The Evolution of fishes
How to become a better fish
• Add paired appendages
• Better tail (symmetrical)
• Add jaws
• Loose the armor
• Strengthen internal
skeleton
Adding jaws
1. Clearly from gills
2. Mouth lined with
ectoderm
3. Movement of markers:
nasal opening,
4. Why jaws= value?
5. How can you get rid of a
gill if gills are useful? =
structural improvement
6. Note: teeth over much of
mouth = ectoderm,
same structure as
dermal armor = bone,
dentine, enamel.
Jaw types and jaw structure
primitively; cartilagenous inner structure
= gill bar
Outer structure = dermal armor
Paired appendages
1. Why? – stabilize swimming
2. some primitive forms with fin fold
Acanthodians = primitive bony fish with spines
for fins, more than 2 pair primitively.
Advanced fish –
2 pairs of fins (why two?? Functions?)
Ray fin structures – found in all sharks, skates,
rays.
Bony fish skeletons
Actinopterygians = ray fins
Note change in position of pelvic
Fin pair. Why??
Crossopterygian or sarcopterygian: bony fish with a
fleshy fin = central bone elements and muscle with a ray
fringe.
Note fleshy fin types:
Symmetrical = lungfish
Assymetrical =
crossopterygians (extinct)
And coelacanth – marine living
Primitive: heterocercal tail
Advanced bony fish =
homocercal = symmetric
Placoderms
Head and
shoulder armor
Primitive bony fish = Amia
fairly solid outer skull.
Advanced bony fish
Skull reduced to barin case, jaw
and gill supports – no heavy
outer armor.
Fish scales
Primitive = ganoid =
bone and enamel form
a tile mosaic- solid.
Placoid = similar but
isolated in sharks
Cycloid, ctenoid =
reduced = thin, light
weight.
Material of the internal skeleton
Old view – since cartilage replaced by bone, cart fish are older, gave rise
to bony fish
New view – Placoderms, with external bony armor and internal cartilage,
arise from Agnatha –
Both bony and cartilagenous fish arise from placoderms
So: make sense out of it for classification
1. How to strengthen skeleton and lighten skeleton
bony fish = osteichthyes – turn internal cartilagenous skeleton into
bone for greater strength, and lighten external armor
cartilagenous fish = chondrichthyes – keep internal cartilagenous
skeleton, but add some calcium salt, turn external armor into ‘denticles’
= tooth like scales
note; teeth = denticles in mouth.
So
Placoderms = armored = ancestral
Notochord – jelly filled,
hydrostatic skeleton
cartilage
Chondrocytes
Matrix of collagen
Haversian system
Haversian canal
Osteocyte (lacunae, space where cell is)
Canaliculi
Matrix is hydroxyapatite + collagen
Typical bone
Origin of bone by replacement of cartilage
Origin of intermembranous bone = direct ossification in tissue, no
cartilage precursor
Fish- placoid scale
Enamel on outside = solid
Dentine inside = has canals
Bone at base = canals and cells
Chondrichthyes
sharks, skates, rays and
ratfish (Chimaera)
Ratfish weird = no scales,
mollusc eaters, deep sea, not
too closely related
Bony fish
Four fin types: ray fin
spine fin
flesh fin – symmetrical
flesh fin - assymetrical