Structural stress response of segmented natural shells: a numerical case study on the clypeasteroid echinoidEchinocyamus pusillus

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.

A Total-Evidence Dated Phylogeny of Echinoids and the Evolution of Body Size across Adaptive Landscape

“…S22 of SI File 4). Recent studies show that these structures are necessary for the stability of flat tests (Grun et al 2018), and they likely evolved independently in different flattened lineages. Other traits, however, appear to have originated in the last common ancestor of sand dollars and sea biscuits, and were subsequently lost by some of its descendants (e.g., the presence of Aristotle’s lantern in adults; Fig.…”

Echinoid skeleton: an insight on the species-specific pattern of the Paracentrotus lividus plate and its microstructural variability

“…These plates are arranged in ten double vertical columns, representing alternating five ambulacral zones and five interambulacral zones [11]. Individual plates are significantly variable in morphology and their form is closely related to specific functional aspects [9,12,13]. The outer surfaces of plates display rounded tubercles on which the spines of variable shapes and functions are articulated by ball-and-socket joints.…”

Hexagonal Voronoi pattern detected in the microstructural design of the echinoid skeleton

“…Echinoids possess a hierarchically organized lightweight endoskeleton with a unique mechanical design [34][35][36][37]. Voronoi diagrams have been used to model echinoderm embryogenesis (e.g.…”

“…The latter constitutes the intrastereomic organic matrix, which include different proteins, glycoproteins, and sulfated polysaccharides [57][58][59][60][61]. The stereom is organized in a three-dimensional mesh of trabeculae and its architecture is highly variable, reflecting its differentiated biological function [35,36,46,54,56,62,63]. During the 1970s, studies investigated the two-dimensional characteristics of the outer surface of the test plate stereom in some regular echinoid species [62,64].…”