Lab 8 Handout

Theyhaveunjointedappendagespelvicandpectoralfins

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 5.
Well
developed
circulatory
system:
This
system
includes
a
chambered
heart.

 
 The
Jawless
Fishes
(Myxini
and
Cephalaspidomorphi)

 The
earliest
vertebrates
were
small,
aquatic,
fishlike
animals
without
jaws.
There
are
few
 species
of
jawless
fish
living
today,
grouped
into
two
major
taxa:
hagfishes
(Myxini)
and
 lampreys
(Cephalaspidomorphi).
These
fish,
often
placed
in
the
class
Agnatha,
have
tough
 skins
and
lack
paired
appendages.

 
 1.
How
do
you
think
jawless
fishes
obtain
their
food?
 
 The
Evolution
of
Jaws
(Infraphylum
Gnathostomata)

 The
origin
of
jaws
was
a
major
adaptive
event
in
early
vertebrate
phylogeny.
Vertebrate
 jaws
evolved
by
modification
the
skeletal
rods
that
had
previously
supported
the
gill
slit
in
 fishes
about
400
million
years
ago,
during
the
Devonian.
The
origin
of
vertebrate
jaws
from
 these
skeletal
parts
illustrates
a
general
feature
of
evolutionary
change:
new
adaptations
 usually
evolve
by
the
modification
of
existing
structures.
The
advent
of
jaws
is
perhaps
the
 greatest
of
all
advances
in
vertebrate
history.
The
Devonian
has
been
called
the
“age
of
 fishes,”
because
of
the
explosive
radiation
of
aquatic
vertebrates
during
this
period.
 
 
 
 Fig.
2
Evolution
of
the
vertebrate
jaw
 
 The
Cartilaginous
Fish
(Class
Chondrichthyes)

 
 Early
jawed
fishes
are
replaced
today
by
two
lines
of
descendants:

 1.
The
cartilaginous
fishes

 2.
The
bony
fishes

 
 The
cartilaginous
fish
lack
bone
completely,
and
their
endoskeleton
is
composed
of
 cartilage.
We
know
fromthe
fossil
record
that
the
ancestors
of
modern
cartilaginous
fish
 had
bone.
Therefore,
lack
of
bone
is
not
a
primitive
trait
(i.e.
it
is
secondarily
derived).
The
 most
common
members
of
the
Chondrichthyes
are
the
sharks
and
rays

 
 Sharks
are
generally
large,
active
predators
with
teeth,
teeth,
and
more
teeth!
In
fact
their
 teeth
are
continually
replaced
as
they
wear
out.
The
largest
fish
on
earth
is
the
whale
shark
 (up
to
18m).
These
giants
aren’t
dangerous
to
us
because
they
feed
on
zooplankton.

 
 1.
What
do
you
think
may
have
been
the
selective
advantage
for
the
loss
of
bone?
 
 The
Bony
Fishes

 This
group
of
fish
is
distinguished
by
having
an
endoskeleton
of
bone,
and
these
species
are
 therefore
grouped
together.
This
is,
howeve...
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