Morphology and mechanics of the teeth and jaws of white-spotted bamboo sharks (Chiloscyllium plagiosum)

The teeth of white-spotted bamboo sharks (Chiloscyllium plagiosum) are used to clutch soft-bodied prey and crush hard prey; however, the dual function is not evident from tooth morphology alone. Teeth exhibit characteristics that are in agreement with a clutching-type tooth morphology that is well suited for grasping and holding soft-bodied prey, but not for crushing hard prey. The dual role of this single tooth morphology is facilitated by features of the dental ligament and jaw joint. Tooth attachment is flexible and elastic, allowing movement in both sagittal and frontal planes. During prey capture spike-like tooth cusps pierce the flesh of soft prey, thereby preventing escape. When processing prey harder than the teeth can pierce the teeth passively depress, rotating inward towards the oral cavity such that the broader labial faces of the teeth are nearly parallel to the surface of the jaws and form a crushing surface. Movement into the depressed position increases the tooth surface area contacting prey and decreases the total stress applied to the tooth, thereby decreasing the risk of structural failure. This action is aided by a jaw joint that is ventrally offset from the occlusal planes of the jaws. The offset joint position allows many teeth to contact prey simultaneously and orients force vectors at contact points between the jaws and prey in a manner that shears or rolls prey between the jaws during a bite, thus, aiding in processing while reducing forward slip of hard prey from the mouth. Together the teeth, dental ligament, and jaws form an integrated system that may be beneficial to the feeding ecology of C. plagiosum, allowing for a diet that includes prey of varying hardness and elusiveness.     

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.     

Making up the numbers: The molecular control of mammalian dental formula

Teeth develop in the mammalian embryo via a series of interactions between odontogenic epithelium and neural crest-derived ectomesenchyme of the early jaw primordia. The molecular interactions required to generate a tooth are mediated by families of signalling molecules, which often act reiteratively in both a temporal and spatial manner. Whilst considerable information is now available on how these molecules interact to produce an individual tooth, much less is known about the processes that control overall tooth number within the dentition. However, a number of mouse models are now starting to provide some insight into the mechanisms that achieve this. In particular, co-ordinated restriction of signalling molecule activity is important in ensuring appropriate tooth number and there are different requirements for this suppression in epithelial and mesenchymal tissues, both along different axes of individual jaws and between the jaws themselves. There are a number of fundamental mechanisms that facilitate supernumerary tooth formation in these mice. A key process appears to be the early death of vestigial tooth primordia present in the embryo, achieved through the suppression of Shh signalling within these early teeth. However, restriction of WNT signalling is also important in controlling tooth number, with increased transduction being capable of generating multiple tooth buds from the oral epithelium or existing teeth themselves, in both embryonic and adult tissues. Indeed, uncontrolled activity of this pathway can lead to the formation of odontogenic tumours containing multiple odontogenic tissues and poorly formed teeth. Finally, disrupted patterning along the buccal–lingual aspect of the jaws can produce extra teeth directly from the oral epithelium in a duplicated row. Together, all of these findings have relevance for human populations, where supernumerary teeth are seen in association with both the primary and permanent dentitions. Moreover, they are also providing insight into how successional teeth form in both embryonic and post-natal tissues of the jaws.     

Postcanine microstructure in Cricodon metabolus,a Middle Triassic gomphodont cynodont from south‐eastern Africa

Cricodon metabolus is a trirachodontid cynodont from the Anisian (Middle Triassic) of eastern and southern Africa. It has labiolingually expanded (gomphodont) postcanines but also a sectorial tooth in the last postcanine locus. In this paper, we examine the crown microstructure of isolated sectorial and gomphodont postcanines belonging to the holotype specimen of this taxon using scanning electron microscopy. The enamel of both teeth is prismless and composed of discontinuous columnar divergence units, supporting the consistent presence of synapsid columnar enamel in cynognathians. Abundant tubules and numerous irregularly spaced incremental lines are also visible in the enamel and dentine layers in each tooth. This study reveals that the enamel thickness varies along the tooth row in Cricodon as the enamel layer of the gomphodont postcanines is 11.5 times thicker than that of the sectorial crown. It is likely that this difference reflects occlusal stresses and fewer replacements in gomphodont postcanines relative to sectorial teeth. Approximately 100 incremental growth lines of von Ebner are present in the dentine layer, indicating that the deposition of the dentine by odontoblasts occurred for three months before the animal's death.     

Eating with a saw for a jaw: Functional morphology of the jaws and tooth‐whorl in Helicoprion davisii

The recent reexamination of a tooth‐whorl fossil of Helicoprion containing intact jaws shows that the symphyseal tooth‐whorl occupies the entire length of Meckel's cartilage. Here, we use the morphology of the jaws and tooth‐whorl to reconstruct the jaw musculature and develop a biomechanical model of the feeding mechanism in these early Permian predators. The jaw muscles may have generated large bite‐forces; however, the mechanics of the jaws and whorl suggest that Helicoprion was better equipped for feeding on soft‐bodied prey. Hard shelled prey would tend to slip anteriorly from the closing jaws due to the curvature of the tooth‐whorl, lack of cuspate teeth on the palatoquadrate (PQ), and resistance of the prey. When feeding on soft‐bodied prey, deformation of the prey traps prey tissue between the two halves of the PQ and the whorl. The curvature of the tooth‐whorl and position of the exposed teeth relative to the jaw joint results in multiple tooth functions from anterior to posterior tooth that aid in feeding on soft‐bodied prey. Posterior teeth cut and push prey deeper into the oral cavity, while middle teeth pierce and cut, and anterior teeth hook and drag more of the prey into the mouth. Furthermore, the anterior‐posterior edges of the teeth facilitate prey cutting with jaw closure and jaw depression. The paths traveled by each tooth during jaw depression are reminiscent of curved pathways used with slashing weaponry such as swords and knifes. Thus, the jaws and tooth‐whorl may have formed a multifunctional tool for capturing, processing, and transporting prey by cyclic opening and closing of the lower jaw in a sawing fashion. J. Morphol. 276:47–64, 2015. © 2014 Wiley Periodicals, Inc.     

The evolution of specialized dentition in the deep-sea lanternfishes (Myctophiformes)

The evolution of heterodonty, the possession of varied tooth morphologies on the jaws of animals, has been relatively unexplored in ray-finned fishes compared to terrestrial vertebrates, and to an even lesser degree in deep-sea fish lineages. Lanternfishes (Myctophiformes) are an abundant and species-rich group endemic to deep-sea pelagic habitats. In this study, we document the presence of heterodonty on the oral jaws of lanternfishes, identifying differing anatomical and positional variations of dentition. We survey the anatomical variation in tooth morphology on the oral jaws of 114 lanternfish species across 37 genera and integrate our findings with a hypothesis of evolutionary relationships of lanternfishes to infer the number of times heterodonty evolved in this lineage. Our results indicate that heterodonty evolved at least six separate times on the oral jaws of lanternfishes, occurring as variable tooth morphologies in combination with villiform teeth. These combinations of tooth types include villiform plus hooked teeth, villiform plus hooked and recurved teeth, villiform plus spade, tricuspid, and hooked teeth, and villiform plus caniniform teeth. The reoccurring evolution of hooked teeth on the premaxilla and dentary in lanternfishes suggests heterodonty may serve an important functional role in their pelagic deep-sea environment. Hooked teeth could aid in securing and retaining prey in the oral cavity and allow for species to specialize on differing food resources, vital attributes for organisms living in open-ocean habitats.     

Vertebrate dentitions at the origin of jaws: when and how pattern evolved

New evidence shows that teeth evolved with a greater degree of independence from jaws than previously considered. Pharyngeal denticles occur in jawless fish and also in early gnathostomes and precede jaw teeth in phylogeny. Many of these denticles form joined polarized sets on each branchial arch; these resemble whorl-shaped tooth sets on the jaws of stem and crown gnathostomes and are proposed as homologous units. Therefore, the source of patterning of these pharyngeal denticle and tooth sets is conserved from jawless conditions. It is proposed that developmental regulatory systems, responsible for all such tooth patterns on the jaws, are co-opted from the pharyngeal region and not from the skin as classically understood. This strongly implicates embryonic endoderm as opposed to ectoderm in the genetic control of dentition patterning. New interpretations of ontogenetic data on patterning dentitions of extant sharks are proposed, together with those of osteichthyan fish. Two entirely fossil groups, placoderms and acanthodians, at the base of gnathostome phylogeny are reassessed on the basis of a new model. It is concluded that within stem group and crown group gnathostomes several different strategies, unique to each taxon, were adopted to produce different developmental models of dentition patterning from pharyngeal denticles. One shared developmental pattern is that of initiation from primordial tooth sites, independently in each dentate zone of the jaws. The new model is proposed as a framework for data on evolutionary developmental genetics.     

Tooth growth and replacement in Ctenolucius hujeta, a neotropical characoid fish

The predaceous neotropical characoid fish Ctenolucius has an essentially homodont dentition, the number of teeth increasing linearly with age. The basic manner of tooth replacement suggests that Ctenolucius is a primitive characoid. Tooth replacement continues throughout life and is similar to that of tetrapods, involving replacement waves which pass from the back to the front of the jaws. The waves containing the greatest number of teeth are found just anterior to the middle of the jaws. In the upper jaw the increase in the number of teeth is restricted to the anterior portion (premaxillary) whereas the number on the posterior part (maxillary) remains constant. In specimens measuring from 68–230 mm in standard length the posterior portion of the upper jaw doubles in length whereas the anterior portion triples. It is suggested that the area immediately anterior to the middle of the jaw, where replacement waves are longest, is where most of the increase in tooth numbers occurs. During growth of the teeth the absolute height is always greater than the absolute width as the shape changes. The final shape of the recurved conical teeth is determined only in the last stages of tooth formation when the main axis of growth abruptly changes.     

Pathologic tooth deformities in modern and fossil chondrichthians: a consequence of feeding-related injury

Deformed teeth are found as rare components of the dentitions of both modern and fossil chondrichthians. Tooth deformities occur as bent or twisted tooth crowns, missing or misshaped cusps, atypical protuberances, perforations, and abnormal root structures. Deformed tooth files consisting of unusually overlapped or small teeth, or teeth misaligned in the jaw also occur in modern forms, but deformed tooth files generally are not recognizable in fossils due to post-mortem dissociation of teeth and jaws. A survey of 200 modern lamniform and carcharhiniform sharks as well as literature sources indicate that such deformities are produced by feeding-related injury to the tooth-forming tissue of the jaws, particularly by impaction of chondrichthian and teleost fin and tail spines. Tooth counts for several late Cretaceous genera, based on material recovered from coastal plain sites from New Jersey to Alabama, suggest that the frequency of occurrence of deformed teeth in a species varies from about 0.015% in Squalicorax kaupi to about 0.36% in Paranomotodon sp. Tooth counts for modern lamniform and carcharhiniform sharks yield a comparable range in frequency of tooth deformities. Variation in frequency of tooth deformity may reflect interspecific differences in feeding behavior and dietary preferences. There is no suggestion in our data of any strong patterns of temporal variation in tooth deformity frequency, or of patterns ­reflecting chondrichthian phylogenetic history and evolution. Skeletal components of the probable prey of the Cretaceous species are preserved in the same horizons as the deformed teeth, and also are found within co-occurring chondrichthian coprolites.     

Oral ontogeny of the Ayu, Plecoglossus altivelis and comparisons with the jaws of other salmoniform fishes

The Japanese Ayu, Plecoglossus altivelis Temminck & Schlegel is the sole representative of the salmoniform family Plecoglossidae. The Ayu is remarkable for its dentition which in adults comprises groups of diagonally arranged comb-like teeth in the outer tissue of the jaws. In juveniles (below 63 mm SL) the teeth are attached normally to the jaws. The transition of tooth form is correlated with a switch from zooplanktivory to algal or aufwuchs grazing. The present study follows the development of the teeth, jaws, oral cavity ethmoid and suspensorial elements in specimens ranging in size from 41–70 mm SL. The possible mode of function of the adult dentition is discussed. Comparisons are made with the jaws of other salmoniform fishes and a suite of supposed apomorphic characters are identified which are also shared with certain genera of the family Osmeridae, thus supporting the ideas of others that the Osmeridae is a paraphyletic assemblage.     

Tooth reorientation affects tooth function during prey processing and tooth ontogeny in the lesser electric ray, Narcine brasiliensis

The dental anatomy of elasmobranch fishes (sharks, rays and relatives) creates a functional system that is more dynamic than that of mammalian dentition. Continuous dental replacement (where new teeth are moved rostrally to replace older ones) and indirect fibrous attachment of the dentition to the jaw allow teeth to reorient relative to the jaw over both long- and short-term scales, respectively. In this study, we examine the processing behavior and dental anatomy of the lesser electric ray Narcine brasiliensis (Olfers, 1831) to illustrate that the freedom of movement of elasmobranch dentition allows a functional flexibility that can be important for complex prey processing behaviors. From static manipulations of dissected jaws and observations of feeding events in live animals, we show that the teeth rotate during jaw protrusion, resulting in a secondary grasping mechanism that likely serves to hold prey while the buccal cavity is flushed free of sediment. The function of teeth is not always readily apparent from morphology; in addition to short-term reorientation, the long-term dental reorientation during replacement allows a given tooth to serve multiple functions during tooth ontogeny. Unlike teeth inside the mouth, the cusps of external teeth (on the portion of the tooth pad that extends past the occlusal plane) lay flat, such that the labial faces act as a functional battering surface, protecting the jaws during prey excavation.     

The eruption times of permanent teeth in boys and girls in the Stormarn District, Schleswig-Holstein (Germany)

In a longitudinal study in two small towns in southern Schleswig-Holstein (Ammersbek and Ahrensburg, District Stormarn; 9155 inhabitants) we investigated 2832 oral findings of 1396 patients (711 males, 685 females). The minimum age was 1.51 years, and the maximum age was 25.50 years. The dental findings were collected over a period of about 20 years (1982-2002). The oral findings per child were assessed between one and eight times. The eruption times of teeth in females are earlier than those for the same teeth in males. Further, the permanent dentition in females is completed earlier than in males. In both sexes the tooth eruption occurs symmetrically in both jaws. The comparison of both jaws revealed a slightly advanced eruption of the lower jaw teeth in both sexes. There is a noteworthy change in the eruption sequence of the teeth. In contrast to other reports we observed that the eruption of the canine proceeds the eruption of the second molar. We found no acceleration of the dentition when compared with other reports and could confirm the rules of tooth eruption in man. Conclusion: Oral examination of teeth is a simple tool to calculate tooth eruption intervals. This first investigation on a population of Schleswig-Holstein revealed a change in the eruption sequence of permanent teeth. These findings are relevant for dental treatment planning and should be reconfirmed at certain intervals.     

A revised hypothesis on the evolutionary origin of the vertebrate dentition

Despite claims to the contrary, the evolutionary origin of teeth has not been definitely established. The classical ‘outside in’ theory stating that teeth derive from odontodes that invaded the oral cavity in conjunction with the origin of jaws has been challenged by an alternative, ‘inside out’, hypothesis suggesting that teeth evolved from pharyngeal denticles, as endodermal derivatives, prior to the origin of jaws. We propose a third scenario, a revised ‘outside in’ hypothesis ( Huysseune et al., 2009 ). Our hypothesis is consistent with the current data and avoids speculations about convergent tooth evolution. We suggest that teeth may indeed have arisen before the origin of jaws, a pillar of the ‘inside out’ hypothesis, but not from the endodermally lined posterior pharynx. Rather, teeth would have been the result of competent, odontode‐forming ectoderm invading the oropharyngeal cavity through the mouth as well as through the gill slits, interacting with neural‐crest derived mesenchyme. Arguments in support of this hypothesis are: (i) the observation that pharyngeal teeth are present only in species known to possess gill slits, and disappear from the pharyngeal region in early tetrapods concomitant with the closure of gill slits; (ii) the assumption that endoderm alone, together with neural crest, cannot form teeth; (iii) observations on pharyngeal tooth and gill slit formation in extant species; (iv) the observation that the dental lamina (sensu Reif, 1982 ) is not a prerequisite for tooth formation; (v) evidence that patterning does not distinguish pharyngeal from skin denticles, and (vi) the observation on zebrafish mutants affected in the dermal skeleton. This ‘modified outside in’ hypothesis can be tested both on paleontological data (it predicts a correlation of the presence of pharyngeal teeth and of gill slits), and on developmental data in extant species (it predicts the necessity of an ectodermal signal to make [pharyngeal] teeth).     

Interspecific and intraspecific relationships between tooth size and jaw size in primates

The association between mandibular robusticity, postcanine megadontia, and canine reduction in hominins has led to speculation that large and robust jaws might be required to spatially accommodate large canine and molar teeth in hominins and other primates. If so, then variations in mandibular form that are generally regarded as biomechanical adaptations to masticatory demands might instead be incidental effects of functional requirements of tooth support. While the association between large teeth and deep, robust jaws in hominins is well known, the relationship between tooth size and jaw size has not been systematically evaluated in a comparative sample of primates. We evaluate the relationships between molar tooth size, canine tooth size, and mandibular corpus and symphyseal dimensions in a sample of adult anthropoids in interspecific (n=84 species) and intraspecific (n=36 species) contexts. For intraspecific comparisons, tooth size and jaw size are correlated, but for a majority of species this is a function of sexual size dimorphism. Interspecific comparisons lend little direct support to the hypothesis that jaw breadth directly covaries with molar tooth breadth, but they do support the hypothesis that mandibular depth is associated with canine tooth size in males. The latter observation suggests that if there is a causal association between canine size and mandibular depth, it is subject to a threshold effect. In contrast, neither corpus nor symphyseal robusticity, measured as a shape index of breadth/height, are correlated with tooth size. Our results suggest that further studies of the relationship between tooth size and corpus morphology should focus on tooth root size and corpus bony architecture, and that species-specific factors should have a strong impact on such relationships.     

Tooth and consequences: Heterodonty and dental replacement in piranhas and pacus (Serrasalmidae)

Tooth replacement in piranhas is unusual: all teeth on one side of the head are lost as a unit, then replaced simultaneously. We used histology and microCT to examine tooth‐replacement modes across carnivorous piranhas and their herbivorous pacu cousins (Serrasalmidae) and then mapped replacement patterns onto a molecular phylogeny. Pacu teeth develop and are replaced in a manner like piranhas. For serrasalmids, unilateral tooth replacement is not an “all or nothing” phenomenon; we demonstrate that both sides of the jaws have developing tooth rows within them, albeit with one side more mineralized than the other. All serrasalmids (except one) share unilateral tooth replacement, so this is not an adaptation for carnivory. All serrasalmids have interlocking teeth; piranhas interdigitate lateral tooth cusps with adjacent teeth, forming a singular saw‐like blade, whereas lateral cusps in pacus clasp together. For serrasalmids to have an interlocking dentition, their teeth need to develop and erupt at the same time. We propose that interlocking mechanisms prevent tooth loss and ensure continued functionality of the feeding apparatus. Serrasalmid dentitions are ubiquitously heterodont, having incisiform and molariform dentitions reminiscent of mammals. Finally, we propose that simultaneous tooth replacement be considered as a synapomorphy for the family.     

Correlation between the size,shape and position of the teeth on the jaws and the bite force in Theropoda

Theropods have fascinated both paleontologists and the general public due to their large diversity of sizes and morphologies. They also present a large variation in tooth morphologies. Previous studies have estimated the bite force of several specimens. The goal of this study is to determine if there is a correlation between the tooth size, shape and position on the skull and mandible and the bite force of these dinosaurs. Measurements were made on several theropods, including the bending strength of the teeth on the anterior-posterior and the mediolateral axes of the jaws, as well as the bending strength of the mandible, and were compared to fossil and modern Crocodylia. We observed that several bending strength maxima of the teeth trends were aligned with key areas of the mandible, and that the size, shape as well as the position of the teeth on the jaws were correlated with the bite force of both Crocodylia and theropods, which can be related to their diet and feeding habits.     

Trigonognathus kabeyai,a new genus and species of the squalid sharks from Japan

Two specimens of the peculiar squalid shark,Trigonognathus kabeyai gen. et sp. nov., were collected from the coastal waters of Wakayama and Tokushima, Japan, by bottom trawl at depths of 330 and 360 meters. Shape of teeth similar in both jaws; slender, unicuspid, canine-like, without any cusplets or serrations, with weak thin fold on both lingual and labial sides in anterior teeth on both jaws; tooth at symphysis of each jaw longest. Interspace between teeth very wide. Both jaws triangular in shape. Most of dermal denticles on body and head roughly rhombic, swollen very much near central part, with about 10–40 facets on the dorsal surface of its crown. Preoral snout length very short. Many small organs considered to be photophores present mainly on ventral surfaces of head and body.     

Wachstumsanomalie an einem Zahn vonProcarcharodon (Selachii,Chondrichthyes) aus den obereozänen Gehlbergschichten von Helmstedt (Niedersachsen)

In the Upper Eocene Gehlberg Formation of Helmstedt, a tooth ofProcarcharodon Casier, 1960 was discovered which shows a distinct anomaly in growth. This anomaly is thought to be the result of a growth hinderance in the germstage of the tooth. The germ of selachian teeth is a hydroskeleton and is situated in the connective tissue at the lower part of the jaws. Since the hydroskeleton of the germ itself was not damaged, the deformed tooth hardened and later on was pushed in its functional position.     

On the geometry and mechanics of tooth position in the white shark,Carcharodon carcharias

The teeth of captured specimens, of prepared museum specimens, and of high-speed videotape images of the white shark, Carcharodon carcharias, were compared with respect to (1) deviation of each tooth from the animal's midline and (2) the crown angle of the functional teeth along the jaw margin. Tooth position was measured either directly using a meter stick apparatus or derived from tracings of the video footage. Tooth positions were not statistically unique in any region of the upper or lower jaw but demonstrated less variability in crown angle within 30° of the midline (71.48° ± 10°). Videotape analysis of feeding sharks indicated an 8.7° increase in crown angle of the centermost teeth during bites where the jaws were closed through an angle of 20–35° and a 15.7° reduction in this same parameter during jaw adduction through 35° or more. Such changes in tooth orientation (relative to the rear of the buccal cavity) are ascribed to flexure of the cartilaginous jaws and cranium by the cranial musculature and possibly also to sliding of the tooth bed over the jaw. Outward rotation of the teeth and jaw rami describes a plucking action during feeding or prey sampling, while larger bites rotate the frontmost teeth inward towards the gullet. Functionally, this may make the teeth more effective at grasping small prey items or gouging chunks from larger prey. However, testing of the load required to remove teeth showed no significant increase in tensile resistance with reduced crown angle. © 1995 Wiley-Liss, Inc.