A DIAMOND-LIKE MICROLATTICE IN THE DEVONIAN CRINOIDS

A triply periodic minimal surface (TPMS) is a minimal surface, which is periodic in three independent directions. This structure has been found in a variety of living organisms, but examples of TPMS with micrometer length scales in the nature are extremely rare. Owing to the optimized surface and high compression performance properties, TPMS structures have received considerably interests in various research fields and technologies including bio-inspired engineering, catalysis, drug delivery and energy conversion. Although one of the three simplest TPMS type, the so-called P surface, has been long recognized in the stereom microstructure of living and fossil echinoderms, the advanced diamond-type TPMS, which is know to substantially enhance the damage tolerance, has been only recently recognized in the living knobbly starfish Protoreaster nodosus. Here we report a diamond-type TPMS microstructure in a 382 mys old crinoid thecae of Haplocrinites. Microstructural organization and crystallographic properites of the diamond-type TPMS in fossil crinoid closely resemble the TPMS stereom features of modern knobbly starfish. Despite of low Mg/Ca ratio of the calcite Devonian sea, the skeleton of these fossil crinoids is composed of high-Mg calcite, which suggests that biomineral geochemistry is driven by biological rather than environmental factors. These results push back the origins of this highly ordered microstructure considerably into the Middle Devonian. Such an optimization of the trabecular arrangement additionally enriched in magnesium, which both increase the mechanical strength, likely evolved in these crinoids in response to increased predation pressure during the so-called Middle Paleozoic Marine Revolution. During this time a number of durophagous predatory taxa diversified inducing evolution of antipredatory morphologies among invertebrate prey. The diamond-type TPMS may had been acquired independently in at least two echinoderm clades, or may be a genetically encoded feature in all echinoderms that emerges during increased predation/mechanical stress pressure periods.