Vertebrate Adaptations

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Vertebrate Adaptations

• Evolution of the Skeletal System

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General Trends in the Skeleton

• Simplification through bone loss, bone fusion,
  and ossification.
• Reduced bone mass, and therefore, less energy
  invested in skeleton (important because mammals
  are endotherms).
• Increased skeletal strength.

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General Trends in the Skeleton

• Improved articulations.
• Loss of indeterminant growth and consequent
  improved articulations and strength.
• Consider the consequences of allometric growth
  for an animal with indeterminant growth.

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Evolutionary Trends Involving the Skull

• Neurocranium (cartilage bone)
• In early chordates, the neurocranium served as a
  support for the brain.
• With the formation of sensory capsules
  (olfactory, optic, and otic) it assumed a
  protective function.

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Neurocranium, Dermocranium, and Splanchnocranium
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Sensory Capsules
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Evolutionary Trends Involving the Skull

• Bones contributed by the neurocranium
• supraoccipital (nim not in mammals).
• Exoccipitals
• Basioccipital (nim - except fused).
• Occipital condyles (amphibians and reptiles have
  1, mammals have 2)
• Basisphenid (contains sella turcica)
• Presphenoid
• Mesethmoid (nasal septum)

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Evolutionary Trends Involving the Skull

• Bones contributed by the neurocranium
• Petrous (houses inner ear)
• Mastoid
• Turbinate bones

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Evolutionary Trends Involving the Skull

• Dermocranium (dermal bone)
• Protection for neurocranium
• Aid in capturing food
• Bones contributed by the dermocranium
• Dorsal series
• premaxilla
• nasal
• septomaxilla
• maxilla

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Evolutionary Trends Involving the Skull

• Bones contributed by the dermocranium
• Dorsal series continued
• frontal
• parietal
• postparietal
• jugal
• squamosal
• quadratojugal
• tabulare

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Evolutionary Trends Involving the Skull

• Bones contributed by the dermocranium
• Ventral series
• premaxilla
• prevomer
• macilla
• palatine
• pterygoid
• ectopterygoid
• jugal
• quadratojugal
• parasphenoid

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Evolutionary Trends Involving the Skull

• Splanchnocranium (cartilage bone)
• Composed of palatquadrate cartilage and Meckels
  cartilage.
• The palatoquadrate becomes the quadrate in
  non-mammals, and the incus and alisphenoid in
  mammals.
• Meckels cartialge becomes the articular in
  non-mammals, and the malleus in mammals.

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Evolutionary Trends Involving the Skull

• Overview
• There is a progressive assimilation of cranial
  components.
• Multiplication of chondral elements.
• Willistons law (reduction in dermal bone)
• Reduction of visceal jaws.
• Evolution of sound conduction routes.
• Evolution of mandibular suspensorium.

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Evolutionary Trends Involving the Skull

• Overview continued
• Dissociation of skull and pectoral girdle.
• Reduction of interorbital space.
• Progressive compounding of bones.
• Division of occipital condyles.
• Formation of temporal fossae.
• Formation of secondary palate.

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Formation of Temporal Fenestrae

• Dermocranium is laid down over the neurocranium.
• All cranial musculature is thus between the
  dermocranium and the neurocranium.

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Evolution of Temporal Musculature
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Formation of Temporal Fenestrae

• With the advent of the amphibians, the
  dermocrnium began to interfere with the operation
  of the jaw musculature.
• To allow for the belly of the adductor mandibulae
  during contraction, the amphibians evolved a
  temporal notch (this anapsid solution also occurs
  in the chelonians)

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Formation of Temporal Fenestrae

• Muscle attachment shifts from the neurocranium to
  the edges of the temporal fenestrae.
• Crocodilians progressed little.
• Lizards and snakes edge of fenestrae and top of
  dermocranium.
• Mammals top of dermocranium.

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Evolution of the Secondary Palate

• Primary palate
• Forms complete roof of mouth.
• Broken only by intenal nares.
• Retained in fishes and amphibians.
• Problems
• terrestrialization results in breathing problems
  when the mouth is open.
• Impossible to breathe when food is in mouth.

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Evolution of the Secondary Palate

• Bones of the primary palate
• prevomers
• parasphenoid
• palatines
• ectopterygoids

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Evolution of the Secondary Palate

• Solution to the problem
• Snakes the trachea extends far forward
  ventrally.
• Turtles and lizards a shelf is formed over the
  primary palate anteriorly - includes the
  maxillary and premaxillary bones, also the
  palatine to some degree.

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Evolution of the Secondary Palate

• Solution to the problem continued
• Crocodilians the secondary palate extends
  completely over the primary palate, even more
  completely than mammals. i.e., in mammals the
  last 1/2 to 1/3 is soft. In crocodiles this
  facilitates manipulation of food under water.
  Probably not for breathing since they can go
  several hours without breathing.

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Evolution of the Secondary Palate

• Solution to the problem continued
• Mammals extensive as in crocodilians - permits
  brathing while eating, this is necessary as
  mammals are endotherms and consequently have high
  metabolic rates.
• Birds reduced bony content, but still extensive
  soft tissue.

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Evolution of Teeth.

• All gnathostomes either have teeth, or evolved
  from ancestors with teeth.
• Those without teeth have tooth-like structures.
• True teeth
• Outer layer of enamel.
• Deep layer of dentine.

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Evolution of Teeth.

• True Teeth continued.
• Innermost pulp layer with connective tissue,
  blood vessels, and nerves.
• Enamel equals 96 inorganic materials, very hard
  non-living substance.
• Dentine is very bone-like, has living matter.

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Evolution of Teeth.

• Location of teeth.
• Thought to be modified denticles originally found
  on all integumentary scales or plates over all
  the body in early fishes.
• Denticles could thus occur wherever ectoderm was,
  i.e. as far back as the branchial bars of some
  fishes.
• Trend towards limitation of the area of dispersal.

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Distribution of Teeth
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More on Teeth

• Number of teeth
• trend toward reduction in number, but increase in
  size and anchorage.
• Cycles of Replacement
• Early vertebrates continuous and unlimited
• Primitive verts polymodal replacement (many ways
  of replacement).

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More on Teeth

• Cycles of Replacement cont.
• mammals and some reptiles unimodal replacement.
• Neither of these replacements occur all at once.

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More on Teeth

• Tooth form
• modified with respect to diet.
• Crushing rounded and flattened.
• Grinding flattened (only mammals).
• Slashing canines.
• Poison conducting (snakes, lizards, and Blarina).
• Shearing carnassials (only mammals).

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More on Teeth

• Tooth form cont.
• Occurrence of 2 types heterodont.
• Occurrence of 1 type homodont.
• Types of teeth reflect diet.

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Functional Evolution of the Mandibular
Suspensorium.

• Initial detection of sound was via waves received
  through solids (ie gross structures of the body).
• Derived condition involves detection of air-borne
  sound waves.

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Functional Evolution of the Mandibular
Suspensorium.

• The sound conducting system in all vertebrates
  involved the mandibular arch and its attachments
  to the skull. It thus became necessary to
  consider the evolution of the mandibular
  suspensorium.

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Functional Evolution of the Mandibular
Suspensorium.

• Paleostyly the agnathan condition in which some
  of the visceal arches are directly associated
  with the skull.
• Autostyly exhibited by the placoderms. Here,
  the mandibular arch is suspended from the cranium
  by itself. In this condition there is
  intervention by the hyomandibula.

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Morphology at bottom represents Autostyly.
Paleostyly is not shown.
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Functional Evolution of the Mandibular
Suspensorium.

• Euamphistyly In the primitive post-placoderm
  fishes the epibranchial portion of the second
  visceral arch suspends the rear portion of the
  mandibular arch. This is a true double
  suspension. The hyomandibula is proximal to the
  otic capsule and also to the spirical. In this
  condition and the following, the hyomandibula is
  ideally suited for the transmission of sound
  waves directly to the otic capsule.

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Euamphistyly is the second from from the bottom.
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Functional Evolution of the Mandibular
Suspensorium.

• At this point, we have a division in our
  evolutionary scheme. Elasmobranchs and teleosts
  have a hyostylic suspension which is solely via
  the hyomandibula. This is OK in terms of sound
  conduction since they are aquatic. In this
  condition, the hyomandibula is the only link to
  the otic region of the skull.

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Functional Evolution of the Mandibular
Suspensorium.

• The second evolutionary line contains those with
  the Metaautostyly condition, which is derived
  directly from euamphistyly, and is characteristic
  of non-mammalian tetrapods. In this condition
• hyomandibula no longer serves in jaw suspension.
• Hyomandibula is modified as a columella, the
  inner ear ossicle of non-mammalian tetrapods.

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Metautostyly is at the top left.
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Functional Evolution of the Mandibular
Suspensorium.

• Serves only for the conduction of sound.
• What are the selective pressures? The importance
  of delicate air-borne sound waves and their
  relation to terrestrialiation.
• Recall that all this takes place in the vicinity
  of the spirical and its cavity. One end of the
  hyomandibula butted against the otic capsules,
  the other end against the spiracular cavity. This
  cavity acted as a resonating chamber for
  air-borne sounds (not possible in water?)

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Functional Evolution of the Mandibular
Suspensorium.

• When this cavity becomes covered by a membrane
  (tympanum - ear drum) it becomes known as the
  middle ear cavity and provides no dimunition of
  sound. Also, in this stage, the columella is
  supported via cartilaginous struts by the
  quadrate. It is thus able to detect both ground
  borne and air-borne sound waves. Ground waves
  are via the articular-quadrate articulation,
  struts, columella, and otic capsule.

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Functional Evolution of the Mandibular
Suspensorium.

• From the metautostyly condition of non-mammalian
  tetrapods we see the evolution of
  Cranioamphistyly in mammals (birds still have the
  typical reptilian condition).
• Mammals were no longer ground crawling as were
  reptiles and thus any sound conduction via solids
  is almost completely gone.
• At the same time (circa Therapsids) the jaw was
  becoming shortened to facilitate leverage and
  differend feeding modes.

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Functional Evolution of the Mandibular
Suspensorium.

• The jaw articulation moved anteriorly. The major
  articulation was between the dentary - articular
  and squamosal-quadrate. Eventually the
  articulation became completely dentary -
  squamosal.
• Ultimately, the reduced quadrate and articular
  are functionless except for sound conduction.
  They lie close to the tympanum. The articular
  ultimately lies against the tympanum and becomes
  the malleus. The quadrate and columela
  (hyomandibula) become the icus and stapes
  respectively.

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Cranioamphi-styly is at upper right.
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Evolution of the Postcranial Skeleton.

• Functional units of the post-cranial skeleton.
• Visceal skeleton
• Vertebral column
• Ribs
• Sternum

• Girdles
• Paired appendages
• Unpaired appendages

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Postcranial Skeleton

• We need to know a little more about bone.
• What sorts of forces operate on bony tissue?
• Compression
• Tension
• Shear
• Torsion

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Forces operating on bone

• Examples
• Compression. Graviportal limbs of
  elephants.
• Shear..Greater trochanter of the femur.
• Torsion... Vertebrae Femur
• Tension.. Sternum

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Forces operating on bone

• Bone is living tissue, and accommodates whatever
  forces are applied to it.
• As an example, someone who loses a lot of weight
  quickly will still possess a robust skeleton
  designed to carry a lot of weight. However, with
  time the skeleton will reabsorb a considerable
  amount of tissue and become more gracile.

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Forces acting on bone.

• We can look at cross-sections of bone and
  determine exactly what kinds of forces were
  applied to the bone.
• Note - a bone is not solid in cross section.
• force lines within the bone become ossified for
  increased strength.

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Changes resulting from terrestrialization.

• What are some of the problems associated with a
  terrestrial life style?
• Support
• Stability
• Locomotion
• Respiration
• Dessication.
• Note some of these same issues are faced by
  aquatic forms as well.

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In an aquatic environment, the water acts as a
skeleton. Terrestrial organism often have their
mass arranged over only a few points of
support.Compare and contrast the articulations
of the 2 joints shown here.
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