Springhares, flying and flightless scaly-tailed squirrels (Anomaluromorpha, Rodentia) are the squirrely mouse: comparative anatomy of the masticatory musculature and its implications on the evolution of hystricomorphy in rodents.

Anomaluromorpha is a particularly puzzling suborder of Rodentia. Endemic to Africa, this clade includes the extant genera Idiurus, Anomalurus, Zenkerella, and Pedetes. These rodents present an hystricomorphous condition of the skull, characterized by a large infraorbital foramen, which evolved independently within the mouse-related clade over a span of approximately 57 million years. They exhibit a high disparity in craniomandibular and dental morphology that has kept their phylogenetic affinities disputed for a long time. Given the past significance of masticatory morphotypes in establishing the classification of Rodentia, we propose to explore variations in the masticatory apparatus of Anomaluromorpha in order to evaluate whether its related features can offer additional data for systematics and contribute to our understanding of the complexity of hystricomorphy. In order to do so, we used traditional dissection and diffusible iodine-based contrast-enhanced computed tomography (diceCT) to accurately describe and compare the anatomy of the specimens. We found that the muscle morphology displays clear differentiation among each anomaluromorph taxonomic unit. Specifically, the masseteric complex of Anomaluromorpha exhibits distinctive synapomorphies such as the infraorbital part of the zygomaticomandibularis muscle being separated into a rostral and orbital part and an absence of a posterior part of the zygomaticomandibularis. Additionally, the orbital portion of the infraorbital part originates from a well-marked ridge and fossa at the level of its area of origin on the anteromedial wall of the orbital cavity, a feature that is absent in other members of the mouse-related clade. This evident bony feature, among others, is strongly associated with muscular anatomy and can contribute to ascertaining the taxonomic status of extinct representatives of the clade. Finally, we showed that the hystricomorphy of Anomaluromorpha largely differs from those of Ctenohystrica and Dipodoidea and that the definition of this morphotype is complex and cannot be reduced simply to the size of the opening of the infraorbital foramen.

Pervasive cranial allometry at different anatomical scales and variational levels in extant armadillosAlometría craneal generalizada a diferentes escalas anatómicas y niveles de variación en armadillos actuales.

Allometry, i.e., morphological variation correlated with size, is a major pattern in organismal evolution. Since size varies both within and among species, allometry occurs at different variational levels. However, the variability of allometric patterns across levels is poorly known since its evaluation requires extensive comparative studies. Here, we implemented a 3D geometric morphometric approach to investigate cranial allometry at three main variational levels-static, ontogenetic, and evolutionary-and two anatomical scales-entire cranium and cranial subunits-based on a dense intra- and interspecific sampling of extant armadillo diversity. While allometric trajectories differ among distantly related species, they hardly do so among sister families. This suggests that phylogenetic distance plays an important role in explaining allometric divergences. Beyond trajectories, our analyses revealed pervasive allometric shape changes shared across variational levels and anatomical scales. At the entire cranial scale, craniofacial allometry (relative snout elongation and braincase reduction) is accompanied notably by variations of nuchal crests and postorbital constriction. Among cranial subunits, the distribution of allometry was highly heterogeneous, with the frontal and petrosal bones showing the most pervasive shape changes, some of which were undetected at a more global scale. Evidence of widespread and superimposed allometric variations raises questions on their determinants and anatomical correlates and demonstrates the critical role of allometry in morphological evolution.

Bayesian total-evidence dating revisits sloth phylogeny and biogeography: a cautionary tale on morphological clock analyses.

Combining morphological and molecular characters through Bayesian total-evidence dating allows inferring the phylogenetic and timescale framework of both extant and fossil taxa, while accounting for the stochasticity and incompleteness of the fossil record. Such an integrative approach is particularly needed when dealing with clades such as sloths (Mammalia: Folivora), for which developmental and biomechanical studies have shown high levels of morphological convergence whereas molecular data can only account for a limited percentage of their total species richness. Here, we propose an alternative hypothesis of sloth evolution that emphasizes the pervasiveness of morphological convergence and the importance of considering the fossil record and an adequate taxon sampling in both phylogenetic and biogeographic inferences. Regardless of different clock models and morphological datasets, the extant sloth Bradypus is consistently recovered as a megatherioid, and Choloepus as a mylodontoid, in agreement with molecular-only analyses. The recently extinct Caribbean sloths (Megalocnoidea) are found to be a monophyletic sister-clade of Megatherioidea, in contrast to previous phylogenetic hypotheses. Our results contradict previous morphological analyses and further support the polyphyly of "Megalonychidae", whose members were found in five different clades. Regardless of taxon sampling and clock models, the Caribbean colonization of sloths is compatible with the exhumation of islands along Aves Ridge and its geological time frame. Overall, our total-evidence analysis illustrates the difficulty of positioning highly incomplete fossils, although a robust phylogenetic framework was recovered by an a posteriori removal of taxa with high percentages of missing characters. Elimination of these taxa improved topological resolution by reducing polytomies and increasing node support. However, it introduced a systematic and geographic bias because most of these incomplete specimens are from northern South America. This is evident in biogeographic reconstructions, which suggest Patagonia as the area of origin of many clades when taxa are underrepresented, but Amazonia and/or Central and Southern Andes when all taxa are included. More generally, our analyses demonstrate the instability of topology and divergence time estimates when using different morphological datasets and clock models, and thus caution against making macroevolutionary inferences when node support is weak or when uncertainties in the fossil record are not considered.

Neuroanatomy and pneumaticity of the extinct Malagasy "horned" crocodile Voay robustus and its implications for crocodylid phylogeny and palaeoecology.

Voay robustus, the extinct Malagasy "horned" crocodile, was originally considered to be the only crocodylian representative in Madagascar during most part of the Holocene. However, Malagasy crocodylian remains have had confused taxonomic attributions and recent studies have underlined that Crocodylus and Voay populations coexisted on the island for at least 7500 years. Here, we describe the inner braincase anatomy of Voay robustus using x-ray computed tomography on four specimens, to provide new anatomical information that distinguishes Voay from Crocodylus, especially features of the brain endocast and the paratympanic sinuses. Geometric morphometric analyses are performed on 3D models of the internal organs to compare statistically Voay with a subset of extant Crocodylidae. Following these comparisons, we build an endocranial morphological matrix to discuss the proposed phylogenetic affinities of Voay with Osteolaeminae from an endocranial point of view. Additionally, we discuss the use of internal characters in systematic studies and find that they can have a major impact on morphological analyses. Finally, new radiocarbon data on Voay and subfossil Crocodylus specimens are recovered between 2010 and 2750 cal BP, which confirm the cohabitation of the two species in the same area for a long period of time. We thus assess several extinction scenarios, and propose a slightly different ecology of Voay compared to Crocodylus, which could have allowed habitat partitioning on the island. Our approach complements information obtained from previous molecular and morphological phylogenies, as well as previous radiocarbon dating, together revealing past diversity and faunal turnovers in Madagascar.

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.

Pedomorphosis in the ancestry of marsupial mammals.

Within mammals, different reproductive strategies (e.g., egg laying, live birth of extremely underdeveloped young, and live birth of well-developed young) have been linked to divergent evolutionary histories. How and when developmental variation across mammals arose is unclear. While egg laying is unquestionably considered the ancestral state for all mammals, many long-standing biases treat the extreme underdeveloped state of marsupial young as the ancestral state for therian mammals (clade including both marsupials and placentals), with the well-developed young of placentals often considered the derived mode of development. Here, we quantify mammalian cranial morphological development and estimate ancestral patterns of cranial shape development using geometric morphometric analysis of the largest comparative ontogenetic dataset of mammals to date (165 specimens, 22 species). We identify a conserved region of cranial morphospace for fetal specimens, after which cranial morphology diversified through ontogeny in a cone-shaped pattern. This cone-shaped pattern of development distinctively reflected the upper half of the developmental hourglass model. Moreover, cranial morphological variation was found to be significantly associated with the level of development (position on the altricial-precocial spectrum) exhibited at birth. Estimation of ancestral state allometry (size-related shape change) reconstructs marsupials as pedomorphic relative to the ancestral therian mammal. In contrast, the estimated allometries for the ancestral placental and ancestral therian were indistinguishable. Thus, from our results, we hypothesize that placental mammal cranial development most closely reflects that of the ancestral therian mammal, while marsupial cranial development represents a more derived mode of mammalian development, in stark contrast to many interpretations of mammalian evolution.

Ontogenetic variability of the intertympanic sinus distinguishes lineages within Crocodylia.

The phylogenetic relationships within crown Crocodylia remain contentious due to conflicts between molecular and morphological hypotheses. However, morphology-based datasets are mostly constructed on external characters, overlooking internal structures. Here, we use 3D geometric morphometrics to study the shape of the intertympanic sinus system in crown crocodylians during ontogeny, in order to assess its significance in a taxonomic context. Intertympanic sinus shape was found to be highly correlated with size and modulated by cranial shape during development. Still, adult sinus morphology distinguishes specimens at the family, genus and species level. We observe a clear distinction between Alligatoridae and Longirostres, a separation of different Crocodylus species and the subfossil Malagasy genus Voay, and a distinction between the Tomistoma and Gavialis lineages. Our approach is independent of molecular methods but concurs with the molecular topologies. Therefore, sinus characters could add significantly to morphological datasets, offering an alternative viewpoint to resolve problems in crocodylian relationships.

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.

Size and shape regional differentiation during the development of the spine in the nine-banded armadillo (Dasypus novemcinctus).

Xenarthrans (armadillos, anteaters, sloths, and their extinct relatives) are unique among mammals in displaying a distinctive specialization of the posterior trunk vertebrae-supernumerary vertebral xenarthrous articulations. This study seeks to understand how xenarthry develops through ontogeny and if it may be constrained to appear within pre-existing vertebral regions. Using three-dimensional geometric morphometrics on the neural arches of vertebrae, we explore phenotypic, allometric, and disparity patterns of the different axial morphotypes during the ontogeny of nine-banded armadillos. Shape-based regionalization analyses showed that the adult thoracolumbar column is divided into three regions according to the presence or absence of ribs and the presence or absence of xenarthrous articulations. A three-region division was retrieved in almost all specimens through development, although younger stages (e.g., fetuses, neonates) have more region boundary variability. In size-based regionalization analyses, thoracolumbar vertebrae are separated into two regions: a prediaphragmatic, prexenarthrous region, and a postdiaphragmatic xenarthrous region. We show that posterior thoracic vertebrae grow at a slower rate, while anterior thoracics and lumbars grow at a faster rate relatively, with rates decreasing anteroposteriorly in the former and increasing anteroposteriorly in the latter. We propose that different proportions between vertebrae and vertebral regions might result from differences in growth pattern and timing of ossification.

New Middle Eocene proboscidean from Togo illuminates the early evolution of the elephantiform-like dental pattern.

Africa has played a pivotal role in the evolution of early proboscideans (elephants and their extinct relatives), yet vast temporal and geographical zones remain uncharted on the continent. A long hiatus encompassing most of the Eocene (Ypresian to the Early Priabonian, around 13 Myr timespan) considerably hampers our understanding of the early evolutionary history of the group. It is notably the case with the origin of its most successful members, the Elephantiformes, i.e. all elephant-like proboscideans most closely related to modern elephants. Here, we describe a proboscidean lower molar discovered in Lutetian phosphate deposits from Togo, and name a new genus and species, Dagbatitherium tassyi. We show that Dagbatitherium displays several elephantiform dental characteristics such as a three-layered Schmelzmuster, the presence of a mesoconid, transversely enlarged buccal cusps and the individualization of a third lophid closely appressed to a minute distal cingulid. Dagbatitherium represents a stem Elephantiformes, pushing back the origin of the group by about 10 Myr, i.e. a third of its currently known evolutionary history. More importantly, Dagbatitherium potentially unlocks the puzzle of the origin of the unique elephantiform tooth crown organization by bridging a critical temporal and morphological gap between early bunodont incipiently bilophodont proboscidean taxa and more derived elephantiforms.

The role and impact of Zootaxa in mammalogy in its first 20 years.

Zootaxa came as a new and innovative publication medium for taxonomy, amidst a scenario of devaluation of this important biological science. After 20 years, it has ascertained itself as one of the main journals in animal taxonomy. However, the contribution of the journal to the taxonomy of Mammalia (mammals), one of the most studied groups of animals with a long-standing, dedicated spectrum of specialized journals (mammalogy), could have been expected as minor. All the current and former editors of the Mammalia section of Zootaxa analyzed the relative contribution of the journal to the description of new species of mammals since 2001. We also analyzed the contribution of Zootaxa by taxon, geographic origin of taxa, and geographic origin of first authors. The taxonomic methodology of authors in species description is described as well as the temporal trends in publications and publication subjects. We highlight the editors' picks and eventually, the challenges for the future. We found that Zootaxa has had a significant contribution to mammalogy, being the second journal (the first being Journal of Mammalogy) in terms of number of new species described (76; 10.6% of the new mammalian species described between 2001 and 2020). The majority of the new species were described following an integrative taxonomic approach with at least two sources of data (86%). The analysis of published taxa, their geographic origin, and the country of origin of first authors shows a wide coverage and exhaustive representation, except for the species from the Nearctic. We conclude that Zootaxa has likely responded to a repressed demand for an additional taxonomic journal in mammalogy, with as possible appeals the absence of publication fees and an established publication speed. With 246 articles published in the past 20 years, the Mammalia section of Zootaxa embraces a large spectrum of systematic subjects going beyond alpha taxonomy. The challenges for the future are to encourage publications of authors from the African continent, still poorly represented, and from the palaeontology community, as the journal has been open to palaeontology since its early days.

Genetic structure in Orkney island mice: isolation promotes morphological diversification.

Following human occupation, the house mouse has colonised numerous islands, exposing the species to a wide variety of environments. Such a colonisation process, involving successive founder events and bottlenecks, may either promote random evolution or facilitate adaptation, making the relative importance of adaptive and stochastic processes in insular evolution difficult to assess. Here, we jointly analyse genetic and morphometric variation in the house mice (Mus musculus domesticus) from the Orkney archipelago. Genetic analyses, based on mitochondrial DNA and microsatellites, revealed considerable genetic structure within the archipelago, suggestive of a high degree of isolation and long-lasting stability of the insular populations. Morphometric analyses, based on a quantification of the shape of the first upper molar, revealed considerable differentiation compared to Western European populations, and significant geographic structure in Orkney, largely congruent with the pattern of genetic divergence. Morphological diversification in Orkney followed a Brownian motion model of evolution, suggesting a primary role for random drift over adaptation to local environments. Substantial structuring of human populations in Orkney has recently been demonstrated, mirroring the situation found here in house mice. This synanthropic species may thus constitute a bioproxy of human structure and practices even at a very local scale.

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.

Developmental origins and homologies of the hyracoid dentition.

Understanding the origins of morphological specializations in mammals is a key goal in evolutionary biology. It can be accomplished by studying dental homology, which is at the core of most evolutionary and developmental studies. Here, we focused on the evolution and development of the specialized dentition of hyraxes for which dental homologies have long been debated, and could have implications on early placental evolution. Specifically, we analysed dental mineralization sequences of the three living genera of hyraxes and 17 fossil species using X-ray computed microtomography. Our results point out the labile position of vestigial upper teeth on jaw bones in extant species, associated with the frequently unusual premolar shape of deciduous canines over 50 Ma of hyracoid evolution. We proposed two evolutionary and developmental hypotheses to explain these original hyracoid dental characteristics. (a) The presence of a vestigial teeth on the maxilla in front of a complex deciduous canine could be interpreted as extra-teeth reminiscent of early placental evolution or sirenians, an order phylogenetically close to hyracoids and showing five premolars. (b) These vestigial teeth could also correspond to third incisors with a position unusually shifted on the maxilla, which could be explained by the dual developmental origin of these most posterior incisors and their degenerated condition. This integrative study allows discussion on the current evolutionary and developmental paradigms associated with the mammalian dentition. It also highlights the importance of nonmodel species to understand dental homologies.

Ancient Mitogenomes Reveal the Evolutionary History and Biogeography of Sloths.

Living sloths represent two distinct lineages of small-sized mammals that independently evolved arboreality from terrestrial ancestors. The six extant species are the survivors of an evolutionary radiation marked by the extinction of large terrestrial forms at the end of the Quaternary. Until now, sloth evolutionary history has mainly been reconstructed from phylogenetic analyses of morphological characters. Here, we used ancient DNA methods to successfully sequence 10 extinct sloth mitogenomes encompassing all major lineages. This includes the iconic continental ground sloths Megatherium, Megalonyx, Mylodon, and Nothrotheriops and the smaller endemic Caribbean sloths Parocnus and Acratocnus. Phylogenetic analyses identify eight distinct lineages grouped in three well-supported clades, whose interrelationships are markedly incongruent with the currently accepted morphological topology. We show that recently extinct Caribbean sloths have a single origin but comprise two highly divergent lineages that are not directly related to living two-fingered sloths, which instead group with Mylodon. Moreover, living three-fingered sloths do not represent the sister group to all other sloths but are nested within a clade of extinct ground sloths including Megatherium, Megalonyx, and Nothrotheriops. Molecular dating also reveals that the eight newly recognized sloth families all originated between 36 and 28 million years ago (mya). The early divergence of recently extinct Caribbean sloths around 35 mya is consistent with the debated GAARlandia hypothesis postulating the existence at that time of a biogeographic connection between northern South America and the Greater Antilles. This new molecular phylogeny has major implications for reinterpreting sloth morphological evolution, biogeography, and diversification history.

Morphometric models for estimating bite force in Mus and Rattus: mandible shape and size perform better than lever-arm ratios.

Morphological traits are frequently used as proxies for functional outputs such as bite force performance. This allows researchers to infer and interpret the impacts of functional variation, notably in adaptive terms. Despite their mechanical bases, the predictive power of these proxies for performance is not always tested. In particular, their accuracy at the intraspecific level is rarely assessed, and they have sometimes been shown to be unreliable. Here, we compared the performance of several morphological proxies in estimating in vivo bite force, across five species of murine rodents, at the interspecific and intraspecific levels. Proxies used included the size and shape of the mandible, as well as individual and combined muscular mechanical advantage (temporalis, superficial masseter and deep masseter). Maximum voluntary bite force was measured in all individuals included. To test the accuracy of predictions allowed by the proxies, we combined linear regressions with a leave-one-out approach, estimating an individual's bite force based on the rest of the dataset. The correlations between estimated values and the in vivo measurements were tested. At the interspecific and intraspecific levels, size and shape were better estimators than mechanical advantage. Mechanical advantage showed some predictive power at the interspecific level, but generally not within species, except for the deep masseter in Rattus In a few species, size and shape did not allow us to predict bite force. Extrapolations of performance based on mechanical advantage should therefore be used with care, and are mostly unjustified within species. In the latter case, size and shape are preferable.

Evolutionary Tinkering of the Mandibular Canal Linked to Convergent Regression of Teeth in Placental Mammals.

Loss or reduction of teeth has occurred independently in all major clades of mammals [1]. This process is associated with specialized diets, such as myrmecophagy and filter feeding [2, 3], and led to an extensive rearrangement of the mandibular anatomy. The mandibular canal enables lower jaw innervation through the passage of the inferior alveolar nerve (IAN) [4, 5]. In order to innervate teeth, the IAN projects ascending branches directly through tooth roots [5, 6], bone trabeculae [6], or bone canaliculi (i.e., dorsal canaliculi) [7]. Here, we used micro-computed tomography (µ-CT) scans of mandibles, from eight myrmecophagous species with reduced dentition and 21 non-myrmecophages, to investigate the evolutionary fate of dental innervation structures following convergent tooth regression in mammals. Our observations provide strong evidence for a link between the presence of tooth loci and the development of dorsal canaliculi. Interestingly, toothless anteaters present dorsal canaliculi and preserve intact tooth innervation, while equally toothless pangolins do not. We show that the internal mandibular morphology of anteaters has a closer resemblance to that of baleen whales [7] than to pangolins. This is despite masticatory apparatus resemblances that have made anteaters and pangolins a textbook example of convergent evolution. Our results suggest that early tooth loci innervation [8] is required for maintaining the dorsal innervation of the mandible and underlines the dorsal canaliculi sensorial role in the context of mediolateral mandibular movements. This study presents a unique example of convergent redeployment of the tooth developmental pathway to a strictly sensorial function following tooth regression in anteaters and baleen whales.

One skull to rule them all? Descriptive and comparative anatomy of the masticatory apparatus in five mouse species.

Murine rodents display a unique cranial morphology and masticatory musculature. Yet detailed myological descriptions are scarce, especially considering the great diversity of the subfamily and the use of the house mouse and brown rat as model organisms. The masticatory musculature in these two species has been thoroughly described, which allows comparisons with other wild species. Description and comparison of a wide range of species constitutes a necessary step to fully understand how ecological factors may influence the morphology and myology of the skull in the Murinae and vice versa. In this study, we describe the masticatory musculature of five mouse species: Mus caroli, Mus cervicolor, Mus pahari, Mus fragilicauda, and Mus minutoides. For each species, one to five specimens were dissected, and their muscle weights and volumes calculated. One specimen was selected for iodine-enhanced CT-scanning, which allowed us to digitally reconstruct the musculature. We then compared the different masticatory arrangements between these species, as well as with the previous descriptions of the house mouse and brown rat. We show that interspecific differences especially involved the temporalis muscle and zygomatico-mandibularis muscular groups, although some differences were also seen in the pterygoid muscle and masseter muscle groups. We then propose some ecological interpretations for these differences, by interpreting them in terms of functional differences.

Phylogenetic and functional implications of the ear region anatomy of Glossotherium robustum (Xenarthra, Mylodontidae) from the Late Pleistocene of Argentina.

Several detailed studies of the external morphology of the ear region in extinct sloths have been published in the past few decades, and this anatomical region has proved extremely helpful in elucidating the phylogenetic relationships among the members of this mammalian clade. Few studies of the inner ear anatomy in these peculiar animals were conducted historically, but these are increasing in number in recent years, in both the extinct and extant representatives, due to wider access to CT-scanning facilities, which allow non-destructive access to internal morphologies. In the present study, we analyze the extinct ground sloth Glossotherium robustum and provide a description of the external features of the ear region and the endocranial side of the petrosal bone, coupled with the first data on the anatomy of the bony labyrinth. Some features observable in the ear region of G. robustum (e.g., the shape and size of the entotympanic bone and the morphology of the posteromedial surface of the petrosal) are highly variable, both intraspecifically and intraindividually. The form of the bony labyrinth of G. robustum is also described, providing the first data from this anatomical region for the family Mylodontidae. The anatomy of the bony labyrinth of the genus Glossotherium is here compared at the level of the superorder Xenarthra, including all available extant and extinct representatives, using geometric morphometric methods. In light of the new data, we discuss the evolution of inner ear anatomy in the xenarthran clade, and most particularly in sloths, considering the influence of phylogeny, allometry, and physiology on the shape of this highly informative region of the skull. These analyses show that the inner ear of Glossotherium more closely resembles that of the extant anteaters, and to a lesser extent those of the giant ground sloth Megatherium and euphractine armadillos, than those of the extant sloths Bradypus and Choloepus, further demonstrating the striking morphological convergence between the two extant sloth genera.

Skull Size and Biomechanics are Good Estimators of In Vivo Bite Force in Murid Rodents.

Rodentia is a species-rich group with diversified modes of life and diets. Although rodent skull morphology has been the focus of a voluminous literature, the functional significance of its variations has yet to be explored in live animals. Myomorphous rodents, including murids, have been suggested to represent "high-performance generalists." We measured in vivo bite force in 14 species of wild and lab-reared murid rodents of various sizes and diets to investigate potential morphofunctional differences between them. We dissected their skulls and computed a biomechanical model to estimate bite force. We first tested if our model allowed good estimation of in vivo data. Then, using morphological, in vivo and estimated bite force data in a phylogenetic context, we aimed to find the drivers of bite force differences among species. Estimated and in vivo bite forces were strongly correlated, which indicates that (a) biomechanical models allow a good estimation of real performance, and that (b) size and muscular changes (increased mass, fiber length, and PCSA) are the main drivers of bite performance differences. Myomorphous rodents, therefore, may have evolved high bite force through a combination of changes in size and musculature, which gave them a great versatility in their ability to process food. We found mixed results at the intraspecific level, with only some species displaying a good fit between estimated and in vivo measurements. We suggest that limited variation in size and muscular organization, and increased behavioral variation might decrease the precision of bite force estimates within species. Anat Rec, 301:256-266, 2018. © 2018 Wiley Periodicals, Inc.