populations of widespread and diverse organisms. Sea levels rose and flooded
large portions of the land. The extended shoreline spurred the explosion of
new species. Life-forms rapidly evolved, and unique and bizarre creatures
roamed the ocean depths.
The dominant animals were the coelenterates.These were radially sym-
metrical invertebrate animals, including giant jellyfishlike floaters up to 3 feet
wide and colonial feathery forms, possibly predecessors of the corals. They
were attached to the ocean floor and grew more than 3 feet long.The remain-
ing organisms were mostly marine worms, unusual naked arthropodlike ani-
mals, and a tiny curious-looking naked starfish with three rays instead of the
customary five. The vase-shaped archaeocyathans (Fig. 35) resembled both
sponges and corals and built the earliest limestone reefs, eventually becoming
extinct in the Cambrian.Few of these creatures looked anything remotely likeFigure 34The
supercontinent Rodinia
700 million years ago.
Historical Geology