Grun TB and Nebelsick JH (2018), Structural design of the minute clypeasteroid e...

ECB-ART-46402

R Soc Open Sci 2018 May 01;55:171323. doi: 10.1098/rsos.171323.

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Structural design of the minute clypeasteroid echinoid Echinocyamus pusillus.

Grun TB , Nebelsick JH .

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The clypeasteroid echinoid skeleton is a multi-plated, light-weight shell construction produced by biomineralization processes. In shell constructions, joints between individual elements are considered as weak points, yet these echinoid skeletons show an extensive preservation potential in both Recent and fossil environments. The remarkable strength of the test is achieved by skeletal reinforcement structures and their constructional layouts. Micro-computed tomography and scanning electron microscopy are used for microstructural and volumetric analyses of the echinoid''s skeleton. It is shown that strengthening mechanisms act on different hierarchical levels from the overall shape of the skeleton to skeletal interlocking. The tight-fitting and interlocking plate joints lead to a shell considered to behave as a monolithic structure. The plate''s architecture features distinct regions interpreted as a significant load-transferring system. The internal support system follows the segmentation of the remaining skeleton, where sutural layout and stereom distribution are designed for effective load transfer. The structural analysis of the multi-plated, yet monolithic skeleton of Echinocyamus pusillus reveals new aspects of the micro-morphology and its structural relevance for the load-bearing behaviour. The analysed structural principles allow E. pusillus to be considered as a role model for the development of multi-element, light-weight shell constructions.

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Genes referenced: irak1bp1 LOC100893907 LOC115918707 LOC590297 stk36

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	Figure 1. Echinocyamus pusillus. Micro-CT-based volume rendering of the denuded test [GPIT/EC/00740:gg-al-1.73]. (a) View of the aboral side. (b) Horizontal section with view onto the internal aboral surface. (c) View of the oral side. (d) Horizontal section with view onto the internal oral surface. (e) Lateral view, anterior to the left. (f) Lateral section showing internal test structures. Abbreviations: au, auricle; bc, basicoronal ring; bu, buttress; db, distal buttress; gp, genital pore; gt, glassy tubercle; hp, hydropore; hr, horizontal rib; is, inner surface; os, outer surface; pb, proximal buttress; ps, peristome; pt, petal; pp, periproct; tu,tubercle; up, unipore.
	Figure 2. Load conditions in E. pusillus. (a) Test rendered in a hypothetical environment, buried within the sediment. Arrows indicate the loaded areas [GPIT/EC/00740:gg-al-1.73]. (b) Sketch of E. pusillus in lateral view. Radial lines indicate the course of radial stress. Horizontal lines represent the course of latitudinal stress. The red line indicates the ambitus. (c) Sketch of a part of the test in lateral view. External arrows indicate downward stress that turns into lateral thrust within the plates.
	Figure 3. Analyses of test thickness. (a) Horizontal µCT section of E. pusillus indicating measurement points for plate thickness. Terminologies following Lovén [74] [GPIT/EC/00740:gg-al-1.73]. (b) Boxplot diagram comparing thicknesses of perradial, adradial and interradial sutures with the thickness of ambulacral and interambulacral plates. (c) Boxplot diagram showing the thicknesses of plates. Ambulacral plates a and b are pooled to a single value (I – V). Interambulacral buttress length is given for each plate individually for plates a and b (1–5). Plate nomenclature following Lovén [74].
	Figure 4. Stereom differentiation in E. pusillus [GPIT/EC/00740:gg-al-1.73]. (a) Micro-CT section of the oral side showing the mosaic of plates. (b) Close-up indicating three analysed regions of the plate: c, unordered labyrinthic stereom at the plate's centre; r, directional galleried stereom at the plate's rim; s, directional galleried stereom within sutures. (c) Comparison between three prominent areas of the plates.
	Figure 5. Plate shape of E. pusillus [GPIT/EC/00740:gg-al-1.73]. (a) Three-dimensional surface rendering of a single segmented plate. Sutures follow a sinuous path in all directions. White arrows indicate the sinuous course in horizontal plane, black arrows in lateral direction. (b) Micro-CT sections of the segmented plate showing the different plate outlines at three intervals. Arrows correspond to black arrows in (a).
	Figure 6. SEM micrographs of plate joints in E. pusillus. (a) Sinuous course of sutures within a perradial boundary [GPIT/EC/00741:RI-n.4]. (b) Sutural interlocking in detail. Knobs reaching from one into another plate can increase test strength [GPIT/EC/00741:RI-n.4]. (c) Trabecular interlocking at a thickened perradial suture [GPIT/EC/00741:RI-n.5]. (d) Trabecular interlocking in detail, where trabeculae from one plate protrude into the stereom interspace of an adjoining plate [GPIT/EC/00741:RI-n.5]. (e) Sutural interlocking in a three-dimensional mosaic of surrounding plates. Arrows indicate plate boundaries [GPIT/EC/00741:RI-1.6]. (f) Orthogonal view onto a suture. Fractured trabeculae are attached and penetrate into the interspace of another plate; a and b indicate the affiliation of the involved plates. Arrows indicate narrow depressions for knob-like interlocking [GPIT/EC/00741:RI-n.4].
	Figure 7. Internal buttress system in E. pusillus. (a) Lateral section of a µCT-based volume rendering showing the internal supports emerging from fused auricles. Arrows indicate sinuous course of plate boundaries [GPIT/EC/00740:gg-al-1.73]. (b) SEM micrograph of segmented buttresses. Arrows indicate plate sutures [GPIT/EC/00741:RI-1.19]. (c) Micro-CT section of a single buttress showing its widest extent at the ambitus. (d) SEM micrograph of two internal buttresses showing the supports as protrusions of interambulacral plates [GPIT/EC/00741:RI-1.2].
	Figure 8. Horizontal section of buttresses in E. pusillus. (a) SEM micrograph of a buttress suture. Individual buttress segments are interconnected by knob-like structures [GPIT/EC/00741:RI-1.29]. (b) Micro-CT section indicating the stereom density distribution within a buttress. Location of rectangles indicates areas used for stereom density comparison [GPIT/EC/00740:gg-al-1.73]. db, distal buttress; pb, proximal buttress.

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Currey, The design of mineralised hard tissues for their mechanical functions. 1999, Pubmed, Echinobase