Crinoid - A Marine Invertebrate Fossil

Introduction

A marine invertebrate belonging to the Crinoidea class (phylum Echinodermata) with a cup-shaped body and five or more flexible and active arms. The reproductive organs are housed in the arms, which are bordered with feathery projections (pinnules) and hold many tube feet with sensory functions. Cilia (minor, hair-like projections) whisk food particles toward the mouth through open grooves in the tentacles.

Figure 1 Periechocrinus, a Silurian crinoid. BGS © UKRI.

They are an early fossil group that originally arose 300 million years before dinosaurs in the mid-Cambrian oceans. They thrived during the Paleozoic and Mesozoic periods, and some still exist now. Living forms provide insights regarding how prehistoric crinoids must have lived, even if they differ in appearance from their fossil predecessors.

Anatomy

A ring of branching arms (brachia) wraps around the top of a globe-shaped, cup-like structure (calyx), which houses the animal's primary body. The calyx was linked to a flexible stem that was fastened to the sea bed in numerous ancient forms.

Figure 2 A stemmed crinoid's anatomy and feeding posture. The water is flowing from left to right. BGS © UKRI.

The skeleton is formed of calcite and is made up of hundreds of separate plates of various sizes and forms. Complete examples are rare because of the decay of the soft tissue that kept many of these plates together, although sections of the stem are common fossils.

Figure 3 A living crinoid in the Red Sea's Gulf of Aqaba. It's in the public domain. (Aqaba, NOOA, Mohammed Al Momany.)

Calyx

The calyx is made up of polygonal plates that are organized in different ways in different crinoids groups. It housed the mouth, which received food through grooves in the brachia. The top of the calyx was a flexible membrane in some fossil crinoids, but it is preserved as a firm dome in others, and it may feature an elongated anal tube for waste disposal.

Figure 4 Amphoracrinus portlocki calyx of Carboniferous (Wright, 1995). 

Stem

The stem was usually made up of disc-like plates (columnals) that were piled on top of one another. Individual columnals were spherical, oval, square, five-sided, or star-shaped, with petal-like decorations on certain plates. Crinoid stem plates come in a variety of morphologies, although some fossil crinoids, like many extant species, don't have stems.

Figure 5 - 1. Crotalocrinites (Silurian), 2. Pentacrinites (Jurassic), and 3. Bourgueticrinus (Jurassic) (Cretaceous).

Environment

Crinoids were abundant in shallow water during the late Silurian and early Carboniferous periods. Stemmed forms could bend towards water currents and use their brachia to catch food particles in a net. Some groups' capacity to exploit the best food supply from a range of water depths was increased by side branches to the brachia (called pinnules), and exceptionally long-stemmed forms may have exploited the best food supply from a range of water depths. Crinoid stems having moveable appendages (cirri) or potentially prehensile abilities provided for temporary anchoring in areas where food was abundant.

Figure 6 Uintacrinus and Marsupites may have been living in this location. All rights retained by BGS UKRI.

Crinoids evolved stemless form on rare occasions in the Paleozoic and more frequently in the Mesozoic, allowing them to search the seafloor for better eating opportunities and avoid predatory situations. The small, stemless Saccocoma (Jurassic to Cretaceous) could swim freely, but the much larger stem fewer Uintacrinus and Marsupites (Cretaceous) would have lain on the bottom with their brachia extending as a food-collecting bowl.

Figure 7 Early Jurassic Pentacrinites briareus (Miller, 1821). A crinoid skeleton that has been completely preserved.

Geology

Fossil crinoids indicate that the rocks holding their remains were created in a marine environment, and their abundance in Paleozoic strata suggests that shallow water conditions existed previously. Their abundant remnants (especially stem fragments) consolidated into crinoidal limestone in the early Carboniferous. Completely fossilized crinoids are uncommon, indicating quick burial in quiet, potentially oxygen-depleted seas.

Figure 8 Crinoidal limestone formed by pentacrinites briareus.

Crinoids can occasionally be used to determine the age of the rocks in which they occur. This is true of the Late Cretaceous Chalk Group strata that make up the famed White Cliffs of Dover. Uintacrinus, Marsupites, and Applinocrinus species are abundant enough in the Chalk to be utilized to define biozones and subzones over four limited intervals.

Time Record

Crinoids can be found in Shropshire's Silurian rocks, Derbyshire's early Carboniferous rocks, and the Jurassic rocks of the Dorset and Yorkshire coasts.

Country of Origin

Crinoids in Batu Moncho Island's reef, Indonesia. Crinoids are rarely preserved in their totality because sea currents scatter the skeletal segments after the soft components of the animal die. The stem fragments are by far the most abundant crinoid fossils. These can be found in abundance in the limestones and shales of eastern Kansas. The cuplike calyx is only encountered on rare occasions. However, Kansas is home to Uintacrinus, a beautiful and rare fossil crinoid that has been preserved in its entirety. These fossils were discovered in the Niobrara Chalk of western Kansas and lived around 75 million years ago, during the Cretaceous Period. Uintacrinus is a stemless crinoid, and examples of these well-preserved crinoids from Kansas may be found in several of the world's most prestigious museums.

References

Kolzoff, E. N. (1990) Invertebrates. Sauders College Publishing.

Mladenov, P.V., and Chia, F.S. (1983) Development, settling behavior, metamorphosis and pentacrinoid feeding and growth of the feather star Florometra serratissima. Marine Biology 73:319-323.

Tasch, P. (1973) Paleobiology of the Invertebrates: Data Retrieval from the Fossil Record. John Wiley & Sons, Inc.