The class is divided into the subclass Perischoechinoidea, which has only one order and contains the most taxonomi-cally primitive members of the class (e.g., pencil urchins), and the subclass Euechinoidea, which has 19 orders and contains the "true" echinoids. These are divided into two broad groups based on body shape: the regular urchins, which contain sea urchins, and the irregular urchins, which include both heart urchins and sand dollars. Collectively, there are 46 families and about 900 known species.
Echinoids are members of a much larger group of marine animals called echinoderms. These include asteroids (sea stars), ophiuroids (brittle stars), crinoids (feather stars), concentricy-cloids (sea daisies), and holothurians (sea cucumbers). The last group has the closest fossil linkage to echinoids than any other echinoderm class. The fossil record shows divergence from sea stars nearly 450 million years ago in the late Ordovician period, the oldest of these being the Perischoechinoidea. Regular and irregular urchins may have first evolved in the lower Jurassic, but these early forms remain poorly understood. In particular, the fossil genus Loriolella has an intermediate morphology, showing both irregular and regular characteristics. However, it is believed that during the Cretaceous period, sea level rise may have accounted for a dramatic increase in their diversity. Today, urchins play an important ecological role in many marine communities globally.
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