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Echinoids and starfish

A yellow crinoid (Phylum Echinodermata, Class Crinoidea)

Echinoderms can be recognized by their unusual symmetry.   Although careful study shows that these animals are bilaterally symmetrical with mirror-image left and right sides, they appear to have a body made up of 5 similar units arranged in a radial pattern.  They are thus often termed to be of pentameral symmetry.  This is easier to see in starfish than in echinoids.

The relative position of the mouth and anus determine whether the echinoid is 'regular' or 'irregular'
Loriolia texana, a 'regular' echinoid, is to the left.  To the right of it is Macraster elegans , an 'irregular' echinoid.

Each bump on the echinoid test supported a moveable spine when the animal was alive.

sea urchin (Phylum Echinodermata, Class Echinoidea)

starfish (Phylum Echinodermata, Class Stellaroidea)

These starfish, which are embedded in a limestone slab, belong to the genus Cratinaster (Austinaster) .  The slab, which measures over one meter square, is covered with starfish. 

Something quite unusual must have occurred to kill so many of them at the same time, yet leave so few signs of damage.  The starfish are not fragmentary or curled.  They include a wide range of sizes, and thus presumably of ages.

Can you think of a way this might have happened? sudden death - how were these organisms preserved so magnificently?
clear pentameral symmetry



Frequently used abbreviations: NPL  Non-vertebrate Paleontology Laboratory | TNSC Texas Natural Science Center | UTDGS Department of Geological Sciences | BEG  Bureau of Economic Geology | VPL Vertebrate Paleontology Laboratory | JSG  Jackson School of Geosciences | SUPPORT | VOLUNTEER | GLOSSARY

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