The semicircular canals (SCs), part of the vestibular apparatus of the inner ear, are directly involved in the detection of angular motion of the head for maintaining balance, and exhibit adaptive patterns for locomotor behaviour. Consequently, they are generally believed to show low levels of intraspecific morphological variation, but few studies have investigated this assumption. On the basis of high-resolution computed tomography, we present here, to our knowledge, the first comprehensive study of the pattern of variation of the inner ear with a focus on Xenarthra. Our study demonstrates that extant three-toed sloths show a high level of morphological variation of the bony labyrinth of the inner ear. Especially, the variation in shape, relative size and angles of their SCs greatly differ from those of other, faster-moving taxa within Xenarthra and Placentalia in general. The unique pattern of variation in three-toed sloths suggests that a release of selection and/or constraints on their organ of balance is associated with the observed wide range of phenotypes. This release is coincident with their slow and infrequent locomotion and may be related, among other possible factors, to a reduced functional demand for a precise sensitivity to movement.

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The petrosal anatomy and inner ear structure of Jurassic cladotherian mammals represent the ancestral morphological conditions (groundplan) from which modern therian mammals (marsupials and placentals) have evolved. We present the reconstruction of the petrosal and inner ear features of the Late Jurassic dryolestoid mammal Henkelotherium guimarotae from high-resolution computed tomography and three-dimensional imaging analysis. This study of Henkelotherium revealed a combination of derived and primitive features, including: cladotherian apomorphies, such as the promontorial sulcus for the internal carotid artery and reduced lateral trough; trechnotherian characters, such as an enclosed cochlear canaliculus for the perilymphatic duct, post-promontorial tympanic sinus and caudal tympanic process; in addition to plesiomorphic mammalian features, such as the cavum supracochleare and prootic canal. The inner ear of Henkelotherium shows a division between the utricle and saccule, a cochlear canal coiled through at least 270 ° , a distinctive primary bony lamina for the basilar membrane, and a secondary bony lamina. The development of the primary and secondary bony laminae in the cochlear canal is suggested here to be correlated with the concurrent coiling of the bony canal and membranous duct of the inner ear cochlea, apomorphies of the more inclusive cladotherian clade that also represent the ancestral morphotype of modern therian mammals. Because these features are crucial for high-frequency hearing in extant therian mammals, their early appearance in Late Jurassic cladotherians suggests a more ancient origination for high-frequency hearing in mammalian history than previously thought.

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Primitive mammals are considered macrosmatic. They have very large and complicated nasal capsules, nasal cavities with extensive olfactory epithelia, and relatively large olfactory bulbs. The complicated structures of the nasal capsule follow a relatively conservative "bauplan," which is normally easy to see in earlier fetal stages; especially in altricial taxa it differentiates well into postnatal life. As anteriormost part of the chondrocranium, the nasal capsule is at first cartilaginous. Most of it ossifies endochondrally, but "appositional bone" ("Zuwachsknochen") is also common. Many fetal structures become resorbed. Together, all surviving bone structures form the ethmoid bone, but cartilages of the external nose and of the vomeronasal complex can persist throughout life. We describe in detail the anatomy of Daubentonia madagascariensis based on a fetal stage (41 mm HL) and an adult skull was analyzed by mCT. We found that the nasal capsule of this species is by far the most complicated one of all extant Primates. We also describe older fetuses of Homo sapiens (35 and 63 mm HL) as representative of a derived primate. The most significant feature of man-and probably of all anthropoids-is the complete loss of the recessus frontoturbinalis and its associated structures. It can be demonstrated that the evolutionary reductions within the primate Abbreviations used: ahy 5 ala hypochiasmatica; ane 5 apertura nasi externa; aor 5 ala orbitalis; at 5 atrioturbinale; az 5 arcus zygomaticus; bol 5 bulbus olfactorius; cat 5 concha atrioturbinalis; cdnp 5 cartilago ductus nasopalatini; cna 5 cupula nasi anterior; cnp 5 cupula nasi posterior; con 5 commissura orbitonasalis; cpa 5 cartilago paraseptalis anterior; cpal 5 cartilago palatina; cse 5 concha septalis; css 5 cartilago supraseptalis; ctu 5 cartilago tubaria; dI 5 deciduous incisor; dni 5 ductus nasi inferior; dnl 5 ductus nasolacrimalis; dnp 5 ductus nasopalatinus (Steno); dvn 5 ductus vomeronasalis; et 5 ethmoturbinale; et a 5 ethmoturbinale lamina anterior; et p 5 ethmoturbinale lamina posterior; fop 5 foramen opticum; fr 5 frontale; ft 5 frontoturbinale; gnl 5 glandula nasi lateralis; hmx 5 hiatus maxillaris; it 5 interturbinale; ju 5 jugale; la 5 lacrimale; lcr 5 lamina cribrosa; lh 5 lamina horizontalis; hsl 5 hiatus semilunaris; lor 5 lamina orbitalis; lsc 5 lamina semicircularis; lta 5 lamina transversalis anterior; lte 5 lamina terminalis; ltp 5 lamina transversalis posterior; ltr 5 lamina trabecularis; mt (mat) 5 marginoturbinale; met 5 mesethmoid; mx 5 maxillare; mxt 5 maxilloturbinale; na 5 nasale; nph 5 ductus nasopharyngeus; nt (nat) 5 nasoturbinale; nop 5 nervus opticus; osp 5 orbitosphenoid; ovn 5 organon vomeronasale (Jacobson); pal 5 palatinum; pao 5 planum antorbitale; pas 5 processus alaris superior; pet 5 processus ethmoidalis orbitosphenoidei; pmp 5 processus maxillaris posterior; pmx 5 praemaxillare; pna 5 paries nasi; ppp 5 processus paraseptalis posterius; ppt 5 processus pterygoideus; prl 5 prominentia lateralis; psp 5 praespheno...

The semicircular canals (SCs) of the inner ear detect angular acceleration and are located in the bony labyrinth of the petrosal bone. Based on highresolution computed tomography, we created a size-independent database of the bony labyrinth of 50 mammalian species especially rodents of the squirrel-related clade comprising taxa with fossorial, arboreal and gliding adaptations. Our sampling also includes gliding marsupials, actively flying bats, the arboreal tree shrew and subterranean species. The morphometric anatomy of the SCs was correlated to the locomotion mode. Even if the phylogenetic signal cannot entirely be excluded, the main significance for functional morphological studies has been found in the diameter of the SCs, whereas the radius of curvature is of minor interest. Additionally, we found clear differences in the bias angle of the canals between subterranean and gliding taxa, but also between sciurids and glirids. The sensitivity of the inner ear correlates with the locomotion mode, with a higher sensitivity of the SCs in fossorial species than in flying taxa. We conclude that the inner ear of flying and gliding mammals is less sensitive due to the large information flow into this sense organ during locomotion.

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The coiled cochlea is a key evolutionary innovation of modern therian mammals. We report that the Late Jurassic mammal Dryolestes, a relative to modern therians, has derived bony characteristics of therian-like innervation, but its uncoiled cochlear canal is less derived than the coiled cochlea of modern therians. This suggests a therian-like innervation evolved before the fully coiled cochlea in phylogeny. The embryogenesis of the cochlear nerve and ganglion in the inner ear of mice is now known to be patterned by neurogenic genes, which we hypothesize to have influenced the formation of the auditory nerve and its ganglion in Jurassic therian evolution, as shown by their osteological correlates in Dryolestes, and by the similar base-to-apex progression in morphogenesis of the ganglion in mice, and in transformation of its canal in phylogeny. The cochlear innervation in Dryolestes is the precursory condition in the curve-tocoil transformation of the cochlea in mammalian phylogeny. This provides the timing of the evolution, and where along the phylogeny the morphogenetic genes were co-opted into patterning the cochlear innervation, and the full coiling of the cochlea in modern therians.