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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
Advanced fish –
2 pairs of fins (why two?? Functions?)
Ray fin structures – found in all sharks, skates, rays.
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
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
Haversian system
Haversian canal
Osteocyte (lacunae, space where cell is)
Canaliculi
Matrix is hydroxyapatite + collagen
Typical bone
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