A Jurassic Sea Lily
Seirocrinus subangularus
Seirocrinus subangularus Jurassic, Germany YPM catalog no. 210500. Gift of Barbara Vasquez

Animal or plant?

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 In the living animal these plates are composed of the mineral calcite (calcium carbonate, which in this specimen has been replaced by pyrite).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.

 

Upside down?

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.