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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primitively; cartilagenous inner structure = gill bar
Outer structure = dermal armor
1. Why? – stabilize swimming
2. some primitive forms with fin fold
Acanthodians = primitive bony fish with spines for fins, more than 2 pair primitively.
2 pairs of fins (why two?? Functions?)
Ray fin structures – found in all sharks, skates, rays.
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.
Symmetrical = lungfish
Assymetrical = crossopterygians (extinct)
And coelacanth – marine living
Advanced bony fish = homocercal = symmetric
Head and shoulder armor
fairly solid outer skull.
Advanced bony fish
Skull reduced to barin case, jaw and gill supports – no heavy outer armor.
Primitive = ganoid = bone and enamel form a tile mosaic- solid.
Placoid = similar but isolated in sharks
Cycloid, ctenoid = reduced = thin, light weight.
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
Matrix of collagen
Osteocyte (lacunae, space where cell is)
Matrix is hydroxyapatite + collagen
Origin of intermembranous bone = direct ossification in tissue, no cartilage precursor
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
Four fin types: ray fin
flesh fin – symmetrical
flesh fin - assymetrical