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  • trilobites
    • cambrian
      • planktonic/ pelagic
        • cephalon and pygidium are same size - aids balance
        • small (< 1 cm) - floats easily
        • small thorax - aids balance
      • benthic
        • eyes on top of head suggests sea floor dweller
        • large genal spines for support on sea floor
    • ordovician
      • burrower
        • head shield - protected head from sediment
        • large genal spines - could've been used as a shovel or used to keep the trilobite in sediment
        • absence of eyes - no need for them due to deep water and lack of sunlight
      • living on sediment, just below surface
        • small and streamlined - water would've flowed easily across it
        • eyes on top of stalks that were level with sea bed - would've been able to see if covered
    • protection
      • many trilobites could curl up into a ball
      • some evolved a series of sharp spines projecting from the back of the trilobite
    • had a chitinous exoskeleton made of calcium phosphate and calcium carbonate - hard for preservation
    • sized from 1 mm to 720 mm
    • nektonic
      • large glabella - probably filled with fat for buoyancy
      • light exoskeleton - stays afloat easier
      • streamlined - moves through water easily
    • closest living relatives : horseshoe crabs, evolved in ordovician (450 ma)
    • devonian
      • probably benthic
        • three pronged tridents attached to their cephalons
          • sensory/radar function?
          • disguise or protection?
          • horns ?
      • lived in shallow, sun lit marine conditions
        • eyes on turrets with a flange on top - active during day
        • eyes had 500 lenses and 360 vision


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