Temporal range: Lower Cretaceous–recent
L. Agassiz, 1840
Their body is a somewhat elongated oval in form, and is distinguished by the mouth being placed towards one end of the animal, and the anus towards the other. As a result, heart urchins, unlike most other sea urchins, are bilaterally symmetrical, and have a distinct anterior surface. The presence and position of the mouth and anus typically give members of this group the distinct "heart" shape from which they get their name.
Heart urchins have no feeding lantern, and often have petaloids sunk into grooves. They are a relatively diverse order, with a number of varying species.
According to World Register of Marine Species :
- sub-order Brissidina Stockley, Smith, Littlewood, Lessios & MacKenzie-Dodds, 2005
- family Asterostomatidae Pictet, 1857
- family Brissidae Gray, 1855
- family Palaeotropidae Lambert, 1896
- super-family Spatangidea Fischer, 1966
- family Hemiasteridae H. L. Clark, 1917
- sub-order Micrasterina Fischer, 1966
- family Palaeostomatidae Lovén, 1868
- sub-order Paleopneustina Markov & Solovjev, 2001
- family Somaliasteridae Wagner & Durham, 1966a †
- family Toxasteridae Lambert, 1920a †