Loss of teeth and enamel in tetrapods: fossil record, genetic data and morphological adaptations

Common developmental origins of beak shapes and evolution in theropods

Vertebrate beaks show a remarkable diversity of forms, epitomized by birds and non-avian theropods. Few studies have investigated how underlying developmental processes influence beak shape. The power cascade is a model of growth describing the log-log linear relationship of the beak radius with distance from the tip. We measured beak and toothed snout shapes in 127 species across 120 families of extant birds and extinct non-avian theropods and found that 95% followed the power cascade model. Ancestral state estimation suggests that the power cascade constitutes a fundamental growth pattern of the theropod rostrum, and perhaps all vertebrate rostra. The morphospace defined by the power cascade shows how bird beak shape explores the geometries associated with ecological specializations while adhering to the growth model. We show that the power cascade influences the macroevolutionary exploration of rostrum morphospace, enabling extant birds to inhabit all components of Earth's biosphere.

Dental Anomalies and Cranio-Dental Ontogeny in a Captive Wild Boar Population From France

Dental anomalies are frequent in boars and pigs, and they generally affect the first premolar loci. The prevalence of these dental anomalies was investigated in a large number of populations around the world. These studies mainly focused on the influence of domestication, size, sexual dimorphism or food hardness on these anomalies. However, they rarely considered ontogenetic aspects, while these are crucial for understanding their aetiology during animal growth and how the dental row-jaw complex is affected. Here, we studied the incidence of missing first upper and lower premolars in a French population of captive wild boars to discuss the functional and developmental reasons for missing teeth and to assess the impact of missing teeth on the growth of the dental row-jaw complex. Using the CT-scan data of the cranium and mandible of 24 wild boars investigated six times each during their growth, and presenting a balanced sex ratio, we recorded the number of missing teeth. We then quantified the shape of the upper and lower jaws using 3D geometric morphometrics. We found a similar prevalence of missing first premolar (37.5%) between the upper and the lower jaws, which is higher than the frequencies observed in most continental populations of wild boars. The increasing number of anomalies during ontogeny suggests a relaxed constraint on the dentition associated with a different feeding behaviour in captivity. The absence of first premolars does not appear to be associated with size variation or sexual dimorphism, nor does it affect the place of the dentition within the jaw, the latter being more influenced by the dimorphic shape of the canines and the timing of dental eruption.

Cellular, Molecular, and Genetic Mechanisms of Avian Beak Development and Evolution

Diverse research programs employing complementary strategies have been uncovering cellular, molecular, and genetic mechanisms essential to avian beak development and evolution. In reviewing these discoveries, I offer an interdisciplinary perspective on bird beaks that spans their derivation from jaws of dinosaurian reptiles, their anatomical and ecological diversification across major taxonomic groups, their common embryonic origins, their intrinsic patterning processes, and their structural integration. I describe how descriptive and experimental approaches, including gene expression and cell lineage analyses, tissue recombinations, surgical transplants, gain- and loss-of-function methods, geometric morphometrics, comparative genomics, and genome-wide association studies, have identified key constituent parts and putative genes regulating beak morphogenesis and evolution. I focus throughout on neural crest mesenchyme, which generates the beak skeleton and other components, and describe how these embryonic progenitor cells mediate species-specific pattern and link form and function as revealed by 20 years of research using chimeras between quail and duck embryos.

Bank vole genomics links determinate and indeterminate growth of teeth

BACKGROUND

Continuously growing teeth are an important innovation in mammalian evolution, yet genetic regulation of continuous growth by stem cells remains incompletely understood. Dental stem cells responsible for tooth crown growth are lost at the onset of tooth root formation. Genetic signaling that initiates this loss is difficult to study with the ever-growing incisor and rooted molars of mice, the most common mammalian dental model species, because signals for root formation overlap with signals that pattern tooth size and shape (i.e., cusp patterns). Bank and prairie voles (Cricetidae, Rodentia, Glires) have evolved rooted and unrooted molars while retaining similar size and shape, providing alternative models for studying roots.

RESULTS

We assembled a de novo genome of Myodes glareolus, a vole with high-crowned, rooted molars, and performed genomic and transcriptomic analyses in a broad phylogenetic context of Glires (rodents and lagomorphs) to assess differential selection and evolution in tooth forming genes. Bulk transcriptomics comparisons of embryonic molar development between bank voles and mice demonstrated overall conservation of gene expression levels, with species-specific differences corresponding to the accelerated and more extensive patterning of the vole molar. We leverage convergent evolution of unrooted molars across the clade to examine changes that may underlie the evolution of unrooted molars. We identified 15 dental genes with changing synteny relationships and six dental genes undergoing positive selection across Glires, two of which were undergoing positive selection in species with unrooted molars, Dspp and Aqp1. Decreased expression of both genes in prairie voles with unrooted molars compared to bank voles supports the presence of positive selection and may underlie differences in root formation.

CONCLUSIONS

Our results support ongoing evolution of dental genes across Glires and identify candidate genes for mechanistic studies of root formation. Comparative research using the bank vole as a model species can reveal the complex evolutionary background of convergent evolution for ever-growing molars.

Macroscopic, microscopic, and immunofluorescent characterization of the Greek tortoise (Testudo graeca graeca) oropharyngeal floor with concern to its feed adaptation as a herbivorous land reptile

The current investigation focuses on gross anatomy, light, and scanning electron microscopy (SEM) of the Testudo graeca oropharyngeal floor, with particular reference to the immunofluorescence technique to examine its tongue. The T. graeca oropharyngeal floor showed many anatomical structures: the lower rhamphotheca, paralingual ridge, lower alveolar ridge, tongue, laryngeal mound, and glottis. The lower rhamphotheca appeared as a V-shaped jaw line with a highly serrated edge and a median tomium (beak). SEM observations of the lingual apex and the lingual body showed rectangular and conical filiform papillae with porous surfaces and taste pores. Meanwhile, the lingual root had two wings that carried papillae with different shapes: dagger-shaped, conical, bifurcated, and leaf-like papillae, and these papillae lacked taste pores. The laryngeal mound had openings for the laryngeal mucus gland and its secretions. Light microscopy findings showed mucous glands in the propria submucosa and near the mucosal surface of the lingual apex. The lingual root had lingual papillae and two hyaline cartilaginous skeletons between skeletal muscles, and the lingual papillae were elongated filiform, rectangular filiform papillae, and fungiform papillae. The lamina propria constituted the core of the lingual papillae and the mucous gland, they had a positive reaction with the periodic acid schiff (PAS) reagent. The apical surface of the fungiform papillae had taste pores. Under immunofluorescence, the vimentin was detected in taste bud cells, and synaptophysin reacted to the taste buds and nerve bundles. The current study of the Greek tortoise oropharyngeal floor investigated its herbivorous eating habits using its serrated lower rhamphotheca, a large tongue with differently shaped papillae, and numerous mucous glands. RESEARCH HIGHLIGHTS: The Greek tortoise (T. graeca graeca) oropharyngeal floor showed many anatomical structures: lower rhamphotheca, paralingual ridge, lower alveolar ridge, tongue, laryngeal mound, and glottis. SEM and light microscopy observations of the tongue revealed varied types and shapes of lingual papillae with a porous surface on the tongue apex (rectangular or conical filiform papillae), on the tongue body (filiform and fungiform papillae), and on the tongue root (dagger-shaped, conical, bifurcated, and leaf-like papillae). Light microscopy findings: the lamina propria constituted the core of the lingual papillae and had numerous mucous glands that had a slightly magenta-red color with PAS reagent. The apical surface of the fungiform papillae had taste pores. Vimentin and synaptophysin gave a reaction to the taste buds.

Macro- and micro-anatomical investigation of the oropharyngeal roof of landform greek tortoise (Testudo graeca graeca) and semi-aquatic red-eared slider turtle (Trachemys scripta elegans)

The present investigation examined the oropharyngeal roof of two turtles having different feeding behaviors: the landform Greek tortoise (Testudo graeca graeca) primarily herbivores and the semi-aquatic red-eared slider turtle (Trachemys scripta elegans) lives in freshwater that opportunistic omnivorous grossly and by scanning and light microscopes. Grossly, the Greek tortoise had a V-shaped roof consisting of the upper rhamphotheca, peri-palatine region, upper alveolar ridge, peripheral palatine ridge, median palatine ridge, vomer, choanae, caudal palatine part, and pharynx. At the same time, the red-eared slider had a semilunar roof consisting of upper rhamphotheca, two peripheral palatine ridges, core of palatine ridges, upper alveolar band, vomer, choanae, caudal palatine part, and pharynx. SEM revealed that the red-eared slider roof appeared more straightforward. The upper rhamphotheca is sharp, with a median premaxillary notch in the red-eared slider that gives a powerful bite for cutting to compensate absence of the teeth. Additionally, the red-eared slider's upper alveolar band is interrupted by a single upper alveolar ridge that appears spiky, pointed, and longer as it needs powerful chewing of prey and there are two types of teeth-like projections at its peri-palatine area for food-crushing and chewing. The Greek tortoise palatine region had numerous ridges and folds to provide roughness for food processing. Greek tortoises had small-sized choanae with two choanal folds to minimize choanal openings when eating dusty grasses. Histologically, Greek tortoise palate was rostrally thicker and more keratinized than caudally, and the caudal palatine region was characterized by a single pair of circumvallate-like papilla with multiple mucous openings and secretions, while red-eared slider palate was slightly keratinized at the peri-choanal region, and the rest of the palate was non-keratinized with few mucous openings. The current investigation found various structural oropharyngeal roof adaptations to feeding behavior in the omnivore red-eared slide compared to the herbivorous Greek turtle.

Size and shape heterodonty in the early Permian synapsid Mesenosaurus efremovi

Paleozoic synapsids represent the first chapter in the evolution of this large clade that includes mammals. These fascinating terrestrial vertebrates were the first amniotes to successfully adapt to a wide range of feeding strategies, reflected by their varied dental morphologies. Evolution of the marginal dentition on the mammalian side of amniotes is characterized by strong, size and shape heterodonty, with the late Permian therapsids showing heterodonty with the presence of incisiform, caniniform, and multicuspid molariform dentition. Rarity of available specimens has previously prevented detailed studies of dental anatomy and evolution in the initial chapter of synapsid evolution, when synapsids were able to evolve dentition for insectivory, herbivory, and carnivory. Numerous teeth, jaw elements, and skulls of the hypercarnivorous varanopid Mesenosaurus efremovi have been recently discovered in the cave systems near Richards Spur, Oklahoma, permitting the first detailed investigation of the dental anatomy of a Paleozoic tetrapod using multiple approaches, including morphometric and histological analyses. As a distant stem mammal, Mesenosaurus is the first member of this large and successful clade to exhibit a type of dental heterodonty that combines size and morphological (shape) variation of the tooth crowns. Here we present the first evidence of functional differentiation in the dentition of this early synapsid, with three distinct dental regions having diverse morphologies and functions. The quality and quantity of preserved materials has allowed us to identify the orientation and curvature of the carinae (cutting edges), and the variation and distribution of the ziphodonty (serrations) along the carinae. The shape-related heterodonty seen in this taxon may have contributed to this taxon's ability to be a successful mid-sized predator in the taxonomically diverse community of early Permian carnivores, but may have also extended the ecological resilience of this clade of mid-sized predators across major faunal and environmental transitions.

Vole genomics links determinate and indeterminate growth of teeth

Continuously growing teeth are an important innovation in mammalian evolution, yet genetic regulation of continuous growth by stem cells remains incompletely understood. Dental stem cells responsible for tooth crown growth are lost at the onset of tooth root formation. Genetic signaling that initiates this loss is difficult to study with the ever-growing incisor and rooted molars of mice, the most common mammalian dental model species, because signals for root formation overlap with signals that pattern tooth size and shape (i.e., cusp patterns). Different species of voles (Cricetidae, Rodentia, Glires) have evolved rooted and unrooted molars that have similar size and shape, providing alternative models for studying roots. We assembled a de novo genome of Myodes glareolus, a vole with high-crowned, rooted molars, and performed genomic and transcriptomic analyses in a broad phylogenetic context of Glires (rodents and lagomorphs) to assess differential selection and evolution in tooth forming genes. We identified 15 dental genes with changing synteny relationships and six dental genes undergoing positive selection across Glires, two of which were undergoing positive selection in species with unrooted molars, Dspp and Aqp1. Decreased expression of both genes in prairie voles with unrooted molars compared to bank voles supports the presence of positive selection and may underlie differences in root formation. Bulk transcriptomics analyses of embryonic molar development in bank voles also demonstrated conserved patterns of dental gene expression compared to mice, with species-specific variation likely related to developmental timing and morphological differences between mouse and vole molars. Our results support ongoing evolution of dental genes across Glires, revealing the complex evolutionary background of convergent evolution for ever-growing molars.

Who was the real sabertooth predator: Thylacosmilus or Thylacoleo?

Sabertoothed mammalian predators, all now extinct, were almost exclusively feloid carnivorans (Eutheria, Placentalia): here a couple of extinct metatherian predators are considered in comparison with the placental sabertooths. Thylacosmilus (the "marsupial sabertooth") and Thylacoleo (the "marsupial lion") were both relatively large (puma-sized) carnivores of the Plio-Pleistocene in the Southern Hemisphere (Argentina and Australia, respectively). Both carnivores have captured the public imagination, especially as predators that were somehow analogous to northern placental forms. But a more detailed consideration of their morphology shows that neither can be simply analogized with its supposed placental counterpart. While Thylacosmilus did indeed have saber-like canines, many aspects of its anatomy show that it could not have killed prey in the manner proposed for the sabertoothed felids such as Smilodon. Rather than being an active predator, it may have been a specialized scavenger, using the hypertrophied canines to open carcasses, and perhaps deployed a large tongue to extract the innards. Thylacoleo lacked canines, and its supposedly "caniniform" incisors could not have acted like a felid's canines. Nevertheless, while its mode of dispatching its prey remains a subject for debate, it was clearly a powerful predator, likely to be capable of bringing down prey bigger than itself while hunting alone. In that regard, it may have filled the ecomorphological role proposed for placental sabertooths, and so despite the lack of canines can be nominated as the true "marsupial sabertooth" out of the two extinct taxa.

From teeth to pad: tooth loss and development of keratinous structures in sirenians

Sirenians are a well-known example of morphological adaptation to a shallow-water grazing diet characterized by a modified feeding apparatus and orofacial morphology. Such adaptations were accompanied by an anterior tooth reduction associated with the development of keratinized pads, the evolution of which remains elusive. Among sirenians, the recently extinct Steller's sea cow represents a special case for being completely toothless. Here, we used μ-CT scans of sirenian crania to understand how motor-sensor systems associated with tooth innervation responded to innovations such as keratinized pads and continuous dental replacement. In addition, we surveyed nine genes associated with dental reduction for signatures of loss of function. Our results reveal how patterns of innervation changed with modifications of the dental formula, especially continuous replacement in manatees. Both our morphological and genomic data show that dental development was not completely lost in the edentulous Steller's sea cows. By tracing the phylogenetic history of tooth innervation, we illustrate the role of development in promoting the innervation of keratinized pads, similar to the secondary use of dental canals for innervating neomorphic keratinized structures in other tetrapod groups.

Developmental origin of the mammalian premaxilla

The evolution of jaws has played a major role in the success of vertebrate expansion into a wide variety of ecological niches. A fundamental, yet unresolved, question in craniofacial biology is about the origin of the premaxilla, the most distal bone present in the upper jaw of all amniotes. Recent reports have suggested that the mammalian premaxilla is derived from embryonic maxillary prominences rather than the frontonasal ectomesenchyme as previously shown in studies of chicken embryos. However, whether mammalian embryonic frontonasal ectomesenchyme contributes to the premaxillary bone has not been investigated and a tool to trace the contributions of the frontonasal ectomesenchyme to facial structures in mammals is lacking. The expression of the Alx3 gene is activated highly specifically in the frontonasal ectomesenchyme, but not in the maxillary mesenchyme, from the beginning of facial morphogenesis in mice. Here, we report the generation and characterization of a novel Alx3CreERT2 knock-in mouse line that express tamoxifen-inducible Cre DNA recombinase from the Alx3 locus. Tamoxifen treatment of Alx3CreERT2/+;Rosa26mTmG/+ embryos at E7.5, E8.5, E9.5, and E10.5, each induced specific labeling of the embryonic medial nasal and lateral nasal mesenchyme but not the maxillary mesenchyme. Lineage tracing of Alx3CreERT2-labeled frontonasal mesenchyme from E9.5 to E16.5 clearly showed that the frontonasal mesenchyme cells give rise to the osteoblasts generating the premaxillary bone. Furthermore, we characterize a Dlx1-Cre BAC transgenic mouse line that expresses Cre activity in the embryonic maxillary but not the frontonasal mesenchyme and show that the Dlx1-Cre labeled embryonic maxillary mesenchyme cells contribute to the maxillary bone as well as the soft tissues lateral to both the premaxillary and maxillary bones but not to the premaxillary bone. These results clearly demonstrate the developmental origin of the premaxillary bone from embryonic frontonasal ectomesenchyme cells in mice and confirm the evolutionary homology of the premaxilla across amniotes.

Show Me Your Teeth And I Will Tell You What You Eat: Differences in Tooth Enamel in Snakes with Different Diets

Teeth are composed of the hardest tissues in the vertebrate body and have been studied extensively to infer diet in vertebrates. The morphology and structure of enamel is thought to reflect feeding ecology. Snakes have a diversified diet, some species feed on armored lizards, others on soft invertebrates. Yet, little is known about how tooth enamel, and specifically its thickness, is impacted by diet. In this study, we first describe the different patterns of enamel distribution and thickness in snakes. Then, we investigate the link between prey hardness and enamel thickness and morphology by comparing the dentary teeth of 63 species of snakes. We observed that the enamel is deposited asymmetrically at the antero-labial side of the tooth. Both enamel coverage and thickness vary a lot in snakes, from species with thin enamel, only at the tip of the tooth to a full facet covered with enamel. There variations are related with prey hardness: snakes feeding on hard prey have a thicker enamel and a lager enamel coverage while species. Snakes feeding on softer prey have a thin enamel layer confined to the tip of the tooth.

Bat teeth illuminate the diversification of mammalian tooth classes

Tooth classes are an innovation that has contributed to the evolutionary success of mammals. However, our understanding of the mechanisms by which tooth classes diversified remain limited. We use the evolutionary radiation of noctilionoid bats to show how the tooth developmental program evolved during the adaptation to new diet types. Combining morphological, developmental and mathematical modeling approaches, we demonstrate that tooth classes develop through independent developmental cascades that deviate from classical models. We show that the diversification of tooth number and size is driven by jaw growth rate modulation, explaining the rapid gain/loss of teeth in this clade. Finally, we mathematically model the successive appearance of tooth buds, supporting the hypothesis that growth acts as a key driver of the evolution of tooth number and size. Our work reveal how growth, by tinkering with reaction/diffusion processes, drives the diversification of tooth classes and other repeated structure during adaptive radiations.

Evolutionary mechanisms modulating the mammalian skull development

Mammals possess impressive craniofacial variation that mirrors their adaptation to diverse ecological niches, feeding behaviour, physiology and overall lifestyle. The spectrum of craniofacial geometries is established mainly during embryonic development. The formation of the head represents a sequence of events regulated on genomic, molecular, cellular and tissue level, with each step taking place under tight spatio-temporal control. Even minor variations in timing, position or concentration of the molecular drivers and the resulting events can affect the final shape, size and position of the skeletal elements and the geometry of the head. Our knowledge of craniofacial development increased substantially in the last decades, mainly due to research using conventional vertebrate model organisms. However, how developmental differences in head formation arise specifically within mammals remains largely unexplored. This review highlights three evolutionary mechanisms acknowledged to modify ontogenesis: heterochrony, heterotopy and heterometry. We present recent research that links changes in developmental timing, spatial organization or gene expression levels to the acquisition of species-specific skull morphologies. We highlight how these evolutionary modifications occur on the level of the genes, molecules and cellular processes, and alter conserved developmental programmes to generate a broad spectrum of skull shapes characteristic of the class Mammalia. This article is part of the theme issue 'The mammalian skull: development, structure and function'.

Are birds pseudoteeth and denticulations related to touch papilla? An investigation in parrot, goose, and chicken

We first studied the morphology and the development of goose denticulations, which develop mainly by a ripple process, and the touch papillae of the bill tip organ, which appears through an evagination process at the end of the beak. During their development, we observed the specific expression of PAX9, PITX2, and BMP4, while SHH was expressed mainly in the basal layer of the epithelium in a non-specific manner. Adult goose denticulations are associated with numerous columns. The goose denticulations and columns were filled with numerous Herbst and Grandry corpuscles, as well the touch papillae of the bill tip organ. Histological analysis of adult parrot pseudoteeth revealed that the osseous pseudoteeth were extended by similar columns filled with Herbst and Grandry corpuscles. We also examined adult and embryonic chicken beaks. During ontogeny, we observed a process of rostral evagination with folding associated with discrete ripples in the anterior part of the beak rudiment, in which PAX9, PITX2, and BMP4 are expressed. In the corresponding adult areas, there were numerous sensory corpuscles with rostral columns, which were similar to the features observed in goose. These observations support the hypothesis that pseudoteeth and denticulations constitute sensory organs, and that the touch papillae exhibit some similarities with pseudoteeth.

Comparative anatomy and functional implications of variation in the buccal mass in coleoid cephalopods

In contrast to the well-studied articulated vertebrate jaws, the structure and function of cephalopod jaws remains poorly known. Cephalopod jaws are unique as the two jaw elements do not contact one another, are embedded in a muscular mass and connected through a muscle joint. Previous studies have described the anatomy of the buccal mass muscles in cephalopods and have proposed variation in muscle volume depending on beak shape. However, the general structure of the muscles has been suggested to be similar in octopuses, squids, and cuttlefish. Here we provide a quantitative analysis of the variation in the buccal mass of coleoids using traditional dissections, histological sections and contrast-enhanced computed tomography scans. Our results show that the buccal mass is composed of four main homologous muscles present in both decapodiforms and octopodiforms as suggested previously. However, we also report the presence of a muscle uniquely present in octopodiforms (the postero-lateral mandibular muscle). Our three dimensional reconstructions and quantitative analyses of the buccal mass muscles pave the way for future functional analyses allowing to better model jaw closing in coleoids. Finally, our results suggest differences in beak and muscle function that need to be validated using future in vivo functional analyses.

Feeding without teeth: the material properties of rhamphothecae from two species of durophagous sea turtles

The feeding apparatus of sea turtles comprises cornified keratinous rhamphothecae overlaying a bony rostrum. Although keratin is less stiff than the enamel of toothed animals, certain species of sea turtles are capable of withstanding large forces when feeding on hard prey. We aimed to quantify the mineral density, water content and compressive mechanical properties of rhamphothecae from two durophagous species: loggerhead (Caretta caretta) and Kemp's ridley (Lepidochelys kempii) sea turtles. Since loggerheads theoretically produce the greater bite forces of these two species, we predicted that keratin from their rhamphothecae would have a greater mineral density and be stiffer, stronger and tougher compared with Kemp's ridley sea turtles. We found that total water weight of hydrated specimens (20%) was consistent between species. Rhamphotheca mineral density ranged between 0 and 0.069 g cm^-3; loggerheads had significantly greater mineral density compared with Kemp's ridleys, for which several specimens had no mineral detected. Despite the greater mineral density in loggerheads, we found no significant difference in Young's modulus, yield strength or toughness between these species. In addition to mineral density, our findings suggest that other material components, such as sulfur, may be influencing the material properties of keratin from sea turtle rhamphothecae.

Cranial osteology of a juvenile specimen of Acratocnus ye (Mammalia, Xenarthra, Folivora) and its ontogenetic and phylogenetic implications

The present study comprises a description of the skull and jaw anatomy of a juvenile specimen of the Antillean sloth Acratocnus ye, from the Holocene of Haiti. Detailed descriptions and illustrations are provided of the skull bones and their sutural connections, which normally fuse in adults. Descriptions are also provided for the mandible and ear ossicles, as well as endocranial surfaces and sinuses exposed by breaks. The anatomy of our juvenile A. ye is compared to that of adult A. ye to assess ontogenetic changes in the skull. Several of these ontogenetic features are significant new observations that impact the relationships within Xenarthra as a whole, or between Xenarthrans and other placental mammals, most notably, the presence of a separate mesethmoid element, the presence of alveoli for a lower deciduous canine and anterior incisor, and the presence of separate rostral and caudal entotympanic elements. A full list of such changes are provided. In addition, the specimen provides information on phylogenetically relevant characters, including features unique to the genus Acratocnus, and features of the clade Choloepodini, including Acratocnus, the smaller extinct Antillean sloth Neocnus, and the extant two-toed sloth Choloepus. Contrary to previous studies, Acratocnus shares as many features with Choloepus as it does with its fellow Antillean form Neocnus in the present study, which is consistent with current morphology-based phylogenetic hypotheses regarding the relationships within Choloepodini. The current study highlights the need for further anatomical and phylogenetic investigations of Antillean sloths (Megalocnidae/Megalonychidae), and juvenile sloths in general.

Getting out of a mammalian egg: the egg tooth and caruncle of the echidna

In the echidna, after development in utero, the egg is laid in the pouch and incubated for 10 days. During this time, the fetuses develop an egg tooth and caruncle to help them hatch. Using rare and unprecedented access to limited echidna pre- and post-hatching tissues, development of the egg tooth and caruncle were assessed by micro-CT, histology and immunofluorescence. Unlike therian tooth germs that develop by placode invagination, the echidna egg tooth developed by evagination, similar to the first teeth in some reptiles and fish. The egg tooth ankylosed to the premaxilla, rather than forming a tooth root with ligamentous attachment found in other mammals, with loss of the egg tooth associated with high levels of activity odontoclasts and apoptosis. The caruncle formed as a separate mineralisation from the adjacent nasal capsule, and as observed in birds and turtles, the nasal region epithelium on top of the nose expressed markers of cornification. Together, this highlights that the monotreme egg tooth shares many similarities with typical reptilian teeth, suggesting that this tooth has been conserved from a common ancestor of mammals and reptiles.

Flexible conservatism in the skull modularity of convergently evolved myrmecophagous placental mammals

Background

The skull of placental mammals constitutes one of the best studied systems for phenotypic modularity. Several studies have found strong evidence for the conserved presence of two- and six-module architectures, while the strength of trait correlations (integration) has been associated with major developmental processes such as somatic growth, muscle-bone interactions, and tooth eruption. Among placentals, ant- and termite-eating (myrmecophagy) represents an exemplar case of dietary convergence, accompanied by the selection of several cranial morphofunctional traits such as rostrum elongation, tooth loss, and mastication loss. Despite such drastic functional modifications, the covariance patterns of the skull of convergently evolved myrmecophagous placentals are yet to be studied in order to assess the potential consequences of this dietary shift on cranial modularity.

Results

Here, we performed a landmark-based morphometric analysis of cranial covariance patterns in 13 species of myrmecophagous placentals. Our analyses reveal that most myrmecophagous species present skulls divided into six to seven modules (depending on the confirmatory method used), with architectures similar to those of non-myrmecophagous placentals (therian six modules). Within-module integration is also similar to what was previously described for other placentals, suggesting that most covariance-generating processes are conserved across the clade. Nevertheless, we show that extreme rostrum elongation and tooth loss in myrmecophagid anteaters have resulted in a shift in intermodule correlations in the proximal region of the rostrum. Namely, the naso-frontal and maxillo-palatine regions are strongly correlated with the oro-nasal module, suggesting an integrated rostrum conserved from pre-natal developmental processes. In contrast, the similarly toothless pangolins show a weaker correlation between the anterior rostral modules, resembling the pattern of toothed placentals.

Conclusions

These results reveal that despite some integration shifts related to extreme functional and morphological features of myrmecophagous skulls, cranial modular architectures have conserved the typical mammalian scheme.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-022-02030-9.

Morphological evaluation of the orbit, eye tunics, eyelids, and orbital glands in young and adult aardvarks Orycteropus afer, Pallas, 1766 (Tubulidentata: Orycteropodidae) - similarities and differences with representatives of the Afrotheria clade

The Afrotheria clade includes a large group of extant mammals, and the aardvark (Orycteropus afer) is the only representative of the order Tubulidentata in it. Here, we studied the morphological nature of the orbital region, eye tunics, upper and lower eyelids, superficial gland of the third eyelid, the third eyelid, deep gland of the third eyelid, and lacrimal gland in post-mortem specimens obtained from three captive aardvarks, two young and one adult. The obtained samples were analyzed using macroscopic, histological, and histochemical methods. We observed choroidal tapetum lucidum fibrosum in all specimens, which was typical for aardvarks. The superficial gland of the third eyelid was a compound multilobar tubular branched gland of a mucous nature. The deep gland of the third eyelid produced a serous secretion. The seromucous secretion was typical for the lacrimal gland. We compared the morphological data of the O. afer skull with that from other endemic African mammals in the Afrotheria clade. We found that other authors provided different anatomical names for some bones and foramina located within the orbit. The types and function of eyelid glands, as well as eyeball glands of aardvarks, can primarily be connected with their habitat. Our study may constitute an introduction to the ontogenesis of individual eyeball glands in aardvarks. This article is protected by copyright. All rights reserved.

Unraveling the heritage of lost traits

We synthesize ontogenetic work spanning the past century that show evolutionarily lost structures are rarely entirely absent from earlier developmental stages. We discuss morphological and genetic insights from developmental studies reveal about the evolution of trait loss and regain.

Diet, feeding behaviour and echidna beaks: a review of functional relationships within the tachyglossids

Echidnas are commonly known as ‘spiny ant-eaters’, but long-beaked echidnas (Zaglossus spp.) do not eat ants, whereas short-beaked echidnas (Tachyglossus aculeatus) eat other invertebrates as well as ants. The differences in skull morphology between short- and long-beaked echidnas are related to the differences in their diets, and I tested the hypothesis that there would be differences in beak length of short-beaked echidnas from populations with different diets. Published data on diet from echidnas from different parts of Australia show that echidnas from arid and semi-arid areas (subspecies acanthion) and Kangaroo Island (subspecies multiaculeatus) principally eat ants and termites, whereas the main dietary items of echidnas from south-eastern Australia (subspecies aculeatus) and Tasmania (subspecies setosus) are ants and scarab larvae. Using museum specimens and photographs I measured skull dimensions on echidnas from different parts of Australia: acanthion and multiaculeatus have narrower skulls and shorter beaks than aculeatus and setosus, with setosus being the only Australian subspecies where beak length exceeds cranium length. Australian short-beaked echidnas fall into two groups: aculeatus and setosus from the wetter east and south-east, which eat ant and scarab larvae, and the arid and semi-arid zone acanthion and multiaculeatus, with shorter, narrower skulls, and which eat ants and termites.

Pseudogenized Amelogenin Reveals Early Tooth Loss in True Toads (Anura: Bufonidae)

Extant anurans (frogs and toads) exhibit reduced dentition, ranging from a lack of mandibular teeth to complete edentulation, as observed in the true toads of the family Bufonidae. The evolutionary time line of these reductions remains vague due to a poor fossil record. Previous studies have demonstrated an association between the lack of teeth in edentulous vertebrates and the pseudogenization of the major tooth enamel gene amelogenin (AMEL) through accumulation of deleterious mutations and the disruption of its coding sequence. In this study, we have harnessed the pseudogenization of AMEL as a molecular dating tool to correlate loss of dentition with genomic mutation patterns during the rise of the family Bufonidae. Specifically, we have utilized AMEL pseudogenes in three members of the family as a tool to estimate the putative date of edentulation in true toads. Comparison of AMEL sequences from Rhinella marina, Bufo gargarizans and Bufo bufo, with nine extant, dentulous frogs, revealed mutations confirming AMEL inactivation in Bufonidae. AMEL pseudogenes in modern bufonids also exhibited remarkably high 86-93% sequence identity among each other, with only a slight increase in substitution rate and relaxation of selective pressure, in comparison with functional copies in other anurans. Moreover, using selection intensity estimates and synonymous substitution rates, analysis of functional and pseudogenized AMEL resulted in an estimated inactivation window of 46-60 million years ago in the lineage leading to modern true toads, a time line that coincides with the rise of the family Bufonidae.

Multiple evolutionary origins and losses of tooth complexity in squamates

Teeth act as tools for acquiring and processing food, thus holding a prominent role in vertebrate evolution. In mammals, dental-dietary adaptations rely on tooth complexity variations controlled by cusp number and pattern. Complexity increase through cusp addition has dominated the diversification of mammals. However, studies of Mammalia alone cannot reveal patterns of tooth complexity conserved throughout vertebrate evolution. Here, we use morphometric and phylogenetic comparative methods across fossil and extant squamates to show they also repeatedly evolved increasingly complex teeth, but with more flexibility than mammals. Since the Late Jurassic, multiple-cusped teeth evolved over 20 times independently from a single-cusped common ancestor. Squamates frequently lost cusps and evolved varied multiple-cusped morphologies at heterogeneous rates. Tooth complexity evolved in correlation with changes in plant consumption, resulting in several major increases in speciation. Complex teeth played a critical role in vertebrate evolution outside Mammalia, with squamates exemplifying a more labile system of dental-dietary evolution.

Molecular Evolution of Tooth-Related Genes Provides New Insights into Dietary Adaptations of Mammals

Mammals have evolved different tooth phenotypes that are hypothesized to be associated with feeding habits. However, the genetic basis for the linkage has not been well explored. In this study, we investigated 13 tooth-related genes, including seven enamel-related genes (AMELX, AMBN, ENAM, AMTN, ODAM, KLK4 and MMP20) and six dentin-related genes (DSPP, COL1A1, DMP1, IBSP, MEPE and SPP1), from 63 mammals to determine their evolutionary history. Our results showed that different evolutionary histories have evolved among divergent feeding habits in mammals. There was stronger positive selection for eight genes (ENAM, AMTN, ODAM, KLK4, DSPP, DMP1, COL1A1, MEPE) in herbivore lineages. In addition, AMELX, AMBN, ENAM, AMTN, MMP20 and COL1A1 underwent accelerated evolution in herbivores. While relatively strong positive selection was detected in IBSP, SPP1, and DSPP, accelerated evolution was only detected for MEPE and SPP1 genes among the carnivorous lineages. We found positive selection on AMBN and ENAM genes for omnivorous primates in the catarrhini clade. Interestingly, a significantly positive association between the evolutionary rate of ENAM, ODAM, KLK4, MMP20 and the average enamel thickness was found in primates. Additionally, we found molecular convergence in some amino acid sites of tooth-related genes among the lineages whose feeding habit are similar. The positive selection of related genes might promote the formation and bio-mineralization of tooth enamel and dentin, which would make the tooth structure stronger. Our results revealed that mammalian tooth-related genes have experienced variable evolutionary histories, which provide some new insights into the molecular basis of dietary adaptation in mammals.

Non-trophic Functional Ecology of Vertebrate Teeth: A Review

Teeth are critical to the functional ecology of vertebrate trophic abilities, but are also used for a diversity of other non-trophic tasks. Teeth can play a substantial role in how animals move, manipulate their environment, positively interact with conspecifics, antagonistically interact with other organisms, and sense the environment. We review these non-trophic functions in an attempt to place the utility of human and all other vertebrate dentitions in a more diverse framework that emphasizes an expanded view of the functional importance and ecological diversity of teeth. In light of the extensive understanding of the developmental genetics, trophic functions, and evolutionary history of teeth, comparative studies of vertebrate dentitions will continue to provide unique insights into multi-functionality, many-to-one mapping, and the evolution of novel abilities.

Reawakening of Ancestral Dental Potential as a Mechanism to Explain Dental Pathologies

During evolution, there has been a trend to reduce both the number of teeth and the location where they are found within the oral cavity. In mammals, the formation of teeth is restricted to a horseshoe band of odontogenic tissue, creating a single dental arch on the top and bottom of the jaw. Additional teeth and structures containing dental tissue, such as odontogenic tumors or cysts, can appear as pathologies. These tooth-like structures can be associated with the normal dentition, appearing within the dental arch, or in nondental areas. The etiology of these pathologies is not well elucidated. Reawakening of the potential to form teeth in different parts of the oral cavity could explain the origin of dental pathologies outside the dental arch, thus such pathologies are a consequence of our evolutionary history. In this review, we look at the changing pattern of tooth formation within the oral cavity during vertebrate evolution, the potential to form additional tooth-like structures in mammals, and discuss how this knowledge shapes our understanding of dental pathologies in humans.

Rampant tooth loss across 200 million years of frog evolution

Teeth are present in most clades of vertebrates but have been lost completely several times in actinopterygian fishes and amniotes. Using phenotypic data collected from over 500 genera via micro-computed tomography, we provide the first rigorous assessment of the evolutionary history of dentition across all major lineages of amphibians. We demonstrate that dentition is invariably present in caecilians and salamanders, but teeth have been lost completely more than 20 times in frogs, a much higher occurrence of edentulism than in any other vertebrate group. The repeated loss of teeth in anurans is associated with a specialized diet of small invertebrate prey as well as shortening of the lower jaw, but it is not correlated with a reduction in body size. Frogs provide an unparalleled opportunity for investigating the molecular and developmental mechanisms of convergent tooth loss on a large phylogenetic scale.

Molecular phyloecology suggests a trophic shift concurrent with the evolution of the first birds

Birds are characterized by evolutionary specializations of both locomotion (e.g., flapping flight) and digestive system (toothless, crop, and gizzard), while the potential selection pressures responsible for these evolutionary specializations remain unclear. Here we used a recently developed molecular phyloecological method to reconstruct the diets of the ancestral archosaur and of the common ancestor of living birds (CALB). Our results suggest a trophic shift from carnivory to herbivory (fruit, seed, and/or nut eater) at the archosaur-to-bird transition. The evolutionary shift of the CALB to herbivory may have essentially made them become a low-level consumer and, consequently, subject to relatively high predation risk from potential predators such as gliding non-avian maniraptorans, from which birds descended. Under the relatively high predation pressure, ancestral birds with gliding capability may have then evolved not only flapping flight as a possible anti-predator strategy against gliding predatory non-avian maniraptorans but also the specialized digestive system as an evolutionary tradeoff of maximizing foraging efficiency and minimizing predation risk. Our results suggest that the powered flight and specialized digestive system of birds may have evolved as a result of their tropic shift-associated predation pressure.

Macroevolutionary dynamics of dentition in Mesozoic birds reveal no long-term selection towards tooth loss

Several potential drivers of avian tooth loss have been proposed, although consensus remains elusive as fully toothless jaws arose independently numerous times among Mesozoic avialans and dinosaurs more broadly. The origin of crown bird edentulism has been discussed in terms of a broad-scale selective pressure or trend toward toothlessness, although this has never been quantitatively tested. Here, we find no evidence for models whereby iterative acquisitions of toothlessness among Mesozoic Avialae were driven by an overarching selective trend. Instead, our results support modularity among jaw regions underlying heterogeneous tooth loss patterns and indicate a substantially later transition to complete crown bird edentulism than previously hypothesized (∼90 mya). We show that patterns of avialan tooth loss adhere to Dollo's law and suggest that the exclusive survival of toothless birds to the present represents lineage-specific selective pressures, irreversibility of tooth loss, and the filter of the Cretaceous-Paleogene (K–Pg) mass extinction.

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The evolutionary processes underlying tooth loss in Mesozoic birds are debated

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Analyses reveal no long-term selective pressure or trend toward toothlessness

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Tooth loss was likely a result of local selective pressures on individual lineages

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The transition to crown bird toothlessness occurred later than previously hypothesized

Development, structure, and protein composition of the corneous beak in turtles

The beak or rhamphotheca in turtles is a horny lamina that replaces the teeth. Its origin, development, structure, and protein composition are here presented. At mid-development stages, the epidermis of the maxilla and mandible gives rise to placodes that enlarge and merge into laminae through an intense cell proliferation. In these expanding laminae, the epidermis gives rise to 5-8 layers of embryonic epidermis where coarse filaments accumulate for the initial keratinization of cells destined to be sloughed before hatching. Underneath the embryonic epidermis of the beak numerous layers of spindle-shaped beta-cells are produced while they are absent in other skin regions. Beta-cells contain hard corneous material and give rise to the corneous layer of the beak whose external layers desquamate due to wearing and mechanical abrasion. Beta-catenin is present in nuclei of proliferating keratinocytes of the germinal layer likely responding to a wnt signal, but also is part of the adhesive junctions located among beak keratinocytes. The thick corneous layer is made of mature corneocytes connected one to another along their irregular perimeter by an unknown cementing material and junctional remnants. Immunolabeling shows that the main components of the horny beak are Corneous Beta Proteins (CBPs) of 10-15 kDa which genes are located in the Epidermal Differentiation Complex (EDC) of the turtle genome. Specific CBPs, in addition to a lower amount of Intermediate Filament Keratins, accumulate in the horny beak. Compaction of the main proteins with other unknown, minor proteins give rise to the hard corneous material of the beak.

Effects of Chronic Stress on the Fecal Microbiome of Malayan Pangolins (Manis javanica) Rescued from the Illegal Wildlife Trade

Pangolins (scaly anteaters, Pholidota) are among those mammals that are most affected by the international, illegal wildlife trade. Recently, wildlife rescue centers in China became dedicated to rehabilitate confiscated pangolins and prepare them for reintroduction to the wild. Chronic stress is thought to be the main reason for a disturbed microbiota community and a higher mortality rate of pangolin in captivity. In this study, we compared the cortisol levels and the fecal microbiome of Malayan pangolin (Manis javanica) born and reared in captivity (PCB; n = 7) with those rescued from the wildlife trade (PCT; n = 16). Results show that the level of cortisol in PCT was significantly lower than that observed in PCB. There were also significant differences in the composition of the fecal microflora between the two groups, and the diversity of intestinal microbiota was higher in PCB than in PCT. At the phylum level, the bacteria with significant difference between the two groups included Firmicutes and Bacteroides. At the genus level, bacteria such as Bacteroides, Parabacterides, and Clostridium showed significant differences between the two groups. This study proves that chronic stress has a considerable effect on the diversity and composition of fecal microbiota in Malayan pangolin.

Patterns and tempo of PCSK9 pseudogenizations suggest an ancient divergence in mammalian cholesterol homeostasis mechanisms

Proprotein convertase subtilisin/kexin type 9 (PCSK9) plays a central role in cholesterol homeostasis in humans as a major regulator of LDLR levels. PCSK9 is an intriguing protease in that it does not act by proteolysis but by preventing LDLR recirculation from endosomes to the plasma membrane. This, and the inexistence of any other proteolytic substrate but itself could suggest that PCSK9 is an exquisite example of evolutionary fine-tuning. However, the gene has been lost in several mammalian species, and null alleles are present (albeit at low frequencies) in some human populations without apparently deleterious health effects, raising the possibility that the PCSK9 may have become dispensable in the mammalian lineage. To address this issue, we systematically recovered, assembled, corrected, annotated and analysed publicly available PCSK9 sequences for 420 eutherian species to determine the distribution, frequencies, mechanisms and timing of PCSK9 pseudogenization events, as well as the evolutionary pressures underlying the preservation or loss of the gene. We found a dramatic difference in the patterns of PCSK9 retention and loss between Euarchontoglires—where there is strong pressure for gene preservation—and Laurasiatheria, where multiple independent events have led to PCSK9 loss in most species. These results suggest that there is a fundamental difference in the regulation of cholesterol metabolism between Euarchontoglires and Laurasiatheria, which in turn has important implications for the use of Laurasiatheria species (e.g. pigs) as animal models of human cholesterol-related diseases.

ACPT gene is inactivated in mammalian lineages that lack enamel or teeth

Loss of tooth or enamel is widespread in multiple mammal lineages. Although several studies have been reported, the evolutionary mechanisms of tooth/enamel loss are still unclear. Most previous studies have found that some tooth-related genes have been inactivated in toothless and/or enamel-less mammals, such as ENAM, ODAM, C4orf26, AMBN, AMTN, DSPP, etc. Here, we conducted evolutionary analyses on ACPT playing a key role in amelogenesis, to interrogate the mechanisms. We obtained the ACPT sequences from 116 species, including edentulous and enamel-less mammals. The results shows that variant ORF-disrupting mutations were detected in ACPT coding region among nine edentulous baleen whales and three enamel-less taxa (pygmy sperm whale, aardvark, nine-banded armadillo). Furtherly, selective pressure uncovered that the selective constraints have been relaxed among all toothless and enamel-less lineages. Moreover, our results support the hypothesis that mineralized teeth were lost or degenerated in the common ancestor of crown Mysticeti through two shared single-base sites deletion in exon 4 and 5 of ACPT among all living baleen whales. D_N/d_S values on transitional branches were used to estimate ACPT inactivation records. In the case of aardvark, inactivation of ACPT was estimated at ~23.60–28.32 Ma, which is earlier than oldest aardvark fossil record (Orycteropus minutus, ~19 Ma), suggesting that ACPT inactivation may result in degeneration or loss of enamel. Conversely, the inactivation time of ACPT estimated in armadillo (~10.18–11.30 Ma) is later than oldest fossil record, suggesting that inactivation of ACPT may result from degeneration or loss of enamel in these mammals. Our findings suggested that different mechanisms of degeneration of tooth/enamel might exist among toothless and enamel-less lineages during evolution. Our study further considered that ACPT is a novel gene for studying tooth evolution.

Dental enamel structure in long-nosed armadillos (Xenarthra: Dasypus) and its evolutionary implications

Most xenarthrans have a reduced and simplified dentition that lacks enamel. However, the presence of prismatic enamel has been recorded in the Eocene armadillos Utaetus buccatus (Euphractinae) and Astegotherium dichotomus (Astegotheriini). Among extant xenarthrans, the occurrence of enamel has been recognized only in the long-nosed armadillo, Dasypus novemcinctus (Dasypodinae), but its microstructure has never been described. In this contribution, we analyse the enamel microstructure in deciduous and permanent teeth of four Dasypus species. In deciduous molariform teeth of some species, we identify an apical cap of vestigial enamel (without crystalline structure), interpreted as an amorphous ameloblastic secretion. In permanent teeth, a thin layer of true enamel is found in the apical portion of unworn molariforms. The enamel is prismatic in D. novemcinctus, but in Dasypus hybridus, Dasypus sabanicola and Dasypus punctatus it is prismless. Taking into account the Eocene species of armadillos, the ancestral condition of enamel in cingulates could have been more complex (as in other placentals) and undergone progressive reduction, as shown in the Dasypus lineage. In light of previous genetic and developmental studies, we review and briefly discuss the processes that can account for the reduction/loss of enamel in extant and extinct armadillos. The retention of enamel and the fact that this genus is the only living xenarthran with two functional generations of teeth support the early divergence of the Dasypus lineage among living cingulates. This is in agreement with morphological and molecular analyses.

Do Meristic Characters Used in Phylogenetic Analysis Evolve in an Ordered Manner?

The use of ordered characters in phylogenetic analysis has been inconsistent throughout the history of phylogenetic inference. It has become more widespread in recent years, and some have advocated that all characters representing continuous or meristic traits should be ordered as a matter of course. Here, using the example of dental evolution, we examine two factors that may impact on whether meristic characters actually evolve in an ordered manner: the regulatory hierarchy governing the development of teeth that allows large sections of the entire tooth row to be suppressed in a single transition and regionalization of the tooth row where different modules have a degree of independence in their evolution. These are studied using both empirical and simulated data. Models of evolution of such characters are examined over molecular phylogenies to see if ordered or unordered models fit best. Simulations of tooth-row evolution are designed to incorporate changes in region size and multiple levels of developmental control to suppress individual regions or the entire row. The empirical analyses show that in a clade with largely homodont dentition the characters evolve in an ordered manner, but if dentition is heterodont with distinct regionalization their evolution better fits an unordered model. In the simulations, even if teeth are added and removed from the tooth row in an ordered manner, dividing the row into independently evolving modules can lead to characters covering multiple modules better fitting an unordered model of evolution. Adding the ability to suppress regions or the entire tooth row has a variable effect depending on the rates of suppression relative to the rates of addition and subtraction of individual teeth. We therefore advise not following a single policy when deciding whether to order meristic traits but to base the decision on a priori knowledge of the focal clade's evolution and developmental biology. [Discrete characters; ordered characters; phylogeny; teeth.].

Comparative masticatory myology in anteaters and its implications for interpreting morphological convergence in myrmecophagous placentals

BACKGROUND

Ecological adaptations of mammals are reflected in the morphological diversity of their feeding apparatus, which includes differences in tooth crown morphologies, variation in snout size, or changes in muscles of the feeding apparatus. The adaptability of their feeding apparatus allowed them to optimize resource exploitation in a wide range of habitats. The combination of computer-assisted X-ray microtomography (µ-CT) with contrast-enhancing staining protocols has bolstered the reconstruction of three-dimensional (3D) models of muscles. This new approach allows for accurate descriptions of muscular anatomy, as well as the quick measurement of muscle volumes and fiber orientation. Ant- and termite-eating (myrmecophagy) represents a case of extreme feeding specialization, which is usually accompanied by tooth reduction or complete tooth loss, snout elongation, acquisition of a long vermiform tongue, and loss of the zygomatic arch. Many of these traits evolved independently in distantly-related mammalian lineages. Previous reports on South American anteaters (Vermilingua) have shown major changes in the masticatory, intermandibular, and lingual muscular apparatus. These changes have been related to a functional shift in the role of upper and lower jaws in the evolutionary context of their complete loss of teeth and masticatory ability.

METHODS

We used an iodine staining solution (I2KI) to perform contrast-enhanced µ-CT scanning on heads of the pygmy (Cyclopes didactylus), collared (Tamandua tetradactyla) and giant (Myrmecophaga tridactyla) anteaters. We reconstructed the musculature of the feeding apparatus of the three extant anteater genera using 3D reconstructions complemented with classical dissections of the specimens. We performed a description of the musculature of the feeding apparatus in the two morphologically divergent vermilinguan families (Myrmecophagidae and Cyclopedidae) and compared it to the association of morphological features found in other myrmecophagous placentals.

RESULTS

We found that pygmy anteaters (Cyclopes) present a relatively larger and architecturally complex temporal musculature than that of collared (Tamandua) and giant (Myrmecophaga) anteaters, but shows a reduced masseter musculature, including the loss of the deep masseter. The loss of this muscle concurs with the loss of the jugal bone in Cyclopedidae. We show that anteaters, pangolins, and aardvarks present distinct anatomies despite morphological and ecological convergences.

Cell proliferation, adhesion, and differentiation of keratinocytes in the developing beak and egg-tooth of the turtle Emydura macquarii

The development of the beak in turtles is poorly known. Beak development has been analyzed by immunofluorescent methods for studying cell proliferation and localization of specific proteins. The flat two-layered epidermis covering the turtle embryo at mid stage of development becomes columnar in the oral region and is associated with an increase of mesenchymal density as in placodes. Using 5BrdU, an intense cell proliferation is observed in the oral and epidermal cells covering the maxilla and mandibular bones, probably stimulated by the underlying mesenchyme in continuation with maxillary and mandibular bones. Expansion and fusion of these placodes give rise to the corneous beak. Beta catenin, mainly junctional but also sparsely detected in nuclei of the germinal layer of the beak epithelium, maintains united the differentiating keratinocytes that form a stratified corneous epithelium. This is initially composed of some layers of large cells that accumulate intermediate filament keratins (IFKs) and give rise to a keratinized embryonic epidermis destined to slough around hatching. The following corneocytes accumulate IFKs but mainly type I/II corneous beta proteins (CBPs) and form a corneous beak. These CBPs appear present with lower intensity in the beak than in the shell, but the higher intensity obtained with a general antibody against CBPs indicates that other CBPs contribute to the composition and stiffness of beak corneous material. The egg-tooth in continuation with the stratum corneum of the maxillary beak develops from a localized proliferation and comprises a thick embryonic epidermis accumulating IFKs under which large beta-cells connected by adhesion proteins accumulate CBPs contributing to hardening of this temporary organ.

Bite force data suggests relationship between acrodont tooth implantation and strong bite force

Extant and extinct reptiles exhibit numerous combinations of tooth implantation and attachment. Tooth implantation ranges from those possessing roots and lying within a socket (thecodonty), to teeth lying against the lingual wall of the jawbone (pleurodonty), to teeth without roots or sockets that are attached to the apex of the marginal jawbones (acrodonty). Attachment may be ligamentous (gomphosis) or via fusion (ankylosis). Generally speaking, adaptative reasonings are proposed as an underlying driver for evolutionary changes in some forms of tooth implantation and attachment. However, a substantiated adaptive hypothesis is lacking for the state of acrodont ankylosis that is seen in several lineages of Lepidosauria, a clade that is plesiomorphically pleurodont. The convergent evolution of acrodont ankylosis in several clades of lepidosaurs suggests a selective pressure shaped the evolution of the trait. We hypothesize that acrodont ankylosis as seen in Acrodonta and Sphenodon punctatus, is an adaptation either resulting from or allowing for a stronger bite force. We analyzed bite force data gathered from the literature to show that those taxa possessing acrodont dentition possess a stronger bite force on average than those taxa with pleurodont dentition. Dietary specialists with pleurodont dentition may also possess relatively high bite forces, though body size may also play a role in their ability to bite hard. Furthermore, our results have implications for the evolution of acrodont ankylosis and potential behaviors related to strong bite force that influenced the evolution of acrodonty within Acrodonta and Rhynchocephalia.

Tusks, the extra-oral teeth

OBJECTIVE

The present review aims to: a) describe the features that support tusks in extra-oral position, and b) represent distinctive features of tusks, which provide insights into tusks adaptation to ambient conditions.

DESIGN

A comprehensive review of scientific literature relevant to tusks and comparable dental tissues was conducted.

RESULTS

The oral cavity provides a desirable condition which is conducive to tooth health. Therefore, it remains questionable how the bare (exposed) tusks resist the extra-oral conditions. The common features among tusked mammals indicate that the structural (e.g. the peculiar dentinal alignment), cellular (e.g. low or lack of cell populations in the tusk), hormonal (e.g. androgens), and behavioral traits have impact on a tusk's preservation and occurrence.

CONCLUSIONS

Understanding of bare mineralized structures, such as tusks and antlers, and their compatibility with different environments, can provide important insight into oral biology.

An eye for a tooth: Thylacosmilus was not a marsupial "saber-tooth predator"

Background

Saber-toothed mammals, now all extinct, were cats or “cat-like” forms with enlarged, blade-like upper canines, proposed as specialists in taking large prey. During the last 66 Ma, the saber-tooth ecomorph has evolved convergently at least in five different mammalian lineages across both marsupials and placentals. Indeed, Thylacosmilus atrox, the so-called “marsupial saber-tooth,” is often considered as a classic example of convergence with placental saber-tooth cats such as Smilodon fatalis. However, despite its superficial similarity to saber-toothed placentals, T. atrox lacks many of the critical anatomical features related to their inferred predatory behavior—that of employing their enlarged canines in a killing head strike.

Methods

Here we follow a multi-proxy approach using canonical correspondence analysis of discrete traits, biomechanical models of skull function using Finite Element Analysis, and 3D dental microwear texture analysis of upper and lower postcanine teeth, to investigate the degree of evolutionary convergence between T. atrox and placental saber-tooths, including S. fatalis.

Results

Correspondence analysis shows that the craniodental features of T. atrox are divergent from those of placental saber-tooths. Biomechanical analyses indicate a superior ability of T. atrox to placental saber-tooths in pulling back with the canines, with the unique lateral ridge of the canines adding strength to this function. The dental microwear of T. atrox indicates a soft diet, resembling that of the meat-specializing cheetah, but its blunted gross dental wear is not indicative of shearing meat.

Conclusions

Our results indicate that despite its impressive canines, the “marsupial saber-tooth” was not the ecological analogue of placental saber-tooths, and likely did not use its canines to dispatch its prey. This oft-cited example of convergence requires reconsideration, and T. atrox may have had a unique type of ecology among mammals.

Species specificity and sexual dimorphism in tooth shape among the three sympatric haplochromine species in Lake Kivu cichlids

Tooth shape is used to differentiate between morphologically similar species of vertebrates, including fish. This study aimed to quantify tooth shape of three sympatric species: Haplochromis kamiranzovu, H. insidiae, and H. astatodon endemic to Lake Kivu, whose existing identification criteria are currently only qualitative. A quantitative tooth shape analysis was performed based on digitized tooth outline data with a subsequent elliptic Fourier analysis to test for differences among the three species. We looked at crown shape and size differences within H. kamiranzovu and H. insidiae at geographical, habitat, and gender levels. No comparison at habitat level was done for H. astatodon because it is found only in littoral zone. The analysis revealed significant tooth shape differences among the three species. Haplochromis astatodon had a significantly longer major cusp height and a longer and larger minor cusp than that of H. insidiae. It had also a longer major cusp height and a longer and larger minor cusp than that of H. kamiranzovu. Tooth shape differences of H. kamiranzovu and H. insidiae species were not significantly different between littoral and pelagic fish (p > .05) while differences were significant between southern and northern Lake Kivu populations (p < .05). Tooth sizes in H. kamiranzovu and H. insidiae were significantly different, both in height and width as well as in their ratios, and this was true at sex and geographic levels (p < .05), but not at habitat level (p > .05). Tooth shape was also significantly different with sharp teeth for males compared with females of southern populations versus northern ones. These shape- and size-related differences between sexes suggest differences in the foraging strategies toward available food resources in the lake habitat. Further research should explain the genetic basis of the observed pattern.

Prenatal developmental sequence of the skull of minke whales and its implications for the evolution of mysticetes and the teeth-to-baleen transition

Baleen whales (Mysticeti) have an extraordinary fossil record documenting the transition from toothed raptorial taxa to modern species that bear baleen plates, keratinous bristles employed in filter-feeding. Remnants of their toothed ancestry can be found in their ontogeny, as they still develop tooth germs in utero. Understanding the developmental transition from teeth to baleen and the associated skull modifications in prenatal specimens of extant species can enhance our understanding of the evolutionary history of this lineage by using ontogeny as a relative proxy of the evolutionary changes observed in the fossil record. Although at present very little information is available on prenatal development of baleen whales, especially regarding tooth resorption and baleen formation, due to a lack of specimens. Here I present the first detailed description of prenatal specimens of minke whales (Balaenoptera acutorostrata and Balaenoptera bonaerensis), focusing on the skull anatomy and tooth germ development, resorption, and baleen growth. The ontogenetic sequence described consists of 10 specimens of both minke whale species, from the earliest fetal stages to full term. The internal skull anatomy of the specimens was visualized using traditional and iodine-enhanced computed tomography scanning. These high-quality data allow detailed description of skull development both qualitatively and quantitatively using three-dimensional landmark analysis. I report distinctive external anatomical changes and the presence of a denser tissue medial to the tooth germs in specimens from the final portion of gestation, which can be interpreted as the first signs of baleen formation (baleen rudiments). Tooth germs are only completely resorbed just before the eruption of the baleen from the gums, and they are still present for a brief period with baleen rudiments. Skull shape development is characterized by progressive elongation of the rostrum relative to the braincase and by the relative anterior movement of the supraoccipital shield, contributing to a defining feature of cetaceans, telescoping. These data aid the interpretation of fossil morphologies, especially of those extinct taxa where there is no direct evidence of presence of baleen, even if caution is needed when comparing prenatal extant specimens with adult fossils. The ontogeny of other mysticete species needs to be analyzed before drawing definitive conclusions about the influence of development on the evolution of this group. Nonetheless, this work is the first step towards a deeper understanding of the most distinctive patterns in prenatal skull development of baleen whales, and of the anatomical changes that accompany the transition from tooth germs to baleen. It also presents comprehensive hypotheses to explain the influence of developmental processes on the evolution of skull morphology and feeding adaptations of mysticetes.

Odontogenic ameloblast-associated (ODAM) is inactivated in toothless/enamelless placental mammals and toothed whales

Background

The gene for odontogenic ameloblast-associated (ODAM) is a member of the secretory calcium-binding phosphoprotein gene family. ODAM is primarily expressed in dental tissues including the enamel organ and the junctional epithelium, and may also have pleiotropic functions that are unrelated to teeth. Here, we leverage the power of natural selection to test competing hypotheses that ODAM is tooth-specific versus pleiotropic. Specifically, we compiled and screened complete protein-coding sequences, plus sequences for flanking intronic regions, for ODAM in 165 placental mammals to determine if this gene contains inactivating mutations in lineages that either lack teeth (baleen whales, pangolins, anteaters) or lack enamel on their teeth (aardvarks, sloths, armadillos), as would be expected if the only essential functions of ODAM are related to tooth development and the adhesion of the gingival junctional epithelium to the enamel tooth surface.

Results

We discovered inactivating mutations in all species of placental mammals that either lack teeth or lack enamel on their teeth. A surprising result is that ODAM is also inactivated in a few additional lineages including all toothed whales that were examined. We hypothesize that ODAM inactivation is related to the simplified outer enamel surface of toothed whales. An alternate hypothesis is that ODAM inactivation in toothed whales may be related to altered antimicrobial functions of the junctional epithelium in aquatic habitats. Selection analyses on ODAM sequences revealed that the composite dN/dS value for pseudogenic branches is close to 1.0 as expected for a neutrally evolving pseudogene. DN/dS values on transitional branches were used to estimate ODAM inactivation times. In the case of pangolins, ODAM was inactivated ~ 65 million years ago, which is older than the oldest pangolin fossil (Eomanis, 47 Ma) and suggests an even more ancient loss or simplification of teeth in this lineage.

Conclusion

Our results validate the hypothesis that the only essential functions of ODAM that are maintained by natural selection are related to tooth development and/or the maintenance of a healthy junctional epithelium that attaches to the enamel surface of teeth.

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1359-6) contains supplementary material, which is available to authorized users.

Bony pseudoteeth of extinct pelagic birds (Aves, Odontopterygiformes) formed through a response of bone cells to tooth-specific epithelial signals under unique conditions

Modern birds (crown group birds, called Neornithes) are toothless; however, the extinct neornithine Odontopterygiformes possessed bone excrescences (pseudoteeth) which resembled teeth, distributed sequentially by size along jaws. The origin of pseudoteeth is enigmatic, but based on recent evidence, including microanatomical and histological analyses, we propose that conserved odontogenetic pathways most probably regulated the development of pseudodentition. The delayed pseudoteeth growth and epithelium keratinization allowed for the existence of a temporal window during which competent osteoblasts could respond to oral epithelial signaling, in place of the no longer present odontoblasts; thus, bony pseudoteeth developed instead of true teeth. Dynamic morphogenetic fields can explain the particular, sequential size distribution of pseudoteeth along the jaws of these birds. Hence, this appears as a new kind of deep homology, by which ancient odontogenetic developmental processes would have controlled the evolution of pseudodentition, structurally different from a true dentition, but morphologically and functionally similar.

Their loss is our gain: regressive evolution in vertebrates provides genomic models for uncovering human disease loci

Throughout Earth's history, evolution's numerous natural 'experiments' have resulted in a diverse range of phenotypes. Though de novo phenotypes receive widespread attention, degeneration of traits inherited from an ancestor is a very common, yet frequently neglected, evolutionary path. The latter phenomenon, known as regressive evolution, often results in vertebrates with phenotypes that mimic inherited disease states in humans. Regressive evolution of anatomical and/or physiological traits is typically accompanied by inactivating mutations underlying these traits, which frequently occur at loci identical to those implicated in human diseases. Here we discuss the potential utility of examining the genomes of vertebrates that have experienced regressive evolution to inform human medical genetics. This approach is low cost and high throughput, giving it the potential to rapidly improve knowledge of disease genetics. We discuss two well-described examples, rod monochromacy (congenital achromatopsia) and amelogenesis imperfecta, to demonstrate the utility of this approach, and then suggest methods to equip non-experts with the ability to corroborate candidate genes and uncover new disease loci.

Heterochronic truncation of odontogenesis in theropod dinosaurs provides insight into the macroevolution of avian beaks

Beaks are innovative structures characterizing numerous tetrapod lineages, including birds, but little is known about how developmental processes influenced the macroevolution of these important structures. Here we provide evidence of ontogenetic vestigialization of alveoli in two lineages of theropod dinosaurs and show that these are transitional phenotypes in the evolution of beaks. One of the smallest known caenagnathid oviraptorosaurs and a small specimen of the Early Cretaceous bird Sapeornis both possess shallow, empty vestiges of dentary alveoli. In both individuals, the system of vestiges connects via foramina with a dorsally closed canal homologous to alveoli. Similar morphologies are present in Limusaurus, a beaked theropod that becomes edentulous during ontogeny; and an analysis of neontological and paleontological evidence shows that ontogenetic reduction of the dentition is a relatively common phenomenon in vertebrate evolution. Based on these lines of evidence, we propose that progressively earlier postnatal and embryonic truncation of odontogenesis corresponds with expansion of rostral keratin associated with the caruncle, and these progenesis and peramorphosis heterochronies combine to drive the evolution of edentulous beaks in nonavian theropods and birds. Following initial apomorphic expansion of rostral keratinized epithelia in perinatal toothed theropods, beaks appear to inhibit odontogenesis as they grow postnatally, resulting in a sequence of common morphologies. This sequence is shifted earlier in development through phylogeny until dentition is absent at hatching, and odontogenesis is inhibited by beak formation in ovo.