Sole Mates: Evolution of Fishes
The origin of fishes
can be traced to the
first chordates
(something like
Pikaia or the modern
Branchiostoma),
which lacked a
backbone but
possessed a flexible
rod of tissue called a
notochord
Like other chordates,
these have the basic
worm-like body plan,
muscle packs, and a
pharynx
“Finding Nemo”
Primitive cephalochordates: fish-like forms without backbone
(but with well-differentiated head and body)
Pikaia
(Cambrian)
Branchiostoma
(Recent)
General Picture
Length of
“spindles” in
diagram
represent
time ranges of
major groups
Width of
“spindles” in
diagram
denote
diversity of
species within
each group
Amphioxus
Earliest fishes were jawless
From the jawless fishes arose jawed fishes
conodonts
conodont animal
Note: The diversity of major fish groups peaked in the Devonian
Period. This period is therefore often referred to as “The Age of
Fishes”
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First True Fishes:
Jawless Fishes (Agnatha):
Ancient, Armoured Forms (Ostracoderms)
Agnatha (jawless fishes): Modern Forms
Followed by appearance of jawed
fishes (all other groups have jaws)
Range:
Cambrian-Carboniferous
Had heavy external armour,
but soft, cartilaginous
internal skeleton
Evolution of Jaws (Step 1)
The evolution of jaws is an example of evolutionary
modification of existing structures to perform new functions.
Jaws are modified gill arches !
Start with no jaws and many gill slits (supported by gill arches
or “skeletal rods”)
Representatives of the jawless fishes have survived to the
present day, but none of these possess armour.
Evolution of Jaws (Step 2)
Lose first couple gill arches and modify third in line into solid
jaws (upper mandible is upper part of arch becomes attached
to skull, lower mandible remains free)
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First Jawed Fishes (but some disagreement on this):
Acanthodians
Evolution of Jaws (Step 3)
Distinguished by
spines that
supported primitive
“fins” and slightly
hardened internal
skeleton
Range:
Silurian-Permian
Modify next gill arch in line into secondary components
of the upper and lower mandibles (red)
May have been
ancestors of bony
fishes
Cartilaginous Fishes: Chondrichthyes (Sharks, Rays, Skates)
Jawed Fishes, cont’d: Placoderms
Distinguished
by jaws and
thick plates of
bony armour.
“Antiarchs”
Range:
Silurian-Carboniferous
Skin with “denticles”
“Arthrodires”
Distinguished by
cartilaginous skeleton,
exposed gill slits, and skin
with imbedded denticles
Range: Silurian-Recent
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Bony Fishes: (Ray fins, Lobe fins)
Rayfinned Fishes
Fins supported by
thin bones that
radiate out from
body
Ray finned fishes
Fins attached to body by
fleshy lobe with complex
internal bone structure.
Lobe finned fishes
Fins much more muscular
than in ray-finned fish
Range: Devonian- Recent
Lobe finned Fishes
Forms one usually
thinks of as
“fishes”
More diverse
group of presentday fishes,
Lobe finned Fishes
Lobe finned fishes were once fairly diverse
(especially in late Paleozoic).
Three major groups (Coelocanths, Lungfishes and
Rhipidistians)
Coelocanths
A group once
thought to have
gone extinct by
the Cretaceous.
Only two major groups have survived to the present
day: Coelocanths and Lungfishes
Lungfishes
Coelocanths
(Latimeria)
First living coelocanth caught off the coast of South Africa
in 1938.
Rhipidistians
Since then, several more have been caught and/or observed
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Lobe finned Fishes
Lobe finned Fishes: Toward Tetrapods
Lived in shallow
ponds, probably had
lungs (may have
been as sister group
to lungfishes)
Similar skeletal
structure to
amphibians
Rhiphidistians
Lungfishes
Have gills, but can also breathe using lungs
Various species live today in South America, Africa and Australia
During dry season, some lungfish can burrow into mud, make a “cocoon” out of skin
secretions and breathe air through openings to surface.
Labyrinthodont teeth (with folded enamel)
provides close link with early amphibians
These lead us to the next stage of vertebrate
evolution;
leaving the water…
The only fish which
had developed an
opening to the
nostrils inside the
mouth - a feature
which is found in all
the land vertebrates.
Why leave the water ?
-new, and as yet unexploited, food resources in
terrestrial realm
-escape from predators
-drying up of ponds (need to cross land to reach other
water bodies)
Challenges
END OF LECTURE
-support of the body in water is made possible by water,
but
difficult to support on land
-need to strengthen shoulder and hip girdles (that
supported limbs)
-modify oesophagus into lungs to breathe out of water
-develop ears to pick up sound waves in air
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