Cetaceans

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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) Brydes, 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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Rissos 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
• man2.5min. 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
• man2.5min. 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 cant 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
• Brydes 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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ad) Balaenidae, be) Eschrichtiidae, cf)
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 Rissos 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.