Evolution of the middle ear apparatus in Talpid moles

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.

Variations in cochlea shape reveal different evolutionary adaptations in primates and rodents

The presence of a coiled cochlea is a unique feature of the therian inner ear. While some aspects of the cochlea are already known to affect hearing capacities, the full extent of the relationships between the morphology and function of this organ are not yet understood-especially when the effect of body size differences between species is minimized. Here, focusing on Euarchontoglires, we explore cochlear morphology of 33 species of therian mammals with a restricted body size range. Using μCT scans, 3D models and 3D geometric morphometrics, we obtained shape information of the cochlea and used it to build phylogenetically corrected least square models with 12 hearing variables obtained from the literature. Our results reveal that different taxonomic groups differ significantly in cochlea shape. We further show that these shape differences are related to differences in hearing capacities between these groups, despite of similar cochlear lengths. Most strikingly, rodents with good low-frequency hearing display "tower-shaped" cochleae, achieved by increasing the degree of coiling of their cochlea. In contrast, primates present relatively wider cochleae and relative better high frequency hearing. These results suggest that primates and rodents increased their cochlea lengths through different morpho-evolutionary trajectories.

Comparison and Phylogenetic Analysis of Mitochondrial Genomes of Talpidae Animals

Talpidae is a model group for evolutionary studies due to their highly specialized morphologies and diverse lifestyles. Mitochondrial genomes are molecular markers commonly used in species evolution and phylogenetic studies. In this study, the complete mitochondrial genome sequence of Scaptochirus moschatus was obtained by Illumina NovaSeq sequencing. The complete mitochondrial genomes of 14 Talpidae species (including Scaptochirus moschatus obtained in the present study) and the cytochrome b (Cyt b) gene sequences of 48 Talpidae species were downloaded from the NCBI database for comparison and phylogenetic studies to analyze the phylogenetic relationships and to find the possible reasons of the niche differentiation and ecotype specialization of Talpidae animals. The results showed that the mitochondrial genome sequences of 14 species belonging to the family Talpidae were 16,528 to 16,962 bp, all containing 13 protein-coding genes, 22 tRNA, two rRNA, and a non-coding region (control region). The difference in the number of repetitive repeats in the control region is responsible for the difference in the length of Talpidae mitochondrial genome sequences. Combining the divergence time of Talpidae animals with the geological history, it is found that the niche differentiation and ecotype divergence of Talpidae is closely related to historically global climate changes. Semi-aquatic groups diverged in the early Oligocene (about 31.22 MYA), probably in response to the global climate transition from warm to cool. During the early Miocene (about 19.54 MYA), some species of Talpidae moved to underground habitats and formed fossorial groups that were adept at digging due to the effects of the glaciation. In the middle Miocene (about 16.23 MYA), some Talpidae animals returned to the ground and formed semi-fossorial shrew moles as global climate warming again.

Hearing in African pygmy hedgehogs (Atelerix albiventris): audiogram, sound localization, and ear anatomy

The behavioral audiogram and sound localization performance, together with the middle and inner ear anatomy, were examined in African pygmy hedgehogs Atelerix albiventris. Their auditory sensitivity at 60 dB SPL extended from 2 to 46 kHz, revealing a relatively narrow hearing range of 4.6 octaves, with a best sensitivity of 21 dB at 8 kHz. Their noise-localization acuity around the midline (minimum audible angle) was 14°, matching the mean of terrestrial mammals. The African pygmy hedgehog was not able to localize low-frequency pure tones or a 3-kHz amplitude-modulated tone when forced to rely on the interaural phase-difference cue, a trait shared by at least nine other mammals. The middle ear of Atelerix has a configuration including an ectotympanic which is not fused to the surrounding bones, a substantial pars flaccida, a synostosed malleo-ectotympanic articulation and a 'microtype' malleus. The hearing and sound localization of A. albiventris is compared to that of a broad range of other mammals. It is shown that a malleus morphology like that of Atelerix, including a stiff articulation with the ectotympanic, is a consistent feature of other mammals that do not hear frequencies below 400 Hz.

Evoked auditory potentials from African mole-rats and coruros reveal disparity in subterranean rodent hearing

Hearing in subterranean rodents exhibits numerous peculiarities, including low sensitivity and restriction to a narrow range of comparatively low frequencies. Past studies provided two conflicting hypotheses explaining how these derived traits evolved: structural degeneration and adaptive specialization. To further elucidate this issue, we recorded auditory brainstem responses from three species of social subterranean rodents that differ in the degree of specialization to the underground habitat: the naked mole-rat (Heterocephalus glaber) and the Mashona mole-rat (Fukomys darlingi), which represent the ancient lineage of African mole-rats (Bathyergidae), and the coruro (Spalacopus cyanus), a South American rodent (Octodontidae) that adopted a subterranean lifestyle in more recent geological time. Additionally, we measured call amplitudes of social vocalizations to study auditory vocal coupling. We found elevated auditory thresholds and severe hearing range restrictions in the African mole-rats, with hearing in naked mole-rats tending to be more sensitive than in Mashona mole-rats, in which hearing notably deteriorated with increasing age. In contrast, hearing in coruros was similar to that of epigeic rodents, with its range extending into ultrasonic frequencies. However, as in the mole-rats, the coruros' region of best hearing was located at low frequencies close to 1 kHz. We argue that the auditory sensitivity of African mole-rats, although remarkably poor, has been underestimated by recent studies, whereas data on coruros conform to previous results. Considering the available evidence, we propose to be open to both degenerative and adaptive interpretations of hearing physiology in subterranean mammals, as each may provide convincing explanations for specific auditory traits observed.

Morphological and morphometrical aspects of the auditory ossicles in goat (Capra hircus)

The present paper deals with a detailed description of the auditory ossicles in Capra hircus. The paper focuses on the morphological and morphometrical description of the ossicular assembly, formed by malleus, incus and stapes. The malleus (overall length, as average- 8.16 mm) comprises the head of malleus (Caput mallei), a slightly strictured part-neck (Collum mallei) with 3 distinctive processes (lateral, rostral and muscular) (Processus lateralis, Processus rostralis and Processus muscularis) and a handle (Manubrium mallei). The head of malleus has an oval aspect with an obtuse articular surface on its medial surface (Facies articularis). The neck is evident with three bony processes described-the anterior, almost triangular, the muscular one quite reduced and the lateral one which is the most developed one. The manubrium is the longest sector-4.4 mm and appears as a slightly curved piece. The incus presents a body of 1.3 mm and two processes-the short and long crus (Crus breve and Crus longum). The overall shape of the ossicle resembles a biradicular molar. The lenticular process is a continuation of the distal part of the long crus. The stapes-the smallest in size of the three ossicles (2.7 mm), has a head (Caput stapedis), an anterior (Crus rostrale) and a caudal (Crus caudale) arm and a footplate (Basis stapedis). The two processes are slightly different in size and morphology, delimiting the intercrural space that shows the presence of a bony spicule. The footplate (1.6 mm² area) is ellipsoidal, with an anterior narrower extremity.

Functional anatomy of the middle and inner ears of the red fox, in comparison to domestic dogs and cats

Anatomical middle and inner ear parameters are often used to predict hearing sensitivities of mammalian species. Given that ear morphology is substantially affected both by phylogeny and body size, it is interesting to consider whether the relatively small anatomical differences expected in related species of similar size have a noticeable impact on hearing. We present a detailed anatomical description of the middle and inner ears of the red fox Vulpes vulpes, a widespread, wild carnivore for which a behavioural audiogram is available. We compare fox ears to those of the well‐studied and similarly sized domestic dog and cat, taking data for dogs and cats from the literature as well as providing new measurements of basilar membrane (BM) length and hair cell numbers and densities in these animals. Our results show that the middle ear of the red fox is very similar to that of dogs. The most obvious difference from that of the cat is the lack of a fully formed bony septum in the bulla tympanica of the fox. The cochlear structures of the fox, however, are very like those of the cat, whereas dogs have a broader BM in the basal cochlea. We further report that the mass of the middle ear ossicles and the bulla volume increase with age in foxes. Overall, the ear structures of foxes, dogs and cats are anatomically very similar, and their behavioural audiograms overlap. However, the results of several published models and correlations that use middle and inner ear measurements to predict aspects of hearing were not always found to match well with audiogram data, especially when it came to the sharper tuning in the fox audiogram. This highlights that, although there is evidently a broad correspondence between structure and function, it is not always possible to draw direct links when considering more subtle differences between related species.

The middle ear of the pink fairy armadillo Chlamyphorus truncatus (Xenarthra, Cingulata, Chlamyphoridae): comparison with armadillo relatives using computed tomography

The pink fairy armadillo Chlamyphorus truncatus is the smallest extant armadillo and one of the least-known fossorial mammals. The aim of this study was to establish if its middle ear is specially adapted to the subterranean environment, through comparison with more epigeic relatives of the groups Euphractinae (Chaetophractus villosus, Chaetophractus vellerosus, Zaedyus pichiy) and Dasypodinae (Dasypus hybridus). We examined the middle ears using micro-computed tomography and subsequent three-dimensional reconstructions. D. hybridus has a relatively small middle ear cavity, an incomplete bulla and 'ancestral' ossicular morphology. The other species, including Chlamyphorus, have fully ossified bullae and middle ear ossicles, with a morphology between 'transitional' and 'freely mobile', but in all armadillos the malleus retains a long anterior process. Unusual features of armadillo ears include the lack of a pedicellate lenticular apophysis and the presence, in some species, of an element of Paaw within the stapedius muscle. In common with many subterranean mammals, Chlamyphorus has a relatively flattened malleo-incudal articulation and appears to lack a functional tensor tympani muscle. Its middle ear cavity is not unusually enlarged, and its middle ear ossicles seem less robust than those of the other armadillos studied. In comparison with the euphractines, there is no reason to believe that the middle ear of this species is specially adapted to the subterranean environment; some aspects may even be indicative of degeneration. The screaming hairy armadillo, Chaetophractus vellerosus, has the most voluminous middle ear in both relative and absolute terms. Its hypertrophied middle ear cavity likely represents an adaptation to low-frequency hearing in arid rather than subterranean conditions.

Into the dark: patterns of middle ear adaptations in subterranean eulipotyphlan mammals

Evolution of the middle ear ossicles was a key innovation for mammals, enhancing the transmission of airborne sound. Radiation into various habitats from a terrestrial environment resulted in diversification of the auditory mechanisms among mammals. However, due to the paucity of phylogenetically controlled investigations, how middle ear traits have diversified with functional specialization remains unclear. In order to identify the respective patterns for various lifestyles and to gain insights into fossil forms, we employed a high-resolution tomography technique and compared the middle ear morphology of eulipotyphlan species (moles, shrews and hedgehogs), a group that has radiated into various environments, such as terrestrial, aquatic and subterranean habitats. Three-dimensional geometric morphometric analysis was conducted within a phylogenetically controlled framework. Quantitative shapes were found to strongly reflect the degree of subterranean lifestyle and weakly involve phylogeny. Our analyses demonstrate that subterranean adaptation should include a relatively shorter anterior process of the malleus, an enlarged incus, an enlarged stapes footplate and a reduction of the orbicular apophysis. These traits arguably allow improving low-frequency sound transmission at low frequencies and inhibiting the low-frequency noise which disturbs the subterranean animals in hearing airborne sounds.

Anatomical and morphometrical study of middle ear ossicles in 2 to 3-month-old Makouei sheep fetuses

The middle ear ossicles are important due to transmission of sound to the inner ear leading to sound understanding. The aim of present study was to determine the anatomical and morphometrical aspects of middle ear ossicles in Makouei sheep fetuses. For this experimental study, eight sheep fetuses at the age of 2 to 3 months were provided from public slaughterhouse; their middle ear ossicles were removed from tympanic cavity subsequently and assessed anatomically using stereomicroscope. For statistical analysis, one-way ANOVA and Tukey's post-hoc test were used. The results showed that rostral process of malleus doesn't exist, but an osseous lamina extending to the tympanic bulla and tympanic ring is located in this place. Moreover, lenticular bone and muscular process of stapes weren't found. These findings were similar to the other animal's ossicles anatomical features, but there were also some differences that can be useful for study of these ossicles developmental evolution.

Ear Structures of the Naked Mole-Rat, Heterocephalus glaber, and Its Relatives (Rodentia: Bathyergidae)

Although increasingly popular as a laboratory species, very little is known about the peripheral auditory system of the naked mole-rat, Heterocephalus glaber. In this study, middle and inner ears of naked mole-rats of a range of ages were examined using micro-computed tomography and dissection. The ears of five other bathyergid species (Bathyergus suillus, Cryptomys hottentotus, Fukomys micklemi, Georychus capensis and Heliophobius argenteocinereus) were examined for comparative purposes. The middle ears of bathyergids show features commonly found in other members of the Ctenohystrica rodent clade, including a fused malleus and incus, a synovial stapedio-vestibular articulation and the loss of the stapedius muscle. Heterocephalus deviates morphologically from the other bathyergids examined in that it has a more complex mastoid cavity structure, poorly-ossified processes of the malleus and incus, a ‘columelliform’ stapes and fewer cochlear turns. Bathyergids have semicircular canals with unusually wide diameters relative to their radii of curvature. How the lateral semicircular canal reaches the vestibule differs between species. Heterocephalus has much more limited high-frequency hearing than would be predicted from its small ear structures. The spongy bone forming its ossicular processes, the weak incudo-stapedial articulation, the columelliform stapes and (compared to other bathyergids) reduced cochlear coiling are all potentially degenerate features which might reflect a lack of selective pressure on its peripheral auditory system. Substantial intraspecific differences were found in certain middle and inner ear structures, which might also result from relaxed selective pressures. However, such interpretations must be treated with caution in the absence of experimental evidence.

Internally coupled ears in living mammals

It is generally held that the right and left middle ears of mammals are acoustically isolated from each other, such that mammals must rely on neural computation to derive sound localisation cues. There are, however, some unusual species in which the middle ear cavities intercommunicate, in which case each ear might be able to act as a pressure-difference receiver. This could improve sound localisation at lower frequencies. The platypus Ornithorhynchus is apparently unique among mammals in that its tympanic cavities are widely open to the pharynx, a morphology resembling that of some non-mammalian tetrapods. The right and left middle ear cavities of certain talpid and golden moles are connected through air passages within the basicranium; one experimental study on Talpa has shown that the middle ears are indeed acoustically coupled by these means. Having a basisphenoid component to the middle ear cavity walls could be an important prerequisite for the development of this form of interaural communication. Little is known about the hearing abilities of platypus, talpid and golden moles, but their audition may well be limited to relatively low frequencies. If so, these mammals could, in principle, benefit from the sound localisation cues available to them through internally coupled ears. Whether or not they actually do remains to be established experimentally.

Multilocus phylogeny of talpine moles (Talpini, Talpidae, Eulipotyphla) and its implications for systematics

The tribe Talpini is a group of strictly subterranean moles distributed across the Eurasian Continent whose phylogenetic relationships and taxonomy remain unresolved. Here we report a multi-locus nuclear-mitochondrial DNA dataset (9468 bp) from 11 talpine species encompassing all five recognized genera, together with analyses of their divergence times and evolutionary affinities inferred from maximum likelihood and Bayesian approaches. Our results finely resolved all relationships except the root of the four recognized Asian genera, which was placed sister to the genus Talpa. With respect to the Asian clade, we moreover provide the first molecular support for a sister-taxon relationship between Parascaptor and Scaptochirus and confirm that the genus Euroscaptor is paraphyletic. Further, and despite a relatively small sample size (22 specimens), our species delimitation analyses support the existence of at least two genetically distinct, and hence potentially cryptic species. Taken together, these findings argue that generic status should be given to E. mizura and illustrate that the taxonomic diversity of the tribe Talpini in mountainous regions of southwestern China and Southeast Asia is underestimated. Finally, results of our divergence time analyses support a rapid radiation of the endemic Asian genera in the late-Miocene, which temporally corresponds with enhanced aridity and cooling arising from a significant uplift of the Himalayan-Tibetan plateau.

A reexamination of the Carnivora malleus (Mammalia, Placentalia)

Authoritative anatomical references depict domestic dogs and cats as having a malleus with a short rostral (anterior) process that is connected via a ligament to the ectotympanic of the auditory bulla. Similar mallei have been reported for representatives of each of the 15 extant families of Carnivora, the placental order containing dogs and cats. This morphology is in contrast to a malleus with a long rostral process anchored to the ectotympanic that is considered to be primitive for mammals. Our reexamination of extant carnivorans found representatives from 12 families that possess an elongate rostral process anchored to the ectotympanic. Consequently, the malleus also is a component of the bulla. In a subset of our carnivoran sample, we confirmed that the elongate rostral process on the ectotympanic is continuous with the rest of the malleus through a thin osseous lamina. This morphology is reconstructed as primitive for Carnivora. Prior inaccurate descriptions of the taxa in our sample having mallei continuous with the bulla were based on damaged mallei. In addition to coupling to the ectotympanic, the rostral process of the malleus was found to have a hook-like process that fits in a facet on the skull base in representatives from seven families (felids, nandiniids, viverrids, canids, ursids, procyonids, and mustelids); its occurrence in the remaining families could not be ascertained. This feature is named herein the mallear hook and is likewise reconstructed to be primitive for Carnivora. We also investigated mallei in one additional placental order reported to have mallei not connected to the ectotympanic, Pholidota (pangolins), the extant sister group of Carnivora. We found pholidotans to also have anchored mallei with long rostral processes, but lacking mallear hooks. In light of our results, other mammals previously reported to have short rostral processes should be reexamined.

Flexibility within the middle ears of vertebrates

INTRODUCTION AND AIMS

Tympanic middle ears have evolved multiple times independently among vertebrates, and share common features. We review flexibility within tympanic middle ears and consider its physiological and clinical implications.

COMPARATIVE ANATOMY

The chain of conducting elements is flexible: even the 'single ossicle' ears of most non-mammalian tetrapods are functionally 'double ossicle' ears due to mobile articulations between the stapes and extrastapes; there may also be bending within individual elements.

SIMPLE MODELS

Simple models suggest that flexibility will generally reduce the transmission of sound energy through the middle ear, although in certain theoretical situations flexibility within or between conducting elements might improve transmission. The most obvious role of middle-ear flexibility is to protect the inner ear from high-amplitude displacements.

CLINICAL IMPLICATIONS

Inter-ossicular joint dysfunction is associated with a number of pathologies in humans. We examine attempts to improve prosthesis design by incorporating flexible components.

Of mice, moles and guinea pigs: functional morphology of the middle ear in living mammals

The middle ear apparatus varies considerably among living mammals. Body size, phylogeny and acoustic environment all play roles in shaping ear structure and function, but experimental studies aimed ultimately at improving our understanding of human hearing can sometimes overlook these important species differences. This review focuses on three groups of mammals, bringing together anatomical, zoological and physiological information in order to highlight unusual features of their middle ears and attempt to interpret their function. "Microtype" ears, found in species such as mice and bats, are associated with high-frequency hearing. The orbicular apophysis, the focus of some recent developmental studies on mouse ears, is characteristic of microtype mallei but is not found in humans or other "freely mobile" species. The apophysis increases ossicular inertia about the anatomical axis of rotation: its adaptive purpose in a high-frequency ear is still not clear. Subterranean mammals have convergently evolved a "freely mobile" ossicular morphology which appears to favour lower-frequency sound transmission. More unusual features found in some of these animals include acoustically coupled middle ear cavities, the loss of middle ear muscles and hypertrophied ossicles which are believed to subserve a form of inertial bone conduction. Middle ears of the rodent group Ctenohystrica (which includes guinea pigs and chinchillas, important models in hearing research) show some striking characteristics which together comprise a unique type of auditory apparatus requiring a classification of its own, referred to here as the "Ctenohystrica type". These characteristics include a distinctive malleus morphology, fusion of the malleus and incus, reduction or loss of the stapedius muscle, a synovial stapedio-vestibular articulation and, in chinchillas, enormously expanded middle ear cavities. These characteristics may be functionally linked and associated with the excellent low-frequency hearing found in these animals. The application of new experimental and imaging data into increasingly sophisticated models continues to improve our understanding of middle ear function. However, a more rigorous comparative approach and a better appreciation of the complex patterns of convergent and divergent evolution reflected in the middle ear structures of living mammals are also needed, in order to put findings from different species into the appropriate context. This article is part of a special issue entitled "MEMRO 2012".

Middle ear structure and bone conduction in Spalax, Eospalax, and Tachyoryctes mole-rats (Rodentia: Spalacidae)

There is evidence that spalacine, tachyoryctine, and myospalacine mole-rats all communicate with conspecifics through a form of seismic signaling, but the route for the detection of these signals is disputed. It has been proposed that two unusual anatomical adaptations in Spalax allow jaw vibrations to pass to the inner ear via the incus and stapes: a pseudoglenoid (=postglenoid) fossa which accomodates the condylar process of the mandible, and a bony cup, supported by a periotic lamina, through which the incus articulates with the skull. In this study, a combination of dissection and computed tomography was used to examine the ear region in more detail in both Spalax and its subterranean relatives Tachyoryctes and Eospalax, about which much less is known. Tachyoryctes was found to lack a pseudoglenoid fossa, while Eospalax lacks a periotic lamina and bony cup. This shows that these structures need not simultaneously be present for the detection of ground vibrations in mole-rats. Based on the observed anatomy, three hypothetical modes of bone conduction are argued to represent more likely mechanisms through which mole-rats can detect ground vibrations: ossicular inertial bone conduction, a pathway involving sound radiation into the external auditory meatus, and a newly-described fluid pathway between pseudoglenoid fossa and cranial cavity. The caudolateral extension of the tympanic cavity and the presence of a bony cup might represent synapomorphies uniting Spalax and Tachyoryctes, while the loss of the tensor tympani muscle in Spalax and Eospalax may be convergently derived.

A bony connection signals laryngeal echolocation in bats

Echolocation is an active form of orientation in which animals emit sounds and then listen to reflected echoes of those sounds to form images of their surroundings in their brains. Although echolocation is usually associated with bats, it is not characteristic of all bats. Most echolocating bats produce signals in the larynx, but within one family of mainly non-echolocating species (Pteropodidae), a few species use echolocation sounds produced by tongue clicks. Here we demonstrate, using data obtained from micro-computed tomography scans of 26 species (n = 35 fluid-preserved bats), that proximal articulation of the stylohyal bone (part of the mammalian hyoid apparatus) with the tympanic bone always distinguishes laryngeally echolocating bats from all other bats (that is, non-echolocating pteropodids and those that echolocate with tongue clicks). In laryngeally echolocating bats, the proximal end of the stylohyal bone directly articulates with the tympanic bone and is often fused with it. Previous research on the morphology of the stylohyal bone in the oldest known fossil bat (Onychonycteris finneyi) suggested that it did not echolocate, but our findings suggest that O. finneyi may have used laryngeal echolocation because its stylohyal bones may have articulated with its tympanic bones. The present findings reopen basic questions about the timing and the origin of flight and echolocation in the early evolution of bats. Our data also provide an independent anatomical character by which to distinguish laryngeally echolocating bats from other bats.

Petrosal anatomy in the fossil mammal Necrolestes: evidence for metatherian affinities and comparisons with the extant marsupial mole

We present reconstructions of petrosal anatomy based on high-resolution X-ray computed tomography scans for the fossil mammal Necrolestes and for the marsupial mole Notoryctes sp. Compared with other mammals, Necrolestes exhibits a mosaic of plesiomorphic and derived characters, but most of the evidence supports its metatherian status. We revised previous descriptions and report on features of phylogenetic or functional significance. Necrolestes exhibits features that support metatherian affinities, such as the presence of a short and lateral prootic canal, and the loss of the stapedial artery in adults. A deep groove at the anterior pole of the promontorium is present in front of the cochlear housing, a variant on the extrabullar pathway of the internal carotid artery. The promontorium is laterally bordered by a large bony projection resembling the eutherian tegmen tympani [De Beer GR (1937) The Development of the Vertebrate Skull, Oxford, Clarendon Press, p. 391]. Posteromedial to the secondary facial foramen and anterolateral to the fenestra vestibuli is a pronounced fossa for the tensor tympani muscle. On the medial part of the pars canalicularis there is a great inflation of the medial side of the caudal tympanic process, a structure of unknown function. The internal acoustic meatus exhibits a broad transverse septum and is bordered laterally by a broad prefacial commissure. The cochleae of Necrolestes and of Notoryctes have fewer spiral turns (1.1 and 1.6, respectively) than most marsupials. The lateral semicircular canal is more expanded than the posterior semicircular canal in Necrolestes but not in Notoryctes. Both Necrolestes and Notoryctes have a second crus commune, i.e. the lateral semicircular canal opens into the ampulla of the posterior semicircular canal. A stylomastoid foramen enclosed anterodorsally by both the pars cochlearis and pars canalicularis is present in Dasyuridae, Dromiciops gliroides and Notoryctes.

Ossicular density in golden moles (Chrysochloridae)

The densities of middle ear ossicles of golden moles (family Chrysochloridae, order Afrosoricida) were measured using the buoyancy method. The internal structure of the malleus was examined by high-resolution computed tomography, and solid-state NMR was used to determine relative phosphorus content. The malleus density of the desert golden mole Eremitalpa granti (2.44 g/cm3) was found to be higher than that reported in the literature for any other terrestrial mammal, whereas the ossicles of other golden mole species are not unusually dense. The increased density in Eremitalpa mallei is apparently related both to a relative paucity of internal vascularization and to a high level of mineralization. This high density is expected to augment inertial bone conduction, used for the detection of seismic vibrations, while limiting the skull modifications needed to accommodate the disproportionately large malleus. The mallei of the two subspecies of E. granti, E. g. granti and E. g. namibensis, were found to differ considerably from one another in both size and shape.