Arthropods

Summary by Jon Tortomasi

Trilobites

• Ecology
  • benthic, epifaunal, marine
  • primarily deposit feeders, but some filter feeders and carnivores
  • some varieties were pelagic
• Evolutionary History
  • little known about ancestors (as with all arthropods
  • possibly derived from annelids (segmented worms)
  • family diversity was at a maximum in the Upper Cambrian and lower Ordovician
  • diversity highest in the Ordovician
• Morphology
  • cephalon, segmented thorax, pygidium (tail)
  • paired walking legs and gill branches
  • hyposome mouth area
  • glabella housed stomach
  • compound eyes
• Stratigraphic Use
  • most of the Cambrian zoned by trilobites
  • much of Ordovician also zoned by trilobites

Ostracods

• Ecology
  • predominantly marine, some freshwater and terrestrial types
  • benthic or pelagic (mobile)
  • primarily filter feeders
  • can be attached to substrate
  • life habit and body shape reflects the environment
  • depth and salinity controls variation
  • tropical areas show more diversity
• Evolutionary History
  • first appeared in the early Cambrian, but true origins are obscure
  • Early Ordivician radiation
  • Devonian and Carboniferous most varied, with first appearance of freshwater types
  • since Paleocene, diversity has increased substantially

Eurypterids

• Ecology
  • benthic, mobile, predators
  • preyed upon fish and other eurypterids
  • lived in marine, brackish, and freshwater
  • could be from 20 cm to 2 m in length
  • developed different appendages for eating, grasping, etc.
  • can crawl on land for short periods of time because of lung books and `kiemenplatlen' (bronchial chamber with dense array of spines for retaining water)
  • Locomotion
    • those with paddles, rowed themselves up to 2.5 times their body length per second (may have turned upside down to do this)
    • others had long legs for walking
    • some used telson (tail) as a rudder or to provide lift
• Evolutionary History
  • lived from the Ordovician to the Permian
  • Eurypterina had the greatest diversity
  • relatively rare as fossils due to nature of the cuticle (thin)

Malacostreans

• Ecology
  • epifaunal or pelagic
  • freshwater or marine (w/ a few terrestrial)
  • mostly predators (clawed forms were the most successful)
  • live at all depths
• Evolutionary History
  • first appeared in Cambrian, still exist today
  • shrimp-like crustaceans of more modern type 1st appeared in Late Devonian
  • underwent major adaptive radiation in Carboniferous
  • clawed forms first appeared in Triassic, most in early Jurassic

Barnacles

• Ecology
  • immobile, attached
  • filter feeders which surround themselves in a calcite `house' which can survive in the inter-tidal zone by retaining water inside
• Evolutionary History
  • first appeared in Silurian, still exist today
  • well known from Tertiary deposits