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. |
