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DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 401

Box 15.5 Echinoid classifi cation

The traditional split of the class into regular and irregular forms is no longer considered to refl ect
the true phylogeny of the echinoids. Whereas the irregular echinoids are probably monophyletic,
arising only once, the regular echinoids do not form a clade. The group was traditionally subdivided
into three subclasses (Fig. 15.10) – the Perischoechinoidea, Cidaroidea and Euechinoidea – the fi rst,
however, has been shown to be polyphyletic and the term stem-group echinoids is preferred.

Stem-group ECHINOIDEA

• Regulars with ambulacra in more than two columns, interambulacra with many columns; in total
the test is composed of over 20 columns. Lantern with simple grooved teeth and lacking a peri-
gnathic girdle
• Upper Ordovician to Permian

Crown-group ECHINOIDEA
Subclass CIDAROIDEA
• Regulars with test consisting of 20 columns of plates; two columns in each ambulacra and inter-
ambulacral areas. Interambulacral plates have large tubercle. Teeth are crescentic to U-shaped
and the perignathic girdle includes only interambulacral elements
• Lower Permian to Recent

Subclass EUECHINOIDEA

• Post-Paleozoic taxa, both regular and irregular. Both ambulacra and interambulacra with twin
columns. Perignathic girdle composed on ambulacral projections
• Middle Triassic to Recent

crown-group echinoids
Euechinoidea
Acroechinoidea
Perischoechinoidea Cidaroidea Echinothuriacea Diadematacea Irregularia Echinacea

6 7 8
4 5
2 3
1

Figure 15.10 Echinoid classification based mainly on cladistic analysis: 1, 10 ambulacral and 10

interambulacral areas; 2, upright lantern without foramen magnum; 3, distinctive perignathic
girdle; 4, distinctive ambulacral areas; 5, upright lantern with deep foramen magnum; 6, grooved
teeth; 7, stout teeth; 8, keeled teeth.