What they were
Sponges that built reefs. Reefs today are built by corals, but this animal did most of the building in its time. Each one grew as a low dome, two meters across and half as tall, about the size of a stool.
How they built
One sheet of lime at a time. Cut a dome open and the sheets stack like the pages of a closed book. Together with honeycomb corals they made a wall hard enough for the open sea to break on. You can still walk one of those reefs: 350 kilometers of it stand in the Australian desert.
What happened to them
They are gone, every one of them, everywhere. Almost nothing alive today builds the way they did. When they went, the reef's frame went with them, and it stayed missing for something like 120 million years.