L7: The Emergence of Gnathostomes, the Vertebrates with Moveable Jaws

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  • The Emergence of Gnathostomes, the Vertebrates with Moveable Jaws
    • Earliest Jawed Vertebrates: Placodermi
      • Massively developed bony fish
      • Plates hinged or connected by tissue
        • hinge between head shield and thoracic shield
      • Bottom dwellers or active swimmers
      • Fed close to surface of sea floor or chased prey
      • Small female placoderm died during birth with umbilical cord intact
      • Skeletal bones in the upper and lower jaws
        • Premaxilla
        • Maxilla
        • Dentary
        • Covered in a set of dermal bones
          • Teeth attached to dermal bones (bony fish and mammals)
    • Modern phylogenetic analysis
      • Cartilaginous fish have secondarily lost the dermatocranium
    • Diagnostic features
      • Opposing moveable jaws, mostly with teeth
        • Splanchnocranium (jaws belong to this)
        • Dermatocranium (dermal bones make up the outside of the jaws in most Gnathostomes
          • Upper palatoquadrate cartilage
          • Lower Mackel's cartilage
          • Teeth attached to them (cartilaginous fish)
        • (primitive) vertical biting of two endoskeletal elements
        • Jaw origins
          • From anterior pharyngeal arch supporting gills
          • Increased respiration efficiency
            • Pump H2O across gills
          • Feeding as a secondary function before becoming the primary one in many vertebrates
      • Paired nasal sacs
        • Independent of the nasohypophyseal tube and opening separately to exterior through external nostrils
      • Complex gill arches for gill support
        • Lie internally to the gill filaments and branchial blood vessels
        • Counter current system
          • Mainstream tangentially across branchial arches and concentrates particles in the posterior oral cavity; the filtrate exists between gill rakers and passes the gill filaments where gas exchange occurs
      • 3 semi-circular canals
        • Through addition of a lateral horizontal canal to the pre-existing anterior and posterior vertical canals
      • Paired anterior and posterior fins or limbs
      • Vertebral elements occuring underneath the notochord


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