Abstract
The skeleton of Echinocyamus pusillus is considered as an exceptional model organism for structural strength and skeletal integrity within the echinoids as demonstrated by the absence of supportive collagenous fibres between single plates and the high preservation potential of their skeletons. The structural principles behind this remarkably stable, multi-plated, light-weight construction remain hardly explored. In this study, high-resolution X-ray micro-computed tomography, finite-element analysis and physical crushing tests are used to examine the structural mechanisms of this echinoid's skeleton. The virtual model of E. pusillus shows that the material is heterogeneously distributed with high material accumulations in the internal buttress system and at the plate boundaries. Finite-element analysis indicates that the heterogeneous material distribution has no effect on the skeleton's strength. This numerical approach also demonstrates that the internal buttress system is of high significance for the overall skeletal stability of this flattened echinoid. Results of the finite-element analyses with respect to the buttress importance were evaluated by physical crushing tests. These uniaxial compression experiments support the results of the simulation analysis. Additionally, the crushing tests demonstrate that organic tissues do not significantly contribute to the skeletal stability. The strength of the echinoid shell, hence, predominantly relies on the structural design.
Highlights
Sea urchins (Echinodermata, Echinoidea) have increasingly become the focus of structural analyses as their light-weight skeletons feature a remarkable load-carrying property (e.g. [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16])
The skeleton of Echinocyamus pusillus is considered as an exceptional model organism for structural strength and skeletal integrity within the echinoids as demonstrated by the absence of supportive collagenous fibres between single plates and the high preservation potential of their skeletons
The outer surface of the skeletons is of high material density
Summary
Sea urchins (Echinodermata, Echinoidea) have increasingly become the focus of structural analyses as their light-weight skeletons feature a remarkable load-carrying property (e.g. [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]). Sea urchins (Echinodermata, Echinoidea) have increasingly become the focus of structural analyses as their light-weight skeletons feature a remarkable load-carrying property The echinoid skeleton is a hierarchically organized The plates of most echinoid skeletons are securely interconnected by collagenous fibres The clypeasteroid echinoids feature additional strengthening mechanisms that enhance skeletal integrity: in addition to the collagenous fibres, these echinoids developed skeletal protrusions bridging the sutures between plates The minute Echinocyamus pusillus represents a particular case within the clypeasteroid echinoids as collagenous fibres between the plates are entirely absent [3]
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