The Echinoid Directory

 Regular echinoids with:

• test composed of 20 columns of plates; pairs of ambulacral columns alternating with pairs of interambulacral columns
• ambulacral plating simple or at most pseudocompound (i.e. every second or third element enlarged to accommodate a larger tubercle and spine, but individual elements not fused together by tubercle overgrowth)
• interambulacral plates dominated by a single large primary tubercle (except Tetracidaris which has two)
• perignathic girdle of interambulacral elements only (apophyses).
• ambulacral plates continuing as a series across the peristomial membrane
• teeth crescentic to U-shaped in cross-section; pyramids without a foramen magnum
• primary spines with external cortex (except in most primitive members)
• globiferous and tridentate pedicellariae only.

Lower Permian to Recent; worldwide.

Cidaroids represent the sister group to all other extant echinoids (Euechinoidea). They differ from euechinoids by having a perignathic girdle composed of enlarged interambulacral elements (apophyses) and from all but the echinothurioids in having a series of ambulacral plates continuing over the peristomial membrane. The great majority have simple ambulacral plating, stout spines with external cortex and a single large primary tubercle dominating each interambulacral plate.

Smith, A. B. 1984. Echinoid Palaeobiology. George Allen & Unwin, 190 pp.

Kroh, A. & Smith, A. B. 2010. The phylogeny and classification of post-Palaeozoic echinoids. Journal of Systematic Palaeontology 8, 147-212.