Vertebrate Adaptations
Evolution of the Skeletal System
General Trends in the Skeleton
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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.
General Trends in the Skeleton
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Improved articulations.
Loss of indeterminant growth and
consequent improved articulations and
strength.
– Consider the consequences of allometric
growth for an animal with indeterminant
growth.
Evolutionary Trends Involving
the Skull
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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.
Neurocranium, Dermocranium,
and Splanchnocranium
Sensory Capsules
Evolutionary Trends Involving
the Skull
– Bones contributed by the neurocranium:
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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)
Evolutionary Trends Involving
the Skull
– Bones contributed by the neurocranium:
• Petrous (houses inner ear)
• Mastoid
• Turbinate bones
Evolutionary Trends Involving
the Skull
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Dermocranium (dermal bone)
– Protection for neurocranium
– Aid in capturing food
– Bones contributed by the dermocranium:
• Dorsal series
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–
–
–
premaxilla
nasal
septomaxilla
maxilla
Evolutionary Trends Involving
the Skull
– Bones contributed by the dermocranium:
• Dorsal series continued
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–
frontal
parietal
postparietal
jugal
squamosal
quadratojugal
tabulare
Evolutionary Trends Involving
the Skull
– Bones contributed by the dermocranium:
• Ventral series
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–
–
–
–
–
–
–
premaxilla
prevomer
macilla
palatine
pterygoid
ectopterygoid
jugal
quadratojugal
parasphenoid
Evolutionary Trends Involving
the Skull
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Splanchnocranium (cartilage bone)
– Composed of palatquadrate cartilage and
Meckel’s cartilage.
– The palatoquadrate becomes the quadrate
in non-mammals, and the incus and
alisphenoid in mammals.
– Meckel’s cartialge becomes the articular in
non-mammals, and the malleus in
mammals.
Evolutionary Trends Involving
the Skull
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Overview:
– There is a progressive assimilation of
cranial components.
– Multiplication of chondral elements.
– Williston’s law (reduction in dermal bone)
– Reduction of visceal jaws.
– Evolution of sound conduction routes.
– Evolution of mandibular suspensorium.
Evolutionary Trends Involving
the Skull
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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.
Formation of Temporal Fenestrae
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Dermocranium is laid down over the
neurocranium.
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All cranial musculature is thus between
the dermocranium and the
neurocranium.
Evolution of Temporal
Musculature
Formation of Temporal Fenestrae
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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)
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.
Evolution of the Secondary
Palate
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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.
Evolution of the Secondary
Palate
– Bones of the primary palate
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•
•
•
prevomers
parasphenoid
palatines
ectopterygoids
Evolution of the Secondary
Palate
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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.
Evolution of the Secondary
Palate
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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.
Evolution of the Secondary
Palate
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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.
Evolution of Teeth.
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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.
Evolution of Teeth.
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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.
Evolution of Teeth.
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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.
Distribution of Teeth
Fish
Amphibians Reptiles
Mammals
Most
palatal
Most jaw
bones
Splenials
Palatine
Dentary
Dentaries
Pterygoid
Premax.
Dentary
Maxilla
Maxilla
Some
palatines
Premax.
More on Teeth
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Number of teeth:
– trend toward reduction in number, but
increase in size and anchorage.
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Cycles of Replacement
– Early vertebrates: continuous and unlimited
– Primitive verts: polymodal replacement
(many ways of replacement).
More on Teeth
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Cycles of Replacement cont.
– mammals and some reptiles: unimodal
replacement.
– Neither of these replacements occur all at
once.
More on Teeth
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Tooth form
– modified with respect to diet.
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Crushing = rounded and flattened.
Grinding = flattened (only mammals).
Slashing = canines.
Poison conducting (snakes, lizards, and
Blarina).
• Shearing = carnassials (only mammals).
More on Teeth
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Tooth form cont.
– Occurrence of 2 types: heterodont.
– Occurrence of 1 type: homodont.
– Types of teeth reflect diet.
Functional Evolution of the
Mandibular Suspensorium.
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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.
Functional Evolution of the
Mandibular Suspensorium.
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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.
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.
Morphology at
bottom represents
Autostyly.
Paleostyly is not
shown.
Functional Evolution of the
Mandibular Suspensorium.
– Euamphistyly: In the primitive postplacoderm 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.
Euamphistyly is
the second from
from the bottom.
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.
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.
Metautostyly is at
the top left.
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 airborne sounds (not possible in water?)
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.
Functional Evolution of the
Mandibular Suspensorium.
– From the metautostyly condition of nonmammalian 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.
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.
Cranioamphistyly is at upper
right.
Evolution of the Postcranial
Skeleton.
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Functional units of
the post-cranial
skeleton.
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Visceal skeleton
Vertebral column
Ribs
Sternum
– Girdles
– Paired appendages
– Unpaired
appendages
Postcranial Skeleton
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We need to know a little more about
bone.
What sorts of forces operate on bony
tissue?
– Compression
– Tension
– Shear
– Torsion
Forces operating on bone
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Examples
– Compression…….Graviportal limbs of
elephants.
– Shear……………..Greater trochanter of the
femur.
– Torsion…………... Vertebrae & Femur
– Tension………….. Sternum
Forces operating on bone
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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.
Forces acting on bone.
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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.
Changes resulting from
terrestrialization.
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What are some of the problems
associated with a terrestrial life style?
– Support
– Stability
– Locomotion
– Respiration
– Dessication.
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Note: some of these same issues are
faced by aquatic forms as well.
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.