Crinoid

1. Halysiocrinus nodosus
2. Camerate indet.

• Mississippian
• Edwardsville Formation
• Montgomery County, Indiana, USA

Size: 5.8 cm crown

The disparids are a most unusual group of crinoids which contain both the most simplified and most specialized forms. I've posted a few of the former, and here now is an example of the latter. Halysiocrinus nodosus is a disparid and within that group a member of the Calceocrinids, which unusually among crinoids exhibit promient bilateral symmetry. They were thought to have laid with their stems parallel to the seafloor, lifting their crowns up into the water to feed as if opening a sail, enabled by a bizarre hinge between the crown and stem. It seems this strategy (and the strong bilateral symmetry) suggests that Calceocrinids evolved to take advantage of environments with predominantly unidirectional water flow.