Mortensen's (1943 A Monograph of the Echinoidea) classification remains the most comprehensive available. M. Jensen (1981 Videnskabelige Meddelelser, Dansk Naturhistorisk Forening, Kobenhavn 143, 1-99) provided important new information on tooth ultrastructure and suggested some ammendments to the classification. A subsequent cladistic analysis, based on a wider suite of characters, but covering fewer taxa, has been published by A. B. Smith (1988 Molecular Biology and Evolution 5, 345-365).
The higher taxonomy for this group was based by Mortensen almost exlusively on characters of the globiferous pedicellariae and on larval structure. Although both provide some excellent derived characters there is also homoplasy. Molecular work by N. Matsuoka (1987, Zoological Science 4, 339-347) clearly shows Pseudocentrotus to be a member of the Strongylocentrotidae rather than Toxopneustidae where it was placed by Mortensen. Furthermore, Toxopneustidae appear to nest within the Echinoida rather than within the Temnopleuroida. The following classification takes into account the full suite of skeletal and biochemical characters:-
Parechinidae Mortensen, 1903. (plesiomorphic grade). Globiferous pedicellariae with an widely open blade bearing multiple lateral teeth. Interambulacral plates densely tuberculate, with multiple subequal tubercles. Ambulacral pore arrangement variable. Buccal notches extremely feeble.
(a) Parechinus, Psammechinus, Isechinus (trigeminate forms)
(b) Paracentrotus, Loxechinus. (polygeminate forms; possibly the plesiomorphic sister-group to Strongylocentrotidae as in test morphology it is virtually indistinguishable)
Echinidae Gray, 1825. Paired lateral teeth on narrow tubular blade of globiferous pedicellariae. Ambulacra almost entirely trigeminate with pore-pairs arranged either as rather vertical arcs or a dense adradial band. Interambulacral plates with relatively sparse or reduced tuberculation, often highly granulose. Buccal notches extremely feeble.
(Echinus, Gracilechinus, Sterechinus, Stirechinus, Dermechinus, Polyechinus)
Echinometridae Gray, 1855. (Possibly paraphyletic). Single asymmetric lateral tooth on tubular blade of globiferous pedicellariae (absent in Parasalenia). Buccal notches standard. Larva with basket-like structure in early stages.
a) Test circular (plesiomorphic)
(Anthocidaris, Caenocentrotus, Echinostrephus, Evechinus, Heliocidaris, Pachycentrotus, Selenechinus)
(b) Test elongate along II-4 or I-3 axis. Ambulacra generally polygeminate
(Echinometra, Zenocentrotus, Heterocentrotus, Podophora, Colobocentrotus)
Parasaleniidae Mortensen, 1903. Test elongate along the axis III-5. Apical disc with valve-like plates. Ambulacra trigeminate; pore-pairs uniserial above but broadened into small phyllodes adorally
Strongylocentrotidae Gregory, 1900. Ambulacra exclusively polygeminate with pore-pairs forming very distinctive sickle-shaped arcs. Interambulacral plates with primary tubercle and smaller secondary tubercles.
(a) Globiferous pedicellariae without muscular neck or lateral teeth. Buccal notches shallow but with prominent tag.
(b) Globiferous pedicellariae with muscular neck. Buccal notches extremely feeble; no tag. (Strongylocentrotus, Hemicentrotus, Allocentrotus)
ToxopneustidaeTroschel, 1872. Globiferous pedicellariae with a tubular blade and no lateral teeth. Ambulacral plating generally trigeminate, but with up to five pore-pairs in a few taxa. Pore zones forming a single band or three vertical series but not arcs. Buccal notches deep and sharp, typically divided in larger species. Larva with basket-structure in early stages.