleons, Brookesiinae ( Brookesia plus Rhampholeon ), as sister-taxon of a monophyletic clade of arboreal chamaeleons, the Chamaeleoninae. Instead, the phylogenetic interrelationships of chamaeleonine lizards are ( Brookesia ("Rhampholeon" (Bradypodion, Chamaeleo) ). The monophyly of the genus Rhampholeon is not demonstrated. The arboreal chamaeleons form a monophyletic group comprising two genera, Bradypodion and Chamaeleo. Bradypodion may be placed as sister-group of the genus Chamaeleo , but its phyletic position and monophyly cannot at present be corroborated.
This study documents that there is a high degree of intraspecific variability in skull characters which may be due to paedomorphic reduction in the skull, especially of small species. Characters affected by a variable degree of ossification include the separation of a prefrontal fontanelle, the contact of the squamosal with the parietal, the size and position of the fenestra vestibuli, the degree of ossification in the basicranial rim of the fenestra, the extent of ossification of the pterygoid wing, and the shape of the skull roof as indicated by the bones forming the dorsal margin of the orbit. The closure of the lateral aperture of the occipital recess might indicate that miniaturization, and concomitant paedomorphosis, may have played a role in the initial phases of chamaeleonine phylogeny.     

A NEW PARASEMIONOTID-LIKE FISH FROM THE LOWER TRIASSIC OF JURONG, JIANGSU PROVINCE, SOUTH CHINA

Abstract:  Based on a well-preserved specimen from the Early Triassic Lower Qinglong Formation exposed at Qingshan Quarry, Jurong, Jiangsu Province of China, the new taxon Peia jurongensis gen. et sp. nov. is named and described with anatomical details of its dermal skull, neurocranium and postcranial skeleton. The new taxon is characterized by the following set of characters: shallow V-shaped, tuber-like rostral; anterior part of frontal widened to roughly same width as its posterior part; roughly squared parietal; supraorbital absent; wide and slightly posteriorly inclined preopercle; five small ossifications present in dorsal half of preopercle; preopercular canal shifting anteriorly in ventral half of the preopercle, and reaching the anteroventral corner of this bone; dermosphenotic joining the skull roof and with a descending lamina; numerous branchiostegal rays. Comparisons are made between the new taxon and several parasemionotids from the same locality; a discussion is carried out on the distribution and evolution of several characters in halecomorphs and other closely related neopterygians.     

A new heterodontosaurid dinosaur (Reptilia: Ornithischia) from the Upper Triassic Red Beds of Lesotho

A new species of ornithischian dinosaur ( Lycorhinus consors sp. nov.) is established on a skull from the Upper Triassic Red Beds of Lesotho. This ornithischian is assigned to the family Heterodontosauridae of the suborder Ornithopoda. The dinosaurs of the family Heterodonto-sauridae are reviewed: Geranosaurus atavus Broom (1911) is considered a nomendubium and the genus name Heterodontosaurus Crompton & Charig (1962) is held to be a junior synonym for Lycorhinus Haughton (1924).
Functional and palaeoecological implications of the heterodontosaurid dentition are discussed. The pattern of tooth wear may reflect a highly specialized jaw action which involved protraction and retraction of the mandible to produce a grinding effect between upper and lower cheek teeth. Lycorhinus consors is presumed to be a female heterodontosaurid because it differs from all other heterodontosaurids in lacking caniniform tusks. It is suggested that the tusks of heterodontosaurids were functionally analogous to those of tayassuids and tragulids and that they were employed as weapons for intra-specific combat and defence. Dental peculiarities indicate that tooth replacement processes were suppressed in heterodontosaurids; replacement of the teeth seems to have been restricted to a brief period each year (presumably when heterodontosaurids underwent aestivation or hibernation).
A new diagnosis is formulated for the family Heterodontosauridae. The relationships of early ornithopod dinosaurs are briefly reviewed and a new classification is proposed. Ten families of ornithopod dinosaurs are recognized; these are ranked in two grades-one (named Dolichopoda) representing the conservative main stem of the ornithischian phylogenetic tree and the other (named Brachypoda) comprising the several more advanced lines of ornithopod evolution.     

Homology between the recessus lateralis and cephalic sensory canals, with the proposition of additional synapomorphies for the Clupeiformes and the Clupeoidei

The recessus lateralis , a complex structure in the otic region of the skull that is probably associated with detection and analysis of small vibrational pressures and displacements, is widely recognized as a synapomorphy of the Clupeiformes. The Clupeiformes includes the Denticipitoidei, with one living species, Denticeps clupeoides , and the Clupeoidei, with about 360 living species commonly known as herrings and anchovies. Comparisons between details of the recessus lateralis of the Clupeoidei and Denticipitoidei, and the sensory cephalic canals of other teleosts, lead to hypotheses of a series of transformations of the cephalic sensory canals . Treating that complex as a single binary 'presence vs. absence' character as was traditional practice obscures important phylogenetically informative variation. Specific synapomorphies in that system exist for the Clupeiformes and the Clupeoidei. Hypothesized synapomorphies in the recessus lateralis for the Clupeiformes are the presence of a dilated internal temporal sensory canal in the pterotic, a postorbital branch of the supraorbital sensory canal located in a bony groove in the lateral wing of the frontal, and the terminal portions of preopercular and infraorbital sensory canals closely positioned. Hypothesized synapomorphies for the Clupeoidei are the presence of a postorbital branch of the supraorbital sensory canal located deep within the body of the lateral wing of the frontal, with the distal portion of that branch totally internal on the cranium, and the expanded distal portion of the postorbital branch of the supraorbital sensory canal. The homology of the sinus temporalis of Clupeoidei, and of the dermosphenotic of both Denticeps and the Clupeoidei, with those of other teleosts is also considered.  © 2004 The Linnean Society of London, Zoological Journal of the Linnean Society , 2004, 141 , 257–270.     

Integument structure in ornithischian dinosaurs (Hypsilophodontia,Ornithopoda) from the Late Jurassic of Transbaikalia

Integumentary structures of ornithischain dinosaurs of the taxon Hypsilophodontia (Ornithopoda) from the Ukureiskaya Formation (Upper Jurassic) of the Kulinda locality (Transbaikal Region, Russia) are described in detail. It is shown that members of this group had so-called bristle scales, integumentary appendages previously unknown in ornithischian dinosaurs. These are relatively small horn plates embedded in the skin, the distal margin of which has several long, flat, and probably constantly growing bristles. The monobristle variant of bristle scale is probably homologous to the protofeather of theropods; if this is the case, it is possible to reconstruct the protofeather as an elongated and constantly growing scale.     

Skeletal heterochrony is associated with the anatomical specializations of snakes among squamate reptiles

Snakes possess a derived anatomy, characterized by limb reduction and reorganization of the skull and internal organs. To understand the origin of snakes from an ontogenetic point of view, we conducted comprehensive investigations on the timing of skeletal elements, based on published and new data, and reconstructed the evolution of the ossification sequence among squamates. We included for the first time Varanus, a critical taxon in phylogenetic context. There is comprehensive delay in the onset of ossification of most skeletal elements in snakes when compared to reference developmental events through evolution. We hypothesize that progressing deceleration accompanied limb reduction and reorganization of the snake skull. Molecular and morphological studies have suggested close relationship of snakes to either amphisbaenians, scincids, geckos, iguanids, or varanids. Likewise, alternative hypotheses on habitat for stem snakes have been postulated. Our comprehensive heterochrony analyses detected developmental shifts in ossification for each hypothesis of snake origin. Moreover, we show that reconstruction of ancestral developmental sequences is a valuable tool to understand ontogenetic mechanisms associated with major evolutionary changes and test homology hypotheses. The “supratemporal” of snakes could be homolog to squamosal of other squamates, which starts ossification early to become relatively large in snakes.     

Evolution and homology of the astragalus in early amniotes: new fossils, new perspectives

The reorganization of the ankle in basal amniotes has long been considered a key innovation allowing the evolution of more terrestrial and cursorial behavior. Understanding how this key innovation arose is a complex problem that largely concerns the homologizing of the amniote astragalus with the various ossifications in the anamniote tarsus. Over the last century, several hypotheses have been advanced homologizing the amniote astragalus with the many ossifications in the ankle of amphibian-grade tetrapods. There is an emerging consensus that the amniote astragalus is a complex structure emerging via the co-ossification of several originally separate elements, but the identities of these elements remain unclear. Here we present new fossil evidence bearing on this contentious question. A poorly ossified, juvenile astragalus of the large captorhinid Moradisaurus grandis shows clear evidence of four ossification centers, rather than of three centers or one center as posited in previous models of astragalus homology. Comparative material of the captorhinid Captorhinikos chozaensis is also interpretable as demonstrating four ossification centers. A new, four-center model for the homology of the amniote astragalus is advanced, and is discussed in the context of the phylogeny of the Captorhinidae in an attempt to identify the developmental transitions responsible for the observed pattern of ossification within this clade. Lastly, the broader implications for amniote phylogeny are discussed, concluding that the neomorphic pattern of astragalus ossification seen in all extant reptiles (including turtles) arose within the clade Diapsida.     

Reossification of the orbital wall following ventral translocation of the fronto-orbital bar and cranial vault remodeling.

The purposes of this study were to determine the extent of ossification of the orbit following ventral translocation of the fronto-orbital bar and to find out whether age at the time of the procedure and presence of a concomitant syndrome adversely affect ossification. A retrospective review of 27 patients with craniosynostosis was conducted at the St. Louis Children's Hospital and the Children's Hospital of Oklahoma. Patients with preoperative, perioperative, and postoperative three-dimensional computed tomography scans were included. Eighty-eight percent of the lateral orbital wall defects and 92 percent of the defects within the roof of the orbit ossified completely in the postoperative period. When syndromic patients were compared with nonsyndromic patients (based on clinical findings only), three of the 19 syndromic defects and three of the 30 nonsyndromic defects demonstrated incomplete ossification in the lateral orbital wall (p > 0.05). Similarly, two of the 19 syndromic defects and two of the 30 nonsyndromic defects demonstrated incomplete ossification within the roof of the orbit (p > 0.05). With respect to age at the time of the procedure, four of the 37 defects and two of the 12 defects demonstrated incomplete ossification in the lateral orbital wall for age at the time of the procedure less than 12 months and greater than 12 months, respectively (p > 0.05). Similarly, two of the 37 defects and two of the 12 defects had incomplete ossification within the roof of the orbit for age at the time of the procedure less than 12 months versus more than 12 months, respectively (p > 0.05). Ossification of the orbital wall and roof is complete in the majority of cases within 1 year after the procedure, and neither age at the time of the procedure nor presence of a concomitant syndrome adversely affects ossification of the orbit after ventral translocation of the fronto-orbital bandeau.     

The Lower Jurassic ornithischian dinosaur Heterodontosaurus tucki Crompton & Charig, 1962: cranial anatomy,functional morphology,taxonomy, and relationships

The cranial anatomy of the Lower Jurassic ornithischian dinosaur Heterodontosaurus tucki Crompton & Charig, 1962 is described in detail for the first time on the basis of two principal specimens: the holotype (SAM‐PK‐K337) and referred skull (SAM‐PK‐K1332). In addition several other specimens that have a bearing on the interpretation of the anatomy and biology of Heterodontosaurus are described. The skull and lower jaw of Heterodontosaurus are compact and robust but perhaps most notable for the heterodont dentition that merited the generic name. Details of the cranial anatomy are revealed and show that the skull is unexpectedly specialized in such an early representative of the Ornithischia, including: the closely packed, hypsodont crowns and ‘warping’ of the occlusal surfaces (created by progressive variation in the angulation of wear on successive crowns) seen in the cheek dentition; the unusual sutural relationships between the bones along the dorsal edge of the lower jaw; the very narrow, deeply vaulted palate and associated structures on the side wall of the braincase; and the indications of cranial pneumatism (more commonly seen in basal archosaurs and saurischian dinosaurs). Evidence for tooth replacement (which has long been recognized, despite frequent statements to the contrary) is suggestive of an episodic, rather than continuous, style of tooth replacement that is, yet again, unusual in diapsids generally and particularly so amongst ornithischian dinosaurs. Cranial musculature has been reconstructed and seems to conform to that typically seen in diapsids, with the exception of the encroachment of M. adductor mandibulae externus superficialis across the lateral surface of the temporal region and external surface of the lower jaw. Indications, taken from the unusual shape of the occlusal surfaces of the cheek dentition and jaw musculature, are suggestive of a novel form of jaw action in this dinosaur. The taxonomy of currently known late Karoo‐aged heterodontosaurids from southern Africa is reviewed. Although complicated by the inadequate nature of much of the known material, it is concluded that two taxa may be readily recognized: H. tucki and Abrictosaurus consors. At least one additional taxon is recognized within the taxa presently named Lanasaurus and Lycorhinus; however, both remain taxonomically problematic and their status needs to be further tested and may only be resolved by future discoveries. The only other named taxon, Geranosaurus atavus, represents an invalid name. The recognition of at least four distinct taxa indicates that the heterodontosaurids were speciose within the late Karoo ecosystem. The systematics of Heterodontosaurus and its congeners has been analysed, using a restricted sample of taxa. A basal (nongenasaurian) position within Ornithischia is re‐affirmed. There are at least four competing hypotheses concerning the phylogenetic placement of the Heterodontosauridae, so the evidence in support of the various hypotheses is reviewed in some detail. At present the best‐supported hypothesis is the one which places Heterodontosauridae in a basal (non‐genasaurian) position; however, the evidence is not fully conclusive and further information is still needed in respect of the anatomy of proximate outgroups, as well as more complete anatomical details for other heterodontosaurids. Heterodontosaurids were not such rare components of the late Karoo ecosystem as previously thought; evidence also suggests that from a phylogenetic perspective they occupied a potentially crucial position during the earliest phases of ornithischian dinosaur evolution. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011.     

The development of the connective tissue in the human orbit.

The development of the mesenchymal structures of the human orbit was studied using 10 mu or 60 mu serial sections of orbits of foetuses of 35 to 320 mm stages (C.R. length; 2-9 months). The ontogenesis of the orbital walls, the eye muscles, the blood vessels and the connective tissue was compared. This comparison revealed that the eye muscles and their fasciae together with the adventitial layers of the larger blood vessels and nerves develop first, closely linked in time to the progress of ossification and growth of most of the orbital bones. Only the (endochondral) ossification of the ethmoid starts much later. The orbital connective tissue septa development commences later, i.e. from the third month onwards, concurrently with the development of special mesenchymal condensations, a particular capillary system and adipose tissue. About five months later the adult configuration is attained.     

纤细滇美龙(Dianmeisaurus gracilis Shang & Li,2015)新材料

根据一件产自云南罗平中三叠世关岭组Ⅱ段的新标本并结合产自相同地点和地层中的模式标本对纤细滇美龙(Dianmeisaurus gracilis Shang & Li,2015)进行了详细研究.原模式标本暴露其腹而,而新标本暴露其背面,两者互相补充提供了更完整、精确的纤细滇美龙解剖学信息.新材料揭示该种具有非常短小的吻部,眶前区的长度不仅短于眶后区长度,甚至短于眼眶的长度;外鼻孔小且位置靠前,即鼻孔前区的长度短于鼻孔后缘与眼眶前缘之间的距离;由两额骨构成的眼眶间隔非常狭窄,宽度小于顶骨平台宽度的1/3;额骨前后两端均具渐尖的突起;顶骨后部不收缩,顶骨平台后缘呈深V型.补充了新信息和包含更多属种(如Dawazisaurus)后的系统发育学分析支持了之前滇美龙和滇东龙互为姊妹群的结论,同时它们和马家山龙、滇肿龙、贵州龙和大洼子龙一起构成了一个仅由中国的属种组成的单系类群.与欧洲肿肋龙类群(Dactylosaurus,Anarosaurus,Serpianosaurus和Neusticosaurus)相比,这一单系类群与幻龙类有更近的亲缘关系.     

The mixosaurid ichthyosaur Contectopalatus from the Middle Triassic of the German Basin

The skull of the mixosaurid species Contectopalatus atavus (Quenstedt, 1851-52) is the most bizarre of any known ichthyosaur. It possesses a very high sagittal crest formed by the nasal, frontal and parietal bones which grows higher during ontogeny. This skull structure - found to a lesser extent in the other mixosaurid genera Mixosaurus and Phalarodon - is a synapomorphy of the family Mixosauridae. It is here interpreted as correlated with a unique arrangement of the jaw adductor musculature among tetrapods, with the internal jaw adductors extending over most of the skull roof up to the external narial opening. This reconstruction would increase the biting force considerably and the hypothesis is supported by peculiarities of the dentition and jaws of Contectopalatus. Contectopalatus probably reached a length of about 5 meters. It is therefore the largest known mixosaurid and one of the largest Triassic ichthyosaurs. The general text-book picture of mixosaurs as small, rather unspecialized, primitive ichthyosaurs is incorrect. Mixosaurs were a highly specialized, uniquely adapted and very diverse ichthyosaur family, some members of which rank among the marine top predators of their time.     

Dental histology of Coelophysis bauri and the evolution of tooth attachment tissues in early dinosaurs

Studies of dinosaur teeth have focused primarily on external crown morphology and thus, use shed or in situ tooth crowns, and are limited to the enamel and dentine dental tissues. As a result, the full suites of periodontal tissues that attach teeth to the jaws remain poorly documented, particularly in early dinosaurs. These tissues are an integral part of the tooth and thus essential to a more complete understanding of dental anatomy, development, and evolution in dinosaurs. To identify the tooth attachment tissues in early dinosaurs, histological thin sections were prepared from the maxilla and dentary of a partial skull of the early theropod Coelophysis bauri from the Upper Triassic (Rhaetian‐ 209–201 Ma) Whitaker Quarry, New Mexico, USA. As one of the phylogenetically and geologically oldest dinosaurs, it is an ideal candidate for examining dental tissues near the base of the dinosaurian clade. The teeth of C. bauri exhibited a fibrous tooth attachment in which the teeth possessed five tissues: enamel, dentine, cementum, periodontal ligament (PDL), and alveolar bone. Our findings, coupled with those of more recent studies of ornithischian teeth, indicate that a tripartite periodontium, similar to that of crocodilians and mammals, is the plesiomorphic condition for dinosaurs. The occurrence of a tripartite periodontium in dinosaurs adds to the growing consensus that the presence of these tissues is the plesiomorphic condition for the major amniote clades. Furthermore, this study establishes the relative timing of tissue development and growth directions of periodontal tissues and provides the first comparative framework for future studies of dinosaur periodontal development, tooth replacement, and histology. J. Morphol. 277:916–924, 2016. © 2016 Wiley Periodicals, Inc.     

A tale of two cities: The genetic mechanisms governing calvarial bone development

The skull bones must grow in a coordinated, three‐dimensional manner to coalesce and form the head and face. Mammalian skull bones have a dual embryonic origin from cranial neural crest cells (CNCC) and paraxial mesoderm (PM) and ossify through intramembranous ossification. The calvarial bones, the bones of the cranium which cover the brain, are derived from the supraorbital arch (SOA) region mesenchyme. The SOA is the site of frontal and parietal bone morphogenesis and primary center of ossification. The objective of this review is to frame our current in vivo understanding of the morphogenesis of the calvarial bones and the gene networks regulating calvarial bone initiation in the SOA mesenchyme.