the evolution of fishes
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The Evolution of fishes. Add paired appendages Better tail (symmetrical) Add jaws Loose the armor Strengthen internal skeleton. How to become a better fish. Adding jaws Clearly from gills Mouth lined with ectoderm Movement of markers: nasal opening, Why jaws= value?

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how to become a better fish
Add paired appendages

Better tail (symmetrical)

Add jaws

Loose the armor

Strengthen internal skeleton

How to become a better fish

Adding jaws

  • Clearly from gills
  • Mouth lined with ectoderm
  • Movement of markers: nasal opening,
  • Why jaws= value?
  • How can you get rid of a gill if gills are useful? = structural improvement
  • 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


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



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

  • 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


Matrix of collagen



Haversian system

Haversian canal

Osteocyte (lacunae, space where cell is)


Matrix is hydroxyapatite + collagen

Typical bone


Fish- placoid scale

Enamel on outside = solid

Dentine inside = has canals

Bone at base = canals and cells



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