more classes of this animal exist than of any phylum both living and extinct.
The major groups of echinoderms include starfish, brittle stars, sea urchins, sea
cucumbers,and crinoids (Fig. 49), known as sea lilies because of their plant-
like appearance.
Surprisingly, according to genetic studies, early echinoderms and chor-
dates were more closely related to each other than to other major phyla.
Apparently, echinoderms and chordates branched away from the arthropods,
annelids, and mollusks as early as 1.2 billion years ago.This was long beforeFigure 48Fossil corals
from Bikini Atoll,
Marshall Islands.
(Photo by J.W.Wells,
courtesy USGS)
Historical Geology