Article: Rapid evolution in echinoids
The evolution of the irregular echinoid and of the sand dollar occurred in a very short time. The first irregular echinoid appears abruptly in the Early Jurassic (Sinemurian); and by the Toarcian, only ten million years later, irregular echinoids possess all the features necessary to permit them to live buried in the sediment. The first clypeasteroid appears in the Paleocene. By the middle Eocene its very specialized descendants, the sand dollars, have a worldwide distribution. This rapid evolution and diversification seem to result from a sudden adaptive breakthrough. The presence of so few intermediates indicates the evolutionary steps must have been large.