Sumrall, Colin D. Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee.
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A diverse class of extinct, marine, stalked echinoderms whose stratigraphic range extends from the Late Cambrian (505 MYA) through Upper Devonian (360 MYA). The class is named for rhombus-shaped structures (rhombs) that circulated seawater or body fluids through the body for the respiratory extraction of dissolved oxygen. The main body, or theca, held the visceral mass and was attached to the seafloor by a short to long stem. Atop the theca were variously developed small appendages that were used for suspension feeding in slowly moving currents. This class is composed of three distantly related groups: Glyptocystitida, Hemicosmititida, and Caryocystitida (see illustration). A fourth group, Polycosmititida, is too poorly known to assign to Rhombifera with certainty.
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