The submicroscopic morphology of magnesian calcite skeletal units of echinoids, revealed by scanning electron microscopy, was compared with crystal orientation data obtained by x-ray methods and with macroscopic morphology. The origin of sand dollars, one of its most distinctive clades, is also unclear due to an unstable phylogenetic. Despite a remarkable fossil record, the emergence of their crown group is documented by few specimens of unclear affinities, rendering their early history uncertain. Echinoids are key components of modern marine ecosystems. Irregular echinoids are the sand dollars, which are generally found on sandy or soft ground.
Regular echinoids are the sea urchins they are generally found on rocky substrates. Echinoids Echinoids are commonly grouped as regular or irregular, with the greatest differences pertaining to the oral structure, shape of the organism, and location of the anus.
