Stromatoporoids
Stromatoporoids are an extinct animal. They were once thought to be a coral, but we now believe they are a type of calcareous sea sponge. They were important reef builders.
The sponges grew upwards in small horizontal layers, and secreted calcium carbonate to form their skeletons, sometimes in the form of calcite like in Tyndall Stone, and sometimes in the form of aragonite. They were filter-feeding animals, similar to modern sponges.
Stromatoporoids come in lots of different shapes depending on the environment they are in. Tidal and seasonal fluctuations, currents, changes in temperature and nutrients, water turbulence, and the consistence of the sea floor all influenced their growth.