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Cetaceans. Cetacean Evolution. The Cetacea probably originated in the Palaeocene, and had an Eocene differentiation. We have 2 questions: 1) From which mammalian group did the Cetacea evolve? 2) Do the 2 modern suborders share a common ancestor?. Cetaceans: Evolution.

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Cetaceans


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Cetacean Evolution

  • The Cetacea probably originated in the Palaeocene, and had an Eocene differentiation.

  • We have 2 questions:

    • 1) From which mammalian group did the Cetacea evolve?

    • 2) Do the 2 modern suborders share a common ancestor?


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Cetaceans: Evolution

  • The earliest cetacean fossils date to the Eocene of Pakistan and belong to the suborder: Archaeoceti.

  • Other early fossils are from the middle Eocene of Egypt and southern Nigeria.

  • These fossils are members of the suborder Archaeoceti (sometimes referred to as Zeuglodont).


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  • This suborder includes Pakicetus and Ambulocetus, species associated with shallow seas.

  • The ancestral group, as we noted in our discussion of ungulates and subungulates, is probably the Condylarthran family Mesonychidae.


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Cetaceans: Evolution

  • Recall that the Condylarthrans also gave rise to the subungulates and ungulates, particularly the Perissodactyla. Condylarthran Mesonychids were carnivorous - scavenging ungulates.


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Cetacean Evolution

  • By the mid to late Eocene, most Archaeocetes were so specialized that they were probably not ancestral to the Odontocetes and Mystecetes.

  • Archaeocete skulls are characteristic of early Eocene Creodonts (ancestral group for the Carnivora - wait, what is going on?).


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Cetacean Evolution

  • Archaeocete skull characteristics:

    • Slightly modified tribosphenic teeth.

    • Presence of turbinal bones

    • Incisors, canines, premolars, and molars are primitive: 3/3, 1/1, 4/4, 3/3.

    • Posterior extension of palate via pterygoid and palatines.

    • Sagital crest on parietals.


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Cetacean Evolution

  • External nares lie halfway to orbit, inline with first premolars.

  • Rostrum is narrowed posteriorly.

  • Nasals are much narrower than Creodonts.

  • Now, what is the connection with the Creodonts?


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    Cetacean Evolution

    • In the Cretaceous and Paleocene, there was considerable differentiation in important mammalian groups, probably derived from the insectivores.

    • These groups were probably closely related to the Ungulata.

    • Suborder Arctocyonia was probably ancestral to the Ungulates.


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    Cetacean Evolution

    • A related order, the Mesonychia, was probably ancestral to the Cetacea (NOTE: taxonomy has changed - now the order containing the Arctocyonia and Mesonychia is the Condylarthra, containing the family Mesonychidae.

    • As early as 1969, VanValen (Evol. 23: 118-130) did serological studies demonstrating a close affinity between Artiodactyla and Cetacea.


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    Cetacean Evolution

    • Zeuglodonts (and perhaps all other Cetacea) probably diverged from the Mesonychidae at the end of the Cretaceous, taking to the sea in the early Paleocene.

      • Skulls of Zeuglodonts and Mesonychidae are very similar in cranial and dental characters.

      • Mesonychids were differentiated and widespread in the late Cretaceous.


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    Cetaceans: Evolution

    • Basilosaurus had functional hind limb elements. Other species were clearly transitional between terrestrial and aquatic. By the mid-Miocene, the Archaeoceti were fully aquatic.


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    2 Zeuglodonts: Basilosaurus and Zeuglodon osiris. Note the remnants of the pelvic girdle and hind-limb elements in Zeuglodon, elongation in Basilosaurus, dentition, and elongation of both skulls.


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    Cetacean Evolution

    • Conclusion:

      • Archeoceti (Zeuglodonts) probably diverged from Mesonychids at the end of the cretaceous. Mesonychids were closely related to the Arctocyonia, which probably gave rise to the Ungulates. Mesonychids actually gave rise to the Perissodactyla.


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    Colonization of the Sea

    • Early Zeuglodont fossils are associated with relatively restricted western arm of the Tethyan Sea (approximately Mediterranean - Persian Gulf) in the Paleocene, and dispersed through the warm shallow coastal waters of the greatly re-enlarged Tethys during the Eocene.


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    Colonization of the Sea

    • Tethys sea was shallow warm water basin throughout the Mesozoic.

    • During the Paleocene, western arm of Tethys became constricted and semi-enclosed.

    • Condylarthrans probably utilized riverbanks and shores of the Tethys, feeding on aquatic invertebrates and fish.


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    Compartmentalized stomach: evodence for ungulate origins?


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    Colonization of the Sea

    • Natural selection may have favored those individuals which avoided intense inter- and intra-specific competition by foraging in deeper mud and waters.

    • Those individuals which had forms of variation which enabled them to exploit food resources in deeper waters probably had greater reproductive success.


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    Colonization of the Sea

    • Perissodactyls graze, and are limited by availability of food - or so we imagine.

    • Diversity of Perissodactyls was much greater in the Eocene than it is now.

    • Warm shallow seas are extremely productive for both plant and animals.

    • Foraging in shallow water makes sense if other resources are limiting.


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    Colonization of the Sea

    • If you forage in the water, what kinds of morphological attributes might be favorable?

      • Longer and narrower rostrum for use in catching fish.

      • Webbed appendages.

      • Migration of nares to top of the skull.


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    Major morphological developments in the transition from terrestrial to fully aquatic marine mammal.


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    Colonization of the Sea

    • Could a small rodent or insectivore have done this?


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    Colonization of the Sea

    • Why are there no transitional forms to bare out this hypothesis?

      • Evolutionary event took place over a very restricted area.

      • Event was probably very rapid (in geological time scale).

      • Fragmentation of skeletons after death.

      • Perhaps limited sediment deposition.


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    Cetaceans: Evolution

    • The transition to aquatic feeders is not difficult to imagine. It has been done before:

      • Ichthyosaurs

      • Plesiosaurs

      • Other reptile groups.

    • Aquatic reptilian groups went extinct by the end of the Cretaceous.


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    Colonization of the Sea

    • Last Archaeocetes are from the middle Miocene of France.

    • Early Odontocetes and Mysticetes were present in the middle Oligocene, and completely replaced the Archaeocetes by the middle Miocene.


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    Note: most modern families of cetaceans are present by the Miocene.


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    Cetacean Evolution

    • Characteristics of the suborders with living representatives:

      • Resistance to lactic acid accumulation.

      • Tolerance of oxygen debt in muscle tissue.

      • High titre of muscle myoglobin for rapid transfer of oxygen to the cells.

      • Hypodermal blubber layer for energy storage, thermoregulation (?)


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    Cetacean Evolution

    • Oil storage in bones for energy.

    • Development of flukes for locomotion.

    • Development of dorsal fin for stability and thermoregulation in smaller forms.

    • External nares located on top of skull with means of sealing out water.

    • Modification of tracheal system and lungs to withstand high pressure.


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    Gray Whale


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    Baleanoptera: Blue Whale


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    Cetacean Evolution- Loss of pelvic appendages & girdles


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    Cetacean Evolution- Loss of mobility of the neck.


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    Cetacean Evolution

    • Modification of the eyes to tolerate salt water and extreme pressure.

    • Modification of sound conducting routes and sound production routes.

    • Modification of dentition to reflect a filterfeeding or piscivorous diet.


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    Genital grooves in a) male and b) female. Forelimbs of c) pilot whale, d) right whale, and e) human.


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    Cetaceans: Evolution

    • Unresolved is the question of how the 2 extant cetacean suborders are related to one another, or how either suborder is related to the Archaeoceti.

    • Are they polyphyletic? Probably not.

    • Odontocetes and Mysticetes are clearly differentiated by the Oligocene.


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    Spinner Dolphin


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    Delphinidae: Lagenorhynchus


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    Evolutionary Patterns Within the Odontoceti

    • How do odontocetes differ from the Zeuglodonts?

      • Odontocete lachrymal bones abut onto the ventral area of the maxillaries, not on to their lateral surfaces.

      • The maxillaries have migrated posteriad to lie over the supraorbital region of the frontal bones.


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    Evolutionary Patterns Within the Odontoceti

    • Significant telescoping of skull with accomodation for melon, nasal diverticula, and spermaceti organ associated with sound production and sound reception.

    • Odontocetes have homodont dentition.


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    HorseBasilosaurusDelphinusBalaenoptera


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    Tursiops truncatus


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    Homodont dentition of an odontocete.


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    Evolutionary Patterns Within the Mysticeti

    • Mysticete skulls have great forward extension of the upper margin of the occipital shield. This results from forces operating on anterior portion of the animals:

      • forward motion against water resistance.

      • Strain on cranial and mandibular system each time animal opens its mouth.


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    Evolutionary Patterns Within the Mysticeti

    • Mysticetes have teeth, embryonically. (the first recognizable mysticete (Aetiocetidae: Aetiocetus) from the Oligocene does not have baleen, but teeth instead.)

    • Baleen is of secondary dermal origin.

    • Long nasal bones are partially enveloped by the premaxillaries and maxillaries.


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    a) Minke, b) Sei, c) Bryde’s, d) pygmy right, e) gray, f) humpback, g) fin, h) blue, i) right, j) bowhead.


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    Baleanoptera: Minke Whale


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    Gray Whale


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    Humpback


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    Humpback


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    Baleanoptera: Fin Whale


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    Baleanoptera: Blue Whale


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    Are the Cetacea monophyletic or polyphyletic?

    • Many published works favor a polyphyletic origin for the Odontocetes, Mysticetes, and Archaeocetes.

    • What is the liklihood of 3 separate lines invading the aquatic environment at roughly the same geological time?

    • Is this parsimonoius?


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    Polyphyly: Anatomical considerations.

    • Similarities (result of supposed convergence in an aquatic environment)

      • loss of true vocal cords.

      • Loss of pelage

      • lung shape and oblique position of diaphragm.

      • Streamlined body shape.

      • Dorsal migration of external nares.


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    Polyphyly: Anatomical considerations.

    • Differences (result of diphyletic origin, supposedly)

      • biochemical differences in the blubber.

      • Lower jaw is symphysial in Odontocetes, but not in Mysticetes.

      • Skull is symmetrical in Mysticetes but not in Odontocetes.


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    Risso’s dolphin and northern right whale.


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    Polyphyly: Anatomical considerations.

    • Ethmoturbinals are present in Mysticetes but not in Odontocetes.

    • Females are the larger sex in Mysticetes while for the most part, males are larger in the Odontocetes.

  • Note: if you look long enough, you can find an impressive list of skeletal characters which are similar, and for which the Archaeocetes are intermediate between Odontocetes and Mysticetes.


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    Echolocation in odontocetes


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    Karyotypic considerations: Major argument in favor of a monophyletic origin.

    • Both suborders share the same characteristic distribution of C-heterochromatin in the chromosomes. (However, several divergent and probably secondary karyotypes were found in the odontocetes)

    • Both have the same diploid number of 22 chromosomes.


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    Physiological attributes of the Cetacea: metabolic rates and energy budgets.

    • Some primary factors which have governed the evolution of modern Cetaceans.

      • Food sources are discontinuously distributed in the world oceans.

      • Within areas of food availability, the food is frequently available only seasonally.


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    Physiological attributes of the Cetacea: metabolic rates and energy budgets.

    • Even when food is present and abundant, it is discontinuously distributed from the viewpoint of an individual whale.

    • Presuming that an animal can locate and stay with optimal feeding conditions, these conditions are probably not optimal for reproductive requirements.


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    Diving Adaptations in Mammals

    • Occurs in the Pinnipedia, Sirenia, and Cetacea.

    • Maximum duratin of dive in minutes for varioius mammals:

      • man=2.5min.Dog = 4.5min

      • Beaver = 15min.Seal = 15min

      • Muskrat = 12min.Gray seal = 20min

      • White rat = 3.1min.Elephant seal =


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    Diving Adaptations

    • Maximum duratin of dive in minutes for varioius mammals:

      • man=2.5min.Weddells seal = 43min.

      • Beaver = 15min.Sperm whale = 75min.

      • Muskrat = 12min.Bottle nosed whale = 120min at

      • White rat = 3.1min.A depth of 2.5mi.

      • Dog = 4.5min.

      • Seal = 15min.

      • Gray seal = 20min.

      • Elephant seal = 30min.

      • Manatee = 16min.


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    Diving Adaptations

    • Problems:

      • Brain and heart must have oxygen at all times.

      • You can’t take the air with you, you must hold your breath.

        • Apnia = holding breath

        • Asphixia = going without oxygen

        • Eupea = normal breathing.


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    Diving Adaptations

    • Problems cont:

      • Must avoid the bends. The bends are caused by nitrogen in the blood. The greatest portion of the atmosphere is composed of N2. Under pressure, nitrogen is forced through the lun and into the blood. When you come up too fast, the nitrogen expands in muscle tissue etc and causes great pain.


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    Diving Adaptations

    • You must watch out for CO2 levels. When CO2 level is high enough, the vagus nerve causes you to breathe.

  • Solutions as determined by Sholander for Harbor Seals

    • Harbor seals display bradycardia (reduce heart rate).


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    Diving Adaptations

    • They have a rete mirabile system surrounding the spinal cord and vertebral column. During dives, blood is shunted away from the periphery of the body and into the rete mirabile surrounding the spinal cord. Thus all the O2 now surrounds the spinal cord, the heart, and the brain.


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    Diving Adaptations

    • Some solutions for whales:

      • Bradycardia

      • Myoglobin

      • Can tolerate a high O2 debt.

      • Vasoconstrict and put blood into the rete mirabile surrounding the vertebral column.

      • Exhale before diving. The typical whale has a lung volume of 100,000 liters. After having exhaled, there is a residual 10,000 liters of gas in the trachea.


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    Diving Adaptations

    • The trachea are reinforced with cartilaginous/bone rings which prevent the trachea from collapsing at great depths. However, the lungs collapse.

    • In sperm whales, there is no sternum and the ribs can pivot on their articulation with the vertebra, thus all the air can be exhaled from the lungs and the lungs can collapse.


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    Diving Adaptations

    • Whales have a very high CO2 tolerance.

    • Whales have a relatively low O2 demand.


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    Globicephela


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    Mysticetes: Balaenidae

    • 2 genera and 3 species.

      • Bowhead whale

      • Northern right whale

      • Southern right whale

    • Lack throat grooves and dorsal fin.

    • Callosities on head.

    • Feed by skimming or gulping just below surface.


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    Mysticetes: Balaenidae

    • Easy to whale because they are slow, they float a long time, and they contain a lot of blubber.

    • Overexploited, and populations have not recovered.


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    Mysticetes: Balaenopteridae

    • 2 genera and 5 species of rorquals.

      • Fin whale

      • Sei whale

      • Blue whale

      • Bryde’s whale

      • Minke whale

      • Humpback whale

    • They all have throat grooves for bucal expansion during feeding.


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    Mysticetes: Balaenopteridae

    • Humpbacks occur closer to shore. They have numerous bumps on head, each containing a sensory hair.

    • Humpbacks use ‘bubblenetting’ while others use gulping or skimming.

    • Humpbacks have complex vocalizations, with regional dialects. Songs throughout the season.


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    Baleanoptera: Minke Whale


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    Baleanoptera


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    Baleanoptera: Fin Whale


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    Humpback


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    Humpback


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    Humpback


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    Mysticetes: Eschrichtiidae

    • Monotypic, only Eschrichtius robustus.

    • Crenulations on back, few throat grooves.

    • Feed in arctic in summer, then migrate 18000km to Baja or Sea of Japan, where they calve.

    • Why migrate? Males do not necessarily migrate. Do not feed on southward migration.


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    Mysticetes: Neobalaenidae

    • Monotypic: pygmy right whale.

    • Only in temperate, southern hemisphere waters.

    • Unlike other right whales, it has 2 shallow throat grooves.

    • These are small, only about 6m in length.


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    a&d) Balaenidae, b&e) Eschrichtiidae, c&f) Balaenopteridae.


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    Odontocete vs. Mysticete


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    Odontocetes: Delphinidae

    • 17 genera and 32 species.

    • Size ranges from 1.7m to Killer whale at 9m.

    • Spinner dolphins are species most often caught in tuna nets.


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    Tooth number in Risso’s dolphin and spinner dolphin.


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    Spotted Dolphin


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    Delphinidae


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    Spinner Dolphin


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    Tursiops truncatus


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    Odontocetes: Monodontidae

    • 2 genera and 2 species: narwhal and beluga.

    • Lack dorsal fin.

    • Circumarctic distribution.

    • Both have robust bodies and heads.

    • Narwhals have 2 incisors: right incisor does not erupt in males, left incisor erupts w/ counterclockwise spiral. Neither incisor erupts in females.


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    Beluga, Laganorhynchus, and Globicephala.


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    Odontocetes: Phocoenidae

    • 4 genera and 6 species of porpoises: they differ from dolphins in that dolphins generally have a beak while porpoises do not.

    • Porpoise teeth are blunt crowned, while dolphin teeth are sharp and conical.


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    Homodont dentition of an odontocete.


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    Odontocetes: Physeteridae

    • 2 genera and 3 species of Sperm whales.

    • Head consitutes 1/3 of total length.

    • Possess a spermaceti organ to regulate bouyancy.

    • Dive to 3.2km for 2 hrs.


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    Spermaceti organ in the Sperm Whale: May function to modify bouyancy, or as a lens to focus outgoing soundwaves.


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    Sperm Whale


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    Sperm Whale


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    Sperm Whale


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    Odontocetes: Platanistidae

    • 4 genera and 5 species of river dolphin, 2-3m in length.

    • Found in Amazon, Yangtze, La Plata river, and the Ganges and Indus river dolphins of India, Pakistan, and Bangladesh.

    • Eyes lack lenses, and are functionally blind - find prey via echolocation.


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    Odontocetes: Ziphiidae

    • 6 general and 19 species of beaked whales - slender, 4 to 13m.

    • Reverse sexual size dimorphism - like baleens.

    • Very reduced number of teeth, and usually found only in males.


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    Ziphiidae


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    More on whales:

    • Check out the web site for the Los Angeles County Museum of Natural History.


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