Cephalopod fossil within Shedd's foyer limestone floorThe last day of our cephalopod celebration coincides with National Fossil Day. And that’s awesomely appropriate because most cephalopod species—about 17,000—are extinct and have been memorialized in stone. Limestone, to be exact.

And you might find a few in the polished limestone floor of the aquarium’s foyer and rotunda.

Limestone is a sedimentary rock made of calcium carbonate, much of which came from the bodies of prehistoric marine invertebrates such as corals, brachiopods and—you guessed it—shelled cephalopods. Limestone also contains many whole fossil shells and fossil molds of these animals.

The Midwest has many major limestone deposits because it was a seafloor covered with coral reefs hundreds of millions of years ago. A huge deposit in Carthage, Missouri, was the source of interior and exterior building stone for private and civic projects across the country, including Shedd Aquarium.

While the stone that makes up the floor of our foyer and rotunda is called Missouri marble or Carthage marble, it’s an incredibly fine-grained limestone. How do we know? Because it’s loaded with small fossils. Marble is limestone that has been metamorphosed, or tortuously changed, by incredible heat and pressure, which would obliterate fossils. It turns out that in the building trades, any large slabs of finished stone are called marble.

Carthage marble is an exceptionally prized building stone because it takes a high polish that brings out its uniform light gray color and a pattern of charcoal gray veins. It was formed in the early Carboniferous period, between 359 and 318 million years ago. That’s a little past the heyday of the giant straight-shelled cephalopods; many were knocked out by the massive extinction event in the late Devonian period, but other shelled cephalopod groups, including many nautiloids, soldiered on.

We’re not sure if the fossil pictured above—found in the rotunda across from the entrance to the Abbott Oceanarium—is a segment of a small straight-shelled cephalopod or something else. The hollow disks surrounding it are sections of crinoid, one of the most common fossil animals found in Carthage marble.

Our fossil-flecked floor is just one of the callouts in Shedd’s brand-new Ten Fun Finds map devoted to our architecture. See what other aquatic animals at Shedd are underfoot, overhead and even scaling the walls!

Karen Furnweger, web editor