The Echinoid Directory

Classification and phylogeny of spatangoids


The classification of spatangoids has traditionally been based on the pattern of fascioles developed. However, a major complication is that fascioles can often be secondarily lost during development. A full account of the phylogeny and classification used here is in preparation.

At the base of the spatangoids lie the toxasterids, probably a paraphyletic grouping as currently constituted, but with some monophyletic clades. Toxasterids primitively lack fascioles and have sternal plates that are clearly asymmetrical.

The Hemiasterina is made up of three families, all of which possess a single peripetalous fasciole passing between the periproct and apical disc over the posterior interambulacrum, and supraambitally over plates 7 or above in the anterior interambulacra. Sternal plates may be asymmetric in the more basal members, but become symmetric in the extant forms.

By contrast both Micrasterina and Paleopneustina have a fasciole band that is initiated much earlier on in development and which crosses plate 4 in the posterior interambulacrum. In the Micrasterina this band curves adorally on entering ambulacra I and V and crosses the posterior interambulacrum either over the rear part of plate 2 (Micrasteridae) or mid way through plate 3 (Brissidea) to form a closed subanal fasciole. In the Paleopneustina the fasciole on plate 4 extends across ambulacra I and V and continues marginally crossing interambulacral plates 4 or 5 in interambulacra 1 and 4. This fasciole either merges with the peripetalous band immediately behind the anterior petals (Schizasteridea) or maintains a separate course from the higher peripetalous fasciole (Paleopneustidea).

In the keys, if fascioles are absent, as is the case with almost all deep-sea taxa, other test architecture features have been used to establish relationships. For example, the characteristic way episternal plates are indented by ambulacral plates towards their rear is a synapomorphy of some Micrasterina and a sure sign that these taxa are evolved from forms that once possessed a subanal fasciole.