Limestone is a sedimentary rock, and along with shale, is one of the best preservers of fossils. Over time, sedimentary rock presses down around what were once living things to preserve the basic outline of their appearance and physical characteristics. Crack open a large piece of limestone or walk a beach covered with limestone rocks and you are almost guaranteed to find a fossil. If you don't find one, rest assured that they are there, lending the calcium carbonate from bones, exoskeletons and shells to the limestone itself.
Crinoids are often called sea lilies, but they are animals, not plants. Complete crinoid fossils bear a strong resemblance to flowers. Their stem of small calcite disks are stacked like poker chips, and five feeding brachioles, or arms, extend from the top and resemble flower petals. The oldest crinoids are around 490 million years old and their relatives include sea cucumbers, sea urchins, starfishes and other echinoderms. After the animal died, its calcite disks would fall apart and scatter around the sea floor. Today, these incomplete crinoid parts are commonly found in lumps of limestone. Crinoid fossils appear as discs (when they are flat) or slightly rectangular (when they are fossilised on their side. Some of the discs may still be connected.
Ammonites are creatures with hard shells that lived in the ocean millions of years ago. These animals are the ancestors of octopus, squid, cuttlefish and the nautilus, and look a lot like today's nautilus, even in fossil form. The earliest ammonites lived 415 million years ago. They moved by jet propulsion and had shells comprised of multiple linked chambers. Their tentacles extended outward from the head chamber to catch and devour their prey. As the ammonite grew, new chambers would grow behind the head. Fossilised ammonites often display what are called "sutures," intricate patterned details on the outer surface of the shell that mark where the chambers met the outer wall of the shell. Not all ammonites are round. Some are straight, but still display distinct chambers along the shell.
Trilobites are arthropods, the ancient ancestors of lobsters, shrimp, barnacles, mosquitos and cockroaches. Trilobites appeared in the Cambrian period and were extinct before the appearance of dinosaurs. A mineralised dorsal exoskeleton is the most common fossilised trilobite, and it is usually found in pieces, where they fell as the animal shed it. Complete skeletons are found when an animal died and was buried whole. Trilobite fossils resemble large bugs or lobsters, but they lack the distinction between different sections of their body. When found in individual pieces they are carefully reassembled to form the creature's entire exoskeleton, but if an imprint is found, the surrounding rock is removed to uncover the whole fossil.