From PhysOrg:

The Cambrian explosion marked a major blossoming in the tree of life around 540 million years ago. Nearly all of the major phyla in the animal kingdom appeared in a sudden burst of evolution. One of the drivers of this rapid diversification was likely the appearance of skeletal-bearing animals a mere 10 million years before. Researchers are studying these first skeletons to gain a better understanding of what led to their development…. more.