Ammonoids and the Evolution of Shell Form

Ecdysozoa: The second major bilaterian monophyletic group.

With the exception of Arthropoda, these are minute creatures with no fossil record. Ecdysozoa is diagnosed by one morphological synapomorphy: An external cuticle that is regularly shed.

Arthropoda is united by the following synapmorphies:

Issues of the arthropod cuticle:

Major Arthropod Groups


Regrettably, a fifty-minute survey does no justice to the variation in these diverse animals. Many invite speculation on the function of their conspicuous specializations. Not surprising given that, during the early Paleozoic, their role was to be as abundant and diverse as "the fish in the sea."

Finally: additional information.