[B] Gnathostomata - Chondrichthyes

• Placoderms:

  • Bony plates on skin 
  • Head shield made of large, fused dermal bone plates 
  • Jaws with bony projections or teeth 
  • Biting → shift to larger prey 
  • Notochord with ossified neural and hemal arches 
  • Synarcual
  • Allow movements of head
  • Sturdy composite bone 
  • Neural + hemal arches just behind the head 
• Chondrichthyes

  • Freshwater, marine, estuarine habitats
  • Predominantly cartilaginous skeleton
  • Secondary loss of bone
  • Tooth Replacement System
  • Placoid scales from beneath skin
  • Pelvic claspers in males
  • Cephalic clasper in chimaeras
  • Need for constant swimming in some sp. 
  • Ram ventilation 
  • Some sharks need to keep swimming