 Ammonites are most closely related to today's squid and octopi. Chris says millions of years ago, their shells made of calcium sunk to the ocean floor after they died and fossilized. The sea, you know, moves around over time. Geologic processes mean that what was under the ocean is now on land, right? Preserved in limestone used to construct many of D.C.'s buildings, remains like this can be unearthed all around the district if you know where to look.