ABSTRACT
The arrangement of skeletal elements shows that Echinocyamus pusillus (Clypeasteroida: Fibulariidae) is built to resist a combination of membrane stresses, bending and rotational forces. At the ambitus, horizontal spreading (thrust) is resisted in tension by the oral surface. The test plates increase in thickness from crown to ambitus and continue into the oral surface with almost constant thickness. Internally the interambulacral plates are extended into paired, curved buttresses which extend into the vault of the dome to the expected inflexion point of the bending moments. Thus they are perfectly placed to resist negative bending of the test and vertical displacement of the floor due to rotational forces. The denser stereom of the buttresses increases their stiffness so that they preferentially carry stress to the oral surface. The material between the supports (the ambulacra) contributes significantly to the stiffness and strength of the structure. The ambulacral plates are thickened towards the center to form horizontal components which resist hoop stresses and bending in the ambital region. Where the ambulacral plates join radially, the junction is reinforced by a slight thickening which provides a secondary buttress. A comparison of the morphology of this and other fibulariid species is made. For stress tests, Echinocyamus was loaded at the crown. In almost every case failure occurred around the apical system. The genital and terminal pores of the petaloids concentrate stress so that fracture occurs between them, and the center is punched in. The spines are able to absorb about 15 - 20% of the force before the load is applied directly to the test. These findings are interpreted in light of the known behavior and habitat of Echinocyamus pusillus.
