Evolutionary Trends

• Changes in Number of Stripes

  1 in the Silurian up to 8 in Ordivician
• Changes in Altitude of stripe

  Downwards (Pendant)

  Horizontal (Outward)

  Upwards (Scandent)
• Changes in shape of Theccae

  Simple to more elaborate (curved and hooked ends)
• Changes in arrangement of Theccae

  Early Ordovician - Uniserial (one side)

  Early Silurian - Biserial (both sides)

  Theccae also became more widely spread