Smith, Andrew B. Department of Paleontology, Natural History Museum, London, United Kingdom.
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An extinct order of regular sea urchins (class Echinoidea) characterized by having a large monocyclic apical disk that forms a ring indented by periproctal plates. Members of the Phymosomatoida have compound ambulacral plating with three or more elements bound together by a single large tubercle. Their jaw apparatus is of the stirodont design, having teeth that are keeled and unfused epiphyses. They comprise two families, one with perforate tubercles and the other with imperforate tubercles, the former being the ancestor to the latter. They first appeared in the Lower Jurassic and became extinct at the end of the Eocene. The morphology of Phymosomatoida suggests that they were epifaunal grazers and lived in shallow shelf seas. See also: Echinodermata; Echinoidea
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