Revisiting the evolutionary trend toward the mammalian lower jaw in non-mammalian synapsids in a phylogenetic context.

The mammalian lower jaw comprises a single bone, the dentary, which is a unique feature among vertebrates. The lower jaws of extinct non-mammalian synapsids were composed of the dentary and several postdentary bones. Synapsid fossils exhibit variation in the dentary size relative to the overall lower jaw. An evolutionary trend toward dentary enlargement and postdentary reduction in non-mammalian synapsids has long been documented but has not been established using modern phylogenetic comparative methods. In this study, we examine the evolutionary pattern of dentary size relative to the lower jaw through phylogenetic analyses of measurements in a broad range of non-mammalian synapsid taxa. Our analyses revealed an evolutionary trend toward dentary area enlargement relative to the overall lower jaw in the lateral view across all non-mammalian synapsids. This trend is likely due to vertical expansion of the dentary given that the same trend is not evident when looking at anterior to posterior measurements of the dentary relative to the lower jaw as a whole in lateral view. Ancestral character reconstructions revealed that the evolution of the measurements was not unidirectional in non-mammalian synapsids. Our results provide no evidence of an evolutionary trend toward the dentary enlargement at the expense of postdentary bones across non-mammalian synapsids. This implies that the evolutionary origin of the mammalian lower jaw is not adequately explained by the evolutionary trend of dentary enlargement throughout non-mammalian synapsids. Instead, selection that occurred during the transition from non-mammalian cynodonts to early mammals may have produced the mammalian lower jaw.

Armed to the teeth: The underestimated diversity in tooth shape in snakes and its relation to feeding behavior and diet.

The structure, composition, and shape of teeth have been related to dietary specialization in many vertebrate species, but comparative studies on snakes' teeth are lacking. Yet, snakes have diverse dietary habits that may impact the shape of their teeth. We hypothesize that prey properties, such as hardness and shape, as well as feeding behavior, such as aquatic or arboreal predation, or holding vigorous prey, impose constraints on the evolution of tooth shape in snakes. We compared the morphology of the dentary teeth of 63 species that cover the phylogenetic and dietary diversity of snakes, using 3D geometric morphometrics and linear measurements. Our results show that prey hardness, foraging substrate, and the main feeding mechanical challenge are important drivers of tooth shape, size, and curvature. Overall, long, slender, curved teeth with a thin layer of hard tissue are observed in species that need to maintain a grip on their prey. Short, stout, less curved teeth are associated with species that undergo high or repeated loads. Our study demonstrates the diversity of tooth morphology in snakes and the need to investigate its underlying functional implications to better understand the evolution of teeth in vertebrates.

Evolution and development of the mammalian jaw joint: Making a novel structure.

A jaw joint between the squamosal and dentary is a defining feature of mammals and is referred to as the temporomandibular joint (TMJ) in humans. Driven by changes in dentition and jaw musculature, this new joint evolved early in the mammalian ancestral lineage and permitted the transference of the ancestral jaw joint into the middle ear. The fossil record demonstrates the steps in the cynodont lineage that led to the acquisition of the TMJ, including the expansion of the dentary bone, formation of the coronoid process, and initial contact between the dentary and squamosal. From a developmental perspective, the components of the TMJ form through tissue interactions of muscle and skeletal elements, as well as through interaction between the jaw and the cranial base, with the signals involved in these interactions being both biomechanical and biochemical. In this review, we discuss the development of the TMJ in an evolutionary context. We describe the evolution of the TMJ in the fossil record and the development of the TMJ in embryonic development. We address the formation of key elements of the TMJ and how knowledge from developmental biology can inform our understanding of TMJ evolution.

Modeling the age-related shift in the mineral content of hard tissues in two Mediterranean deer species.

OBJECTIVE: In this paper we aim to provide baseline data and model the changes of Ca, P and Mg throughout life in the mandibular bone, enamel and dentin of red (Cervus elaphus) and fallow deer (Dama dama) in Mediterranean ecosystems. DESIGN: Through a cross-sectional study of cervids from 1.5 to 20 yrs old, hunted between 1990 and 1997, we apply generalized additive models (GAMs) with data from scanning-electron-microscope with energy-dispersive X-ray (FESEM-EDX) and inductively coupled plasma-mass spectrometry (ICP-MS) analyses. RESULTS: The mineral content varied in a similar range to that reported for other ruminants. However, we detected lower Ca content values, while more similar results were obtained for P and Mg contents, which led to relatively lower Ca/P ratios and higher Ca/Mg in our deer at that time. A significantly lesser pattern of decreasing mineral content with aging was detected in the fallow deer males, similarities were found between the sexes, and significantly less resistance to demineralization was observed in dentin compared to bone. We discuss how the basic macromineral elements involved in the biomineralization process vary with age throughout life depending on deer species, sex and hard tissues. CONCLUSION: Allowing for possible inferences of differential changes in the mineralization state at the main stages in life history, our methodological approach opens up new possibilities in zooarchaeological, paleontological, and wildlife research.

Thecodont tooth attachment and replacement in bolosaurid parareptiles.

Permian bolosaurid parareptiles are well-known for having complex tooth crowns and complete tooth rows in the jaws, in contrast to the comparatively simple teeth and frequent replacement gaps in all other Paleozoic amniotes. Analysis of the specialized dentition of the bolosaurid parareptiles Bolosaurus from North America and Belebey from Russia, utilizing a combination of histological and tomographic data, reveals unusual patterns of tooth development and replacement. The data confirm that bolosaurid teeth have thecodont implantation with deep roots, the oldest known such example among amniotes, and independently evolved among much younger archosauromorphs (including dinosaurs and crocodilians) and among synapsids (including mammals). High-resolution CT scans were able to detect the density boundary between the alveolar bone and the jawbone, as confirmed by histology, and revealed the location and size of developing replacement teeth in the pulp cavity of functional teeth. Evidence provided by the paratype dentary of Belebey chengi indicates that replacement teeth are present along the whole tooth row at slightly different stages of development, with the ontogenetically more developed teeth anteriorly, suggesting that tooth replacement was highly synchronized. CT data also show tooth replacement is directly related to the presence of lingual pits in the jaw, and that migration of tooth buds occurs initially close to these resorption pits to a position immediately below the functional tooth within its pulp cavity. The size and complex shape of the replacement teeth in the holotype of Bolosaurus grandis indicate that the replacement teeth can develop within the pulp cavity to an advanced stage while the previous generation remains functional for an extended time, reminiscent of the condition seen in other amniotes with occluding dentitions, including mammals.

Morphological study and mineral analysis of the lower mandible of adult Atlantic salmon (Salmo salar) from Scotland with mandibular deformation / Estudio morfológico y análisis de minerales de la mandíbula inferior de salmones del Atlántico (Salmo salar) adultos con deformación mandibular, provenientes de Escocia

Mandibular deformity is a condition that affects the jaw bone of adult salmon and has been observed in Norway and Chile, causing weight loss, poor quality of farmed fish and increased mortality. The causes range from high temperatures of the state of eggs, to poor nutrition phosphorus or vitamin C. This work aims to analyze this deformity by histochemical and mineral analysis technique during an episode presented in centers of the Scotia Sea. Jaw and spinal segments of 21 Atlantic salmon in Scotland were used. These samples were classified into three groups: Group 1: Severely deformed. Group 2: Mildly affected. Group 3: Normal controls. Four jaws per group were fixed in 10 % formalin and embedded in Paraplast, sections of 5 microns were performed using a Microm® microtome histochemical technique Von Kossa was used for the detection of calcium deposits, which highlights the calcium osteoid black and red color. For proximate analysis, and in order to obtain and compare levels of calcium, phosphorus, zinc and magnesium in total 9 bone jaws (6 affected with DM and 3 controls) and 9 body sections the Mann-Whitney test was used to compare these values between misshapen salmon and controls. To correlate values, jaw and body segment a Spearman corrrelation was applied. Fish group 1 presented a ventral deviation of the alveolar bone body. In fish group 2 prominence of the visible joint on both sides or unilaterally was observed. Comparing the values of % Ca, % P, % Mg and Zn jaws with DM and healthy ones with Mann Witney method it was found that the values of these minerals vary between salmon and controls affected. There was a significant difference in the percentage of P, which indicates that there is less P in affected fish vertebrae. Spearman correlation noted that the percentages of the minerals studied in dental bone and vertebral segments are uncorrelated. Rather, Von Kossa distribution indicates that Ca/P is not homogeneous in the dental bone, as a result of mineral resorption from the skeleton including the operculum, articular bone and dental towards kype. This paper states that Von Kossa histochemical technique showed significant differences between deformed fish and controls and also showed differences between the various segments of the dental bone. The alveolar bone is a dynamic structure adapted to continuous histological changes may be involved in MD, phosphorus deficient diets, coupled with the initial formation of Kippe.

La deformación mandibular es una patología que afecta al hueso dentario de salmones adultos, se observó en Noruega y en Chile, ocasionando disminución de peso, baja calidad de peces cultivados y aumento de la mortalidad. Las causas varían desde temperaturas elevadas al estado de ovas, hasta alimentación deficitaria en fósforo o vitamina C. Este trabajo tiene como propósito analizar esta deformación mediante una técnica histoquímica y de análisis de minerales durante un episodio presentado en centros de mar de Escocia. Se utilizó la mandíbula y segmento vertebral de 21 salmones del Atlántico de Escocia. Estas muestras se clasificaron en tres grupos Grupo 1: Severamente deformes. Grupo 2: Levemente afectados. Grupo 3: Controles normales. Cuatro mandíbulas por grupo fueron fijadas en formalina al 10 % y se incluyeron en paraplast, se realizaron cortes de 5 µm utilizando un micrótomo Microm®. Se utilizó la técnica histoquímica de Von Kossa para la detección de depósitos de calcio la cual destaca al calcio de color negro y el osteoide de color rojo. Para el análisis químico proximal, y con el propósito de obtener y comparar niveles de calcio, fósforo, zinc y magnesio en los huesos se utilizó un total de 9 mandíbulas (6 afectadas con DM y 3 controles) y sus 9 secciones corporales. Para comparar estos valores entre salmones deformes y controles se utilizó la prueba de Mann-Whitney. Para correlacionar los valores de mandíbula y segmento corporal se hizo una correlación por jerarquías de Spearman. Los peces del grupo 1, presentaron una desviación ventral del cuerpo del hueso dentario. En los peces del grupo 2 se observó la prominencia de la articulación visible en ambos lados o unilateralmente. Al comparar los valores de % Ca, % P, % Mg y Zn de las mandíbulas con DM y sanas con el método de Mann Witney se encontró que los valores de estos minerales no varían entre salmones afectados y controles. Hubo una diferencia significativa en el porcentaje de P, lo cual indica que existe menos P en vértebras de peces afectados. La correlación de Spearman señaló que los porcentajes de los minerales estudiados en huesos dentarios y segmentos vertebrales no están correlacionados. Por el contrario, la técnica Von Kossa mostró que la distribución de Ca/ P no es homogénea en el hueso dental producto de la reabsorción mineral desde el esqueleto incluyendo el opérculo, hueso articular y dental hacia la kype. El presente trabajo establece que la técnica histoquímica de Von Kossa fue la que permitió observar diferencias importantes entre peces deformados y controles, además mostró diferencias entre los distintos segmentos del hueso dentario. El hueso dentario es una estructura dinámica adaptada a continuos cambios histológicos pudiendo estar involucrados en la DM, dietas deficientes de fósforo, sumado a la formación inicial del Kippe.

The dentary of Australovenator wintonensis (Theropoda, Megaraptoridae); implications for megaraptorid dentition.

Megaraptorid theropods were an enigmatic group of medium-sized predatory dinosaurs, infamous for the hypertrophied claw on the first manual digit. Megaraptorid dentition is largely restricted to isolated teeth found in association with skeletal parts; however, the in situ maxillary dentition of Megaraptor was recently described. A newly discovered right dentary pertaining to the Australovenator holotype preserves in situ dentition, permitting unambiguous characterisation of the dentary tooth morphology. The new jaw is virtually complete, with an overall elongate, shallow profile, and fifteen visible in situ teeth at varying stages of eruption. In situ teeth confirm Australovenator exhibited modest pseudoheterodonty, recurved lateral teeth with a serrate distal carina and reduced mesial carina, similar to other megaraptorids. Australovenator also combines of figure-of-eight basal cross-section with a lanceolate shape due to the presence of labial and lingual depressions and the lingual twist of the distal carina. Computed tomography and three-dimensional imagery provided superior characterisation of the dentary morphology and enabled an accurate reconstruction to a pre-fossilised state. The newly established dental morphology also afforded re-evaluation of isolated theropod teeth discovered at the Australovenator holotype locality and from several additional Winton Formation localities. The isolated Winton teeth are qualitatively and quantitatively similar to the in situ dentary teeth of Australovenator, but are also morphometrically similar to Abelisauridae, Allosauridae, Coelophysoidea, Megalosauridae and basal Tyrannosauroidea. Qualitative characters, however, clearly distinguish the teeth of Australovenator and the isolated Winton teeth from all other theropods. Evidence from teeth suggests megaraptorids were the dominant predators in the Winton Formation, which contrasts with other penecontemporaneous Gondwanan ecosystems.

Species-specific modifications of mandible shape reveal independent mechanisms for growth and initiation of the coronoid.

BACKGROUND: The variation in mandibular morphology of mammals reflects specialisations for different diets. Omnivorous and carnivorous mammals posses large mandibular coronoid processes, while herbivorous mammals have proportionally smaller or absent coronoids. This is correlated with the relative size of the temporalis muscle that forms an attachment to the coronoid process. The role of this muscle attachment in the development of the variation of the coronoid is unclear. RESULTS: By comparative developmental biology and mouse knockout studies, we demonstrate here that the initiation and growth of the coronoid are two independent processes, with initiation being intrinsic to the ossifying bone and growth dependent upon the extrinsic effect of muscle attachment. A necessary component of the intrinsic patterning is identified as the paired domain transcription factor Pax9. We also demonstrate that Sox9 plays a role independent of chondrogenesis in the growth of the coronoid process in response to muscle interaction. CONCLUSIONS: The mandibular coronoid process is initiated by intrinsic factors, but later growth is dependent on extrinsic signals from the muscle. These extrinsic influences are hypothesised to be the basis of the variation in coronoid length seen across the mammalian lineage.

Combined function of HoxA and HoxB clusters in neural crest cells.

The evolution of chordates was accompanied by critical anatomical innovations in craniofacial development, along with the emergence of neural crest cells. The potential of these cells to implement a craniofacial program in part depends upon the (non-)expression of Hox genes. For instance, the development of jaws requires the inhibition of Hox genes function in the first pharyngeal arch. In contrast, Hox gene products induce craniofacial structures in more caudal territories. To further investigate which Hox gene clusters are involved in this latter role, we generated HoxA;HoxB cluster double mutant animals in cranial neural crest cells. We observed the appearance of a supernumerary dentary-like bone with an endochondral ossification around a neo-Meckel's cartilage matrix and an attachment of neo-muscle demonstrating that HoxB genes enhance the phenotype induced by the deletion of the HoxA cluster alone. In addition, a cervical and hypertrophic thymus was associated with the supernumerary dentary-like bone, which may reflect its ancestral position near the filtrating system. Altogether these results show that the HoxA and HoxB clusters cooperated during evolution to lead to present craniofacial diversity.