Convergent evolution in toothed whale cochleae

The comparative anatomy of the petrosal bone and bony labyrinth of four small-sized deer

Here we provide complete 3D reconstructions of the petrosal bone and bony labyrinth of four kinds of small-sized deer (Elaphodus cephalophus, Muntiacus reevesi, Muntiacus muntjak, Hydropotes inermis) based on high-resolution CT scanning, and select one musk deer (Moschus moschiferus) as a comparative object. The petrosal bone and bony labyrinth of E. cephalophus are illustrated for the first time, as well as the petrosal bones of M. reevesi and H. inermis. Some morphological characters of petrosal bone and bony labyrinth can be used to distinguish the above-mentioned species. For example, M. moschiferus shows a prominent transpromontorial sulcus and a ventral basicapsular groove on the petrosal bone; there is a bifurcate cochlear aqueduct on the bony labyrinth of E. cephalophus; there is a distinct fusion between the lateral and posterior semicircular canals on the bony labyrinth of H. inermis. Meanwhile, there are some intraspecific variations on the subarcuate fossa, the tegmen tympani, the cochlear aqueduct, as well as the endolymphatic sac. Our results further confirm that the petrosal bone and bony labyrinth have enormous potential for taxonomy. This work will provide new anatomical data for the phylogenetic study of ruminants in the future, and it will be very practical to identify the isolated ruminants' petrosal bones that are frequently unearthed from paleontological or archeological sites.

Drivers of morphological evolution in the toothed whale jaw

Toothed whales (odontocetes) emit high-frequency underwater sounds (echolocate)-an extreme and unique innovation allowing them to sense their prey and environment. Their highly specialized mandible (lower jaw) allows high-frequency sounds to be transmitted back to the inner ear. Echolocation is evident in the earliest toothed whales, but little research has focused on the evolution of mandibular form regarding this unique adaptation. Here, we use a high-density, three-dimensional geometric morphometric analysis of 100 living and extinct cetacean species spanning their ∼50-million-year evolutionary history. Our analyses demonstrate that most shape variation is found in the relative length of the jaw and the mandibular symphysis. The greatest morphological diversity was obtained during two periods of rapid evolution: the initial evolution of archaeocetes (stem whales) in the early to mid-Eocene as they adapted to an aquatic lifestyle, representing one of the most extreme adaptive transitions known, and later on in the mid-Oligocene odontocetes as they became increasingly specialized for a range of diets facilitated by increasingly refined echolocation. Low disparity in the posterior mandible suggests the shape of the acoustic window, which receives sound, has remained conservative since the advent of directional hearing in the aquatic archaeocetes, even as the earliest odontocetes began to receive sounds from echolocation. Diet, echolocation, feeding method, and dentition type strongly influence mandible shape. Unlike in the toothed whale cranium, we found no significant asymmetry in the mandible. We suggest that a combination of refined echolocation and associated dietary specializations have driven morphology and disparity in the toothed whale mandible.

Ruminant inner ear shape records 35 million years of neutral evolution

Extrinsic and intrinsic factors impact diversity. On deep-time scales, the extrinsic impact of climate and geology are crucial, but poorly understood. Here, we use the inner ear morphology of ruminant artiodactyls to test for a deep-time correlation between a low adaptive anatomical structure and both extrinsic and intrinsic variables. We apply geometric morphometric analyses in a phylogenetic frame to X-ray computed tomographic data from 191 ruminant species. Contrasting results across ruminant clades show that neutral evolutionary processes over time may strongly influence the evolution of inner ear morphology. Extant, ecologically diversified clades increase their evolutionary rate with decreasing Cenozoic global temperatures. Evolutionary rate peaks with the colonization of new continents. Simultaneously, ecologically restricted clades show declining or unchanged rates. These results suggest that both climate and paleogeography produced heterogeneous environments, which likely facilitated Cervidae and Bovidae diversification and exemplifies the effect of extrinsic and intrinsic factors on evolution in ruminants.

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.

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.

Reassessment of the enigmatic ruminant Miocene genus Amphimoschus Bourgeois, 1873 (Mammalia, Artiodactyla, Pecora)

Amphimoschus is an extinct Eurasian ruminant genus, mostly recorded in Europe, without a close living relative and, hence, an unknown systematic position. This genus is known from around 50 localities from the late early to the middle Miocene. Two species were described during 180 years, but since their first description during the late 19^th century and early 20^th century, hardly any detailed taxonomic work has been done on the genus. Over the years, extensive collecting and excavating activities have enriched collections with more and more complete material of this still rare and enigmatic animal. Most interestingly, a number of skull remains have been unearthed and are promising in terms of providing phylogenetic information. In the present paper, we describe cranial material, the bony labyrinth, the dentition through 780 teeth and five skulls from different ontogenetic stages. We cannot find a clear morphometric distinction between the supposedly smaller and older species Amphimoschus artenensis and the supposedly younger and larger species A. ponteleviensis. Accordingly, we have no reason to retain the two species and propose, following the principle of priority (ICZN chapter 6 article 23), that only A. ponteleviensis Bourgeois, 1873 is valid. Our studies on the ontogenetic variation of Amphimoschus does reveal that the sagittal crest may increase in size and a supraorbital ridge may appear with age. Despite the abundant material, the family affiliation is still uncertain.

The so-called foramen singulare in cetacean periotics is actually the superior vestibular area

It is nearly 100 years ago that the "foramen singulare" was first identified in cetacean periotics. Since then, the "foramen singulare" has been recognized in periotics of many cetacean species, extant or extinct. Surprisingly, however, it has never been confirmed if the foramen singulare in cetacean periotics is really homologous to that in other mammals. It is known that in mammals including humans the posterior ampullary nerve, which innervates the posterior semicircular duct, passes through the foramen singulare. We use an X-ray micro-CT scan to examine endocasts of the bony labyrinth of the inner ear of cetacean periotics, showing that the osseous canal extending from the so-called foramen singulare goes toward the anterior bony ampulla, meaning that the alleged foramen singulare in cetacean periotics is really the superior vestibular area, through which the utriculoampullary nerve enters. The transverse crest is quite significant to identify each quadrant of the fundus of the internal acoustic meatus, but in many cetacean species the transverse crest is poorly developed, almost imperceptible in some species, and this could have brought confusion into the interpretation over the superior vestibular area and the foramen singulare. The bony septum separating the cerebral aperture of the facial canal from the foramen singulare is not the transverse crest, but the perpendicular crest. The foramen singulare is not a distinct foramen separated from the inferior vestibular area. Instead, the true foramen singulare opens near the inferior vestibular area in the internal acoustic meatus in cetacean periotics.

Wonky whales: the evolution of cranial asymmetry in cetaceans

BACKGROUND

Unlike most mammals, toothed whale (Odontoceti) skulls lack symmetry in the nasal and facial (nasofacial) region. This asymmetry is hypothesised to relate to echolocation, which may have evolved in the earliest diverging odontocetes. Early cetaceans (whales, dolphins, and porpoises) such as archaeocetes, namely the protocetids and basilosaurids, have asymmetric rostra, but it is unclear when nasofacial asymmetry evolved during the transition from archaeocetes to modern whales. We used three-dimensional geometric morphometrics and phylogenetic comparative methods to reconstruct the evolution of asymmetry in the skulls of 162 living and extinct cetaceans over 50 million years.

RESULTS

In archaeocetes, we found asymmetry is prevalent in the rostrum and also in the squamosal, jugal, and orbit, possibly reflecting preservational deformation. Asymmetry in odontocetes is predominant in the nasofacial region. Mysticetes (baleen whales) show symmetry similar to terrestrial artiodactyls such as bovines. The first significant shift in asymmetry occurred in the stem odontocete family Xenorophidae during the Early Oligocene. Further increases in asymmetry occur in the physeteroids in the Late Oligocene, Squalodelphinidae and Platanistidae in the Late Oligocene/Early Miocene, and in the Monodontidae in the Late Miocene/Early Pliocene. Additional episodes of rapid change in odontocete skull asymmetry were found in the Mid-Late Oligocene, a period of rapid evolution and diversification. No high-probability increases or jumps in asymmetry were found in mysticetes or archaeocetes. Unexpectedly, no increases in asymmetry were recovered within the highly asymmetric ziphiids, which may result from the extreme, asymmetric shape of premaxillary crests in these taxa not being captured by landmarks alone.

CONCLUSIONS

Early ancestors of living whales had little cranial asymmetry and likely were not able to echolocate. Archaeocetes display high levels of asymmetry in the rostrum, potentially related to directional hearing, which is lost in early neocetes-the taxon including the most recent common ancestor of living cetaceans. Nasofacial asymmetry becomes a significant feature of Odontoceti skulls in the Early Oligocene, reaching its highest levels in extant taxa. Separate evolutionary regimes are reconstructed for odontocetes living in acoustically complex environments, suggesting that these niches impose strong selective pressure on echolocation ability and thus increased cranial asymmetry.

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.