Trilobites

Trilobites

Morphology
1. Cephalon
  1. Made up of eyes, facial sutures, free cheeks, fixed cheeks and glabella
  2. Composed eyes - small lenses of calcite, allowed wide-angled vision
  3. Spines attached to glabella at genal angle. Defense or spread mass, bigger surface area
2. Thorax
  1. Made up thoracic segments, each have pair appendages and gills, 2 pleurae and segment from axis.
  2. Spines depending on mode of life
  3. Skeleton, many plates articulated together, thorax very flexible
  4. Some trilobites curl up into ball or enroll, defensive measure
3. Pygidium
  1. composed several segments fused together
  2. Micropygus - Pygidium smaller than head
  3. Isopygus - Pygidium same size as head
  4. Macropygus - Pygidium bigger than head
Ecdysis
1. Exoskeleton didn't grow, would shed various times of life. Fractured along lines of weaknesses (facial sutures)
2. After moulting, period of rapid growth
3. Leaves animal vulnerable to predators, skeleton temporarily flexible to allow growth
Adaptations/mode of life
1. Benthonic (Calymene)
  1. Many pleura
    1. Supports many pairs of legs for walking
    2. Supports pairs of gills for respiration
    3. Thorax flexible and ability to enroll. Vulnerable soft underside and mouth hidden
  2. Crescent-shaped compound eyes, high on cheeks
    1. Gave 360 vision, lived on sea floor so didn't need to look underneath
    2. May mean it was active hunter or scavenger
  3. Large, not streamlined
    1. No need as animal walked on sea floor
    2. Trace fossils of trilobite tracks - walking, resting and foraging show movements on sea floor
2. Planktonic (Agnostus)
  1. No eyes/very small eyes
    1. Bline/very blind, animal did not hunt, filtered from sea water or organic rich sediment
  2. Inflated/large glabella and large pygidium
    1. Filled with fat or gas, possible floatation device. Buoyancy in water column
  3. Very small size, 2mm --> 10mm
    1. Stay afloat in water column
  4. Few pleura
    1. Few legs, used to paddle or steer animal
    2. Limited flexibility and movement restricted
3. Nektonic (Deiphon)
  1. Eyes on stalks
    1. See forwards, backwards, sideways and look down/underneath
    2. Complex eyes, was active hunter or scavenger
  2. Inflated/large glabella and large pygidium
    1. Filled with fat or gas, possible floatation device. Buoyancy in water column
  3. Very small size
    1. Stay afloat in water column
  4. Numerous separated pleura, spines
    1. Many legs for swimming
    2. Gave higher surface area, aids buoyancy
4. Burrowing (Trinucleus)
  1. No eyes
    1. Unlikely to hunt, food source organic-rich sediment
  2. Wide cephalic fringe
    1. Spread mass in substrate or digging tool make burrows
  3. Extended genal spines
    1. Spread mass on soft substrate
    2. Defence against predators
  4. Pitted cephalic fringe
    1. Housed sensory hairs in life
    2. Able to detect movement of prey and water currents on sea floor, chemicals or temp.
Phylum: Arthropoda Class: Trilobita

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