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Fossil Crinoid Pentacrinites Stem.jpg

Dpaul7

By Dpaul7

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Fossil Crinoid Pentacrinites Stem
 
Lincoln County, Wyoming, USA
TIME PERIOD: Jurassic Period (145.5 to 199.6 million years)
Pentacrinites is an extinct genus of crinoids that lived from the Middle Triassic to the Eocene of Asia, Europe, North America, and New Zealand. Their stems are pentagonal to star-shaped in cross-section and are the most commonly preserved parts. Pentacrinites are commonly found in the Pentacrinites Bed of the Early Jurassic (Lower Lias) of Lyme Regis, Dorset, England. Pentacrinites can be recognized by the extensions (or cirri) all around the stem, which are long, unbranching, and of increasing length further down, the very small cup and 5 long freely branching arms. Like most echinoderms, Pentacrinites was composed of numerous calcite plates which were arranged into different body parts. Pentacrinites had 3 kinds of body parts: arms, cup (calyx or theca) and stem. The stem consisted of a stack of numerous 5-sided beads (or columnal plates) with a canal at their centre. The stem had flexible appendages (or cirri) that were used to attach an individual. These cirri themselves were connected to specialized columnals called nodals, leaving oval scars after breaking off. The cirri consisted of diamond-shaped plates with a central canal, less flatted further from the stem. The cup-shaped calyx was very small and consisted of two bands of five plates. These were the bases of the five arms. The top of the calyx was covered by numerous small polygonal plates and the mouth and anus were found on this surface. The arms divide frequently, like tree branches, so that at the top end there could be over than 50 branches in all. The arms were formed of piles of calcite plates. The arms carried many thin feeding branches (or pinnae, like a fern frond). These pinnae had tube feet, that were covered in mucus, reached into the water and caught plankton. These arms were not very mobile. The arms plates of the arms have an insertion, that formed a grove that ran along the length of the arm and onto the calyx. This served to transport the food particles to the mouth.
Kingdom: Animalia
Phylum: Echinodermata
Class: Crinozoa
Order: Isocrinida
Family: †Pentacrinitidae
Genus: †Pentacrinites

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