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Why Are So Many Sea Fossils Found in Limestone?
Limestone makes for prime fossil hunting, due to its biochemical composition. A sedimentary stone found near sea reefs as well as modern-day and prehistoric water sources, limestone is composed largely of calcite, a mineral created by the remains of bottom-dwelling sea animals. Most of these organisms are shelled, so when the animals inhabiting these shells die, their former homes collect into layers and piles of what's known as "shelly debris." This marine detritus form beds of sea fossil-rich limestone.
The types of fossils you might encounter vary by region as well as by the age of the limestone. For example, the sea fossils that dominated the Cambrian Period evolved into different organisms during the Cretaceous Period.
Corals
A variety of coral species are preserved in limestone, including early corals that grew in a solitary manner without forming reefs. Many other corals developed colonies, and demonstrate a narrow, branching pattern of growth. Favosites are another type of colonial coral commonly discovered in limestone. These corals are distinctive groupings of hollow-looking tubes, bunched together so that, from a topside view, they resemble honeycombs.
Brachiopods
Shellfish known as brachiopods, also called lamp shells, feature two valves; however, when discovered as limestone fossils, it's extremely rare to find both valves together, as the organic tissue that connects the valves decays after the animal's death. In life, brachiopods feed by attaching themselves to the ocean floor with a stalk that in many ways resembles a ligament. From their anchored position, they filter and feed on plankton and other tiny food sources in the water that pass over them.
Crinoids
Crinoids are beautiful and curious animals. With stunning and colorful feather-like arms that radiate from a stem, they look like underwater flowers. They draw nutrients, such as plankton from the sea, around them by using their feathered arms to collect these tidbits, which move the food into the creature's mouth. Their delicate skeletons are made of a series of tiny, circular bones. When the animal dies and its connective materials decay, these little circles scatter in the current, often settling into a forming bed of limestone.
Teeth
Since limestone tends to develop near marine reefs, it's fairly common to find evidence of the predators that once swam these reefs in search of an easy meal. In most cases, these discoveries involve teeth. In fact, teeth from all types of sharks, as well as rays, are often discovered in limestone outcroppings.
Trace Fossils
Rather than the remains of an animal, trace fossils reveal a permanent marking of the animal's movements. In limestone, tracks from trilobites, the burrows of ancient worms and trails blazed by snails, are commonly found. And if you're really lucky, you could find an animal's footprint.