Evolution of the Middle and Inner Ears of Mammaliaforms: The Approach to Mammals

Is cranial anatomy indicative of fossoriality? A case study of the mammaliaform Hadrocodium wui

Determining the ecology of fossil species presents considerable challenges due to the often fragmentary preservation of specimens. The mammaliaform Hadrocodium wui from the Jurassic of China is known only from the cranium and mandible but may have had a fossorial lifestyle. It shares morphological similarities with talpid moles and soricid shrews and is closely related to other fossorial mammaliaforms. However, the lack of postcranial elements has so far precluded a definitive assessment regarding its fossorial behavior. Using a combination of geometric morphometric analysis of the lower mandible and finite element analyses of the cranium, comparisons between H. wui and extant groups are made. H. wui resembles talpid moles more closely than shrews in mandible shape. However, there are more similarities between H. wui and semi-fossorial/semi-aquatic moles than fully fossorial moles. The skull of H. wui is particularly weak in every tested biomechanical scenario when compared to the related mammaliaform Morganucodon oehleri and talpid moles. This weakness is potentially a result of the reduction in thickness of the zygomatic arch. In contrast, the shrew crania show similar stress magnitudes and distribution. These results imply that H. wui was not fully fossorial and fed on a diet of softer invertebrates. Skull morphology may therefore not be sufficient to infer fossoriality in forelimb-digging species.

Brazilian fossils reveal homoplasy in the oldest mammalian jaw joint

The acquisition of the load-bearing dentary-squamosal jaw joint was a key step in mammalian evolution1-5. Although this innovation has received decades of study, questions remain over when and how frequently a mammalian-like skull-jaw contact evolved, hindered by a paucity of three-dimensional data spanning the non-mammaliaform cynodont-mammaliaform transition. New discoveries of derived non-mammaliaform probainognathian cynodonts from South America have much to offer to this discussion. Here, to address this issue, we used micro-computed-tomography scanning to reconstruct the jaw joint anatomy of three key probainognathian cynodonts: Brasilodon quadrangularis, the sister taxon to Mammaliaformes6-8, the tritheledontid-related Riograndia guaibensis9 and the tritylodontid Oligokyphus major. We find homoplastic evolution in the jaw joint in the approach to mammaliaforms, with ictidosaurs (Riograndia plus tritheledontids) independently evolving a dentary-squamosal contact approximately 17 million years before this character first appears in mammaliaforms of the Late Triassic period10-12. Brasilodon, contrary to previous descriptions6-8, lacks an incipient dentary condyle and squamosal glenoid and the jaws articulate solely using a plesiomorphic quadrate-articular joint. We postulate that the jaw joint underwent marked evolutionary changes in probainognathian cynodonts. Some probainognathian clades independently acquired 'double' craniomandibular contacts, with mammaliaforms attaining a fully independent dentary-squamosal articulation with a conspicuous dentary condyle and squamosal glenoid in the Late Triassic. The dentary-squamosal contact, which is traditionally considered to be a typical mammalian feature, therefore evolved more than once and is more evolutionary labile than previously considered.

Ontogeny of the malleus in Mesocricetus auratus (Mammalia, Rodentia): Systematic and functional implications for the muroid middle ear

The three mammalian auditory ossicles enhance sound transmission from the tympanic membrane to the inner ear. The anterior anchoring of the malleus is one of the key characters for functional classification of the auditory ossicles. Previous studies revealed a medial outgrowth of the mallear anterior process, the processus internus praearticularis, which serves as an anchor for the auditory ossicle chain but has been often missed due to its delicate nature. Here we describe the development and morphology of the malleus and its processus internus praearticularis in the cricetine rodent Mesocricetus auratus, compared to selected muroid species (Cricetus cricetus, Peromyscus maniculatus, and Mus musculus). Early postnatal stages of Mesocricetus show the formation of the malleus by fusion of the prearticular and mallear main body. The processus internus praearticularis forms an increasing broad lamina fused anteriorly to the ectotympanic in adult stages of all studied species. Peromyscus and Mus show a distinct orbicular apophysis that increases inertia of the malleus and therefore these species represent the microtype of auditory ossicles. In contrast, the center of mass of the malleus in the studied Cricetinae is close to the anatomical axis of rotation and their auditory ossicles represent the transitional type. The microtype belongs to the grundplan of Muroidea and is plesiomorphic for Cricetidae, whereas the transitional type evolved several times within Muroidea and represents an apomorphic feature of Cricetinae.

A morphometric comparison of the ductus reuniens in humans and guinea pigs, with a note on its evolutionary importance

The mammalian inner ear contains the sensory organs responsible for balance (semicircular canals, utricle, and saccule) and hearing (cochlea). While these organs are functionally distinct, there exists a critical structural connection between the two: the ductus reuniens (DR). Despite its functional importance, comparative descriptions of DR morphology are limited, hindering our understanding of the evolutionary diversification of hearing and balance systems among mammals. Using virtual 3D models derived from micro-CT, we examine the morphology of the DR and its relationship to the bony labyrinth in humans compared to that in a commonly used animal model, the guinea pig. Anatomical reconstructions and univariate measurements were carried out in the software 3D Slicer. Data indicate similarities in DR morphology between humans and guinea pigs in terms of overall shape. However, there are considerable differences in relative DR length and width between humans and guinea pigs. Humans possess a relatively shorter and narrower DR but with wider openings to the saccule and cochlear duct. This results in a relatively more constricted DR lumen in humans which may differentially limit fluid transfer between the saccule and cochlea. Our results reveal previously hidden morphological diversity in the communication between the hearing and balance systems of the mammalian inner ear which may indicate alternative strategies for isolating the Organ of Corti from the peripheral vestibular system throughout mammalian evolution.

Early synapsids neurosensory diversity revealed by CT and synchrotron scanning

Non-mammaliaform synapsids (NMS) represent the closest relatives of today's mammals among the early amniotes. Exploring their brain and nervous system is key to understanding how mammals evolved. Here, using CT and Synchrotron scanning, we document for the first time three extreme cases of neurosensory and behavioral adaptations that probe into the wide range of unexpected NMS paleoneurological diversity. First, we describe adaptations to low-frequency hearing and low-light conditions in the non-mammalian cynodont Cistecynodon parvus, supporting adaptations to an obligatory fossorial lifestyle. Second, we describe the uniquely complex and three-dimensional maxillary canal morphology of the biarmosuchian Pachydectes elsi, which suggests that it may have used its cranial bosses for display or low-energy combat. Finally, we introduce a paleopathology found in the skull of Moschognathus whaitsi. Since the specimen was not fully grown, this condition suggests the possibility that this species might have engaged in playful fighting as juveniles-a behavior that is both social and structured. Additionally, this paper discusses other evidence that could indicate that tapinocephalid dinocephalians were social animals, living and interacting closely with one another. Altogether, these examples evidence the wide range of diversity of neurological structures and complex behavior in NMS.

Endocranial anatomy of the early prozostrodonts (Eucynodontia: Probainognathia) and the neurosensory evolution in mammal forerunners

Prozostrodon brasiliensis and Therioherpeton cargnini are non-mammaliaform cynodonts that lived ~233 million years ago (late Carnian, Late Triassic) in western Gondwana. They represent some of the earliest divergent members of the clade Prozostrodontia, which includes "tritheledontids", tritylodontids, "brasilodontids", and mammaliaforms (including Mammalia as crown group). Here, we studied the endocranial anatomy (cranial endocast, nerves, vessels, ducts, ear region, and nasal cavity) of these two species. Our findings suggest that during the Carnian, early prozostrodonts had a brain with well-developed olfactory bulbs, expanded cerebral hemispheres divided by the interhemispheric sulcus, and absence of an unossified zone and pineal body. The morphology of the maxillary canal represents the necessary condition for the presence of facial vibrissae. A slight decrease in encephalization is observed at the origin of the clade Prozostrodontia. This new anatomical information provides evidence for the evolution of endocranial traits of the first prozotrodonts, a Late Triassic lineage that culminated in the origin of mammals.

Middle ear innovation in Early Cretaceous eutherian mammals

The middle ear ossicles in modern mammals are repurposed from postdentary bones in non-mammalian cynodonts. Recent discoveries by palaeontological and embryonic studies have developed different models for the middle ear evolution in mammaliaforms. However, little is known about the evolutionary scenario of the middle ear in early therians. Here we report a detached middle ear preserved in a new eutherian mammal from the Early Cretaceous Jehol Biota. The well-preserved articulation of the malleus and incus suggest that the saddle-shaped incudomallear joint is a major apomorphy of Early Cretaceous eutherians. By contrast to the distinct saddle-like incudomallear articulation in therians, differences between the overlapping versus the half-overlapping incudomallear joints in monotremes and stem mammals would be relatively minor. The middle ear belongs to the microtype by definition, indicating its adaptation to high-frequency hearing. Current evidence indicates that significant evolutionary innovations of the middle ear in modern therians evolved in Early Cretaceous.

A switch in jaw form-function coupling during the evolution of mammals

The evolutionary shift from a single-element ear, multi-element jaw to a multi-element ear, single-element jaw during the transition to crown mammals marks one of the most dramatic structural transformations in vertebrates. Research on this transformation has focused on mammalian middle-ear evolution, but a mandible comprising only the dentary is equally emblematic of this evolutionary radiation. Here, we show that the remarkably diverse jaw shapes of crown mammals are coupled with surprisingly stereotyped jaw stiffness. This strength-based morphofunctional regime has a genetic basis and allowed mammalian jaws to effectively resist deformation as they radiated into highly disparate forms with markedly distinct diets. The main functional consequences for the mandible of decoupling hearing and mastication were a trade-off between higher jaw stiffness versus decreased mechanical efficiency and speed compared with non-mammals. This fundamental and consequential shift in jaw form-function underpins the ecological and taxonomic diversification of crown mammals. This article is part of the theme issue 'The mammalian skull: development, structure and function'.

Craniodental anatomy in Permian-Jurassic Cynodontia and Mammaliaformes (Synapsida, Therapsida) as a gateway to defining mammalian soft tissue and behavioural traits

Mammals are diagnosed by more than 30 osteological characters (e.g. squamosal-dentary jaw joint, three inner ear ossicles, etc.) that are readily preserved in the fossil record. However, it is the suite of physiological, soft tissue and behavioural characters (e.g. endothermy, hair, lactation, isocortex and parental care), the evolutionary origins of which have eluded scholars for decades, that most prominently distinguishes living mammals from other amniotes. Here, we review recent works that illustrate how evolutionary changes concentrated in the cranial and dental morphology of mammalian ancestors, the Permian-Jurassic Cynodontia and Mammaliaformes, can potentially be used to document the origin of some of the most crucial defining features of mammals. We discuss how these soft tissue and behavioural traits are highly integrated, and how their evolution is intermingled with that of craniodental traits, thus enabling the tracing of their previously out-of-reach phylogenetic history. Most of these osteological and dental proxies, such as the maxillary canal, bony labyrinth and dental replacement only recently became more easily accessible-thanks, in large part, to the widespread use of X-ray microtomography scanning in palaeontology-because they are linked to internal cranial characters. This article is part of the theme issue 'The mammalian skull: development, structure and function'.

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

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

Functional reorganisation of the cranial skeleton during the cynodont-mammaliaform transition

Skeletal simplification occurred in multiple vertebrate clades over the last 500 million years, including the evolution from premammalian cynodonts to mammals. This transition is characterised by the loss and reduction of cranial bones, the emergence of a novel jaw joint, and the rearrangement of the jaw musculature. These modifications have long been hypothesised to increase skull strength and efficiency during feeding. Here, we combine digital reconstruction and biomechanical modelling to show that there is no evidence for an increase in cranial strength and biomechanical performance. Our analyses demonstrate the selective functional reorganisation of the cranial skeleton, leading to reduced stresses in the braincase and the skull roof but increased stresses in the zygomatic region through this transition. This cranial functional reorganisation, reduction in mechanical advantage, and overall miniaturisation in body size are linked with a dietary specialisation to insectivory, permitting the subsequent morphological and ecological diversification of the mammalian lineage.

Insights into Inner Ear Function and Disease Through Novel Visualization of the Ductus Reuniens, a Seminal Communication Between Hearing and Balance Mechanisms

The sensory end-organs responsible for hearing and balance in the mammalian inner ear are connected via a small membranous duct known as the ductus reuniens (also known as the reuniting duct (DR)). The DR serves as a vital nexus linking the hearing and balance systems by providing the only endolymphatic connection between the cochlea and vestibular labyrinth. Recent studies have hypothesized new roles of the DR in inner ear function and disease, but a lack of knowledge regarding its 3D morphology and spatial configuration precludes testing of such hypotheses. We reconstructed the 3D morphology of the DR and surrounding anatomy using osmium tetroxide micro-computed tomography and digital visualizations of three human inner ear specimens. This provides a detailed, quantitative description of the DR's morphology, spatial relationships to surrounding structures, and an estimation of its orientation relative to head position. Univariate measurements of the DR, inner ear, and cranial planes were taken using the software packages 3D Slicer and Zbrush. The DR forms a narrow, curved, flattened tube varying in lumen size, shape, and wall thickness, with its middle third being the narrowest. The DR runs in a shallow bony sulcus superior to the osseus spiral lamina and adjacent to a ridge of bone that we term the "crista reuniens" oriented posteromedially within the cranium. The DR's morphology and structural configuration relative to surrounding anatomy has important implications for understanding aspects of inner ear function and disease, particularly after surgical alteration of the labyrinth and potential causative factors for Ménière's disease.

Neurosensory anatomy of Varanopidae and its implications for early synapsid evolution

Varanopids are a group of Palaeozoic terrestrial amniotes which represent one of the earliest-diverging groups of synapsids, but their palaeoneurology has gone largely unstudied and recent analyses have challenged their traditional placement within synapsids. We utilized computed tomography (CT) to study the virtual cranial and otic endocasts of six varanopids, including representative taxa of both mycterosaurines and varanodontines. Our results show that the varanopid brain is largely plesiomorphic, being tubular in shape and showing no expansion of the cerebrum or olfactory bulbs, but is distinct in showing highly expanded floccular fossae. The housing of the varanopid bony labyrinth is also distinct, in that the labyrinth is bounded almost entirely by the supraoccipital-opisthotic complex, with the prootic only bordering the ventral portion of the vestibule. The bony labyrinth is surprisingly well-ossified, clearly preserving the elliptical, sub-orthogonal canals, prominent ampullae, and the short, undifferentiated vestibule; this high degree of ossification is similar to that seen in therapsid synapsids and supports the traditional placement of varanopids within Synapsida. The enlarged anterior canal, together with the elliptical, orthogonal canals and enlarged floccular fossa, lend support for the fast head movements indicated by the inferred predatory feeding mode of varanopids. Reconstructed neurosensory anatomy indicates that varanopids may have a much lower-frequency hearing range compared to more derived synapsids, suggesting that, despite gaining some active predatory features, varanopids retain plesiomorphic hearing capabilities. As a whole, our data reveal that the neuroanatomy of pelycosaur-grade synapsids is far more complex than previously anticipated.

New Skull Material of Taeniolabis taoensis (Multituberculata, Taeniolabididae) from the Early Paleocene (Danian) of the Denver Basin, Colorado

Taeniolabis taoensis is an iconic multituberculate mammal of early Paleocene (Puercan 3) age from the Western Interior of North America. Here we report the discovery of significant new skull material (one nearly complete cranium, two partial crania, one nearly complete dentary) of T. taoensis in phosphatic concretions from the Corral Bluffs study area, Denver Formation (Danian portion), Denver Basin, Colorado. The new skull material provides the first record of the species from the Denver Basin, where the lowest in situ specimen occurs in river channel deposits ~730,000 years after the Cretaceous-Paleogene boundary, roughly coincident with the first appearance of legumes in the basin. The new material, in combination with several previously described and undescribed specimens from the Nacimiento Formation of the San Juan Basin, New Mexico, is the subject of detailed anatomical study, aided by micro-computed tomography. Our analyses reveal many previously unknown aspects of skull anatomy. Several regions (e.g., anterior portions of premaxilla, orbit, cranial roof, occiput) preserved in the Corral Bluffs specimens allow considerable revision of previous reconstructions of the external cranial morphology of T. taoensis. Similarly, anatomical details of the ascending process of the dentary are altered in light of the new material. Although details of internal cranial anatomy (e.g., nasal and endocranial cavities) are difficult to discern in the available specimens, we provide, based on UCMP 98083 and DMNH.EPV 95284, the best evidence to date for inner ear structure in a taeniolabidoid multituberculate. The cochlear canal of T. taoensis is elongate and gently curved and the vestibule is enlarged, although to a lesser degree than in Lambdopsalis.

Jurassic mammaliaform petrosals from Western Siberia (Russia) and implications for early mammalian inner-ear anatomy

Five partially preserved mammaliaform petrosals recovered from Middle Jurassic sediments of the Berezovsk coal mine (Krasnoyarsk Krai, Russia) show similarities to other early mammaliaforms like the morganucodontan Morganucodon and the docodontan Haldanodon in having an inflated promontorium and a curved and apically inflated cochlear canal, but they are distinct from dryolestoid and derived mammalian petrosals by the weak coiling of the cochlear duct and the presence of a perilymphatic foramen with an open perilymphatic sulcus. The two larger and robust specimens exhibit striking similarities to docodontan petrosals. Inside the bone an intricate circumpromontorial venous plexus was discovered, as recently described for the docodontan Borealestes, confirming that this structure is consistently present in basal non-mammalian mammaliaforms. The three smaller and slender petrosals probably belong to haramiyidans and are unique in showing a septum-like structure medially along the cochlear nerve entrance. The protruding perforated bony bar, which is preserved in two of the three, is interpreted here to be a remnant of a bony septum with multiple foramina for cochlear nerve fibres, representing an autapomorphic feature of Haramiyida. This newly described passageway for nerve fibres shows that the formation of the osteological structure surrounding the nervous pathways of the cochlea is more plastic among the non-mammalian mammaliaforms than previously thought.

The evolution of anteriorly directed molar occlusion in mammals

In non-mammalian synapsids and early mammals, evolutionary transformations in the feeding and hearing apparatuses are posited to have been prerequisites for the radiation of extant mammals. Unlike most vertebrates, including many early synapsids, mammals have precise dental occlusion, a lower jaw composed of one bone, and middle ear ossicles derived from ancestral jaw bones. We illuminate a related functional transition: therian mammals (eutherians and metatherians) evolved anteriorly directed chewing strokes, which are absent in other synapsid lineages. Anteriorly directed jaw movement during occlusion necessitates anteriorly directed muscle force vectors, and we posit that a shift in muscle orientation is reflected in the fossil record by the evolutionary appearance of a posteriorly positioned angular process in cladotherians (therians and their close kin). Anteriorly directed occlusion might have been absent in earlier synapsids because of the presence of attached middle ear elements in the posterior region of the jaw that prohibited the posterior insertion of jaw musculature. These changes to the masticatory apparatus in cladotherians are likely to have permitted the evolution of novel masticatory movements, including grinding in both the anterior and medial directions (e.g. rodents and ungulates, respectively). Thus, this evolutionary transition might have been a crucial prerequisite for the dietary diversification of therians.

Evolution of vision and hearing modalities in theropod dinosaurs

Owls and nightbirds are nocturnal hunters of active prey that combine visual and hearing adaptations to overcome limits on sensory performance in low light. Such sensory innovations are unknown in nonavialan theropod dinosaurs and are poorly characterized on the line that leads to birds. We investigate morphofunctional proxies of vision and hearing in living and extinct theropods and demonstrate deep evolutionary divergences of sensory modalities. Nocturnal predation evolved early in the nonavialan lineage Alvarezsauroidea, signaled by extreme low-light vision and increases in hearing sensitivity. The Late Cretaceous alvarezsauroid Shuvuuia deserti had even further specialized hearing acuity, rivaling that of today's barn owl. This combination of sensory adaptations evolved independently in dinosaurs long before the modern bird radiation and provides a notable example of convergence between dinosaurs and mammals.

A comparative study on auditory and hyoid bones of Jurassic euharamiyidans and contrasting evidence for mammalian middle ear evolution

The holotypes of euharamiyidan Arboroharamiya allinhopsoni and Arboroharamiya jenkinsi preserve the auditory and hyoid bones, respectively. With additional structures revealed by micro-computerized tomography (CT) and X-ray micro-computed laminography (CL), we provide a detailed description of these minuscule bones. The stapes in the two species of Arboroharamiya are similar in having a strong process for insertion of the stapedius muscle. The incus is similar in having an almond-shaped body and a slim short process, in addition to a robust stapedial process with a short lenticular process preserved in A. allinhopsoni. The plate-like ectotympanic in the two species of Arboroharamiya is similar and comparable to that of Qishou jizantang. The surangular in the two species has a fan-shaped body and a needle-shaped anterior process. The malleus, ectotympanic, and surangular are fully detached from the dentary and should have functioned exclusively for hearing. All the auditory bones of Arboroharamiya display unique features unknown in other mammaliaforms. Moreover, hyoid elements are found in the two species of Arboroharamiya and co-exist with the five auditory bones in the holotype of A. allinhopsoni. The element interpreted as the stylohyal is similar to the bone identified as the ectotympanic in Vilevolodon. We reconstruct the auditory apparatus of Arboroharamiya and compare it with that of Vilevolodon as well as those in extant mammals and basal mammaliaforms. The comparison shows diverse morphological patterns of the auditory region in mammaliaforms. In particular, those of Vilevolodon and Arboroharamiya differ significantly: the former has a mandibular middle ear, whereas the latter possesses a definitive mammalian middle ear. It is puzzling that the two sympatric and dentally similar taxa have such different auditory apparatuses. In light of the available evidence, we argue that the mandibular middle ear reconstructed in Vilevolodon encounters many problems, and the so-called ectotympanic in Vilevolodon may be interpreted as a stylohyal; thus, the dilemma can be resolved.

Comparative analysis of the shape and size of the middle ear cavity of turtles reveals no correlation with habitat ecology

The middle ear of turtles differs from other reptiles in being separated into two distinct compartments. Several ideas have been proposed as to why the middle ear is compartmentalized in turtles, most suggesting a relationship with underwater hearing. Extant turtle species span fully marine to strictly terrestrial habitats, and ecomorphological hypotheses of turtle hearing predict that this should correlate with variation in the structure of the middle ear due to differences in the fluid properties of water and air. We investigate the shape and size of the air-filled middle ear cavity of 56 extant turtles using 3D data and phylogenetic comparative analysis to test for correlations between habitat preferences and the shape and size of the middle ear cavity. Only weak correlations are found between middle ear cavity size and ecology, with aquatic taxa having proportionally smaller cavity volumes. The middle ear cavity of turtles exhibits high shape diversity among species, but we found no relationship between this shape variation and ecology. Surprisingly, the estimated acoustic transformer ratio, a key functional parameter of impedance-matching ears in vertebrates, also shows no relation to habitat preferences (aquatic/terrestrial) in turtles. We suggest that middle ear cavity shape may be controlled by factors unrelated to hearing, such as the spatial demands of surrounding cranial structures. A review of the fossil record suggests that the modern turtle ear evolved during the Early to Middle Jurassic in stem turtles broadly adapted to freshwater and terrestrial settings. This, combined with our finding that evolutionary transitions between habitats caused only weak evolutionary changes in middle ear structure, suggests that tympanic hearing in turtles evolved as a compromise between subaerial and underwater hearing.

The vertebrate middle and inner ear: A short overview

The evolution of the various hearing adaptations is connected to major structural changes in nearly all groups of vertebrates. Besides hearing, the detection of acceleration and orientation in space are key functions of this mechanosensory system. The symposium "show me your ear - the inner and middle ear in vertebrates" held at the 11th International Congress of Vertebrate Morphology (ICVM) 2016 in Washington, DC (USA) intended to present current research addressing adaptation and evolution of the vertebrate otic region, auditory ossicles, vestibular system, and hearing physiology. The symposium aimed at an audience with interest in hearing research focusing on morphological, functional, and comparative studies. The presented talks and posters lead to the contributions of this virtual issue highlighting recent advances in the vertebrate balance and hearing system. This article serves as an introduction to the virtual issue contributions and intends to give a short overview of research papers focusing on vertebrate labyrinth and middle ear related structures in past and recent years.

A Functional Perspective on the Evolution of the Cochlea

This review summarizes paleontological data as well as studies on the morphology, function, and molecular evolution of the cochlea of living mammals (monotremes, marsupials, and placentals). The most parsimonious scenario is an early evolution of the characteristic organ of Corti, with inner and outer hair cells and nascent electromotility. Most remaining unique features, such as loss of the lagenar macula, coiling of the cochlea, and bony laminae supporting the basilar membrane, arose later, after the separation of the monotreme lineage, but before marsupial and placental mammals diverged. The question of when hearing sensitivity first extended into the ultrasonic range (defined here as >20 kHz) remains speculative, not least because of the late appearance of the definitive mammalian middle ear. The last significant change was optimizing the operating voltage range of prestin, and thus the efficiency of the outer hair cells' amplifying action, in the placental lineage only.

Disconnecting bones within the jaw-otic network modules underlies mammalian middle ear evolution

The origin of the mammalian middle ear ossicles from the craniomandibular articulation of their synapsid ancestors is a key event in the evolution of vertebrates. The richness of the fossil record and the multitude of developmental studies have provided a stepwise reconstruction of this evolutionary innovation, highlighting the homology between the quadrate, articular, pre-articular and angular bones of early synapsids with the incus, malleus, gonial and ectotympanic bones of derived mammals, respectively. There are several aspects involved in this functional exaptation: (i) an increase of the masticatory musculature; (ii) the separation of the quadrate bone from the cranium; and (iii) the disconnection of the post-dentary bones from the dentary. Here, we compared the jaw-otic complex for 43 synapsid taxa using anatomical network analysis, showing that the disconnection of mandibular bones was a key step in the mammalian middle ear evolution, changing the skull anatomical modularity concomitant to the acquisition of new functions. Furthermore, our analysis allows the identification of three types of anatomical modules evolving through five evolutionary stages during the anatomical transformation of the jawbones into middle ear bones, with the ossification and degradation of Meckel's cartilage in mammals as the key ontogenetic event leading the change of anatomical modularity.

Oldest known multituberculate stapes suggests an asymmetric bicrural pattern as ancestral for Multituberculata

Middle ear ossicles (malleus, incus, stapes) are known for few multituberculate taxa, and three different stapedial morphotypes have been suggested: (i) slender, columelliform and microperforate, (ii) robust and rod-like, and (iii) bicrural. Reinvestigation of Upper Jurassic (Kimmeridgian) mammalian petrosals from the Guimarota coal mine in central Portugal (Western Europe) revealed an asymmetric bicrural stapes (ABS) in the paulchoffatiid Pseudobolodon oreas The middle ear ossicles displaced inside the osseous vestibule were detected by a µCT analysis. The Kimmeridgian age of the Guimarota stapes exceeds the stapes from the Early Cretaceous (Barremian) of Asia (about 122-124 Ma) by approximately 30 Myr, and is only slightly younger than the stapes of the recently described Oxfordian euharamiyidan Arboroharamiya allinhopsoni The Guimarota stapes indicates that the stapes of Lambdopsalis, described as columelliform and microperforate (small stapedial foramen), does not represent a general condition for multituberculates. The stapes of Pseudobolodon is bicrural, the anterior crus sits centrally on the oval footplate, and the stapedial head is simple and smaller than the footplate. We hypothesize that the ABS evolved from the symmetric bicrural stapes (SBS) of non-mammaliaform cynodonts. The ABS appears to be the ancestral morphotype of the mammalian SBS, and the mammalian columelliform imperforate stapes.

Synchrotron scanning reveals the palaeoneurology of the head-butting Moschops capensis (Therapsida, Dinocephalia)

Dinocephalian therapsids are renowned for their massive, pachyostotic and ornamented skulls adapted for head-to-head fighting during intraspecific combat. Synchrotron scanning of the tapinocephalid Moschops capensis reveals, for the first time, numerous anatomical adaptations of the central nervous system related to this combative behaviour. Many neural structures (such as the brain, inner ear and ophthalmic branch of the trigeminal nerve) were completely enclosed and protected by bones, which is unusual for non-mammaliaform therapsids. The nearly complete ossification of the braincase enables precise determination of the brain cavity volume and encephalization quotient, which appears greater than expected for such a large and early herbivore. The practice of head butting is often associated with complex social behaviours and gregariousness in extant species, which are known to influence brain size evolution. Additionally, the plane of the lateral (horizontal) semicircular canal of the bony labyrinth is oriented nearly vertically if the skull is held horizontally, which suggests that the natural position of the head was inclined about 60–65°to the horizontal. This is consistent with the fighting position inferred from osteology, as well as ground-level browsing. Finally, the unusually large parietal tube may have been filled with thick conjunctive tissue to protect the delicate pineal eye from injury sustained during head butting.