RESUMEN

Ontocetus is one of the most notable extinct odobenines owing to its global distribution in the Northern Hemisphere. Originating in the Late Miocene of the western North Pacific, this lineage quickly spread to the Atlantic Ocean during the Pliocene, with notable occurrences in England, Belgium, The Netherlands, Morocco and the eastern seaboard of the United States. Reassessment of a pair of mandibles from the Lower Pleistocene of Norwich (United Kingdom) and a mandible from the Upper Pliocene of Antwerp (Belgium) that were referred to as Ontocetus emmonsi reveals existences of features of both Ontocetus and Odobenus. The presence of four post-canine teeth, a lower canine larger than the cheek-teeth and a lower incisor confirms the assignment to Ontocetus; simultaneously, characteristics such as a fused and short mandibular symphysis, a well-curved mandibular arch and thin septa between teeth align with traits usually found in Odobenus. Based on a combination of these characters, we describe Ontocetus posti, sp. nov. Its mandibular anatomy suggests, a better adaptation to suction-feeding than what was previously described in the genus suggesting that Ontocetus posti sp. nov. likely occupied a similar ecological niche to the extant walrus Odobenus rosmarus. Originating from the North Pacific Ocean, Ontocetus most likely dispersed via the Central American Seaway. Although initially discovered in the Lower Pliocene deposits of the western North Atlantic, Ontocetus also left its imprint in the North Sea basin and Moroccan Plio-Pleistocene deposits. The closure of the Isthmus of Panama during the Mio-Pliocene boundary significantly impacted the contemporary climate, inducing global cooling. This event constrained Ontocetus posti in the North Sea basin leaving the taxon unable to endure the abrupt climate changes of the Early Pleistocene, ultimately going extinct before the arrival of the extant counterpart, Odobenus rosmarus.

Asunto(s)

RESUMEN

The neurocranial elevation generated by axial muscles is widespread among aquatic gnathostomes. The mechanism has two functions: first, it contributes to the orientation of the mouth gape, and second, it is involved in suction feeding. To provide such mobility, anatomical specialization of the anterior part of the vertebral column has evolved in many fish species. In modern chimaeras, the anterior part of the vertebral column develops into the synarcual. Possible biological roles of the occipital-synarcual joint have not been discussed before. Dissections of the head of two species of ratfishes (Chimaera monstrosa and Chimaera phantasma) confirmed the heterocoely of the articulation surface between the synarcual and the neurocranium, indicating the possibility of movements in the sagittal and frontal planes. Muscles capable of controlling the movements of the neurocranium were described. The m. epaxialis is capable of elevating the head, the m. coracomandibularis is capable of lowering it if the mandible is anchored by the adductor. Lateral flexion is performed by the m. lateroventralis, for which this function was proposed for the first time. The first description of the m. epaxialis profundus is given, its function is to be elucidated in the future. Manipulations with joint preparations revealed a pronounced amplitude of movement in the sagittal and frontal planes. Since chimaeras generate weak decrease in pressure in the oropharyngeal cavity when sucking in prey, we hypothesised the primary effect of neurocranial elevation, in addition to the evident lateral head mobility, is accurate prey targeting.

Asunto(s)

RESUMEN

The evolution of the tongue in tetrapods is associated with feeding in the terrestrial environment. This study analyzes the tongue morphology of two closely related frog species, Telmatobius oxycephalus and T. rubigo, which exhibit contrasting feeding mechanisms. Telmatobius oxycephalus, a semi-aquatic species, relies on its tongue to capture terrestrial prey whereas T. rubigo, a secondarily aquatic species, uses suction feeding not involving the tongue. Through anatomical, histological and scanning electron microscopy analyses, we revealed remarkable differences in tongue morphology between these species. Telmatobius oxycephalus exhibits a well-developed tongue whose dorsal epithelium has numerous and slender filiform papillae. The epithelial cells of the papillae are protruded and have a complex array of microridges. In contrast, T. rubigo possesses a reduced tongue with flat and less numerous filiform papillae. The epithelial cells are completely flat and lack microridges. These findings highlight the remarkable adaptability of lingual morphology in Telmatobius to respond to the contrasting ecological niches and prey capture mechanisms. This study sheds light on the relationship between tongue shape and the different functional demands, contributing to our understanding of the evolution of prey capture mechanisms in amphibians.

Asunto(s)

RESUMEN

The fully aquatic Japanese giant salamander (Andrias japonicus) is a member of the Cryptobranchidae, and is currently distributed in western Japan, with other members of this group restricted to China and North America. Their feeding behaviour is characterized by a form of suction feeding that includes asymmetric movements of the jaw and hyobranchial apparatus. Previous studies on the North American species, Cryptobranchus alleganiensis, have suggested that this specialized jaw movement is produced by a flexible quadrate-articular joint combined with a loosely connected lower jaw symphysis including two small fibrocartilaginous pads. However, little is known about this feeding behaviour in the Asian species, nor have the three-dimensional asymmetric jaw movements been fully investigated in any member of Cryptobranchidae. In this study, we explore the asymmetric jaw movements in A. japonicus using three methods: (1) dissection of musculoskeletal structures; (2) filming of feeding behaviour to understand in which situations asymmetric feeding is used; (3) analysis of 3D movement of jaws and skull. In the third component, fresh (from frozen) specimens of A. japonicus were manipulated to replicate asymmetric and symmetric jaw movements, with the specimens CT scanned after each step to obtain the 3D morphology of the jaws at different positions. These positions were combined and their Euler angles from resting (closed) jaw position were calculated for asymmetric or symmetric jaw positions. Our filming revealed that asymmetric jaw movements are linked to the position of the prey in relation to the snout, with the jaw closest to the prey opening asymmetrically. Moreover, this action allows the salamander to simultaneously grasp prey in one side of the mouth while ejecting water on the other side, if the first suction attempt fails. The asymmetric jaw movements are performed mainly by rotation of the mandible about its long axis, with very limited lateral jaw movements. During asymmetric and symmetric jaw movements, the posterior ends of the maxilla and quadrate move slightly. The asymmetric jaw movements are permitted by a mobile quadrate-articular joint formed by wide, round cartilages, and by two small fibrocartilage pads within the jaw symphysis that act as cushions during jaw rotation. Some of these soft tissue structures leave traces on the jaws and skull, allowing feeding mode to be reconstructed in fossil taxa. Understanding cryptobranchid asymmetric jaw movement thus requires a comprehensive assessment of not only the symphysial morphology but also that of other cranial and hyobranchial elements.

Asunto(s)

RESUMEN

Throughout evolution, organisms repeatedly developed elastic elements to power explosive body motions, overcoming ubiquitous limits on the power capacity of fast-contracting muscles. Seahorses evolved such a latch-mediated spring-actuated (LaMSA) mechanism; however, it is unclear how this mechanism powers the two complementary functions necessary for feeding: rapidly swinging the head towards the prey, and sucking water into the mouth to entrain it. Here, we combine flow visualization and hydrodynamic modelling to estimate the net power required for accelerating the suction feeding flows in 13 fish species. We show that the mass-specific power of suction feeding in seahorses is approximately three times higher than the maximum recorded from any vertebrate muscle, resulting in suction flows that are approximately eight times faster than similar-sized fishes. Using material testing, we reveal that the rapid contraction of the sternohyoideus tendons can release approximately 72% of the power needed to accelerate the water into the mouth. We conclude that the LaMSA system in seahorses is powered by two elastic elements, the sternohyoideus and epaxial tendons. These elements jointly actuate the coordinated acceleration of the head and the fluid in front of the mouth. These findings extend the known function, capacity and design of LaMSA systems.

Asunto(s)

RESUMEN

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.

Asunto(s)

RESUMEN

The Carboniferous (358.9 to 298.9 Ma) saw the emergence of marine ecosystems dominated by modern vertebrate groups, including abundant stem-group holocephalans (chimaeras and relatives). Compared with the handful of anatomically conservative holocephalan genera alive today-demersal durophages all-these animals were astonishingly morphologically diverse, and bizarre anatomies in groups such as iniopterygians hint at specialized ecological roles foreshadowing those of the later, suction-feeding neopterygians. However, flattened fossils usually obscure these animals' functional morphologies and how they fitted into these important early ecosystems. Here, we use three-dimensional (3D) methods to show that the musculoskeletal anatomy of the uniquely 3D-preserved iniopterygian Iniopera can be best interpreted as being similar to that of living holocephalans rather than elasmobranchs but that it was mechanically unsuited to durophagy. Rather, Iniopera had a small, anteriorly oriented mouth aperture, expandable pharynx, and strong muscular links among the pectoral girdle, neurocranium, and ventral pharynx consistent with high-performance suction feeding, something exhibited by no living holocephalan and never clearly characterized in any of the extinct members of the holocephalan stem-group. Remarkably, in adapting a distinctly holocephalan anatomy to suction feeding, Iniopera is more comparable to modern tetrapod suction feeders than to the more closely related high-performance suction-feeding elasmobranchs. This raises questions about the assumed role of durophagy in the evolution of holocephalans' distinctive anatomy and offers a rare glimpse into the breadth of ecological niches filled by holocephalans in a pre-neopterygian world.

Asunto(s)

RESUMEN

A simple hydrodynamic model of predator-prey interactions between larval clownfish and copepod prey is used to elucidate how larval fish capture highly evasive copepods. Fish larvae are considered to be suction feeders; however, video observations revealed that successful captures by clownfish larvae were preceded by rapidly accelerating lunges (ram), while the role of suction to draw prey into the fish's mouth was less clear. Simulations were made of the fish's strike, varying strengths of ram and suction to characterize optimal strategies for copepod capture given known evasive capabilities. Our results suggest that, contrary to expectations, suction feeding is dominant only in older larvae, whereas ram feeding is the dominant mode for early larvae. Despite the relatively weak suction produced by smaller larvae, it still plays a crucial role in prey capture through hydrodynamic stealth. Escape-triggering water deformations from the strike can be cancelled through controlled suction. Experimental data obtained from larval clownfish agree with model results, suggesting that the primary role of suction in early larvae is providing hydrodynamic stealth rather than capture.

RESUMEN

Diversity of feeding mechanisms is a hallmark of reef fishes, but the history of this variation is not fully understood. Here, we explore the emergence and proliferation of a biting mode of feeding, which enables fishes to feed on attached benthic prey. We find that feeding modes other than suction, including biting, ram biting, and an intermediate group that uses both biting and suction, were nearly absent among the lineages of teleost fishes inhabiting reefs prior to the end-Cretaceous mass extinction, but benthic biting has rapidly increased in frequency since then, accounting for about 40% of reef species today. Further, we measured the impact of feeding mode on body shape diversification in reef fishes. We fit a model of multivariate character evolution to a dataset comprising three-dimensional body shape of 1,530 species of teleost reef fishes across 111 families. Dedicated biters have accumulated over half of the body shape variation that suction feeders have in just 18% of the evolutionary time by evolving body shape ∼1.7 times faster than suction feeders. As a possible response to the ecological and functional diversity of attached prey, biters have dynamically evolved both into shapes that resemble suction feeders as well as novel body forms characterized by lateral compression and small jaws. The ascendance of species that use biting mechanisms to feed on attached prey reshaped modern reef fish assemblages and has been a major contributor to their ecological and phenotypic diversification.

Asunto(s)

RESUMEN

Suction feeding in ray-finned fishes requires substantial muscle power for fast and forceful prey capture. The axial musculature located immediately behind the head has been long known to contribute some power for suction feeding, but recent XROMM and fluoromicrometry studies found nearly all the axial musculature (over 80%) provides effectively all (90-99%) of the power for high-performance suction feeding. The dominance of axial power suggests a new framework for studying the musculoskeletal biomechanics of fishes: the form and function of axial muscles and bones should be analysed for power production in feeding (or at least as a compromise between swimming and feeding), and cranial muscles and bones should be analysed for their role in transmitting axial power and coordinating buccal expansion. This new framework is already yielding novel insights, as demonstrated in four species for which suction power has now been measured. Interspecific comparisons suggest high suction power can be achieved in different ways: increasing the magnitude of suction pressure or the rate of buccal volume change, or both (as observed in the most powerful of these species). Our framework suggests that mechanical and evolutionary interactions between the head and the body, and between the swimming and feeding roles of axial structures, may be fruitful areas for continued study.

Asunto(s)

RESUMEN

Suction-feeding in fishes is a ubiquitous form of prey capture whose outcome depends both on the movements of the predator and the prey, and on the dynamics of the surrounding fluid, which exerts forces on the two organisms. The inherent complexity of suction-feeding has challenged previous efforts to understand how the feeding strikes are modified when species evolve to feed on different prey types. Here, we use the concept of dynamic similarity, commonly applied to understanding the mechanisms of swimming, flying, walking and aquatic feeding. We characterize the hydrodynamic regimes pertaining to (i) the forward movement of the fish (ram), and (ii) the suction flows for feeding strikes of 71 species of acanthomorph fishes. A discriminant function analysis revealed that feeding strikes of zooplanktivores, generalists and piscivores could be distinguished based on their hydrodynamic regimes. Furthermore, a phylogenetic comparative analysis revealed that there are distinctive hydrodynamic adaptive peaks associated with zooplanktivores, generalists and piscivores. The scaling of dynamic similarity across species, body sizes and feeding guilds in fishes indicates that elementary hydrodynamic principles govern the trophic evolution of suction-feeding in fishes.

Asunto(s)

RESUMEN

Nectar-feeding birds employ unique mechanisms to collect minute liquid rewards hidden within floral structures. In recent years, techniques developed to study drinking mechanisms in hummingbirds have prepared the groundwork for investigating nectar feeding across birds. In most avian nectarivores, fluid intake mechanisms are understudied or simply unknown beyond hypotheses based on their morphological traits, such as their tongues, which are semi-tubular in sunbirds, frayed-tipped in honeyeaters and brush-tipped in lorikeets. Here, we use hummingbirds as a case study to identify and describe the proposed drinking mechanisms to examine the role of those peculiar traits, which will help to disentangle nectar-drinking hypotheses for other groups. We divide nectar drinking into three stages: (1) liquid collection, (2) offloading of aliquots into the mouth and (3) intraoral transport to where the fluid can be swallowed. Investigating the entire drinking process is crucial to fully understand how avian nectarivores feed; nectar-feeding not only involves the collection of nectar with the tongue, but also includes the mechanisms necessary to transfer and move the liquid through the bill and into the throat. We highlight the potential for modern technologies in comparative anatomy [such as microcomputed tomography (µCT) scanning] and biomechanics (such as tracking BaSO4-stained nectar via high-speed fluoroscopy) to elucidate how disparate clades have solved this biophysical puzzle through parallel, convergent or alternative solutions.

Asunto(s)

RESUMEN

Fishes possess an impressive repertoire of feeding and locomotor behaviors that in many cases rely on the same power source: the axial musculature. As both functions employ different skeletal systems, head versus body, integrating these functions would likely require modular motor control. Although there have been many studies of motor control in feeding or locomotion in fishes, only one study to date has examined both functions in the same individuals. To characterize bilateral motor control of the epaxial musculature in feeding and locomotion, we measured muscle activity and shortening in bluegill sunfish (Lepomis macrochirus) using electromyography and sonomicrometry. We found that sunfish recruit epaxial regions in a dorsal-to-ventral manner to increase feeding performance, such that high-performance feeding activates all the epaxial musculature. In comparison, sunfish seemed to activate all three epaxial regions irrespective of locomotor performance. Muscle activity was present on both sides of the body in nearly all feeding and locomotor behaviors. Feeding behaviors used similar activation intensities on the two sides of the body, whereas locomotor behaviors consistently used higher intensities on the side undergoing muscle shortening. In all epaxial regions, fast-starts used the highest activation intensities, although high-performance suction feeding occasionally showed near-maximal intensity. Finally, active muscle volume was positively correlated with the peak rate of body flexion in feeding and locomotion, indicating a continuous relationship between recruitment and performance. A comparison of these results with recent work on largemouth bass (Micropterus salmoides) suggests that centrarchid fishes use similar motor control strategies for feeding, but interspecific differences in peak suction-feeding performance are determined by active muscle volume.

Asunto(s)

RESUMEN

What is the functional effect of prolonged development? By controlling for size, we quantify first-feeding performance and hydrodynamics of zebrafish and guppy offspring (5 ± 0.5 mm in length), which differ fivefold in developmental time and twofold in ontogenetic state. By manipulating water viscosity, we control the hydrodynamic regime, measured as Reynolds number. We predicted that if feeding performance were strictly the result of hydrodynamics, and not development, feeding performance would scale with Reynolds number. We find that guppy offspring successfully feed at much greater distances to prey (1.0 vs. 0.2 mm) and with higher capture success (90 vs. 20%) compared with zebrafish larvae, and that feeding performance was not a result of Reynolds number alone. Flow visualization shows that zebrafish larvae produce a bow wave ~0.2 mm in length, and that the flow field produced during suction does not extend beyond this bow wave. Due to well-developed oral jaw protrusion, the similar-sized suction field generated by guppy offspring extends beyond the horizon of their bow wave, leading to successful prey capture from greater distances. These findings suggest that prolonged development and increased ontogenetic state provides first-feeding fish time to escape the pervasive hydrodynamic constraints (bow wave) of being small.

Asunto(s)

RESUMEN

Aquatic bladderworts (Utricularia gibba and U. australis) capture zooplankton in mechanically triggered underwater traps. With characteristic dimensions less than 1 mm, the trapping structures are among the smallest known to capture prey by suction, a mechanism that is not effective in the creeping-flow regime where viscous forces prevent the generation of fast and energy-efficient suction flows. To understand what makes suction feeding possible on the small scale of bladderwort traps, we characterised their suction flows experimentally (using particle image velocimetry) and mathematically (using computational fluid dynamics and analytical mathematical models). We show that bladderwort traps avoid the adverse effects of creeping flow by generating strong, fast-onset suction pressures. Our findings suggest that traps use three morphological adaptations: the trap walls' fast release of elastic energy ensures strong and constant suction pressure; the trap door's fast opening ensures effectively instantaneous onset of suction; the short channel leading into the trap ensures undeveloped flow, which maintains a wide effective channel diameter. Bladderwort traps generate much stronger suction flows than larval fish with similar gape sizes because of the traps' considerably stronger suction pressures. However, bladderworts' ability to generate strong suction flows comes at considerable energetic expense.

Asunto(s)

RESUMEN

The aquatic bladderwort Utricularia gibba captures zooplankton in mechanically triggered underwater traps. With characteristic dimensions <1 mm, the trapping structures are among the smallest known that work by suction-a mechanism that would not be effective in the creeping-flow regime. To understand the adaptations that make suction feeding possible on this small scale, we have measured internal flow speeds during artificially triggered feeding strikes in the absence of prey. These data are compared with complementary analytical models of the suction event: an inviscid model of the jet development in time and a steady-state model incorporating friction. The initial dynamics are well described by a time-dependent Bernoulli equation in which the action of the trap door is represented by a step increase in driving pressure. According to this model, the observed maximum flow speed (5.2 m/s) depends only on the pressure difference, whereas the initial acceleration (3 × 104 m/s2 ) is determined by pressure difference and channel length. Because the terminal speed is achieved quickly (~0.2 ms) and the channel is short, the remainder of the suction event (~2.0 ms) is effectively an undeveloped viscous steady state. The steady-state model predicts that only 17% of power is lost to friction. The energy efficiency and steady-state fluid speed decrease rapidly with decreasing channel diameter, setting a lower limit on practical bladderwort size.

Asunto(s)

RESUMEN

Whales use baleen, a novel integumentary structure, to filter feed; filter feeding itself evolved at least five times in tetrapod history but demonstrably only once in mammals [1]. Living baleen whales (mysticetes) are born without teeth, but paleontological and embryological evidence demonstrate that they evolved from toothed ancestors that lacked baleen entirely [2]. The mechanisms driving the origin of filter feeding in tetrapods remain obscure. Here we report Maiabalaena nesbittae gen. et sp. nov., a new fossil whale from early Oligocene rocks of Washington State, USA, lacking evidence of both teeth and baleen. The holotype possesses a nearly complete skull with ear bones, both mandibles, and associated postcrania. Phylogenetic analysis shows Maiabalaena as crownward of all toothed mysticetes, demonstrating that tooth loss preceded the evolution of baleen. The functional transition from teeth to baleen in mysticetes has remained enigmatic because baleen decays rapidly and leaves osteological correlates with unclear homology; the oldest direct evidence for fossil baleen is ∼25 million years younger [3] than the oldest stem mysticetes (∼36 Ma). Previous hypotheses for the origin of baleen [4, 5] are inconsistent with the morphology and phylogenetic position of Maiabalaena. The absence of both teeth and baleen in Maiabalaena is consistent with recent evidence that the evolutionary loss of teeth and origin of baleen are decoupled evolutionary transformations, each with a separate morphological and genetic basis [2, 6]. Understanding these macroevolutionary patterns in baleen whales is akin to other macroevolutionary transformations in tetrapods such as scales to feathers in birds.

Asunto(s)

RESUMEN

Understanding the physical mechanics behind morphological systems can offer insights into their evolution. Recent work on linkage systems in fish and crustaceans has suggested that the evolution of such systems may depend on mechanical sensitivity, where geometrical changes to different parts of a biomechanical system have variable influence on mechanical outputs. While examined at the evolutionary level, no study has directly explored this idea at the level of the mechanism. We analyse the mechanical sensitivity of a fish cranial linkage to identify the influence of linkage geometry on the kinematic transmission (KT) of the suspensorium, hyoid and lower jaw. Specifically, we answer two questions about the sensitivity of this linkage system: (i) What changes in linkage geometry affect one KT while keeping the other KTs constant? (ii) Which geometry changes result in the largest and smallest changes to KT? Our results show that there are ways to alter the morphology that change each KT individually, and that there are multiple ways to alter a single link that have variable influence on KT. These results provide insight into the morphological evolution of the fish skull and highlight which structural features in the system may have more freedom to evolve than others.

RESUMEN

Among over 30 000 species of ray-finned fishes, seahorses and pipefishes have a unique feeding mechanism whereby the elastic recoil of tendons allows them to rotate their long snouts extremely rapidly in order to capture small elusive prey. To understand the evolutionary origins of this feeding mechanism, its phylogenetic distribution among closely related lineages must be assessed. We present evidence for elastic recoil-powered feeding in snipefish (Macroramphosus scolopax) from kinematics, dynamics and morphology. High-speed videos of strikes show they achieve extremely fast head and hyoid rotational velocities, resulting in rapid prey capture in as short a duration as 2 ms. The maximum instantaneous muscle-mass-specific power requirement for head rotation in snipefish was above the known vertebrate maximum, which is evidence that strikes are not the result of direct muscle power. Finally, we show that the over-centre conformation of the four-bar linkage mechanism coupling head elevation to hyoid rotation in snipefish can function as a torque reversal latch, preventing the head from rotating and providing the opportunity for elastic energy storage. The presence of elastic recoil feeding in snipefish means that this high-performance mechanism is not restricted to the Syngnathidae (seahorses and pipefish) and may have evolved in parallel.

Asunto(s)

RESUMEN

Living baleen whales, or Mysticeti, lack teeth and instead feed using keratinous baleen plates to sieve prey-laden water. This feeding strategy is profoundly different from that of their toothed ancestors, which processed prey using the differentiated dentition characteristic of mammals. The fossil record of mysticetes reveals stem members that include extinct taxa with dentition, illuminating the morphological states that preceded the loss of teeth and the subsequent origin of baleen. The relationships among stem mysticetes, including putative clades such as Mammalodontidae and Aetiocetidae, remain debatable. Aetiocetids are among the more species-rich clade of stem mysticetes, and known only from fossil localities along the North Pacific coastline. Here, we report a new aetiocetid, Salishicetus meadi gen. et sp. nov, from the late Oligocene of Washington State, USA. Salishicetus preserves a near-complete lower dentition with extensive occlusal wear, indicating that it processed prey using shearing cheek teeth in the same way as its stem cetacean ancestors. Using a matrix with all known species of aetiocetids, we recover a monophyletic Aetiocetidae, crownward of a basal clade of Mammalodontidae. The description of Salishicetus resolves phylogenetic relationships among aetiocetids, which provides a basis for reconstructing ancestral feeding morphology along the stem leading to crown Mysticeti.