Vertebrates

The third major type of animal was not closely related to the molluscs or arthropods. It is not clear which of the invertebrates was their ancestor, perhaps early flatworms. Although not so successful in terms of numbers they were far more successful in other ways. They first appeared around 500 million years ago, and are called the vertebrates, meaning "with backbone".
Bone was a key factor in the success of the vertebrates.