They were like sponges

This is an overturned silicified stromatoporoid in the Devonian Manlius Limestone in upstate New York. The stromatoporoid is not the little black thing at the end of the finger; it is the blobby object in the middle of the field of view. Stromatoporoids have been extinct for hundreds of millions of years, but they appear to have been a lot like sponges. They lived in warm shallow water, couldn't move around, and were structurally very simple. This one had some bad luck one day. Something, perhaps a storm, turned it upside down and it got buried in mud. Later, it was partly turned to silica (the major component in glass and the most abundant mineral near the Earth's surface). The silica is the lighter yellowish squiggly stuff inside the stromatoporoid.