At Shedd Aquarium, you’ll find belugas and bluegills, stingrays and sturgeons, sea stars and sea otters—in fact, aquatic animals from the poles to the tropics and back. If you look up at the main entrance and in the vestibule and foyer, you’ll also find terra-cotta turtles, bronze octopuses and other architectural portrayals of marine animals. But have you ever looked down—at the floor? If you look closely, you’ll spot sea life from more than 300 million years ago.
From ancient tropical seas
Fossils of small reef invertebrates dot the polished limestone floors of Shedd’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 shelled cephalopods, both the extinct straight ones and the coiled nautiluses that continue virtually unchanged today. Limestone also contains many whole fossil shells and fossil molds of these animals.
Much, if not most, of what would become North America was underwater from 500 to 300 million years ago. The land did a lot of shape-shifting as sea levels rose and fell, but large swaths of the future Midwest, including Chicago, were consistently covered by shallow tropical seas. During that time, the Midwest was considerably closer to the tropics, based on evidence from plate tectonics and vast limestone deposits that were originally reef-covered seafloor.
From the mid-19th century until the end of the 20th, a huge limestone deposit in Carthage, Missouri, was the source of interior and exterior building stone for private and civic projects across the country, including Shedd Aquarium.
When is marble not marble?
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 intense 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’s hard enough to take 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 323 million years ago.
The fossil pictured at the top—located in the rotunda across from the entrance to the Abbott Oceanarium— might be a split segment of a straight-shelled cephalopod. The hollow disks surrounding it are sections of crinoid, one of the most common fossil animals found in Carthage marble.
Shedd’s grand entrance, foyer and rotunda, built with the finest materials and lavishly embellished by world-class artisans, was designed to be just a preview of the living wonders within. That includes these living fossils.
You can have a hands-on encounter with a fish species that outlived the dinosaurs. At the Sturgeon Touch pool, you’ll get an idea of how lake sturgeons have survived for more than 200 million years as you feel the protective bony plates encasing these 4-foot bottom-dwelling fish.
Don’t miss the spotted ratfish in the Oceans gallery. The ratfish and its clan, the chimaeras (kih-MEER-uhz), are the oldest order of fishes alive today. They belong to the taxonomic class Chondrichthyes (kahn-drik-THEEZ, literally “cartilaginous fishes”), along with sharks and rays, but they split from the ancestral shark line early on, about 400 million years ago—about 40 million years before the limestone in the foyer floor was a living reef.
—Karen Furnweger, web editor
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