The better to eat you with: morphological disparity and enamel ultrastructure in odontocetes.

Variations in the shape and size of teeth have been associated with changes in enamel ultrastructure across odontocetes. Characterizing these features in extinct taxa can elucidate their functional morphology and feeding strategy, while also shedding light into macroevolutionary patterns during the evolutionary history of cetaceans. This study aimed to (1) describe the enamel and dentine ultrastructure of the Early Miocene odontocetes Notocetus vanbenedeni and Phoberodon arctirostris from Patagonia (Argentina) and (2) quantify tooth and enamel ultrastructure morphological disparity among odontocetes. Enamel was predominantly prismatic, thin in the anterior tooth of N. vanbenedeni and P. arctirostris; whilst thick on the posterior tooth of N. vanbenedeni. Together with skull morphology, data suggests a raptorial feeding strategy for P. arctirostris and a combination suction feeding method for N. vanbenedeni. Statistical analyses supported these inferences, indicating that enamel characters are useful for paleoecological research. Morphological disparity analyses showed that extant odontocetes occupy a larger morphospace and have more disparate morphologies, whilst extinct odontocetes were more similar among each other than with the extant group. There was no clear phylogenetic-based grouping, suggesting that tooth and enamel ultrastructure disparity were mainly driven by ecological pressures. These results highlight enamel ultrastructure as a source for broader-scale paleoecological studies in cetaceans.

New heterodont odontocetes from the Oligocene Pysht Formation in Washington State, U.S.A., and a reevaluation of Simocetidae (Cetacea, Odontoceti).

Odontocetes first appeared in the fossil record by the early Oligocene, and their early evolutionary history can provide clues as to how some of their unique adaptations, such as echolocation, evolved. Here, three new specimens from the early to late Oligocene Pysht Formation are described further increasing our understanding of the richness and diversity of early odontocetes, particularly for the North Pacific. Phylogenetic analysis shows that the new specimens are part of a more inclusive, redefined Simocetidae, which now includes Simocetus rayi, Olympicetus sp. 1, Olympicetus avitus, O. thalassodon sp. nov., and a large unnamed taxon (Simocetidae gen. et sp. A), all part of a North Pacific clade that represents one of the earliest diverging groups of odontocetes. Amongst these, Olympicetus thalassodon sp. nov. represents one of the best known simocetids, offering new information on the cranial and dental morphology of early odontocetes. Furthermore, the inclusion of CCNHM 1000, here considered to represent a neonate of Olympicetus sp., as part of the Simocetidae, suggests that members of this group may not have had the capability of ultrasonic hearing, at least during their early ontogenetic stages. Based on the new specimens, the dentition of simocetids is interpreted as being plesiomorphic, with a tooth count more akin to that of basilosaurids and early toothed mysticetes, while other features of the skull and hyoid suggest various forms of prey acquisition, including raptorial or combined feeding in Olympicetus spp., and suction feeding in Simocetus. Finally, body size estimates show that small to moderately large taxa are present in Simocetidae, with the largest taxon represented by Simocetidae gen. et sp. A with an estimated body length of 3 m, which places it as the largest known simocetid, and amongst the largest Oligocene odontocetes. The new specimens described here add to a growing list of Oligocene marine tetrapods from the North Pacific, further promoting faunistic comparisons across other contemporaneous and younger assemblages, that will allow for an improved understanding of the evolution of marine faunas in the region.

Cetacean tongue mobility and function: A comparative review.

Cetaceans are atypical mammals whose tongues often depart from the typical (basal) mammalian condition in structure, mobility, and function. Their tongues are dynamic, innovative multipurpose tools that include the world's largest muscular structures. These changes reflect the evolutionary history of cetaceans' secondary adaptation to a fully aquatic environment. Cetacean tongues play no role in mastication and apparently a greatly reduced role in nursing (mainly channeling milk ingestion), two hallmarks of Mammalia. Cetacean tongues are not involved in drinking, breathing, vocalizing, and other non-feeding activities; they evidently play no or little role in taste reception. Although cetaceans do not masticate or otherwise process food, their tongues retain key roles in food ingestion, transport, securing/positioning, and swallowing, though by different means than most mammals. This is due to cetaceans' aquatic habitat, which in turn altered their anatomy (e.g., the intranarial larynx and consequent soft palate alteration). Odontocetes ingest prey via raptorial biting or tongue-generated suction. Odontocete tongues expel water and possibly uncover benthic prey via hydraulic jetting. Mysticete tongues play crucial roles driving ram, suction, or lunge ingestion for filter feeding. The uniquely flaccid rorqual tongue, not a constant volume hydrostat (as in all other mammalian tongues), invaginates into a balloon-like pouch to temporarily hold engulfed water. Mysticete tongues also create hydrodynamic flow regimes and hydraulic forces for baleen filtration, and possibly for cleaning baleen. Cetacean tongues lost or modified much of the mobility and function of generic mammal tongues, but took on noteworthy morphological changes by evolving to accomplish new tasks.

Cranial and endocranial anatomy of a three-dimensionally preserved teleosauroid thalattosuchian skull.

Thalattosuchians represent one of the several independent transitions into the marine realm among crocodylomorphs. The extent of their aquatic adaptations ranges from the semiaquatic teleosauroids, superficially resembling extant gharials, to the almost cetacean-like pelagic metriorhynchids. Understanding the suite of osteological, physiological, and sensory changes that accompanied this major transition has received increased attention, but is somewhat hindered by a dearth of complete three-dimensionally preserved crania. Here, we describe the cranial and endocranial anatomy of a well-preserved three-dimensional specimen of Macrospondylus bollensis from the Toarcian of Yorkshire, UK. The trigeminal fossa contains two similar-sized openings separated by a thin lamina of prootic, a configuration that appears unique to a subset of teleosauroids. Macrospondylus bollensis resembles other thalattosuchians in having pyramidal semicircular canals with elongate cochlear ducts, enlarged carotid canals leading to an enlarged pituitary fossa, enlarged orbital arteries, enlarged endocranial venous sinuses, reduced pharyngotympanic sinuses, and a relatively straight brain with a hemispherical cerebral expansion. We describe for the first time the olfactory region and paranasal sinuses of a teleosauroid. A relatively large olfactory region suggests greater capacity for airborne olfaction in teleosauroids than in the more aquatically adapted metriorhynchoids. Additionally, slight swellings in the olfactory region suggest the presence of small salt glands of lower secretory capacity than those of metriorhynchoids. The presence of osteological correlates for salt glands in a teleosauroid corroborates previous hypotheses that these glands originated in the common ancestor of Thalattosuchia, facilitating their rapid radiation into the marine realm.

Early giant reveals faster evolution of large body size in ichthyosaurs than in cetaceans.

Body sizes of marine amniotes span six orders of magnitude, yet the factors that governed the evolution of this diversity are largely unknown. High primary production of modern oceans is considered a prerequisite for the emergence of cetacean giants, but that condition cannot explain gigantism in Triassic ichthyosaurs. We describe the new giant ichthyosaur Cymbospondylus youngorum sp. nov. with a 2-meter-long skull from the Middle Triassic Fossil Hill Fauna of Nevada, USA, underscoring rapid size evolution despite the absence of many modern primary producers. Surprisingly, the Fossil Hill Fauna rivaled the composition of modern marine mammal faunas in terms of size range, and energy-flux models suggest that Middle Triassic marine food webs were able to support several large-bodied ichthyosaurs at high trophic levels, shortly after ichthyosaur origins.

The pattern of brain-size change in the early evolution of cetaceans.

Most authors have identified two rapid increases in relative brain size (encephalization quotient, EQ) in cetacean evolution: first at the origin of the modern suborders (odontocetes and mysticetes) around the Eocene-Oligocene transition, and a second at the origin of the delphinoid odontocetes during the middle Miocene. We explore how methods used to estimate brain and body mass alter this perceived timing and rate of cetacean EQ evolution. We provide new data on modern mammals (mysticetes, odontocetes, and terrestrial artiodactyls) and show that brain mass and endocranial volume scale allometrically, and that endocranial volume is not a direct proxy for brain mass. We demonstrate that inconsistencies in the methods used to estimate body size across the Eocene-Oligocene boundary have caused a spurious pattern in earlier relative brain size studies. Instead, we employ a single method, using occipital condyle width as a skeletal proxy for body mass using a new dataset of extant cetaceans, to clarify this pattern. We suggest that cetacean relative brain size is most accurately portrayed using EQs based on the scaling coefficients as observed in the closely related terrestrial artiodactyls. Finally, we include additional data for an Eocene whale, raising the sample size of Eocene archaeocetes to seven. Our analysis of fossil cetacean EQ is different from previous works which had shown that a sudden increase in EQ coincided with the origin of odontocetes at the Eocene-Oligocene boundary. Instead, our data show that brain size increased at the origin of basilosaurids, 5 million years before the Eocene-Oligocene transition, and we do not observe a significant increase in relative brain size at the origin of odontocetes.

The isotopic niche of Atlantic, biting marine mammals and its relationship to skull morphology and body size.

Understanding the trophic niches of marine apex predators is necessary to understand interactions between species and to achieve sustainable, ecosystem-based fisheries management. Here, we review the stable carbon and nitrogen isotope ratios for biting marine mammals inhabiting the Atlantic Ocean to test the hypothesis that the relative position of each species within the isospace is rather invariant and that common and predictable patterns of resource partitioning exists because of constrains imposed by body size and skull morphology. Furthermore, we analyze in detail two species-rich communities to test the hypotheses that marine mammals are gape limited and that trophic position increases with gape size. The isotopic niches of species were highly consistent across regions and the topology of the community within the isospace was well conserved across the Atlantic Ocean. Furthermore, pinnipeds exhibited a much lower diversity of isotopic niches than odontocetes. Results also revealed body size as a poor predictor of the isotopic niche, a modest role of skull morphology in determining it, no evidence of gape limitation and little overlap in the isotopic niche of sympatric species. The overall evidence suggests limited trophic flexibility for most species and low ecological redundancy, which should be considered for ecosystem-based fisheries management.

Genomic and anatomical comparisons of skin support independent adaptation to life in water by cetaceans and hippos.

The macroevolutionary transition from terra firma to obligatory inhabitance of the marine hydrosphere has occurred twice in the history of Mammalia: Cetacea and Sirenia. In the case of Cetacea (whales, dolphins, and porpoises), molecular phylogenies provide unambiguous evidence that fully aquatic cetaceans and semiaquatic hippopotamids (hippos) are each other's closest living relatives. Ancestral reconstructions suggest that some adaptations to the aquatic realm evolved in the common ancestor of Cetancodonta (Cetacea + Hippopotamidae). An alternative hypothesis is that these adaptations evolved independently in cetaceans and hippos. Here, we focus on the integumentary system and evaluate these hypotheses by integrating new histological data for cetaceans and hippos, the first genome-scale data for pygmy hippopotamus, and comprehensive genomic screens and molecular evolutionary analyses for protein-coding genes that have been inactivated in hippos and cetaceans. We identified eight skin-related genes that are inactivated in both cetaceans and hippos, including genes that are related to sebaceous glands, hair follicles, and epidermal differentiation. However, none of these genes exhibit inactivating mutations that are shared by cetaceans and hippos. Mean dates for the inactivation of skin genes in these two clades serve as proxies for phenotypic changes and suggest that hair reduction/loss, the loss of sebaceous glands, and changes to the keratinization program occurred ∼16 Ma earlier in cetaceans (∼46.5 Ma) than in hippos (∼30.5 Ma). These results, together with histological differences in the integument and prior analyses of oxygen isotopes from stem hippopotamids ("anthracotheres"), support the hypothesis that aquatic skin adaptations evolved independently in hippos and cetaceans.

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.

A natural endocast of an early Miocene odontocete and its implications in cetacean brain evolution.

The natural endocast Museo di Geologia e Paleontologia of the Università degli Studi di Torino (MGPT)-PU 13873 is described and analyzed in order to interpret its taxonomic affinities and its potential significance on our understanding of cetacean brain evolution. The endocast is from the early Miocene of Piedmont (between ca. 19 and 16 million years ago), Northwestern Italy, and shows a number of plesiomorphic characters. These include: scarcely rounded cerebral hemispheres, cerebellum exposed in dorsal view with little superimposition by the cerebral hemispheres, short temporal lobe, and long sylvian fissure. The distance between the hypophysis and the rostral pons is particularly high, as it was determined by the calculus of the hypothalamus quotient, suggesting that the development of a deep interpeduncular fossa was not as advanced as in living odontocetes. The encephalization quotient (EQ) of MGPT-PU 13873 is ~1.81; therefore, this specimen shows an EQ in line with other fossil whales of the same geological age (early Miocene). Comparative analysis shows that there is a critical lack of data from the late Miocene and Pliocene that prevents us to fully understand the recent evolution of the EQ diversity in whales. Moreover, the past diversity of brain size and shape in mysticetes is virtually unknown. All these observations point to the need of additional efforts to uncover evolutionary patterns and processes on cetacean brain evolution.

Microscopic anatomical, immunohistochemical, and morphometric characterization of the terminal airways of the lung in cetaceans.

The lungs of cetaceans undergo anatomical and physiological adaptations that facilitate extended breath-holding during dives. Here, we present new insights on the ontogeny of the microscopic anatomy of the terminal portion of the airways of the lungs in five cetacean species: the fin whale (Balaenoptera physalus); the sperm whale (Physeter macrocephalus), the Cuvier's beaked whale (Ziphius cavirostris); the bottlenose dolphin (Tursiops truncatus); and the striped dolphin (Stenella coeruleoalba). We (a) studied the histology of the terminal portion of the airways; (b) used immunohistochemistry (IHC) to characterize the muscle fibers with antibodies against smooth muscle (sm-) actin, sm-myosin, and desmin; (c) the innervation of myoelastic sphincters (MESs) with an antibody against neurofilament protein; and (d) defined the diameter of the terminal bronchioles, the diameter and length of the alveoli, the thickness of the septa, the major and minor axis, perimeter and section area of the cartilaginous rings by quantitative morphometric analyses in partially inflated lung tissue. As already reported in the literature, in bottlenose and striped dolphins, a system of MESs was observed in the terminal bronchioles. Immunohistochemistry confirmed the presence of smooth muscle in the terminal bronchioles, alveolar ducts, and alveolar septa in all the examined species. Some neurofilaments were observed close to the MESs in both bottlenose and striped dolphins. In fin, sperm, and Cuvier's beaked whales, we noted a layer of longitudinal smooth muscle going from the terminal bronchioles to the alveolar sacs. The morphometric analysis allowed to quantify the structural differences among cetacean species by ranking them into groups according to the adjusted mean values of the morphometric parameters measured. Our results contribute to the current understanding of the anatomy of the terminal airways of the cetacean lung and the role of the smooth muscle in the alveolar collapse reflex, crucial for prolonged breath-holding diving.

The oblique extraocular muscles in cetaceans: Overall architecture and accessory insertions.

The oblique extraocular muscles (EOMs) were dissected in 19 cetacean species and 10 non-cetacean mammalian species. Both superior oblique (SO) and inferior oblique (IO) muscles in cetaceans are well developed in comparison to out-groups and have unique anatomical features likely related to cetacean orbital configurations, swimming mechanics, and visual behaviors. Cetacean oblique muscles originate at skeletal locations typical for mammals: SO, from a common tendinous cone surrounding the optic nerve and from the medially adjacent bone surface at the orbital apex; IO, from the maxilla adjacent to lacrimal and frontal bones. However, because of the unusual orbital geometry in cetaceans, the paths and relations of SO and IO running toward their insertions onto the temporal ocular sclera are more elaborate than in humans and most other mammals. The proximal part of the SO extends from its origin at the apex along the dorsomedial aspect of the orbital contents to a strong fascial connection proximal to the preorbital process of the frontal bone, likely the cetacean homolog of the typical mammalian trochlea. However, the SO does not turn at this connection but continues onward, still a fleshy cylinder, until turning sharply as it passes through the external circular muscle (ECM) and parts of the palpebral belly of the superior rectus muscle. Upon departing this "functional trochlea" the SO forms a primary scleral insertion and multiple accessory insertions (AIs) onto adjacent EOM tendons and fascial structures. The primary SO scleral insertions are broad and muscular in most cetacean species examined, while in the mysticete minke whale (Balaenoptera acutorostrata) and fin whale (Balaenoptera physalus) the muscular SO bellies transition into broad fibrous tendons of insertion. The IO in cetaceans originates from an elongated fleshy attachment oriented laterally on the maxilla and continues laterally as a tubular belly before turning caudally at a sharp bend where it is constrained by the ECM and parts of the inferior rectus which form a functional trochlea as with the SO. The IO continues to a fleshy primary insertion on the temporal sclera but, as with SO, also has multiple AIs onto adjacent rectus tendons and connective tissue. The multiple IO insertions were particularly well developed in pygmy sperm whale (Kogia breviceps), minke whale and fin whale. AIs of both SO and IO muscles onto multiple structures as seen in cetaceans have been described in humans and domesticated mammals. The AIs of oblique EOMs seen in all these groups, as well as the unique "functional trochleae" of cetacean SO and IO seem likely to function in constraining the lines of action at the primary scleral insertions of the oblique muscles. The gimble-like sling formed by SO and IO in cetaceans suggest that the "primary" actions of the cetacean oblique EOMs are not only to produce ocular counter-rotations during up-down pitch movements of the head during swimming but also to rotate the plane containing the functional origins of the rectus muscles during other gaze changes.

Inner ear sensory system changes as extinct crocodylomorphs transitioned from land to water.

Major evolutionary transitions, in which animals develop new body plans and adapt to dramatically new habitats and lifestyles, have punctuated the history of life. The origin of cetaceans from land-living mammals is among the most famous of these events. Much earlier, during the Mesozoic Era, many reptile groups also moved from land to water, but these transitions are more poorly understood. We use computed tomography to study changes in the inner ear vestibular system, involved in sensing balance and equilibrium, as one of these groups, extinct crocodile relatives called thalattosuchians, transitioned from terrestrial ancestors into pelagic (open ocean) swimmers. We find that the morphology of the vestibular system corresponds to habitat, with pelagic thalattosuchians exhibiting a more compact labyrinth with wider semicircular canal diameters and an enlarged vestibule, reminiscent of modified and miniaturized labyrinths of other marine reptiles and cetaceans. Pelagic thalattosuchians with modified inner ears were the culmination of an evolutionary trend with a long semiaquatic phase, and their pelagic vestibular systems appeared after the first changes to the postcranial skeleton that enhanced their ability to swim. This is strikingly different from cetaceans, which miniaturized their labyrinths soon after entering the water, without a prolonged semiaquatic stage. Thus, thalattosuchians and cetaceans became secondarily aquatic in different ways and at different paces, showing that there are different routes for the same type of transition.

Comparative morphology, histology, and cytology of odontocete cetaceans prostates.

The prostate is the only male accessory gland in cetaceans. However, little is known about this organ in these species. Anatomical and histological characteristics of the prostate have been described in only a few cetacean species, further, one study reported a high incidence of prostatic pathologies in cetaceans that may impair reproduction. The objective of this work was to describe and compare the morphological, histological, and cytological characteristics of the prostate in different odontocete cetaceans. To this end, the prostate glands of 47 animals from nine different species of cetaceans were macroscopically and microscopically studied. Members of the families Delphinidae, Ziphiidae, and Physeteridae were included. In general, the prostate appeared as a musculo-glandular organ with two distinct parts-the Corpus prostatae and the Pars disseminata prostatae. In the pygmy sperm whale (Kogia breviceps) and the Cuvier's beaked whale (Ziphius cavirostris), the prostate was a discrete gland with a small Corpus prostatae. Microscopically, the prostates of different delphinids species shared similarities; however, the prostate of the pygmy sperm whale revealed significant histological differences compared to those of the delphinids. Immunohistochemical analysis was performed using low- and high-molecular-weight cytokeratin, vimentin, and prostatic specific antigen commercial antibodies. Electron microscopy analysis was performed on the prostate of a bottlenose dolphin and the cytomorphological differences among the major epithelial components of the prostatic epithelium were described. Anat Rec, 2019. © 2019 American Association for Anatomy Anat Rec, 303:2036-2053, 2020. © 2019 American Association for Anatomy.

Cetacean Orbital Muscles: Anatomy and Function of the Circular Layers.

Dissections of cetacean orbits identified two distinct circular muscle layers that are uniquely more elaborate than the orbitalis muscles described in numerous mammals. The circular orbital muscles in cetaceans form layers that lie both external and internal to the rectus extra ocular muscles (EOMs). A cone-shaped external circular muscle (ECM) that invests the external surface of the rectus EOMs was found in all cetacean specimens examined. The cetacean ECM corresponds generally to descriptions of the musculus orbitalis in various mammals but is more strongly developed and has more layers than in noncetaceans. A newly identified internal circular muscle (ICM) is located internal to the rectus EOMs and external to the retractor bulbi (RB). The RB is massive in cetaceans and is encased in a connective tissue layer containing convoluted bundles of blood vessels. The most robust ECM and ICM layers were in sperm whale (Physeter macrocephalus) where they form complete rings. Surprisingly, histological analysis showed the sperm whale ECM to contain both smooth and striated (skeletal) muscle layers while the ICM appeared to contain solely skeletal muscle fibers. The extreme development of the ECM (orbitalis) and RB suggest a co-evolved system mediating high degrees of protrusion and retraction in cetaceans. We know of no homolog of the ICM but its function seems likely related to the complex vascular structures surrounding and deep to the retractor muscle. Skeletal muscle components in orbital circular muscles appear to be highly derived specializations unknown outside of cetaceans. Anat Rec, 2019. © 2019 American Association for Anatomy Anat Rec, 303:1792-1811, 2020. © 2019 American Association for Anatomy.

Accelerated evolution and diversifying selection drove the adaptation of cetacean bone microstructure.

BACKGROUND: The transition from land to sea by the ancestor of cetaceans approximately 50 million years ago was an incredible evolutionary event that led to a series of morphological, physiological, and behavioral adaptations. During this transition, bone microstructure evolved from the typical terrestrial form to the specialized structure found in modern cetaceans. While the bone microstructure of mammals has been documented before, investigations of its genetic basis lag behind. The increasing number of cetaceans with whole-genome sequences available may shed light on the mechanism underlying bone microstructure evolution as a result of land to water transitions. RESULTS: Cetacean bone microstructure is consistent with their diverse ecological behaviors. Molecular evolution was assessed by correlating bone microstructure and gene substitution rates in terrestrial and aquatic species, and by detecting genes under positive selection along ancestral branches of cetaceans. We found that: 1) Genes involved in osteoclast function are under accelerated evolution in cetaceans, suggestive of important roles in bone remodeling during the adaptation to an aquatic environment; 2) Genes in the Wnt pathway critical for bone development and homeostasis show evidence of divergent evolution in cetaceans; 3) Several genes encoding bone collagens are under selective pressure in cetaceans. CONCLUSIONS: Our results suggest that evolutionary pressures have shaped the bone microstructure of cetaceans, to facilitate life in diverse aquatic environments.

Co-evolution of cerebral and cerebellar expansion in cetaceans.

Cetaceans possess brains that rank among the largest to have ever evolved, either in terms of absolute mass or relative to body size. Cetaceans have evolved these huge brains under relatively unique environmental conditions, making them a fascinating case study to investigate the constraints and selection pressures that shape how brains evolve. Indeed, cetaceans have some unusual neuroanatomical features, including a thin but highly folded cerebrum with low cortical neuron density, as well as many structural adaptations associated with acoustic communication. Previous reports also suggest that at least some cetaceans have an expanded cerebellum, a brain structure with wide-ranging functions in adaptive filtering of sensory information, the control of motor actions, and cognition. Here, we report that, relative to the size of the rest of the brain, both the cerebrum and cerebellum are dramatically enlarged in cetaceans and show evidence of co-evolution, a pattern of brain evolution that is convergent with primates. However, we also highlight several branches where cortico-cerebellar co-evolution may be partially decoupled, suggesting these structures can respond to independent selection pressures. Across cetaceans, we find no evidence of a simple linear relationship between either cerebrum and cerebellum size and the complexity of social ecology or acoustic communication, but do find evidence that their expansion may be associated with dietary breadth. In addition, our results suggest that major increases in both cerebrum and cerebellum size occurred early in cetacean evolution, prior to the origin of the major extant clades, and predate the evolution of echolocation.

Odontocete peduncle tendons for possible control of fluke orientation and flexibility.

Dorso-ventral oscillations of cetacean caudal flukes generate lift-based thrust for swimming. Movements of the flukes are actuated by epaxial and hypaxial muscles through caudal tendons inserting onto vertebrate in the peduncle. To determine if the caudal tendons in the peduncle affect the flexibility of the flukes, we must understand how the tendons from axial muscles insert onto the caudal vertebrate. The purpose of this study was to provide a detailed description of the various tendons within the cetacean peduncle with regard to their role in swimming and flexibility. Dissection of the peduncle and flukes of multiple odontocete species showed that there were two distinct epaxial tendon sets within the peduncle: (1) extensor caudae medialis tendon (ECM) and (2) extensor caudae lateralis tendon (ECL). There is one distinct hypaxial tendon set, the medial tendon of the hypaxialis lumborum (MHL). The ECM and MHL tendons inserted serially onto caudal vertebrae while the ECL inserted exclusively onto the terminal vertebrae posterior to the fluke insertion. It is typical that tendons insert onto bone, however, the connection to the core fibrous layer of the flukes suggests an element of active control of the flexibility of the flukes via the axial muscles. Tension from muscular contraction transmitted through the tendons could affect both spanwise and chordwise flexibility. Changing flexibility could modulate thrust and efficiency over an extended operation range of swimming speeds in cetaceans.

Flipper bone distribution reveals flexible trailing edge in underwater flying marine tetrapods.

Hydrofoil-shaped limbs (flipper-hydrofoils) have evolved independently several times in secondarily marine tetrapods and generally fall into two functional categories: (1) those that produce the majority of thrust during locomotion (propulsive flipper-hydrofoils); (2) those used primarily to steer and resist destabilizing movements such as yaw, pitch, and roll (controller flipper-hydrofoils). The morphological differences between these two types have been poorly understood. Theoretical and experimental studies on engineered hydrofoils suggest that flapping hydrofoils with a flexible trailing edge are more efficient at producing thrust whereas hydrofoils used in steering and stabilization benefit from a more rigid one. To investigate whether the trailing edge is generally more flexible in propulsive flipper-hydrofoils, we compared the bone distribution along the chord in both flipper types. The propulsive flipper-hydrofoil group consists of the forelimbs of Chelonioidea, Spheniscidae, and Otariidae. The controller flipper-hydrofoil group consists of the forelimbs of Cetacea. We quantified bone distribution from radiographs of species representing more than 50% of all extant genera for each clade. Our results show that the proportion of bone in both groups is similar along the leading edge (0-40% of the chord) but is significantly less along the trailing edge for propulsive flipper-hydrofoils (40-80% of the chord). Both flipper-hydrofoil types have little to no bony tissue along the very edge of the trailing edge (80-100% of the chord). This suggests a relatively flexible trailing edge for propulsive flipper-hydrofoils compared to controller flipper-hydrofoils in line with findings from prior studies. This study presents a morphological correlate for inferring flipper-hydrofoil function in extinct taxa and highlights the importance of a flexible trailing edge in the evolution of propulsive flipper-hydrofoils in marine tetrapods.

Unexpected evolutionary patterns of dental ontogenetic traits in cetartiodactyl mammals.

Studying ontogeny in both extant and extinct species can unravel the mechanisms underlying mammal diversification and specialization. Among mammalian clades, Cetartiodactyla encompass species with a wide range of adaptations, and ontogenetic evidence could clarify longstanding debates on the origins of modern specialized families. Here, we study the evolution of dental eruption patterns in early diverging cetartiodactyls to assess the ecological and biological significance of this character and shed new light on phylogenetic issues. After investigation of the ontogenetic dental series of 63 extinct genera, our parsimony reconstructions of eruption state evolution suggest that the eruption of molars before permanent premolars represents a plesiomorphic condition within Cetartiodactyla. This result substantially differs from a previous study based on modern species only. As a result, the presence of this pattern in most ruminants might represent an ancestral condition contributing to their specialized herbivory, rather than an original adaptation. In contrast, the late eruption of molars in hippopotamoids is more likely related to biological aspects, such as increases in body mass and slower pace of life. Our study mainly shows that eruption sequences reliably characterize higher level cetartiodactyl taxa and could represent a new source of phylogenetic characters, especially to disentangle the origin of hippopotamoids and cetaceans.