An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tree that contains any multifurcations can be described as a multifurcating tree.
109-658: Aetiocetus is a genus of extinct basal mysticete, or baleen whale that lived 33.9 to 23.03 million years ago , in the Oligocene in the North Pacific ocean, around Japan, Mexico, and Oregon, U.S. It was first described by Douglas Emlong in 1966 and currently contains known four species, A. cotylalveus , A. polydentatus , A. tomitai , and A. weltoni . These whales are remarkable for their retention of teeth and presence of nutrient foramina, indicating that they possessed baleen. Thus, Aetiocetus represents
218-475: A clade of even-toed ungulates (Artiodactyla). Baleen whales split from toothed whales (Odontoceti) around 34 million years ago . Baleen whales range in size from the 6 m (20 ft) and 3,000 kg (6,600 lb) pygmy right whale to the 31 m (102 ft) and 190 t (210 short tons) blue whale , the largest known animal to have ever existed. They are sexually dimorphic . Baleen whales can have streamlined or large bodies, depending on
327-479: A V-shaped blow. They are surrounded by a fleshy ridge that keeps water away while the whale breathes. The septum that separates the blowholes has two plugs attached to it, making the blowholes water-tight while the whale dives. Like other mammals, the skin of baleen whales has an epidermis , a dermis , a hypodermis , and connective tissue . The epidermis, the pigmented layer, is 5 millimeters (0.2 in) thick, along with connective tissue. The epidermis itself
436-411: A coarse sieve. At first glance, the fact that species are known from only one locality, and that may suggest that Aetiocetus was highly endemic. Deméré and Berta consider Aetiocetus to be a lineage endemic to the north Pacific Ocean basin. High endemism would be highly atypical of mysticetes. However, a more likely explanation is that the fossil record for Aetiocetus is poor, or that a sampling bias
545-511: A coastal marine depositional environment, and is considered late late Oligocene in age (Chattian) based on foraminifera and mollusc stages; approximately 24-25 million years in age. A. weltoni is also known from the Yaquina Formation and occurs along the same cliff face as A. cotylalveus , but occurs higher in the stratigraphic section . This specimen was found in situ near the contact of the conformably overlying Nye Formation, which
654-437: A fiber-reinforced structure made of intermediate filaments (proteins). The degree of calcification varies between species, with the sei whale having 14.5% hydroxyapatite , a mineral that coats teeth and bones, whereas minke whales have 1–4% hydroxyapatite. In most mammals, keratin structures, such as wool , air-dry, but aquatic whales rely on calcium salts to form on the plates to stiffen them. Baleen plates are attached to
763-503: A functional mysticete. Lending credence to this interpretation is the presence of mandibular kinesis in Aetiocetus , though they lack the rostral kinesis seen in more derived mysticetes. This cranial kinesis , or ability of the skull bones to move relative to one another, permit the mysticete skull to decrease the strain exerted on the skull during bulk feeding. Fitzgerald argued against the model of tooth-aided filter feeding, based on
872-425: A highly derived archaeocete. Van Valen in his 1968 essay “Monophyly or diphyly in the origin of whales” placed Aetiocetus in its accepted position as a basal, or early, mysticete. In 1995, Lawrence G. Barnes and his co-authors Masaichi Kimura, Hitoshi Furusawa, and Hiroshi Sawamura described three new aetiocetids that allied with the genus Aetiocetus . These finds were unique in that they placed an aetiocetid within
981-425: A large amount of water and sieve the slow-moving prey. Males typically mate with more than one female ( polygyny ), although the degree of polygyny varies with the species. Male strategies for reproductive success vary between performing ritual displays ( whale song ) or lek mating . Calves are typically born in the winter and spring months and females bear all the responsibility for raising them. Mothers fast for
1090-504: A layer of fat, or blubber , under the skin to keep warm in the cold water. Although baleen whales are widespread, most species prefer the colder waters of the Arctic and Antarctic . Gray whales are specialized for feeding on bottom-dwelling crustaceans . Rorquals are specialized at lunge-feeding, and have a streamlined body to reduce drag while accelerating. Right whales skim-feed, meaning they use their enlarged head to effectively take in
1199-468: A lunge-feeding behavior. Lunge-feeding is where a whale rams a bait ball (a swarm of small fish ) at high speed. Rorquals generally have streamlined physiques to reduce drag in the water while doing this. Balaenids rely on their huge heads, as opposed to the rorquals' throat pleats, to feed effectively. This feeding behavior allows them to grow very big and bulky, without the necessity for a streamlined body. They have callosities , unlike other whales, with
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#17328021093751308-468: A member of Aetiocetus at all because of its seemingly derived features in comparison to other members of the genus Aetiocetus , for instance, its polydont dentition and greatly enlarged nasal bones . The debate regarding relationships within the Aetiocetidae highlight the importance of this clade to the understanding of basal mysticete evolution and hypotheses surrounding the loss of adult teeth and
1417-478: A monophyletic Aetiocetidae with three subfamilies: Chonecetinae, which includes Chonecetus spp., Morawanocetinae, which includes Morawanocetus yabukii , and Aetiocetinae, which contains Ashorocetus eguchii and Aetiocetus spp. In 2002, Sanders and Barnes hypothesized that there was a larger superfamily, Aetiocetoidea, which would include all known toothed mysticetes: Aetiocetidae, Llanocetidae, and Mammalodontidae. However, evidence suggests that this “Aetiocetoidea”
1526-401: A monophyletic group. Both results have been supported in previous studies. In an even more recent paper, entitled “A Phylogenetic Blueprint for a Modern Whale”, more than one species in the genus Aetiocetus is used: A. cotylalveus , A. weltoni , and A. polydentatus . These three taxa form a polytomy with Chonecetus , where the relationships of the four taxa cannot be more determined with
1635-566: A relatively long period of time over the period of migration, which varies between species. Baleen whales produce a number of infrasonic vocalizations , notably the songs of the humpback whale . The meat, blubber, baleen, and oil of baleen whales have traditionally been used by the indigenous peoples of the Arctic . Once relentlessly hunted by commercial industries for these products, cetaceans are now protected by international law. These protections have allowed their numbers to recover. However,
1744-444: A relatively small brain compared to their body mass . Like other mammals, their brain has a large, folded cerebrum , the part of the brain responsible for memory and processing sensory information. Their cerebrum only makes up about 68% of their brain's weight, as opposed to human's 83%. The cerebellum , the part of the brain responsible for balance and coordination, makes up 18% of their brain's weight, compared to 10% in humans, which
1853-485: A sensory organ located in the middle of the jaw to regulate these functions. Baleen whales have two flippers on the front, near the head. Like all mammals, baleen whales breathe air and must surface periodically to do so. Their nostrils, or blowholes , are situated at the top of the cranium . Baleen whales have two blowholes, as opposed to toothed whales which have one. These paired blowholes are longitudinal slits that converge anteriorly and widen posteriorly, which causes
1962-471: A soft tissue, does not preserve in the fossil record, whale paleontologists are able to identify evidence for baleen attachment in the palates of mysticetes. These are evident in what are known as nutrient foramina . These nutrient foramina, present on the maxillae of the whale, are associated with grooves and sulci , or fissures, which in life are occupied by branches of the superior alveolar artery and nerve. This superior alveolar artery supplies nutrients to
2071-439: A spherical lens. The retina is surrounded by a reflective layer of cells ( tapetum lucidum ), which bounces light back at the retina, enhancing eyesight in dark areas. However, light is bent more near the surface of the eye when in air as opposed to water; consequently, they can see much better in the air than in the water. The eyeballs are protected by a thick outer layer to prevent abrasions and an oily fluid (instead of tears) on
2180-644: A translation error in early copies of Aristotle 's Historia Animalium (in Ancient Greek ), in which " ὁ μῦς τὸ κῆτος " ( ho mus to kētos , "the mouse, the whale so called") was mistakenly translated as " ὁ μυστικῆτος " ( ho mustikētos , "the Mysticetus"), which D. W. Rice (of the Society for Marine Mammalogy ) in Rice 1998 assumed was an ironic reference to the animals' great size. An alternate name for
2289-460: A wing-like manner similar to penguins and sea turtles . Flipper movement is continuous. While doing this, baleen whales use their tail fluke to propel themselves forward through vertical motion while using their flippers for steering, much like an otter . Some species leap out of the water, which may allow them to travel faster. Because of their great size, right whales are not flexible or agile like dolphins, and none can move their neck because of
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#17328021093752398-662: Is Miocene in age. Thus, A. weltoni is very close to the Oligocene-Miocene boundary. A. tomitai was discovered in the Middle Hard Shale member of the Morawan Formation , Kawakami group in Japan. This is also late Oligocene in age and represents a basinal depositional environment. This specimen was not found in situ, but in a loose concretion, and could potentially be stratigraphically higher than
2507-404: Is a grade taxon and does not actually form a natural group, as the retention of teeth is a symplesiomorphic condition for Cetacea and cannot be used as a synapomorphy for the group. Fitzgerald in 2006 proposed six major toothed mysticete lineages, in which Aetiocetidae was paraphyletic , with a Chonecetus clade and an Aetiocetus clade. A. polydentatus was considered by Fitzgerald to not be
2616-665: Is also in line with its late Oligocene stratigraphic occurrence, where crown Mysticeti had yet to appear in the fossil record. Aetiocetus cotylalveus was discovered in 1966 and described by Emlong, who initially ascribed Aetiocetus to the Archaeoceti based on its plesiomorphic , or primitive, dentition; he felt that the presence of teeth barred Aetiocetus from the Mysticeti . There are many distinct features that separates Aetiocetus from Odontoceti , or toothed whales, and Emlong did not see evidence of remodeling necessary for
2725-780: Is also known from Japan and Mexico. The genus is currently constrained to the Northern hemisphere and has little value in biostratigraphic studies of the Oligocene due to its limited occurrences across the Pacific. The genus name Aetiocetus comes from Ancient Greek αἰτία via Latin 'cause, origin' and Latin cetus 'whale', translating to "original whale". A. cotylalveus approximately means "bowl cavity", κοτύλη meaning 'cup' or 'bowl' in Ancient Greek, and alveus Latin for "hollow" or "cavity". A. tomitai
2834-561: Is an archaic word for "whale", which came from Old French baleine , derived from the Latin word balæna , derived itself from the Ancient Greek φάλλαινα (phállaina) . Right whales got their name because of whalers preferring them over other species; they were essentially the "right whale" to catch. Rorquals use throat pleats to expand their mouths, which allow them to feed more effectively. However, rorquals need to build up water pressure in order to expand their mouths, leading to
2943-734: Is disputed. Baleen whale Baleen whales ( / b ə ˈ l iː n / ), also known as whalebone whales , are marine mammals of the parvorder Mysticeti in the infraorder Cetacea ( whales , dolphins and porpoises ), which use keratinaceous baleen plates (or "whalebone") in their mouths to sieve planktonic creatures from the water. Mysticeti comprises the families Balaenidae ( right and bowhead whales), Balaenopteridae ( rorquals ), Eschrichtiidae (the gray whale ) and Cetotheriidae (the pygmy right whale ). There are currently 16 species of baleen whales. While cetaceans were historically thought to have descended from mesonychians , molecular evidence instead supports them as
3052-455: Is disturbed by vibrations, it triggers sensory hairs which send electric current to the brain, where vibrations are processed into sound. Baleen whales have a small, yet functional, vomeronasal organ . This allows baleen whales to detect chemicals and pheromones released by their prey. It is thought that 'tasting' the water is important for finding prey and tracking down other whales. They are believed to have an impaired sense of smell due to
3161-638: Is known as bulk-feeding, in that the whale is not selecting individual prey items and does not use echolocation to find prey, as odontocetes do. Fossil mysticetes with wide, toothless palates are inferred to bulk-feed and the first occurrence of such whales is in the late Oligocene, approximately 4 to 5 million years after the first toothless mysticetes appeared. Structurally, Aetiocetus possesses teeth that are quite similar to primitive odontocetes, such as Squalodon . These odontocetes have an inferred bite and tear style of eating with limited mastication . Both primitive and extant odontocetes find their prey through
3270-428: Is likely due to climate change which caused seasonally shifting accumulations of plankton in various parts of the world, necessitating travel over long distances, as well as the ability to feed on large baitballs to make such trips worthwhile. A 2017 analysis of body size based on data from the fossil record and modern baleen whales indicates that the evolution of gigantism in baleen whales occurred rather recently, within
3379-430: Is made of keratin (the same material that comprises claws, hooves, nails, and hair) that grows throughout the whale's life. Development of mysticetes indicate that they had a toothed ancestor, as the fetal baleen whale forms tooth buds which are later reabsorbed and do not develop any further. However, Aetiocetus presents the evolutionary biologist with evidence for this transition in the fossil record. While baleen, as
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3488-540: Is named in honor of then-mayor Akio Tomita of Ashoro, Hokkaido in Japan. A. weltoni is named in honor of Doctor Bruce J. Welton, who initially discovered the specimen and directed the excavation of the skeleton. A. polydentatus is named in reference to the increase of teeth (polydont dentition) present in the specimen. There has been some dispute about the relationships between Aetiocetus and stem Mysticetes. Barnes et al. (1995) expanded Emlong's original definition to encompass eight species in four genera. They suggested
3597-471: Is not a transitional form in the strictest sense, that is, it cannot be an ancestor to extant Mysticeti. More derived forms, such the Cetotheriidae, a family of toothless baleen whales, are contemporaneous with Aetiocetus . Hence, whales whose feeding relied entirely on baleen made their stratigraphic appearance before Aetiocetus , meaning that “true” baleen whales existed before Aetiocetus . Baleen
3706-501: Is only 1 millimeter (0.04 in) thick. The dermis, the layer underneath the epidermis, is also thin. The hypodermis, containing blubber, is the thickest part of the skin and functions as a means to conserve heat. Right whales have the thickest hypodermis of any cetacean, averaging 51 centimeters (20 in), though, as in all whales, it is thinner around openings (such as the blowhole) and limbs. Blubber may also be used to store energy during times of fasting. The connective tissue between
3815-583: Is present and not enough work has been done in late Oligocene deposits in the south Pacific Ocean. Perhaps there are more specimens of Aetiocetus that will be discovered as paleontologists continue searching. The other genera in the family Aetiocetidae are Ashorocetus , Chonecetus , Morawanocetus , and Willungacetus . All aetiocetids are known from the North Pacific except the Australian Willungacetus and its taxonomy
3924-472: Is probably due to the great degree of control necessary for constantly swimming. Necropsies on the brains of gray whales revealed iron oxide particles, which may allow them to find magnetic north like a compass . Unlike most animals, whales are conscious breathers. All mammals sleep, but whales cannot afford to become unconscious for long because they may drown. They are believed to exhibit unihemispheric slow-wave sleep , in which they sleep with half of
4033-465: Is remarkable in that the number of teeth are asymmetrical. On the right side of the upper jaw, A. polydentatus has 13 teeth, and on the left it has 14 teeth. The bottom jaw is also asymmetric, with 14 teeth in the right dentary and 15 in the left. This is the first polydont aetiocetid, meaning that it had more teeth than the standard mammalian formula. Embryonic baleen whales have polydont teeth before birth. A. polydentatus indicates that this condition
4142-595: Is repeated annually. The gray whale has the longest recorded migration of any mammal, with one traveling 23,000 kilometers (14,000 mi) from the Sea of Okhotsk to the Baja Peninsula . Polytomy Two types of polytomies are recognized, soft and hard polytomies. Soft polytomies are the result of insufficient phylogenetic information: though the lineages diverged at different times – meaning that some of these lineages are closer relatives than others, and
4251-438: Is still basal and is still the sister taxon to Eomysticetus + Micromysticetus + Diorocetus + Pelocetus + crown Mysticeti, all of which possess baleen and no teeth. There are two competing hypotheses supported by the supermatrix: 1) that Aetiocetus is not the sister group to Chonecetus , suggesting that the Aetiocetidae is a paraphyletic group, a group in which not all descendants are considered, or 2) that they indeed form
4360-414: Is the fastest among baleen whales, having been recorded travelling as fast as 10 m/s (36 km/h; 22 mph), and sustaining a speed of 2.5 m/s (9.0 km/h; 5.6 mph) for an extended period. While feeding, the rorqual jaw expands to a volume that can be bigger than the whale itself; to do this, the mouth inflates. The inflation of the mouth causes the cavum ventrale, the throat pleats on
4469-426: Is the second-most basal mysticete; two undescribed museum specimens, ChM, are considered the most basal mysticetes in this phylogeny. In Geisler et al.’s 2011 study entitled “A supermatrix analysis of genomic, morphological, and paleontological data from crown Cetacea”, there is higher resolution of Aetiocetus ’ phylogenetic relationship with other mysticetes, as well as more taxa considered. In this study, Aetiocetus
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4578-751: Is thought that their size increased with their dependence on baleen. However, the discovery of a skull of the toothed Llanocetus , the second-oldest mysticete, yielded a total length of 8 meters (26 ft), indicating filter feeding was not a driving feature in mysticete evolution. The discovery of Janjucetus and others like it suggests that baleen evolution went through several transitional phases. Species like Mammalodon colliveri had little to no baleen, while later species like Aetiocetus weltoni had both baleen and teeth, suggesting they had limited filter feeding capabilities; later genera like Cetotherium had no teeth in their mouth, meaning they were fully dependent on baleen and could only filter feed. However,
4687-411: Is unknown where this occurred, but it is generally believed that they, like their descendants, followed plankton migrations. These primitive baleen whales had lost their dentition in favor of baleen, and are believed to have lived on a specialized benthic, plankton, or copepod diet like modern baleen whales. Baleen whales experienced their first radiation in the mid-Miocene . It is thought this radiation
4796-454: Is usually much faster than evolution of complex phenotypic traits, it may be that genetic lineages diverge a short time apart from each other, while the actual organism has not changed if the whole ancestral population is considered. Since few if any individuals in a population are genetically alike in any one population – especially if lineage sorting has not widely progressed – it may be that hard polytomies are indeed rare or nonexistent if
4905-518: Is usually reversed, with the males being larger, but the females of all baleen whales are usually five percent larger than males. Sexual dimorphism is also displayed through whale song , notably in humpback whales where the males of the species sing elaborate songs. Male right whales have bigger callosities than female right whales. The males are generally more scarred than females which is thought to be because of aggression during mating season. The unique lungs of baleen whales are built to collapse under
5014-544: The Muséum National d'Histoire Naturelle ) and colleagues suggested that, based on phylogenic criteria, there are four extant genera of rorquals. They recommend that the genus Balaenoptera be limited to the fin whale, have minke whales fall under the genus Pterobalaena , and have Rorqualus contain the Sei whale, Bryde's whale, Eden's whale (and by extension Rice's whale), the blue whale, and Omura's whale. The gray whale
5123-534: The North Atlantic right whale is ranked critically endangered by the International Union for Conservation of Nature . Besides hunting, baleen whales also face threats from marine pollution and ocean acidification . It has been speculated that man-made sonar results in strandings . They have rarely been kept in captivity, and this has only been attempted with juveniles or members of one of
5232-475: The Oligocene are the Mammalodontidae ( Mammalodon and Janjucetus ) from Australia . They were small with shortened rostra , and a primitive dental formula ( 3.1.4.3 3.1.4.3 ). In baleen whales, it is thought that enlarged mouths adapted for suction feeding evolved before specializations for bulk filter feeding . In the toothed Oligocene mammalodontid Janjucetus , the symphysis is short and
5341-405: The anterior and posterior margins of the posterior upper teeth. Their postcanine teeth are somewhat heterodont . The base of the rostrum , or snout, of the whale, is greater than 170% of the width of the occipital condyles where the skull meets the neck. These features are synapomorphies , or shared derived traits, of Aetiocetus . There is a distinct notch by the internal nostrils formed of
5450-403: The epithelial , or surface cells of the body, from which the baleen continuously develops. In all known archaeocetes and odontocetes, nutrient foramina are absent. These nutrient foramina are most apparent in A. weltoni , whose holotype has the best-preserved palate. The development of nutrient foramina and teeth are closely intertwined in mysticetes: first, an alveolar groove on the palate of
5559-560: The fin whale ( B. physalus ), the Sei whale ( B. borealis ), Bryde's whale ( B. brydei ), Eden's whale ( B. edeni ), Rice's whale ( B. ricei ), the blue whale ( B. musculus ), the common minke whale ( B. acutorostrata ), the Antarctic minke whale ( B. bonaerensis ), Omura's whale ( B. omurai ), the humpback whale ( M. novaeangliae ), and the gray whale ( E. robustus ). In a 2012 review of cetacean taxonomy, Alexandre Hassanin (of
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#17328021093755668-421: The palatine , pterygoid , and vomer bones; this is a synapomorphy of Aetiocetus + Chonecetus . Synapomorphies of the aetiocetids present in Aetiocetus are: the coronoid process of the dentary , or lower jaw, is well developed; the zygomatic arch is expanded anteriorly and posteriorly but is narrow at the middle. Aetiocetus also shares several traits with all mysticetes. The mandibular symphysis , or
5777-539: The respiratory tract to hold gas while diving. Doing so may cause serious complications such as embolism . Unlike other mammals, the lungs of baleen whales lack lobes and are more sacculated. Like in humans, the left lung is smaller than the right to make room for the heart. To conserve oxygen, blood is rerouted from pressure-tolerant-tissue to internal organs, and they have a high concentration of myoglobin which allows them to hold their breath longer. The heart of baleen whales functions similarly to other mammals, with
5886-458: The 2018 discovery of the toothless Maiabalaena indicates some lineages evolved toothlessness before baleen. Mystacodon selenensis is the earliest mysticete, dating back to 37 to 33 million years ago ( mya ) in the Late Eocene , and, like other early toothed mysticetes, or "archaeomysticetes", M. selenensis had heterodont dentition used for suction feeding. Archaeomysticetes from
5995-430: The 60 to 100 bpm in humans. When diving, their heart rate will drop to 4 to 15 bpm to conserve oxygen. Like toothed whales, they have a dense network of blood vessels ( rete mirabile ) which prevents heat-loss. Like in most mammals, heat is lost in their extremities , so, in baleen whales, warm blood in the arteries is surrounded by veins to prevent heat loss during transport. As well as this, heat inevitably given off by
6104-596: The Antarctic circumpolar current and its effects on global climate patterns is excluded as being causal for the same reason. Gigantism also was preceded by divergence of different mysticete lineages, meaning multiple lineages arrived at large size independently. It is possible the Plio-Pleistocene increase in seasonally intense upwellings , causing high-prey-density zones, led to gigantism. When swimming, baleen whales rely on their flippers for locomotion in
6213-604: The Middle Hard Shale, but Barnes et al. presume that the animal was not transported far from the location where it died. A. polydentatus was also discovered in the Morawan Formation of Japan, but from the Upper Tuffaceous Siltstone Member, which also represents a basinal depositional environment. The holotype was found in situ in the uppermost part of the member. It is currently the stratigraphically highest occurrence of an aetiocetid from
6322-483: The North Atlantic right whale ( E. glacialis ). Cetotheriidae consists of only one living member: the pygmy right whale ( Caperea marginata ). The first descriptions date back to the 1840s of bones and baleen plates resembling a smaller version of the right whale, and was named Balaena marginata . In 1864, it was moved into the genus Caperea after a skull of another specimen was discovered. Six years later,
6431-456: The ability to distinguish between hard and soft polytomies is limited: if for example a kilobase of DNA sequences that mutate approximately 1% per million years is analyzed, lineages diverging from the same ancestor within the same 100,000 years cannot be reliably distinguished as to which one diverged first. Founder effects and genetic drift may result in different rates of evolution. This can easily confound molecular clock algorithms to
6540-500: The arteries warms blood in the surrounding veins as it travels back into the core . This is otherwise known as countercurrent exchange . To counteract overheating while in warmer waters, baleen whales reroute blood to the skin to accelerate heat-loss. They have the largest blood corpuscles ( red and white blood cells ) of any mammal, measuring 10 micrometers (4.1 × 10 in) in diameter, as opposed to human's 7.1-micrometer (2.8 × 10 in) blood corpuscles. When sieved from
6649-407: The available data does not allow recognition of this. Most polytomies are soft, meaning that they would be resolved into a typical tree of dichotomies if better data were available. In contrast, a hard polytomy represents a true divergence event of three or more lineages. Interpretations for a polytomy depend on the individuals that are represented in the phylogenetic tree. If the lineages in
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#17328021093756758-457: The brain while the other half remains active. This behavior was only documented in toothed whales until footage of a humpback whale sleeping (vertically) was shot in 2014. It is largely unknown how baleen whales produce sound because of the lack of a melon and vocal cords . In a 2007 study, it was discovered that the larynx had U-shaped folds which are thought to be similar to vocal cords. They are positioned parallel to air flow, as opposed to
6867-653: The capacity of their throats, allowing them to filter larger volumes of water at once. Gray whales are bottom-feeders, meaning they sift through sand to get their food. They usually turn on their sides, scoop up sediment into their mouths and filter out benthic creatures like amphipods , which leave noticeable marks on their heads. The pygmy right whale is easily confused with minke whales because of their similar characteristics, such as their small size, dark gray tops, light gray bottoms, and light eye patches. Molecular phylogeny suggests Mysticeti split from Odontoceti (toothed whales) between 26 and 17 million years ago between
6976-510: The connection between both lower jaw bones, is not fused. The descending process of the maxilla becomes a toothless plate below the orbit. They possess a wide rostrum. All these features are functionally related to filter-feeding with baleen and is a hallmark of the Mysticeti. The presence of teeth, as Barnes et al. remark, seems paradoxical. Lastly, Aetiocetus shows some symplesiomorphic traits with more archaic whales. The do not experience
7085-535: The developing mysticete. The deciduous teeth form in the groove, and then are reabsorbed, while development of rudimentary baleen plates begin. The alveolar groove fills with bone until the laterial nutrient foramina form. This close association leads Deméré and Berta to hypothesize that Aetiocetus displays an ancient ontogeny , or growth sequence. These nutrient foramina are also present on A. cotylalveus and another related aetiocetid, Chonecetus goedertorum . Compared to other edentulous , or toothless, mysticetes,
7194-587: The development of baleen. The larger scale phylogenetic placement of Aetiocetus has remained fairly constant throughout modern studies. Geisler and Sanders’ 2003 paper, “Morphological Evidence for the Phylogeny of Cetacea” used the genus in their morphological cladistics study and their results support a monophyletic Aetiocetidae ( Aetiocetus + Chonecetus ). Here, the Aetiocetidae is the sister taxon to Eomysticetus + Micromysticetus + Diorocetus + Pelocetus + crown Mysticeti. In their sample, Aetiocetus
7303-430: The end of the mouth. This is probably because they feed on slow or immobile prey, combined with the fact that most sunlight does not pass 9.1 meters (30 ft), and hence they do not need acute vision. A whale's eye is adapted for seeing both in the euphotic and aphotic zones by increasing or decreasing the pupil 's size to prevent damage to the eye. As opposed to land mammals which have a flattened lens , whales have
7412-458: The entire genome of each individual organism is considered, but rather widespread on the population genetical level if entire species are considered as interbreeding populations (see also species concept ). "Speciation or lineage divergence events occurring at the same time" refers to evolutionary time measured in generations , as this is the only means that novel traits (e.g. germline point mutations ) can be passed on. In practical terms,
7521-439: The exception of the bowhead whale. Rorquals have a higher proportion of muscle tissue and tend to be negatively buoyant, whereas right whales have a higher proportion of blubber and are positively buoyant. Gray whales are easily distinguished from the other rorquals by their sleet-gray color, dorsal ridges (knuckles on the back), and their gray-white scars left from parasites. As with the other rorquals, their throat pleats increase
7630-484: The feeding behavior, and two limbs that are modified into flippers . The fin whale is the fastest baleen whale, recorded swimming at 10 m/s (36 km/h; 22 mph). Baleen whales use their baleen plates to filter out food from the water by either lunge-feeding or skim-feeding. Baleen whales have fused neck vertebrae , and are unable to turn their heads at all. Baleen whales have two blowholes . Some species are well adapted for diving to great depths. They have
7739-636: The first radiation in the late Oligocene. Eomysticetus and others like it showed no evidence in the skull of echolocation abilities, suggesting they mainly relied on their eyesight for navigation. The eomysticetes had long, flat rostra that lacked teeth and had blowholes located halfway up the dorsal side of the snout. Though the palate is not well-preserved in these specimens, they are thought to have had baleen and been filter feeders. Miocene baleen whales were preyed upon by larger predators like killer sperm whales and megalodon . The lineages of rorquals and right whales split almost 20 mya. It
7848-476: The fused cervical vertebrae ; this sacrifices speed for stability in the water. The hind legs are enclosed inside the body, and are thought to be vestigial organs . However, a 2014 study suggests that the pelvic bone serves as support for whale genitalia . Rorquals, needing to build speed to feed, have several adaptions for reducing drag , including a streamlined body; a small dorsal fin, relative to its size; and lack of external ears or long hair. The fin whale
7957-433: The hypodermis and muscles allows only limited movement to occur between them. Unlike toothed whales, baleen whales have small hairs on the top of their head, stretching from the tip of the rostrum to the blowhole, and, in right whales, on the chin. Like other marine mammals , they lack sebaceous and sweat glands. The baleen of baleen whales are keratinous plates. They are made of a calcified, hard α-keratin material,
8066-644: The lack of closely pressed teeth and the presence of simple postcanine crowns. Deméré argues that this assumes a very small prey size (i.e., krill). The distinction here is that Aetiocetus was a bulk feeder, and prey size does not enter into this definition of feeding strategy. There is no reason to assume a priori that all bulk filter-feeders eat small prey, given the large diversity of food items consumed by modern mysticetes. Deméré hypothesizes that Aetiocetus ’ bulk feeding behavior could have targeted large prey, such as schooling fish or squid. With prey items of this size, Aetiocetus ’ teeth would still have served well as
8175-428: The lack of the olfactory bulb , but they do have an olfactory tract . Baleen whales have few if any taste buds, suggesting they have lost their sense of taste. They do retain salt-receptor taste-buds suggesting that they can taste saltiness. Most species of baleen whale migrate long distances from high latitude waters during spring and summer months to more tropical waters during winter months. This migration cycle
8284-413: The last 3 million years. Before 4.5 million years ago, few baleen whales exceeded 10 meters (33 ft) in length; the two largest Miocene species were less than 13 m (43 ft) in length. The initial evolution of baleen and filter feeding long preceded the evolution of gigantic body size, indicating the evolution of novel feeding mechanisms did not cause the evolution of gigantism. The formation of
8393-851: The late Oligocene or middle Miocene , but the earliest Mysticeti fossils date to at least 34 million years ago. Their evolutionary link to archaic toothed cetaceans ( Archaeoceti ) remained unknown until the extinct Janjucetus hunderi was discovered in the early 1990s in Victoria, Australia . While, unlike a modern baleen whale, Janjucetus lacked baleen in its jaw, the anatomy shows sufficient similarity to baleen whales. It appears to have had very limited apparent biosonar capabilities. Its jaw contained teeth, with incisors and canines built for stabbing and molars and premolars built for tearing. These early mysticetes were exceedingly small compared to modern baleen whales, with species like Mammalodon measuring no greater than 3 meters (10 ft). It
8502-1055: The latter. Cetaceans ( whales , dolphins , and porpoises ) and artiodactyls are now classified under the order Cetartiodactyla , often still referred to as Artiodactyla (given that the cetaceans are deeply nested with the artiodactyls). The closest living relatives to baleen whales are toothed whales both from the infraorder Cetacea . Bowhead whale [REDACTED] Southern right whale [REDACTED] North Atlantic right whale [REDACTED] North Pacific right whale [REDACTED] Pygmy right whale [REDACTED] Common minke whale [REDACTED] Southern minke whale [REDACTED] Gray whale [REDACTED] Humpback whale [REDACTED] Fin whale [REDACTED] Blue whale [REDACTED] Omura's whale Eden's whale [REDACTED] Rice's whale [REDACTED] Sei whale [REDACTED] Bryde's whale [REDACTED] Balaenidae consists of two genera: Eubalaena (right whales) and Balaena (the bowhead whale , B. mysticetus ). Balaenidae
8611-406: The lower jaw. The auditory meatus is blocked by connective tissue and an ear plug, which connects to the eardrum . The inner-ear bones are contained in the tympanic bulla , a bony capsule. However, this is attached to the skull, suggesting that vibrations passing through the bone is important. Sinuses may reflect vibrations towards the cochlea . It is known that when the fluid inside the cochlea
8720-435: The major difference being the size. The heart can reach 454 kilograms (1,000 lb), but is still proportional to the whale's size. The muscular wall of the ventricle , which is responsible for pumping blood out of the heart, can be 7.6 to 12.7 centimeters (3 to 5 in) thick. The aorta, an artery , can be 1.9 centimeters (.75 in) thick. Their resting heart rate is 60 to 140 beats per minute (bpm), as opposed to
8829-456: The modern odontocete skull. Notably, Emlong noted that Aetiocetus was some antecedent to the mysticete lineage, due to the degree of telescoping on the skull, indicating that the nostrils of the whale had migrated further back on the skull than seen in archaeocetes. Along with the relation between cranial features, Emlong allied Aetiocetus more closely with Mysticeti than Odontoceti. However, as A. cotylalveus retains teeth, Emlong considered it
8938-551: The most part, Aetiocetus retains a primitive tooth count of 11 upper teeth and 11 lower teeth, abbreviated 11/11. This is interpreted to be the basic placental mammalian dental formula with three incisors , one canine , four premolars , and three molars on both upper and lower jaws. However, A. weltoni and A. polydentatus show variation from the plesiomorphic mammalian dental formula. A. weltoni possesses an 11/12 dentition . True to its name of “many toothed”, A. polydentatus possesses more teeth than any other aetiocetid, and
9047-467: The mouth enlarged, the rostrum is wide, and the edges of the maxillae are thin, indicating an adaptation for suction feeding. The aetiocetid Chonecetus still had teeth, but the presence of a groove on the interior side of each mandible indicates the symphysis was elastic, which would have enabled rotation of each mandible, an initial adaptation for bulk feeding like in modern mysticetes. The first toothless ancestors of baleen whales appeared before
9156-451: The northwestern Pacific Ocean, meaning that it is the youngest known specimen of Aetiocetus . The type species for the genus is Aetiocetus cotylalveus. It is defined as being the monophyletic group encompassing the closest common ancestor of A. cotylalveus and A. polydentatus and all its descendants: the textbook definition for a monophyletic taxon. Aetiocetus is a small, toothed whale with no more than three small denticles on
9265-440: The parvorder is "Mystacoceti" (from Greek μύσταξ "mustache" + κῆτος "whale"), which, although obviously more appropriate and occasionally used in the past, has been superseded by "Mysticeti" ( junior synonym ). Mysticetes are also known as baleen whales for their baleen , which they use to sieve plankton and other small organisms from the water. The term "baleen" ( Middle English baleyn, ballayne, ballien, bellane , etc.)
9374-432: The pattern of nutrient foramina is most similar to extant balaenopterids (blue whales and other rorquals ) and fossil cetotheres . The teeth of Aetiocetus resemble those of archaeocetes and odontocetes, which employ a bite-and-swallow feeding strategy, but they also possessed expanded palates. Modern mysticetes grow their baleen from this expanded palate and use the baleen to trap arthropods and fish in their mouths. This
9483-408: The perpendicular vocal cords of terrestrial mammals. These may control air flow and cause vibrations. The walls of the larynx are able to contract which may generate sound with support from the arytenoid cartilages . The muscles surrounding the larynx may expel air rapidly or maintain a constant volume while diving. The eyes of baleen whales are relatively small for their size and are positioned near
9592-479: The phylogenetic tree stand for species, a polytomy shows the simultaneous speciation of three or more species. In particular situations, they may be common, for example when a species that has rapidly expanded its range or is highly panmictic undergoes peripatric speciation in different regions. An example is the Drosophila simulans species complex . Here, the ancestor seems to have colonized two islands at
9701-584: The present resolution. However, this result suggests monophyly of the Aetiocetidae, or that all aetiocetids are derived from a single common ancestor. In this phylogeny, the Aetiocetidae is the sister taxon to Eomysticetus + Cetotheriidae + crown Mysticeti. Almost all phylogenies agree that Aetiocetus is a stem mysticete with no affiliation with crown Mysticeti. This result is not entirely surprising, given its symplesiomorphic condition, meaning that Aetiocetus still retains many primitive features and few derived ones. Its phylogenetic placement among stem mysticetes
9810-400: The pressure instead of resisting the pressure which would damage the lungs, enabling some, like the fin whale, to dive to a depth of −470 meters (−1,540 ft). The whale lungs are very efficient at extracting oxygen from the air, usually 80%, whereas humans only extract 20% of oxygen from inhaled air. Lung volume is relatively low compared to terrestrial mammals because of the inability of
9919-529: The pygmy right whale was classified under the family Neobalaenidae. Despite its name, the pygmy right whale is more genetically similar to rorquals and gray whales than to right whales. A study published in 2012, based on bone structure, moved the pygmy right whale from the family Neobalaenidae to the family Cetotheriidae, making it a living fossil ; Neobalaenidae was demoted to subfamily level as Neobalaeninae. Rorquals consist of three genera ( Balaenoptera , Megaptera , and Eschrichtius ) and 11 species:
10028-416: The same degree of telescoping as modern whales, so their nares , or nostrils, are still relatively anterior. Contrary to the image of the modern baleen whales, Aetiocetus still possessed developed, enamelized adult teeth . This indicates that loss of functionality in relevant enamel genes, such as ameloblastin (AMBN), enamelin (ENAM), and amelogenin (AMEL), had not yet taken place in Aetiocetus . For
10137-502: The same geologic formation as the type specimen, A. cotylalveus , and also placed new aetiocetids on the western coast of the Pacific, in Japan. This extended the geographic range of Aetiocetus dramatically. In 1998, L.G. Barnes listed a specimen of Aetiocetus within his list of fossil marine mammal assemblages in Mexico. However, this specimen remains aff. Aetiocetus sp., and cannot be ascribed to any particular species. This specimen
10246-473: The same time but independently, yielding two equally old but divergently evolved daughter species If a phylogenetic tree is reconstructed from DNA sequence data of a particular gene, a hard polytomy arises when three or more sampled genes trace their ancestry to a single gene in an ancestral organism. In contrast, a soft polytomy stems from branches on gene trees of finite temporal duration but for which no substitutions have occurred. As DNA sequence evolution
10355-505: The smallest species. Baleen whales are cetaceans classified under the parvorder Mysticeti, and consist of four extant families : Balaenidae ( right whales ), Balaenopteridae (rorquals), Eschrichtiidae ( gray whale ) and Cetotheriidae ( pygmy right whale ). Balaenids are distinguished by their enlarged head and thick blubber , while rorquals and gray whales generally have a flat head, long throat pleats, and are more streamlined than Balaenids. Rorquals also tend to be longer than
10464-435: The surface of the eye. Baleen whales appear to have limited color vision, as they lack S-cones . The mysticete ear is adapted for hearing underwater, where it can hear sound frequencies as low as 7 Hz and as high as 22 kHz , distinct from odontocetes whose hearing is optimized for ultrasonic frequencies. It is largely unknown how sound is received by baleen whales. Unlike in toothed whales, sound does not pass through
10573-437: The third stomach, where it meets fat-digesting enzymes, and is then mixed with an alkaline liquid to neutralize the acid from the fore-stomach to prevent damage to the intestinal tract . Their intestinal tract is highly adapted to absorb the most nutrients from food; the walls are folded and contain copious blood vessels , allowing for a greater surface area over which digested food and water can be absorbed. Baleen whales get
10682-549: The transition from teeth to baleen in Oligocene mysticetes. Baleen is a highly derived character, or synapomorphy , of mysticetes, and is a keratinous structure that grows from the palate, or roof of the mouth, of the whale. The presence of baleen is inferred from the fossil record in the skull of Aetiocetus . Aetiocetus is known from both sides of the Pacific Ocean: it was first documented in Oregon, United States, but it
10791-414: The underside stretching to the navel , to expand, increasing the amount of water that the mouth can store. The mandible is connected to the skull by dense fibers and cartilage ( fibrocartilage ), allowing the jaw to swing open at almost a 90° angle. The mandibular symphysis is also fibrocartilaginous, allowing the jaw to bend which lets in more water. To prevent stretching the mouth too far, rorquals have
10900-410: The upper jaw and are absent in the mid-jaw, forming two separate combs of baleen. The plates decrease in size as they go further back into the jaw; the largest ones are called the "main baleen plates" and the smallest ones are called the "accessory plates". Accessory plates taper off into small hairs. Unlike other whales (and most other mammals), the females are larger than the males. Sexual dimorphism
11009-412: The use of echolocation; however, mysticetes have no evidence in their fossil record of ever evolving or initially possessing the ability to echolocate. Piscivory , or a diet based solely on fish, is likely the primitive condition for Cetacea, and it seems most parsimonious that Aetiocetus fed like an archaeocete, locating fish without the use of echolocation. However, an argument exists that Aetiocetus
11118-434: The water they need from their food; however, the salt content of most of their prey ( invertebrates ) is similar to that of seawater, whereas the salt content of a whale's blood is considerably lower (three times lower) than that of seawater. The whale kidney is adapted to excreting excess salt; however, while producing urine more concentrated than seawater, it wastes a lot of water which must be replaced. Baleen whales have
11227-401: The water, food is swallowed and travels through the esophagus where it enters a three-chambered-stomach. The first compartment is known as the fore-stomach; this is where food gets ground up into an acidic liquid, which is then squirted into the main stomach. Like in humans, the food is mixed with hydrochloric acid and protein-digesting enzymes . Then, the partly digested food is moved into
11336-471: Was also present in tooth-bearing mysticetes, as expected from developmental data. In addition to its polydont dentition, A. polydentatus is unique in that these teeth are not differentiated into different teeth types as seen in other members of Aetiocetus . Paleontologists refer to this condition as being homodont , or “same tooth”. Aetiocetus is unique in its representation of transition from toothed archaeocete to toothless mysticete. However, Aetiocetus
11445-555: Was caused by global climate change and major tectonic activity when Antarctica and Australia separated from each other, creating the Antarctic Circumpolar Current . Balaenopterids grew bigger during this time, with species like Balaenoptera sibbaldina perhaps rivaling the blue whale in terms of size, though other studies disagree that any baleen whale grew that large in the Miocene. The increase in size
11554-526: Was formerly classified in its own family. The two populations, one in the Sea of Okhotsk and Sea of Japan and the other in eastern Pacific are thought to be genetically and physiologically dissimilar. However, there is some discussion as to whether the gray whale should be classified into its own family, or as a rorqual, with recent studies favoring the latter. The taxonomic name "Mysticeti" ( / ˌ m ɪ s t ɪ ˈ s iː t aɪ / ) apparently derives from
11663-625: Was found in Baja California in the El Cien Formation, but as of yet no paper has been published describing this specimen. A. cotylalveus is known from the Yaquina Formation of Oregon. The Yaquina Formation is late Oligocene in age and at the cetacean's locality consists of a fine-grained grey sandstone with alternating layers of medium-grained light-grey sandstone and siltstone . The Yaquina Formation represents
11772-442: Was in fact a bulk feeder, who fed by gulping and straining prey from the water through their interlocking cusped cheek teeth. This is supported by the presence of a lack of mandibular symphysis, meaning the jaw was loosely articulated, and by the presence of the wide palate. This feeding method has an analog in crab-eater seals. This hypothesis combines the idea of bulk feeding and retention of the dentition. Aetiocetus might have been
11881-509: Was thought to have consisted of only one genus until studies done through the early 2000s reported that bowhead whales and right whales are morphologically (different skull shape) and phylogenically different. According to a study done by H. C. Rosenbaum (of the American Museum of Natural History ) and colleagues, the North Pacific ( E. japonica ) and Southern right ( E. australis ) whales are more closely related to each other than to
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