Weight Belts, Diverticula, and the Phylogeny of the Sand Dollars
The intestinal diverticulum found in some juvenile clypeasteroids consists of a series of tubes and pouches in the peripheral part of the test. When distended with sand grains, the diverticulum can form a weight belt. We conducted a survey of the occurrence of the weight belt in the three major clades: the suborders Clypeasterina, Laganina, and Scutellina. Dissections and radiographs of juvenile representatives of all extant clypeasteroid families, the majority of genera, and some fossil species, indicate that the weight belt is found only in the Scutellina. The diverticulum and weight belt are absent throughout the ontogeny of clypeasterines and laganines. The weight belt is therefore a synapomorphy for the extant scutelline sand dollars. Some implications of this discovery for the phylogeny of clypeasteroids and the adaptive significance of the weight belt are discussed. Primary among these are the observations that: 1) the presence of a weight belt in miniaturized taxa such as Sinaechinocyamus and Marginoproctus supports their assignment to the Scutellina, and 2) the distribution of the weight belt character in the Clypeasteroida is best interpreted as a result of phylogenetic history, and not of adaptive factors.
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Document Type: Research Article
Publication date: January 1, 1996
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