Crinoid

Xenocrinus baeri

• Ordovician
• Whitewater Formation
• Warren County, Ohio, USA

Size: ~4 cm crowns

Here is a grouping of unusual Ordovician monobathrid crinoids known as Xenocrinus baeri. This crinoid is similar to Gaurocrinus in its dominant ray ridges with small, irregular interradial plates, except that it is monocyclic and has very thick, unbranched free arms. However, the most notable feature of this crinoid is actually in its stem: Xenocrinus has uniquely quadrangular or "square" columnals, which is more evident in cross-section (seen in the last photo). A square stem is highly unusual considering that echinoderms generally exhibit a combination of pentaradial and bilateral symmetry.