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

cartilageChondrocytes

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