Early vertebrates and Primitive fish

Early vertebrates and Primitive fish

1. What is a fish?

   • All vertebrates which aren't tetrapods
   • Aquatic vertebrate
   • (appendages - fins)
   • Gills
   • Aquatic vertebrate
     • Paraphyletic group
   • Skin with scales of dermal region

2. Early vertebrates

   • Early Cambrian fossils (535mya)
     • China - lack mineralised tissues
   • Heart and fin rays
   • Mineralised tissue - late Cambrian

3. CONODONTS

   • Microfossils made of Apatite: Cambrian-Triassic
     • Similar to dentine
   • Diet: Marine algae/inverts
   • Many vertebrate characteristics
     • (but no gill slits)
       • Diffusion?
     • MINERALISED TISSUE

4. Agnatha

   • (Jawless fish)
     • Hagfish & Lampreys
       • Hagfish (Myxinoidea)
         • ~ 65 spp (all marine)
           • Scavengers & predators
         • Single nasal opening. Single semi-circular canal
         • Degenerate eyes (skin covered)
         • Mucoid bag
         • Skewed sex ratios (100:1)
           • Eggs with tough covering and attachment hooks
         • Colonial/semi-colonial
       • Lampreys (Petromyzontia)
         • 41spp ( ~50% parasitic)
           • Marine & freshwater
         • Cartilage pipe around notochord (Arcualia)
           • Simple digestive system
             • Well developed kidney
           • Oral gland anticoagulent
           • 2 semicircular canals
         • Chloride cells gills : tidal ventilation
   • No scales
   • Eel like bodies
   • No distinct stomach, jaws or vertabrae
   • Cartilaginous
   • 5-16 pairs of gills
   • Distinct Brain

5. Ostracoderms

   • Anapsids
     • relationship in flux
   • Lacked jaws - movable mouth plates
     • filter feed / soft bodied prey
   • midline dorsal fins & some pectoral
   • Extinct: late devonian

6. Jawless to Jaws

   • Major evolutionary step: changes in habitat/behaviourresources
   • 1. Ancestral Agnathan: unjointed branchial arches
     • 2. Jointed branchial arches increase ventilation
       • 3. Mouth closing ventilatory jaws
         • 4. Early jawed Gnathostome
           • Improved locomotion, sensory and ciculatory systems
           • Silurian -> mid Ordovician
           • 2nd Hox duplication event
           • Enlarged forebrain
           • Lateral line system
           • 4 recognised clades
   • Gnathostomata
     • Class: Placoderms
       • Plated skin
         • Plates hinged
           • Internal skeleton: bone & cartilage
       • Plated teeth: Ptyctodontids
         • Early specimens - no teeth. Later on - true teeth
       • Primarily marine but some in estuaries and fresh water
       • Primary benthic - dorsoventrally flattened
         • External pelvic appendages
     • Class: Acanthodians
       • Many short spines and ventral fins
         • Slender bodied
       • Mid Ordovician
       • Scales and heterocercal tail - primarily midwater
       • No teeth - filter feeders
     • Class: Condrychthyes (cartilaginous fish)
       • Subclass: Elasmobranchii
         • Sharks, skates, rays
       • Subclass: Holocephali
         • Chimaeras & ghost fish
     • Osteichthyes (bony fish)
       • Class: Sarcopterygii
         • Lobe finned fish
       • Class: Actinopterygii
         • Ray finned fish