Pentacrinites fossils, Putative Pseudoplanktonic Crinoid from UK

Name: Artculate Pentacrinus crinoid fossils

Phylum Echinodermata; Subphylum Crinozoa; Class Crinoidea; Order Articulata

Geological Time: Lower Jurassic Sinemurian Stage

Size: 115 mm by 100 mm completely engulfed matrix plaque

Fossil Site: Charmouth, Dorset, United Kingdom

The sea cliffs on the Dorset coast of England are remowned for the wonderful Lower Jurassic fossils they have yielded for 200 years. One of the best-known of these is the crioid Pentacrinites fossilis which can occur as large tangled groups such as that seen here. Because Pentacrinus criniods have been associated with coalified driftwood, some postulate that they were attached to floating tree trunks. This belief was not universally accepted, but an important discovery made just over 20 years ago substantiated the claim that Pentacrinites led a pesudoplanktonic existence, only sinking to its death on the anoxic seafloor when the driftwood became waterlogged; this specimen showed an adult Pentacrinites attached to the lower surface of the wood, while numerous tiny crinoid larvae were seen along the side of the section. This seems convincing evidence that the crinoids colonized the lower submerged surface, then began to colonize the surfaces that became exposed to the sea as the log sank lower as it took on water.

click fossil pictures to enlarge

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