Page 374 - Introduction to Paleobiology and The Fossil Record

Chapter 14

Ecdysozoa: arthropods

Key points

• Arthropods – such as lobsters, spiders, beetles and trilobites – have legs, a segmented
body plan with jointed appendages and the ability to molt.

• The first major arthropod faunas of the Early Cambrian appear bizarre by modern
standards but probably were no more morphologically different to each other than are
living faunas.
• A number of arthropod-like animals in the Ediacara biota suggest an ancient origin for
the phylum.
• Trilobites appeared in the Early Cambrian and during the Paleozoic evolved advanced
visual systems and enrolment structures while pursuing a variety of benthic and pelagic
life styles.
• The largest arthropods were the chelicerates and included the giant eurypterids that
patrolled marine marginal environments during the Silurian and Devonian.
• Myriapods represent the earliest terrestrial body fossils in the Mid Ordovician, but
trackways indicate euthycarcinoids (i.e. stem-group mandibulates) moved onto land
even earlier, in the Late Cambrian.
• Insects fi rst appeared during the Early Devonian and diversifi ed rapidly; there are prob-
ably 10 million species of living insects.

• Insects had probably already evolved flight before the Mid Carboniferous, when giant
dragonflies patrolled the forests.

• The crustaceans include many familiar groups such as crabs, lobsters and shrimps,
together with the barnacles and ostracodes.
• Much of our knowledge of the early history of the phylum has come from exceptionally
preserved fossils from the Cambrian Burgess Shale, Chengjiang and Sirius Passet
faunas.

Whence we see spiders, flies, or ants entombed and preserved forever in amber, a more
than royal tomb.

Francis Bacon, English philosopher (1561–1626)