Part of the early Cambrian explosion included the development of the archaeocyathids, somewhat conical animals with calcareous skeletons rather like corals. But they are not corals. Their affiliations are not 100% certain, and some scientists put them in their own phylum, but I think most taxonomists are classifying them as early sponges.
They appeared about 525 million years ago, part of a famous fossil assemblage called the Tommotian fauna for the specimens found along the Tommot River in Siberia. They’ve been found all over the world, and they are distinctive enough that they serve as index fossils for the early Cambrian. That means rocks can be correlated across large distances using the particular varieties of archaeocyathids that are found in them.
Just nine or ten million years after they appeared and proliferated, the archaeocyathids went into a sharp decline in numbers and diversity about 516 million years ago. Interestingly, this time is when more modern sponges began a rapid diversification, another pulse in the Cambrian explosion. The sponges that evolved then have survived with variations to this day, but the archaeocyathids were all extinct before the end of the Cambrian.
—Richard I. Gibson
Drawing of reconstructed archaeocyathids by Stanton F. Fink, via Wikipedia under GNU free documentation license.