Abstract
This paper analyzes an aberrant group of echinoids in terms of constructional morphology, i.e., as modification of an established “Bauplan” by a set of new functional and morphogenetic constraints and possibilities. The characteristics of sand dollars (flat test, spine differentiation, branched food grooves, lunules) are related to a particular combination of burrowing and sieve feeding in sandy sediments. It has independently evolved from less specialized Clypeasteroids in at least three lineages (Scutellina, Rotulidae, Arachnoididae), which have solved inherent problems differently (sutural interlocking; growth patterning of food grooves and canal systems; lunule formation; weight belts). These three groups have radiated in different degrees due to their different palegeographic histories.
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