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Devonian Crinoids and Blastoids at the Falls of the Ohio and Surrounding Areas

by Alan Goldstein

 

           

           Figure 1. Crinoid - Dolatocrinus venustus                    Figure 2. Blastoid - Elaeacrinus verneuili

 

              Crinoids and blastoids are two kinds of fossils that may be observed at the fossil beds at the Falls of the Ohio. These fossils belong to the phylum Echinodermata.  Starfish, sand dollars and sea urchins are echinoderms that are commonly observed along the sea shore today. Echinoderm means "spiny skin." If you have felt a starfish or sand dollar, you are probably familiar with their scratchy texture. They lack an outer skin like we have. Crinoids are still living today, but usually occur is water a thousand feet (300 meters) or greater. One important characteristic of living (and most fossil) echinoderms is their pentameral symmetry. That means their body is organized in multiples of five. We have bilateral symmetry (a left and a right side).

              Fossils of these animals might be mistaken for plants, since both had a long, narrow "stem." Superficially, the column may be compared to a stalk, and the "head" (which is actually the body) is often compared to a flower. These animals did not carry out photosynthesis. They did eat microscopic plankton filtered from the sea water. Crinoids and blastoids had a gut, muscles, nerves, a reproductive system and other features of animals. Oxygen is distributed to tissue through a water vascular system. Their "blood" was sea water - just like echinoderms today! As adults, most of these Devonian echinoderms did not have the ability to move. Their relatives - sea stars, urchins and sea cucumbers - could. If a storm buried a sea star, it could wiggle out and crawl away. Immobile crinoids and blastoids would be buried, and quite possibly become preserved as a fossil.

 

           

                                   Figure 3: Model of Crinoid                  Figure 4: Model of Blastoid

Comparing Crinoids and Blastoids

 

     Crinoids and blastoids share some common characteristics (see Figures 3 and 4). There are some important differences leading to placing them in separate classes (Class Crinoidea and Class Blastoidea). Perhaps the most important relates to the body (sometimes called the "head") of the animal. Echinoderm skeletal material consist plates or ossicles. With crinoids, these plates are held together with muscle and ligaments (Figure 1). Upon death, the tissue which held the skeletal material decayed within several weeks and the plates became disarticulated. Blastoids had fused plates which usually held together after death (Figure 2), although they may have been shattered by hitting rocks or crushed upon burial. Refer to the illustrations to identify important characteristics.

                       

Crinoid Columns

                             Figure 5A. Steinkern (internal mold)      Figure 5B. Star-shaped Lumen

     Crinoids and blastoids have a stem, called a column (Figure 5A). It consisted of hundreds or thousands of disk-like columnals. The small disk or wheel-like columnals are very common fossils and can be found abundantly on the upper fossil beds at the Falls. Like any growing organism, the stem would start small (both in diameter and length) and increase in size as the crinoid or blastoid grew. Crinoid stems can get very thick (over one inch / 2.5 cm) in diameter. They are usually thickest towards the end buried in the sea floor sediment. Close scrutiny will reveal that crinoid columnals are usually ridged. This allowed adjacent columnals to interlock securely. Not all columnals are round. Dolatocrinus has "flanges" positioned at 120 degree angles and Himerocrinus columnals resemble cogwheels. The central axial canal or lumen contained a fluid-filled sac and nerve that extended the length of the column. A circular cross-section is most common. Star, four or five-leaf clover-shaped lumens are not unusual (Figure 5B). Two and five holes may be observed. (The columnal with five lumens is usually square, not round!)  Columns must be buried within a matter of weeks after the animal's death, or the individual columnals would fall away and become mixed with the surrounding sediment on the sea floor.

         

Figure 6. Crinoid Holdfasts

          A. Ancyrocrinus - Grapnel                   B.  Multicirri Holdfast              C. Eucalyptocrinites - Spreading

     Crinoids were known to use a variety of methods to anchor them in place. Blastoids are only known to use one. The most common means of "staying put" is to develop "roots" called cirri (Figure 6B). Cirri radiate from the column and were imbedded into the soft sediment, much like the trees on the edge of the Ohio River. Blastoids used this technique exclusively (as far as the fossil record shows us). One Devonian crinoid called Ancyrocrinus (Figure 6A) developed a unique grappling hook. It could have swiveled on the open sea floor or get hooked on coral or other debris to keep it from being swept away when ocean currents were swift. This form can be seen in the Interpretive Center exhibits. A third type of holdfast, was a button-like disk that was cemented to a hard surface. These interesting holdfasts may be found attached to corals.

     

Blastoid Theca

Figure 7A                    Figure 7B.

     Crinoids and blastoids differ markedly with the nature of their body. The character of the plates has already been described. The blastoid "head" resembles a flower bud and is called a theca it contains the vital organs. Five petal- or zipper-like ambulacral grooves (Figure 7) moved food to the blastoid's mouth at the top. There are five six openings at the top of a blastoid. The mouth is at the apex and is usually indistinct. There are four spiracles and an anus. The tentacles, made of skeletal ossicles are called brachioles. They probably contained tube feet to move the captured plankton down to the mouth. Six species of blastoids have been described from the Falls area (see table 1).

 

    

Crinoid Calices (Crowns)

                                                Figure 8A.                                     Figure 8B.

     The "head" (actually the body) of the crinoid also contain the vital organs and is called the calyx. Attached to the calyx is a set of arms. The calyx + arms together are called the crown (Figure 8). The tentacle-like arms of crinoids were composed of skeletal plates. Each arm contained rows of smaller tentacle-like pinnules. Combined, an individual arm resembles a moving feather. Food is captured by tube feet and transported down the ambulacral groove on the side of the arm to the mouth. Crinoid arms are in multiples of five. A simple crinoid may have only five arms. One species at the Falls, Himerocrinus, had 80 arms! The blastoid anal opening was adjacent to the mouth, while most crinoids had theirs elevated. Crinoids were more efficient at feeding than blastoids. Over 650 species of crinoids inhabit the world's oceans. Blastoids became extinct at the end of the Permian Period some 200 million years ago. Eighteen species of crinoids are known from the Falls (see Species List below).

     Both crinoid calices and blastoid thecae are very rare at the Falls. Less than a dozen or each have been observed in outcrops by interpretive staff over the years. The muscles and ligaments holding the plates of a crinoid calyx together would rot soon after the animal’s death. Unless it was buried within a month, the calyx would disintegrate. The theca of the blastoid would usually be pulverized in strong ocean currents. Consequently, well-preserved bodies of crinoid and blastoids are highly sought after by fossil collectors. If you locate one in the park, please tell the interpretive staff and remember - never remove them from the Falls area.

Suggested Reading

Boardman, R.S., Cheetham A.H. and Rowell, A.J., 1987, Fossil Invertebrates, Blackwell Scientific Publications, 713 p.

Greene, G. K., 1898 - 1904, Contributions to Indiana Paleontology, Ewing & Zeller, New Albany.

Moore, R.C. and Teichert, C., 1978, Treatise on Invertebrate Paleontology, Part T, Echinodermata.

Springer, Frank, 1921. The Fossil Crinoid Genus Dolatocrinus and its Allies, U.S. National Museum Bulletin 115.

 

Devonian Crinoid and Blastoid Species Lists

 

Created July 28, 2010, Updated December 14