Crinoids:
Crinoids, also known as sea lilies, appear to be sea plants but are not plants at all. Crinoids are from the echinoderm species - a group of invertebrate animals that includes sea urchins, brittle stars, sand dollars, starfish, and sea cucumbers. They are passive suspension-feeding animals that live attached to the sea bottom by a thin stem (or stalk). At the top of the stem is a plate-covered cup shaped body bearing five usually branched and commonly feather-like arms. Several kinds of crinoids still live today although modern crinoids have lost their thin stem. Usually within hours of death, crinoids will fall to pieces as the ligaments, which hold the skeleton together, break down. Therefore, although crinoids are among the most abundant fossil echinoderms, they are usually poorly preserved because the skeletal plates have become disarticulated. You will probably find sections of stem - they look like stacks of small coins with a star shape in the center.
Crinoids anchor themselves to the sea floor by many different methods. Some coiled their stems or attachment arms (cirri) around other stationary sea floor objects. Others used different styles of specialized anchoring structures or "holdfasts", including those resembling roots, grappling hooks, weighted globes and cemented discs.
Thier distinctive holdfast grappling hook had 4 or 5 back-directed spines. The crinoid would snag this hook-like holdfast on seaweed or other firmly attached crinoids. Such an attachment may have been useful on sea bottoms composed of shifting sediment.
Most of a crinoid's body, in fact usually at least 80% or so, consists of a skeleton of calcium carbonate pieces, or ossicles, held together by ligaments and muscles. This skeleton explains both why crinoids make good fossils (calcium carbonate is basically limestone) and why not too many creatures subsist on a crinoid diet (they're highly crunchy). The crinoid body consists of three basic parts. A segmented stem (1) supports a cup-like calyx or aboral cup (2), which contains or supports the viscera and from which radiate five segmented and usually branched arms (3). Stems consist of a series of ossicles called columnals held together by ligaments, plus a variety of holdfast structures. Their arms wave in the water as they capture plankton which is their main food source. Living crinoids have a delicate structure of many skeletal plates composed of calcium carbonate.
Crinoids were most abundant during the Missisipian Period, which ranged from 375 to 320 million years ago. During most of this time Pennsylvania was covered by warm, shallow seas that provided favorable conditions for crinoid growth.
Trilobites were an extremely successful type of paleozoic marine arthropod (arthropods includes arachnids, crustaceans, and insects). Trilobite fossils first appeared in early Cambrian (570 mya) strata and may have first evolved 100 million years before that. Trilobites mysteriously disappeared at the end of the Permian period (245 mya) before dinosaurs even existed.
