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The mechanisms that regulate minor and trace element biomineralization in the echinoid skeleton can be primarily controlled biologically (i.e., by the organism and its vital effects) or by extrinsic environmental factors. Assessing the relative role of those controls is essential for understanding echinoid biomineralization, taphonomy, diagenesis, and their potential as geochemical archives. In this study, we (1) contrast geochemical signatures of specimens collected across multiple taxa and environmental settings to assessin situthe effects of environmental and physiological factors on skeletal biomineralogy; and (2) analyze the nanomechanical properties of the echinoid skeleton to assess potential linkages between magnesium/calcium (Mg/Ca) ratios and skeletal nanohardness. Live specimens of sand dollars and sea biscuits (Mellita tenuis,Encopespp.,Leodia sexiesperforata, andClypeaster subdepressus) were collected from three different salinity regimes: (1) a coastal region of Cedar Key influenced by freshwater input from Suwannee River, with low and fluctuating salinity; (2) St. James Bay with less fluctuating, higher salinity; and (3) Florida Keys with stable, fully marine salinity conditions. No clear relationship was found between the bulk skeletal barium/calcium (Ba/Ca), zinc/calcium (Zn/Ca), sodium/calcium (Na/Ca), cadmium/calcium (Cd/Ca), copper/calcium (Cu/Ca), phosphorous/calcium (P/Ca), lead/calcium (Pb/Ca), boron/calcium (B/Ca), manganese/calcium (Mn/Ca) ratios pooled across all taxa. In contrast, bulk Mg/Ca, strontium/calcium (Sr/Ca), sulfur/calcium (S/Ca) and lithium/calcium (Li/Ca) ratios exhibited notable differences between the three regions, indicating that distribution of these elements can be at least partly influenced by environmental factors such as salinity. However, such patterns were highly variable across taxa and regions, indicating that both environmental and physiological factors influenced geochemical signatures to varying degrees, depending on the species and environmental setting. In addition, regardless of species identity, different types of stereom within single tests were characterized by distinct skeletal Mg/Ca ratios and nanohardness. The inner galleried and coarse labyrinthic stereom typically exhibited a lower Mg/Ca ratio and nanohardness than the outer imperforate stereom layer that locally forms tubercles. Such heterogeneity in Mg distribution within single specimens cannot be ascribed solely to environmental changes, indicating that these echinoids actively regulate their intraskeletal Mg content: the higher magnesium concentration at the tubercles relative to that of the underlying stereom may be interpreted as a strategy for enhancing their mechanical strength to withstand surface friction and wear. The results suggest that the trace element composition of echinoid tests is a complex outcome of environmental and physiological factors. 

The transition from sessile suspension to active mobile detritus feeding in early echinoderms (c.a. 500 Mya) required sophisticated locomotion strategies. However, understanding locomotion adopted by extinct animals in the absence of trace fossils and modern analogues is extremely challenging. Here, we develop a biomimetic soft robot testbed with accompanying computational simulation to understand fundamental principles of locomotion in one of the most enigmatic mobile groups of early stalked echinoderms—pleurocystitids. We show that these Paleozoic echinoderms were likely able to move over the sea bottom by means of a muscular stem that pushed the animal forward (anteriorly). We also demonstrate that wide, sweeping gaits could have been the most effective for these echinoderms and that increasing stem length might have significantly increased velocity with minimal additional energy cost. The overall approach followed here, which we call “Paleobionics,” is a nascent but rapidly developing research agenda in which robots are designed based on extinct organisms to generate insights in engineering and evolution.