Fossils in the limestone layers exposed in the Gorge provide clear evidence of ancient life that once inhabited a shallow sea. More than half of some layers consist of fossils, especially the coral-rich Rapid Biostrome at the Biostrome Plaza, which can be traced over 150 miles from Davenport to the Minnesota border. Corals and stromatoporoid sponges are among the most spectacular fossils, and the abundance of fossil corals gave Coralville its name. Some corals formed complex colonies of over 1000 individual animals, while others were secreted by a single animal as solitary horn and cup corals. Many coral fossils show growth rings that reflect seasonal changes in growth rates, similar to modern tree rings. The extinct stromatoporoid sponges had hard lime skeletons with lumpy or branching forms. A slab containing a bony plate from a large armored arthrodire, an early jawed fish that reached 30 feet in length, was removed from the Gorge after the 1993 flood and is displayed with other Gorge fossils in the Visitor Center, including slabs of crinoids. Careful observers sometimes also find smaller tooth-like and bony plates from other early jawed fish. Brachiopod shells are the most abundant fossils in the Gorge, and Favosites, the honeycomb coral, is one of the most common colonial corals, especially in the Rapid Biostrome. A polished slab of Devonian limestone also shows abundant fossil stromatoporoid sponges with scattered small corals and other fossils. These stromatoporoids, an extinct group of sponges with hard lime skeletons, are common around Coralville Lake in layers of the Cedar Valley Group known as the Idiostroma beds, named for the abundance of small broken branches of a particular stromatoporoid. The slabs also show lumpy and massive stromatoporoid forms, along with twigs and lumps of Favosites, some encrusted by stromatoporoids. The Devonian Diorama at the University of Iowa Museum of Natural History's Iowa Hall shows the colorful profusion of life that once existed here.