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