Although crinoids with long stems
are commonly called “sea lilies,” they are actually echinoderms, the
animal group that includes sea stars, sand dollars, sea urchins and sea
cucumbers.
This crinoid, Seirocrinus subangularis,
was quarried from the 180-million-year-old Jurassic Posidonia Shale of
Holzmaden in southern Germany. The Holzmaden site is well known for its
exceptional fossil preservation, the result of very low oxygen levels
on the sea floor, which inhibited destruction by scavengers and
bacteria. The site is most famous for its fossils of icthyosaurs
(dolphin-like reptiles), which sometimes show the outline of fins.
Echinoderm
skeletons are made up of small plates (see inset) embedded within the
soft tissue (not preserved). In the living animal these plates are
composed of the mineral calcite (calcium carbonate, which in this
specimen has been replaced by pyrite). The stem has round disc-like
plates, while other shapes make up the ornate body. The complete
preservation of this specimen of Seirocrinus indicates that it was blanketed with sediment before its plates could separate and scatter.
Crinoids
with long stems are typically anchored to the sea floor. The ancient
Holzmaden sea floor, however, was apparently unsuitable, and Seirocrinus
is found attached to logs. As the wood drifted with currents at the
surface, the crinoid spread its arms like a tow-net to capture food
particles suspended in the water.
The stem of large specimens of Seirocrinus
reached lengths of several yards. Here much of the stem has been
discarded and a piece of the log inserted to display a specimen of more
manageable dimensions. The log also provided an attachment site for the
clam Pseudomytiloides dubius.
