Anatomical evidence for low frequency sensitivity in an archaeocete whale: comparison of the inner ear of Zygorhiza kochii with that of crown Mysticeti

The tympanoperiotic complex of the blue whale, Balaenoptera musculus

The tympanoperiotic complex of a blue whale Balaenoptera musculus is described and compared to the homologous structures in the other extant and fossil baleen whale species. The periotic and the tympanic bulla represent informative anatomical regions in both functional and phylogenetic studies and for this reason a micro-CT scan of the bones was performed in order to better characterize their external aspect and to reconstruct the inner structures. In particular, the cochlea, the semicircular canals and associated portions of the periotic are reconstructed so that these structures may be used in phylogenetic analyses. We observed that the blue whale periotic is characterized by the presence of a strong dorsal protrusion which is posteriorly bordered by a previously undescribed morphological character that we name the posterotransverse fossa. The peculiar shape of the anterior process and the en echelon organization of the posterior foramina of the pars cochlearis are also described and compared. From a phylogenetic perspective, the blue whale is confirmed to be closely related to the fin whale, Balaenoptera physalus, but it is suggested, based on ear bone characters only, that it diverged before the other balaenopterid species in the phylogeny of Balaenopteridae. This placement supports a series of morphological observations suggesting that the extant blue whale was an early-diverging member of Balaenoptera. Our results help to decipher the evolutionary origin of the blue whale, the largest living animal, by allowing new and more detailed morphological analyses of the balaenopterid fossil record.

Predicting ecology and hearing sensitivities in Parapontoporia-An extinct long-snouted dolphin

Analyses of the cetacean (whale and dolphin) inner ear provide glimpses into the ecology and evolution of extinct and extant groups. The paleoecology of the long-snouted odontocete (toothed whale) group, Parapontoporia, is primarily marine with its depositional context also suggesting freshwater tolerance. As an extinct relative of the exclusively riverine Lipotes vexillifer, Parapontoporia provides insight into a transition from marine to freshwater environments. High-resolution X-ray CT scans (~3 microns or less) of three individual specimens from two species, P. sternbergi and P. pacifica, were acquired. Digital endocasts of the inner ear labyrinths were extracted non-destructively. Nine measurements of the inner ear were compared with an existing dataset covering 125 terrestrial and aquatic artiodactyls. These measurements were then subjected to a principal component analysis to interpret hearing sensitivities among other artiodactyls. Based on our analyses, Parapontoporia was likely to have been able to hear within narrow-band high frequency (NBHF) ranges. This finding indicates another convergence of NBHF-style hearing, or, more intriguingly, suggests that it may be an ancestral characteristic present among the longirostrine dolphins that dominated in the Miocene prior to the evolution of more modern lineages.

Developing echolocation: distinctive patterns in the ontogeny of the tympanoperiotic complex in baleen and toothed whales (Cetacea)

Cetaceans (baleen and toothed whales) present a unique set of adaptations for life in water. Among other abilities, the two living groups can hear and produce different sound frequencies: baleen whales use low frequencies primarily for communication, whereas toothed whales acquired the ability to echolocate using high-frequency sounds. Both groups exhibit modifications to the morphology of the ear bones (tympanic bulla and periotic) that closely track their behaviour and ecology. The evolution of sound reception in whales is being investigated thoroughly, whereas the changes in prenatal development (ontogeny) that generate these disparate ear bone morphologies remain mostly unknown. In this study, we characterize the ontogeny of the ear bones in Cetacea by looking at the progression of ossification and associated changes in morphology using a combination of traditional measurements and an innovative landmark-free method to quantify shape on a newly assembled three-dimensional dataset spanning the ontogeny and phylogeny of extant Cetacea. We have found that the two groups of Cetacea share some aspects of ear ontogeny, such as a common growth trajectory of the periotic. However, differences in ossification, allometry and growth trajectory, particularly in the periotic bone, reflect their divergent inner ear morphology and hearing abilities.

Dental polymorphisms in Crocidura (Soricomorpha: Soricidae) and evolutionary diversification of crocidurine shrew dentition

The upper dentition of Crocidura exhibits polymorphic characters that were revealed for the first time in this study via high-resolution X-ray computed microtomography. Our analyses of 11 Crocidura species and selected Diplomesodon, Suncus and Sylvisorex species from different geographical regions and size groups revealed the most complex character states of upper dentition in the Ethiopian endemic species Crocidura yaldeni. A three-dimensionally based geometric morphometric analysis revealed the dependence of variation in skull muzzle shape on alterations in general upper dentition, such as a reduction in the number of antemolars. Principal components analysis revealed highly significant shape alterations and morphological trajectories in C. yaldeni (and more moderate ones in Suncus murinus) toward the Sorex-like morphotype in the outgroup, and less significant shape alterations in Crocidura obscurior, Crocidura phanluongi and Crocidura sapaensis with double-rooted third antemolar. Cladistic analysis based on a new data matrix for 20 species and 46 characters allowed us to determine the directions of the morphological trajectories: the apomorphic state of the most complex antemolars of C. yaldeni is associated with deviating skull muzzle shape changes, which we determined to be attributable to neomorphosis, and the less significant alterations in the shape of other Crocidura with complex antemolars are attributable to regional adaptation.

Evolution of whale sensory ecology: Frontiers in nondestructive anatomical investigations

Studies surrounding the evolution of sensory system anatomy in cetaceans over the last ~100 years have shed light on aspects of the early evolution of hearing sensitivities, the small relative size of the organ of balance (semicircular canals and vestibule), brain (endocast) shape and relative volume changes, and ontogenetic development of sensory-related structures. Here, I review advances in our knowledge of sensory system anatomy as informed by the use of nondestructive imaging techniques, with a focus on applied methods in computed tomography (CT and μCT), and identify the key questions that remain to be addressed. Of these, the most important are: Is lower frequency hearing sensitivity the ancestral condition for whales? Did echolocation evolve more than once in odontocetes; and if so, when and why? How has the structure of the cetacean brain changed, through the evolution of whales, and does this correspond to changes in hearing sensitivities? Finally, what are the general pathways of ontogenetic development of sensory systems in odontocetes and mysticetes? Answering these questions will allow us to understand important macroevolutionary patterns in a fully aquatic mammalian group and provides baseline data on species for which we have limited biological information because of logistical limitations.

What are the limits on whale ear bone size? Non-isometric scaling of the cetacean bulla

The history of cetaceans demonstrates dramatic macroevolutionary changes that have aided their transformation from terrestrial to obligate aquatic mammals. Their fossil record shows extensive anatomical modifications that facilitate life in a marine environment. To better understand the constraints on this transition, we examined the physical dimensions of the bony auditory complex, in relation to body size, for both living and extinct cetaceans. We compared the dimensions of the tympanic bulla, a conch-shaped ear bone unique to cetaceans, with bizygomatic width—a proxy for cetacean body size. Our results demonstrate that cetacean ears scale non-isometrically with body size, with about 70% of variation explained by increases in bizygomatic width. Our results, which encompass the breadth of the whale fossil record, size diversity, and taxonomic distribution, suggest that functional auditory capacity is constrained by congruent factors related to cranial morphology, as opposed to allometrically scaling with body size.

Alterations to vestibular morphology in highly bred domestic dogs may affect balance

The modern domestic dog (Canis lupus familiaris) provides an excellent model to examine the effects of cranial modification. Extreme variation in skull length among dog breeds due to high levels of selective breeding is known to be linked to disorders of the head and neck. Such alteration may also influence sensory organs including those of the vestibular system (VS), one of the most fundamental sense organs, essential in maintaining balance. Studies in mammals have shown that orientation of ipsilateral semicircular canals (SCCs) of the VS at right angles (orthogonality) is related to angular acceleration sensitivity. Due to their considerable variation in craniofacial form while exhibiting similar locomotion, domestic dogs provide an excellent natural experiment to examine if cranial alteration influences VS functional morphology. Our methods examine how change in cranial base length across dog breeds relates to SCC orthogonality using linear modeling and analyses of variance. The sample studied (29 bony labyrinths of 17 dog breeds) was obtained from a previous study on canid inner ear metrics. Results support the hypothesis that orthogonality between the anterior and posterior SCC + ampulla significantly correlates with cranial base length. This suggests a close relationship between the orientations of SCCs with their ampullae and cranial structure among dog breeds. Specifically, highly derived breeds, such as the brachycephalic pug, have anterior and posterior SCCs and ampullae that deviate the most from orthogonality. Therefore, such highly bred domestic dogs may also have altered vestibular function due to compressed cranial form.

Intraspecific variation in the cochleae of harbour porpoises (Phocoena phocoena) and its implications for comparative studies across odontocetes

In morphological traits, variation within species is generally considered to be lower than variation among species, although this assumption is rarely tested. This is particularly important in fields like palaeontology, where it is common to use a single individual as representative of a species due to the rarity of fossils. Here, we investigated intraspecific variation in the cochleae of harbour porpoises (Phocoena phocoena). Interspecific variation of cochlear morphology is well characterised among odontocetes (toothed whales) because of the importance of the structure in echolocation, but generally these studies use only a single cochlea to represent each species. In this study we compare variation within the cochleae of 18 specimens of P. phocoena with variations in cochlear morphology across 51 other odontocete species. Using both 3D landmark and linear measurement data, we performed Generalised Procrustes and principal component analyses to quantify shape variation. We then quantified intraspecific variation in our sample of P. phocoena by estimating disparity and the coefficient of variation for our 3D and linear data respectively. Finally, to determine whether intraspecific variation may confound the results of studies of interspecific variation, we used multivariate and univariate analyses of variance to test whether variation within the specimens of P. phocoena was significantly lower than that across odontocetes. We found low levels of intraspecific variation in the cochleae of P. phocoena, and that cochlear shape within P. phocoena was significantly less variable than across odontocetes. Although future studies should attempt to use multiple cochleae for every species, our results suggest that using just one cochlea for each species should not strongly influence the conclusions of comparative studies if our results are consistent across Cetacea.

Evidence for convergent evolution of ultrasonic hearing in toothed whales (Cetacea: Odontoceti)

Toothed whales (Cetacea: Odontoceti) are the most diverse group of modern cetaceans, originating during the Eocene/Oligocene transition approximately 38 Ma. All extant odontocetes echolocate; a single origin for this behaviour is supported by a unique facial source for ultrasonic vocalizations and a cochlea adapted for hearing the corresponding echoes. The craniofacial and inner ear morphology of Oligocene odontocetes support a rapid (less than 5 Myr) early evolution of echolocation. Although some cranial features in the stem odontocetes Simocetus and Olympicetus suggest an ability to generate ultrasonic sound, until now, the bony labyrinths of taxa of this grade have not been investigated. Here, we use µCT to examine a petrosal of a taxon with clear similarities to Olympicetus avitus. Measurements of the bony labyrinth, when added to an extensive dataset of cetartiodactyls, resulted in this specimen sharing a morphospace with stem whales, suggesting a transitional inner ear. This discovery implies that either the lineage leading to this Olympicetus--like taxon lost the ability to hear ultrasonic sound, or adaptations for ultrasonic hearing evolved twice, once in xenorophids and again on the stem of the odontocete crown group. We favour the latter interpretation as it matches a well-documented convergence of craniofacial morphology between xenorophids and extant odontocetes.

Intraspecific variation and symmetry of the inner-ear labyrinth in a population of wild turkeys: implications for paleontological reconstructions

The cochlea and semicircular canals (SCCs) of the inner ear are vital neurosensory devices. There are associations between the anatomy of these sensorineural structures, their function, and the function of related biological systems, for example, hearing ability, gaze stabilization, locomotor agility, and posture. The endosseous labyrinth is frequently used as a proxy to infer the performance of the hearing and vestibular systems, locomotor abilities, and ecology of extinct species. Such fossil inferences are often based on single specimens or even a single ear, representing an entire species. To address whether a single ear is representative of a population, we used geometric morphometrics to quantitatively assess the variation in shape and symmetry in a sample of endosseous labyrinths of wild turkeys Meleagris gallopavo of southern Ohio. We predicted that ears would be symmetrical both within individuals and across the sample; that labyrinth shape and size would covary; that labyrinth shape would vary with the size of the brain, measured as width of the endocranium at the cerebellum; and that labyrinths would be morphologically integrated. To test these predictions, we microCT-scanned the heads of 26 cadaveric turkeys, digitally segmented their endosseous labyrinths in Avizo, and assigned 15 manual landmarks and 20 sliding semilandmarks to each digital model. Following Procrustes alignment, we conducted an analysis of bilateral symmetry, a Procrustes regression analysis for allometry and other covariates including side and replicate, and analyses of global integration and modularity. Based on Procrustes distances, no individual’s left and right ears were clearly different from each other. When comparing the ears of different specimens, statistically clear differences in shape were found in only 66 of more than 1,300 contrasts. Moreover, effects of both directional and fluctuating asymmetry were very small—generally, two orders of magnitude smaller than the variance explained by individual variation. Statistical tests disagreed on whether these asymmetric effects crossed the threshold of significance, possibly due to non-isotropic variation among landmarks. Regardless, labyrinths appeared to primarily vary in shape symmetrically. Neither labyrinth size nor endocranial width was correlated with labyrinth shape, contrary to our expectations. Finally, labyrinths were found to be moderately integrated in a global sense, but four weakly separated modules—the three SCCs and cochlea—were recovered using a maximum-likelihood analysis. The results show that both fluctuating and directional asymmetry play a larger role in shape variation than expected—but nonetheless, endosseous labyrinths are symmetrical within individuals and at the level of the population, and their shape varies symmetrically. Thus, inferences about populations, and very possibly species, may be confidently made when only a single specimen, or even a single ear, is available for study.

Associated tympanic bullar and cochlear hypertrophy define adaptations to true deserts in African gerbils and laminate-toothed rats (Muridae: Gerbillinae and Murinae)

Hearing capabilities in desert rodents such as gerbils and heteromyids have been inferred from both anatomical and ecological aspects and tested with experiments and theoretical models. However, very few studies have focused on other desert-adapted species. In this study, a refined three-dimensional morphometric approach was used on three African rodent tribes (Otomyini, Taterillini and Gerbillini) to describe the cochlear and tympanic bullar morphology, and to explore the role of phylogeny, allometry and ecology to better understand the underlying mechanism of any observed trends of hypertrophy in the bulla and associated changes in the cochlea. As a result, desert-adapted species could be distinguished from mesic and semi-arid taxa by the gross cochlear dimensions, particularly the oval window, which is larger in desert species. Bullar and cochlear modifications between species could be explained by environment (bulla and oval window), phylogeny (cochlear curvature gradient) and/or allometry (cochlear relative length, oval window and bulla) with some exceptions. Based on their ear anatomy, we predict that Desmodillus auricularis and Parotomys brantsii should be sensitive to low-frequency sounds, with D. auricularis sensitive to high-frequency sounds, too. This study concludes that in both arid and semi-arid adapted laminate-toothed rats and gerbils there is bulla and associated cochlea hypertrophy, particularly in true desert species. Gerbils also show tightly coiled cochlea but the significance of this is debatable and may have nothing to do with adaptations to any specific acoustics in the desert environment.

Genomic and functional evidence reveals molecular insights into the origin of echolocation in whales

Echolocation allows toothed whales to adapt to underwater habitats where vision is ineffective. Because echolocation requires the ability to detect exceptional high-frequency sounds, fossils related to the auditory system can help to pinpoint the origin of echolocation in whales. However, because of conflicting interpretations of archaeocete fossils, when and how whales evolved the high-frequency hearing correlated with echolocation remain unclear. We address these questions at the molecular level by systematically investigating the convergent evolution of 7206 orthologs across 16 mammals and find that convergent genes between the last common ancestor of all whales (LCAW) and echolocating bats are not significantly enriched in functional categories related to hearing, and that convergence in hearing-related proteins between them is not stronger than that between nonecholocating mammalian lineages and echolocating bats. However, these results contrast with those of parallel analyses between the LCA of toothed whales (LCATW) and echolocating bats. Furthermore, we reconstruct the ancestral genes for the hearing protein prestin for the LCAW and LCATW; we show that the LCAW prestin exhibits the same function as that of nonecholocating mammals, but the LCATW prestin shows functional convergence with that of extant echolocating mammals. Mutagenesis shows that functional convergence of prestin is driven by convergent changes in the prestins S392A and L497M in the LCATW and echolocating bats. Our results provide genomic and functional evidence supporting the origin of high-frequency hearing in the LCAW, not the LCATW, and reveal molecular insights into the origin and evolutionary trajectories of echolocation in whales.

Neuroanatomy and inner ear labyrinths of the narwhal, Monodon monoceros, and beluga, Delphinapterus leucas (Cetacea: Monodontidae)

Narwhals (Monodon monoceros) and belugas (Delphinapterus leucas) are the only extant members of the Monodontidae, and are charismatic Arctic-endemic cetaceans that are at risk from global change. Investigating the anatomy and sensory apparatuses of these animals is essential to understanding their ecology and evolution, and informs efforts for their conservation. Here, we use X-ray CT scans to compare aspects of the endocranial and inner ear labyrinth anatomy of extant monodontids and use the overall morphology to draw larger inferences about the relationship between morphology and ecology. We show that differences in the shape of the brain, vasculature, and neural canals of both species may relate to differences in diving and other behaviors. The cochleae are similar in morphology in the two species, signifying similar hearing ranges and a close evolutionary relationship. Lastly, we compare two different methods for calculating 90var - a calculation independent of body size that is increasingly being used as a proxy for habitat preference. We show that a 'direct' angular measurement method shows significant differences between Arctic and other habitat preferences, but angle measurements based on planes through the semicircular canals do not, emphasizing the need for more detailed study and standardization of this measurement. This work represents the first comparative internal anatomical study of the endocranium and inner ear labyrinths of this small clade of toothed whales.

The bony labyrinth of toothed whales reflects both phylogeny and habitat preferences

The inner ear of toothed whales (odontocetes) is known to have evolved particular shapes related to their abilities to echolocate and move under water. While the origin of these capacities is now more and more examined, thanks to new imaging techniques, little is still known about how informative inner ear shape could be to tackle phylogenetic issues or questions pertaining to the habitat preferences of extinct species. Here we show that the shape of the bony labyrinth of toothed whales provides key information both about phylogeny and habitat preferences (freshwater versus coastal and fully marine habitats). Our investigation of more than 20 species of extinct and modern odontocetes shows that the semi-circular canals are not very informative, in contrast to baleen whales, while the cochlea alone bears a strong signal. Inner ear shape thus provides a novel source of information to distinguish between morphologically convergent lineages (e.g. river dolphins).

Low-frequency hearing preceded the evolution of giant body size and filter feeding in baleen whales

Living baleen whales (mysticetes) produce and hear the lowest-frequency (infrasonic) sounds among mammals. There is currently debate over whether the ancestor of crown cetaceans (Neoceti) was able to detect low frequencies. However, the lack of information on the most archaic fossil mysticetes has prevented us from determining the earliest evolution of their extreme acoustic biology. Here, we report the first anatomical analyses and frequency range estimation of the inner ear in Oligocene (34-23 Ma) fossils of archaic toothed mysticetes from Australia and the USA. The cochlear anatomy of these small fossil mysticetes resembles basilosaurid archaeocetes, but is also similar to that of today's baleen whales, indicating that even the earliest mysticetes detected low-frequency sounds, and lacked ultrasonic hearing and echolocation. This suggests that, in contrast to recent research, the plesiomorphic hearing condition for Neoceti was low frequency, which was retained by toothed mysticetes, and the high-frequency hearing of odontocetes is derived. Therefore, the low-frequency hearing of baleen whales has remained relatively unchanged over the last approximately 34 Myr, being present before the evolution of other signature mysticete traits, including filter feeding, baleen and giant body size.

FOSSIL SECRETS REVEALED: X-RAY CT SCANNING AND APPLICATIONS IN PALEONTOLOGY

X-ray computed tomography (CT) provides a nondestructive means of studying the inside and outside of objects. It allows accurate visualization and measurement of internal features, that are otherwise impossible to obtain nondestructively, and is a lasting digital record that can be made available to future researchers, museums, and the general public. Here, an overview of CT scanning methodologies and protocol is provided, as well as some recent examples of how this technology is allowing paleontologists to make new inroads into understanding the ecology, evolution, and development of both extant and extinct organisms. Lastly, some frontiers and outstanding questions in the acquisition, processing, and storage of digital 3-D morphological data are highlighted.

Virtual reconstruction of the endocranial anatomy of the early Jurassic marine crocodylomorph Pelagosaurus typus (Thalattosuchia)

Thalattosuchians were highly specialised aquatic archosaurs of the Jurassic and Early Cretaceous, and represent a peak of aquatic adaptation among crocodylomorphs. Relatively little is known of their endocranial anatomy or its relevance for the evolution of sensory systems, physiology, and other aspects of biology. Nevertheless, such data have significance for two reasons: (1) thalattosuchians represent an important data point regarding adaptation to marine life in tetrapods; and (2) as early-diverging members of the crocodylian stem-lineage, thalattosuchians provide information on the evolutionary assembly of the brain and other endocranial structures in crocodylomorphs. Here we use µCT data to virtually reconstruct the endocranial anatomy of Pelagosaurus typus, an early thalattosuchian with plesiomorphic traits of relevance to the split between the two major subgroups: Teleosauroidea and Metriorhynchoidea. Interpretation of these data in a broad comparative context indicate that several key endocranial features may be unique to thalattosuchians, including: a pyramidal morphology of the semicircular canals, the presence of an elongate endosseous cochlear duct that may indicate enhanced hearing ability, the presence of large, paired canals extending anteriorly from an enlarged pituitary fossa, a relatively straight brain (possibly due to the presence of large, laterally placed orbits), and an enlarged venous sinus projecting dorsally from the endocast that is confluent with the paratympanic sinus system. Notably, we document a large expansion of the nasal cavity anterior to the orbits in Pelagosaurus as an osteological correlate of an enlarged salt gland previously only documented in Late Jurassic metriorhynchoids. This is the first anatomical evidence of this structure in early thalattosuchians. Pelagosaurus also shares the presence of paired olfactory bulbs with metriorhynchoids, and shows an enlarged cerebrum, which may also be present in teleosauroids. Taken together, our findings indicate that physiological and sensory adaptations to marine life occurred early in thalattosuchian evolution, predating the origins of flippers, tail flukes, and hydrodynamic body forms seen later in metriorhynchoids.

Ultrasonic hearing and echolocation in the earliest toothed whales

The evolution of biosonar (production of high-frequency sound and reception of its echo) was a key innovation of toothed whales and dolphins (Odontoceti) that facilitated phylogenetic diversification and rise to ecological predominance. Yet exactly when high-frequency hearing first evolved in odontocete history remains a fundamental question in cetacean biology. Here, we show that archaic odontocetes had a cochlea specialized for sensing high-frequency sound, as exemplified by an Oligocene xenorophid, one of the earliest diverging stem groups. This specialization is not as extreme as that seen in the crown clade. Paired with anatomical correlates for high-frequency signal production in Xenorophidae, this is strong evidence that the most archaic toothed whales possessed a functional biosonar system, and that this signature adaptation of odontocetes was acquired at or soon after their origin.

The cochlea of the enigmatic pygmy right whale Caperea marginata informs mysticete phylogeny

The pygmy right whale, Caperea marginata, is the least understood extant baleen whale (Cetacea, Mysticeti). Knowledge on its basic anatomy, ecology, and fossil record is limited, even though its singular position outside both balaenids (right whales) and balaenopteroids (rorquals + grey whales) gives Caperea a pivotal role in mysticete evolution. Recent investigations of the cetacean cochlea have provided new insights into sensory capabilities and phylogeny. Here, we extend this advance to Caperea by describing, for the first time, the inner ear of this enigmatic species. The cochlea is large and appears to be sensitive to low-frequency sounds, but its hearing limit is relatively high. The presence of a well-developed tympanal recess links Caperea with cetotheriids and balaenopteroids, rather than balaenids, contrary to the traditional morphological view of a close Caperea-balaenid relationship. Nevertheless, a broader sample of the cetotheriid Herpetocetus demonstrates that the presence of a tympanal recess can be variable at the specific and possibly even the intraspecific level.

Morphological variation among the inner ears of extinct and extant baleen whales (Cetacea: Mysticeti)

Living mysticetes (baleen whales) and odontocetes (toothed whales) differ significantly in auditory function in that toothed whales are sensitive to high-frequency and ultrasonic sound vibrations and mysticetes to low-frequency and infrasonic noises. Our knowledge of the evolution and phylogeny of cetaceans, and mysticetes in particular, is at a point at which we can explore morphological and physiological changes within the baleen whale inner ear. Traditional comparative anatomy and landmark-based 3D-geometric morphometric analyses were performed to investigate the anatomical diversity of the inner ears of extinct and extant mysticetes in comparison with other cetaceans. Principal component analyses (PCAs) show that the cochlear morphospace of odontocetes is tangential to that of mysticetes, but odontocetes are completely separated from mysticetes when semicircular canal landmarks are combined with the cochlear data. The cochlea of the archaeocete Zygorhiza kochii and early diverging extinct mysticetes plot within the morphospace of crown mysticetes, suggesting that mysticetes possess ancestral cochlear morphology and physiology. The PCA results indicate variation among mysticete species, although no major patterns are recovered to suggest separate hearing or locomotor regimes. Phylogenetic signal was detected for several clades, including crown Cetacea and crown Mysticeti, with the most clades expressing phylogenetic signal in the semicircular canal dataset. Brownian motion could not be excluded as an explanation for the signal, except for analyses combining cochlea and semicircular canal datasets for Balaenopteridae. J. Morphol. 277:1599-1615, 2016. © 2016 Wiley Periodicals, Inc.

Best practices for digitally constructing endocranial casts: examples from birds and their dinosaurian relatives

The rapidly expanding interest in, and availability of, digital tomography data to visualize casts of the vertebrate endocranial cavity housing the brain (endocasts) presents new opportunities and challenges to the field of comparative neuroanatomy. The opportunities are many, ranging from the relatively rapid acquisition of data to the unprecedented ability to integrate critically important fossil taxa. The challenges consist of navigating the logistical barriers that often separate a researcher from high-quality data and minimizing the amount of non-biological variation expressed in endocasts - variation that may confound meaningful and synthetic results. Our purpose here is to outline preferred approaches for acquiring digital tomographic data, converting those data to an endocast, and making those endocasts as meaningful as possible when considered in a comparative context. This review is intended to benefit those just getting started in the field but also serves to initiate further discussion between active endocast researchers regarding the best practices for advancing the discipline. Congruent with the theme of this volume, we draw our examples from birds and the highly encephalized non-avian dinosaurs that comprise closely related outgroups along their phylogenetic stem lineage.

Form and function of the mammalian inner ear

The inner ear of mammals consists of the cochlea, which is involved with the sense of hearing, and the vestibule and three semicircular canals, which are involved with the sense of balance. Although different regions of the inner ear contribute to different functions, the bony chambers and membranous ducts are morphologically continuous. The gross anatomy of the cochlea that has been related to auditory physiologies includes overall size of the structure, including volume and total spiral length, development of internal cochlear structures, including the primary and secondary bony laminae, morphology of the spiral nerve ganglion, and the nature of cochlear coiling, including total number of turns completed by the cochlear canal and the relative diameters of the basal and apical turns. The overall sizes, shapes, and orientations of the semicircular canals are related to sensitivity to head rotations and possibly locomotor behaviors. Intraspecific variation, primarily in the shape and orientation of the semicircular canals, may provide additional clues to help us better understand form and function of the inner ear.

Bony labyrinth shape variation in extant Carnivora: a case study of Musteloidea

The bony labyrinth provides a proxy for the morphology of the inner ear, a primary cognitive organ involved in hearing, body perception in space, and balance in vertebrates. Bony labyrinth shape variations often are attributed to phylogenetic and ecological factors. Here we use three-dimensional (3D) geometric morphometrics to examine the phylogenetic and ecological patterns of variation in the bony labyrinth morphology of the most species-rich and ecologically diversified traditionally recognized superfamily of Carnivora, the Musteloidea (e.g. weasels, otters, badgers, red panda, skunks, raccoons, coatis). We scanned the basicrania of specimens belonging to 31 species using high-resolution X-ray computed micro-tomography (μCT) to virtually reconstruct 3D models of the bony labyrinths. Labyrinth morphology is captured by a set of six fixed landmarks on the vestibular and cochlear systems, and 120 sliding semilandmarks, slid at the center of the semicircular canals and the cochlea. We found that the morphology of this sensory structure is not significantly influenced by bony labyrinth size, in comparisons across all musteloids or in any of the individual traditionally recognized families (Mephitidae, Procyonidae, Mustelidae). PCA (principal components analysis) of shape data revealed that bony labyrinth morphology is clearly distinguishable between musteloid families, and permutation tests of the Kmult statistic confirmed that the bony labyrinth shows a phylogenetic signal in musteloids and in most mustelids. Both the vestibular and cochlear regions display morphological differences among the musteloids sampled, associated with the size and curvature of the semicircular canals, angles between canals, presence or absence of a secondary common crus, degree of lateral compression of the vestibule, orientation of the cochlea relative to the semicircular canals, proportions of the cochlea, and degree of curvature of its turns. We detected a significant ecological signal in the bony labyrinth shape of musteloids, differentiating semi-aquatic taxa from non-aquatic ones (the taxa assigned to terrestrial, arboreal, semi-arboreal, and semi-fossorial categories), and a significant signal for mustelids, differentiating the bony labyrinths of terrestrial, semi-arboreal, arboreal, semi-fossorial and semi-aquatic species from each other. Otters and minks are distinguished from non-aquatic musteloids by an oval rather than circular anterior canal, sinuous rather than straight lateral canal, and acute rather than straight angle between the posterior and lateral semicircular canals - each of these morphological characters has been related previously to animal sensitivity for detecting head motion in space.

Cranial symmetry in baleen whales (Cetacea, Mysticeti) and the occurrence of cranial asymmetry throughout cetacean evolution

Odontoceti and Mysticeti (toothed and baleen whales) originated from Eocene archaeocetes that had evolved from terrestrial artiodactyls. Cranial asymmetry is known in odontocetes that can hear ultrasound (>20,000 Hz) and has been linked to the split function of the nasal passage in breathing and vocalization. Recent results indicate that archaeocetes also had asymmetric crania. Their asymmetry has been linked to directional hearing in water, although hearing frequencies are still under debate. Mysticetes capable of low-frequency and infrasonic hearing (<20 Hz) are assumed to have symmetric crania. This study aims to resolve whether mysticete crania are indeed symmetric and whether mysticete cranial symmetry is plesiomorphic or secondary. Cranial shape was analyzed applying geometric morphometrics to three-dimensional (3D) cranial models of fossil and modern mysticetes, Eocene archaeocetes, modern artiodactyls, and modern odontocetes. Statistical tests include analysis of variance, principal components analysis, and discriminant function analysis. Results suggest that symmetric shape difference reflects general trends in cetacean evolution. Asymmetry includes significant fluctuating and directional asymmetry, the latter being very small. Mysticete crania are as symmetric as those of terrestrial artiodactyls and archaeocetes, without significant differences within Mysticeti. Odontocete crania are more asymmetric. These results indicate that (1) all mysticetes have symmetric crania, (2) archaeocete cranial asymmetry is not conspicuous in most of the skull but may yet be conspicuous in the rostrum, (3) directional cranial asymmetry is an odontocete specialization, and (4) directional cranial asymmetry is more likely related to echolocation than hearing.

Disproportionate Cochlear Length in Genus Homo Shows a High Phylogenetic Signal during Apes' Hearing Evolution

Changes in lifestyles and body weight affected mammal life-history evolution but little is known about how they shaped species’ sensory systems. Since auditory sensitivity impacts communication tasks and environmental acoustic awareness, it may have represented a deciding factor during mammal evolution, including apes. Here, we statistically measure the influence of phylogeny and allometry on the variation of five cochlear morphological features associated with hearing capacities across 22 living and 5 fossil catarrhine species. We find high phylogenetic signals for absolute and relative cochlear length only. Comparisons between fossil cochleae and reconstructed ape ancestral morphotypes show that Australopithecus absolute and relative cochlear lengths are explicable by phylogeny and concordant with the hypothetized ((Pan,Homo),Gorilla) and (Pan,Homo) most recent common ancestors. Conversely, deviations of the Paranthropus oval window area from these most recent common ancestors are not explicable by phylogeny and body weight alone, but suggest instead rapid evolutionary changes (directional selection) of its hearing organ. Premodern (Homo erectus) and modern human cochleae set apart from living non-human catarrhines and australopiths. They show cochlear relative lengths and oval window areas larger than expected for their body mass, two features corresponding to increased low-frequency sensitivity more recent than 2 million years ago. The uniqueness of the “hypertrophied” cochlea in the genus Homo (as opposed to the australopiths) and the significantly high phylogenetic signal of this organ among apes indicate its usefulness to identify homologies and monophyletic groups in the hominid fossil record.

Convergence vs. Specialization in the ear region of moles (Mammalia)

We investigated if and how the inner ear region undergoes similar adaptations in small, fossorial, insectivoran-grade mammals, and found a variety of inner ear phenotypes. In our sample, afrotherian moles (Chrysochloridae) and the marsupial Notoryctes differ from most other burrowing mammals in their relatively short radii of semicircular canal curvature; chrysochlorids and fossorial talpids share a relatively long interampullar width. Chrysochlorids are unique in showing a highly coiled cochlea with nearly four turns. Extensive cochlear coiling may reflect their greater ecological dependence on low frequency auditory cues compared to talpids, tenrecids, and the marsupial Notoryctes. Correspondingly, the lack of such extensive coiling in the inner ear of other fossorial species may indicate a greater reliance on other senses to enable their fossorial lifestyle, such as tactile sensation from vibrissae and Eimer's organs. The reliance of chrysochlorids on sound is evident in the high degree of coiling and in the diversity of its mallear types, and may help explain the lack of any semiaquatic members of that group. The simplest mallear types among chrysochlorids are not present in the basal-most members of that clade, but all extant chrysochlorids investigated to date exhibit extensive cochlear coiling. The chrysochlorid ear region thus exhibits mosaic evolution; our data suggest that extensive coiling evolved in chrysochlorids prior to and independently of diversification in middle ear ossicle size and shape.

Baleen boom and bust: a synthesis of mysticete phylogeny, diversity and disparity

A new, fully dated total-evidence phylogeny of baleen whales (Mysticeti) shows that evolutionary phases correlate strongly with Caenozoic modernization of the oceans and climates, implying a major role for bottom-up physical drivers. The phylogeny of 90 modern and dated fossil species suggests three major phases in baleen whale history: an early adaptive radiation (36-30 Ma), a shift towards bulk filter-feeding (30-23 Ma) and a climate-driven diversity loss around 3 Ma. Evolutionary rates and disparity were high following the origin of mysticetes around 38 Ma, coincident with global cooling, abrupt Southern Ocean eutrophication and the development of the Antarctic Circumpolar Current (ACC). Subsequently, evolutionary rates and disparity fell, becoming nearly constant after approximately 23 Ma as the ACC reached its full strength. By contrast, species diversity rose until 15 Ma and then remained stable, before dropping sharply with the onset of Northern Hemisphere glaciation. This decline coincided with the final establishment of modern mysticete gigantism and may be linked to glacially driven variability in the distribution of shallow habitats or an increased need for long-distance migration related to iron-mediated changes in glacial marine productivity.

Early development and orientation of the acoustic funnel provides insight into the evolution of sound reception pathways in cetaceans

Whales receive underwater sounds through a fundamentally different mechanism than their close terrestrial relatives. Instead of hearing through the ear canal, cetaceans hear through specialized fatty tissues leading to an evolutionarily novel feature: an acoustic funnel located anterior to the tympanic aperture. We traced the ontogenetic development of this feature in 56 fetal specimens from 10 different families of toothed (odontocete) and baleen (mysticete) whales, using X-ray computed tomography. We also charted ear ossification patterns through ontogeny to understand the impact of heterochronic developmental processes. We determined that the acoustic funnel arises from a prominent V-shaped structure established early in ontogeny, formed by the malleus and the goniale. In odontocetes, this V-formation develops into a cone-shaped funnel facing anteriorly, directly into intramandibular acoustic fats, which is likely functionally linked to the anterior orientation of sound reception in echolocation. In contrast, the acoustic funnel in balaenopterids rotates laterally, later in fetal development, consistent with a lateral sound reception pathway. Balaenids and several fossil mysticetes retain a somewhat anteriorly oriented acoustic funnel in the mature condition, indicating that a lateral sound reception pathway in balaenopterids may be a recent evolutionary innovation linked to specialized feeding modes, such as lunge-feeding.