Scyphocrinites

The Mysterious Silurian Crinoid

Scyphocrinites is a large Crinoid that is found in many places throughout the world, often in abundance. Although this Crinoid is not part of Ordovician fauna, it is worth mentioning due to its proliferation and unique features. A popular item in the fossil trade, large slabs with many individuals have been collected from Morocco, especially in the Dra Valley. Scyphocrinites appears to be restricted to uppermost Silurian and lowermost Devonian sediments. These Crinoids have been reported also from other countries in northwestern Africa, several countries in Europe, Kazakhstan in central Asia, and in the United States. Some fossils similar to Scyphocrinites have been given their own genus names, such as Marhoumacrinus and Carolicrinus.

Scyphocrinites has a calyx up to 10 cm long, borne at the end of thin stalk that reaches up to 3 m in length. The arms of the crown may reach 30 cm in length. The distal end consists of a bulb-like structure, often called the lobolith. The lobolith may reach 20 cm in diameter. This has chambers in it, and is of two types – the “plate” type and the “cirrus” type. The plate type has larger chambers while the cirrus type has many small chambers and canals. The lobolith was originally classified as a separate species, Camarocrinus, but it has been found attached to the stalks of Scyphocrinites.

Most paleontologists have interpreted the lobolith as a float that kept the crinoid suspended in the water in a pelagic environment. The calyx was the feeding structure, filtering particles from the water up to 3 meters below the float. This interpretation requires that the float be filled with gas, but the source of the gas is not clear. Suggestions have included diffusion from the surrounding water, or production by symbiotic algae, but neither explanation is persuasive.

Another possibility is that the lobolith is not a float, but an anchor. Some paleontologists have studied the structure of the lobolith and concluded that it was too porous to retain gas. It seems more reasonable that the lobolith was anchored in the mud at the seafloor, and the calyx reached upward above the floor, and above most other seafloor organisms. Loboliths are usually discovered separately from the stem, and this could be explained if they functioned as anchors. Another possibility is that the stem was partially recumbent – lying on the surface with the calyx end extending upward into the water. These crinoids are often found in dense lenses, with many individuals tangled together. This seems unlikely if they were floating high above the seafloor. Perhaps they lived in mat-like groups. Another possibility is that they were buried catastrophically, whether in the place they were living or transported together.

Reference:

Hess, H., 1999. Scyphocrinitids from the Silurian-Devonian boundary of Morocco. Fossil Crinoids, pp.93-102. Cambridge: Cambridge University Press. (Accessed online: October 2024.)