1. Adult echinoderms are radially symmetrical, usually with a fivefold pattern, however, they are considered to be bilaterally symmetrical animals because they start off as bilateral larvae, such as the bipinnaria larva (sea star larva, left) and pluteus larva (brittle star larva, right) shown in the engravings:
2. Echinoderms use a water vascular system for moving, feeding and respiration.
3. They have a skeletal system made of limy (calcite) plates.
4. They have special connective tissue that they can make rigid or flexible, allowing them to maintain postures without muscular effort.
There are around 20 classes of echinoderms, of which only five survive today. All five modern classes and a number of extinct classes are represented by specimens in this case. How many can you find?
Echinoderms with a flattened body consisting of a central disk and radially arranged arms. Sea stars (class Asteroidea) are mobile predators, while brittle stars (class Ophiuroidea) are more commonly scavengers or deposit feeders. The skeleton consists of loosely connected calcite plates and small spines.
The sea stars are represented by a single specimen of an unknown species from the Oligocene (Paleogene) of Oregon. The internal anatomy and an arm cross section of a typical sea star can be explored in the linked engravings from A Textbook of Invertebrate Morphology by J. P. McMurrich (1894): sea star internal anatomy; sea star arm cross section.
The brittle stars are represented by two specimens, Furcaster sp. from the Devonian of Germany and Amphiura sp. from the Miocene (Neogene) of California. Brittle star surface morphology can be explored in the linked engraving from A Textbook of Invertebrate Morphology by J. P. McMurrich (1894): brittle star morphology.
Stalked echinoderms with short unbranched arms. Includes eocrinoids (class Eocrinoidea), cystoids (classes Diploporita & Rhombifera), and blastoids (class Blastoidea).
Middle Cambrian – Silurian
Gogia palmeri is one of the earliest stalked echinoderms. These animals lived attached to the sea floor by a plate covered stalk, but did not have stems.
Silurian – Permian
The blastoids are represented by three Mississippian species: Pentremites symmetricus and Pentremites obesus both from Illinois. These specimens show the commonly fossilized theca, or body of the organism in different orientations. The single specimen of Pentremites pyriformis from Kentucky is much rarer since both the arms and part of the stalk are preserved. A comparison of this specimen to the other blastoid theca shows where the arms were attached.
Lower Ordovician – Devonian
Cystoids consist of organisms in two classes, the Rhombifera (represented by a specimen of Caryocrinus sp. in our Silurian case) and the Diploporita (represented by a specimen of Holocystites scutellatus from the Silurian of Indiana). Cystoids are generally stemmed organisms with globular or pear-shaped theca with round or slit piercings.
Crinoids (class Crinoidea) and their relatives are small to very large (up to 20 meters long) echinoderms. Their food-gathering arms are usually branched. Most fossil sea lilies were attached to the seafloor with stalks. The first free moving feather stars appear in the Mesozoic.
Crinoid form and anatomy
Crinoid form and anatomy can be explored in two linked engravings from the 11th edition of the Encyclopedia Britannica (1911): modern crinoid; simple crinoid parts diagram. Crinoid features can be seen and explored in the specimens in this case:
Two plates contain complete and partial specimens of crinoids showing all the major parts:
- A Mississippian plate has numerous partial and complete specimens of Eretmocrinus tentor with both top and side views of the arms/theca along with the stems and occasional holdfasts of these organisms.
- An Oligocene plate with multiple specimens of Isocrinus sp. including holdfasts (the dark object is a leaf fossil).
Details of the arms, body (theca), stems and holdfasts are illustrated in different specimens:
- A Mississippian crinoid Onychocrinus sp. shows branching in the arms and the attachment for the stalk
- Mississippian crinoid heads and arms from Actinicrinites gibsoni & Pachylocrinus sp.
- A theca with feather-ilke arms of the Mississippian crinoid Macrocrinus mundulus.
- The theca and arms of the Mississippian crinoid Cactocrinus sp.
- A Mississippian crinoid, Uperocrinus pyriformis, with the arms removed shows details of the theca.
- A Devonian specimen of the crinoid Arthroacantha carpenteri with the arms removed shows the arm attachments and the upper surface of the theca (compare to the simple crinoid parts diagram intrabasals).
- The structure of the stems with branch attachment sites are seen in specimens from our Mississippian case.
- Finally, a holdfast from an unidentified species of Silurian crinoid is seen from the top.
As with many other fossils crinoids sometimes have their mineral skeletons partially or completely replaced by pyrite (iron sulfide) through bacterial action in anerobic muds.
- Two pyritized specimens of the Devonian crinoid, Arthrocantha sp. with Platycerid snails attached. In the first, one can see the theca and attached arm bases with a small snail on the top.In the second the snail dominates the specimen, covering the top of the crinoid.
- A pyritized specimen of the Devonian crinoid Arthroacantha carpenteri with highly branched arms and a partial stem.
While most crinoids are sessile, attached to the ocean bottom, some later species took on a new lifestyle as pelagic animals living among and feeding on plankton. Two specimens are on display:
This diverse echinoderm group is mostly free-living. Modern groups include urchins, heart urchins, sand dollars (all in class Echinozoa), and sea cucumbers (class Holothuroidea). Some are scavengers, others feed on seaweeds. Some live on rocks, others burrow in mud or sand. Edrioasteroids are an extinct class of echinozoan that lived attached to hard objects such as rocks or shells.
Ordovician – Recent
Five different echinoids are on display in this case, representing some of the diverse shapes of the shells of these organisms. Hemicidaris crenularis is a Jurrasic sea urchin that originally had relatively few spines (note the large bumps on the surfaces), while an Oligocene plate shows multiple specimens of an unidentified species of urchin with spines attached. Mellita sp. and Encope sp., both of the Pleistocene in Mexico, each display the flat anatomy expected in sand dollars, while Clypeaster sp., from the Miocene of Sardinia, is somewhat thicker (sometimes called a sea biscuit). Finally, Eupatagus clevi, from the Oligocene of Florida, shows the deep heart-like shape of heart urchins.
Ordovician – Recent
Sea cucumbers (holothuroidea) are soft bodied echinoderms, and thus rarely fossilized. The fossil remains of Achistrum sp. are therefore very special fossils.
Cambrian – Mississippian
Edrioasteroids (Edrioastroidea), were an extinct, round, sessile form of echinoderm . Note the short, thick, plate-covered stalk on our specimen of Isorophus cincinnatiensis (attached to a rock). Some of the five rayed feeding grooves, which often make these organisms look like starfish, are damaged but discernable.
©1998, HSU NHM | Last modified 26 October 2012