Crown group Euechinoidea are distinguished from Cidaroidea by their perignathic girdle, which is composed of adradial ambulacral projections termed auricles. Cidaroidea by contrast have no ambulacral component to their perignathic girdle, which is composed entirely of apophyses. Stem group Echinoidea have no perignathic girdle but their test is composed of more than 20 columns. However, in the Triassic there a a small group of echinoids that have the crown group test structure (i.e. a test composed of just 20 columns of plates) but which lack anything more than just the most rudimentary perignathic girdle. These may be very advanced stem group Echinoidea, stem group Cidaroidea or stem group Euechinoidea. In Triadocidaridae and Paurocidaridae the most adoral ambulacral plates often show a small adradial swelling which Kier (1984) took to be incipient auricles. Similarly, Serpianotiaridae has small adradial prongs angled towards the ambulacra on its most interambulacral plates. These differ from the more massive apophyses of cidaroids. Kier (1984) and Smith (1994) both placed these families as stem group Euechinoidea.
Kier (1984) first realised the phylogenetic significance of perignathic girdle structure in this group. A phylogenetic analysis of this group is provided by Smith (1994).
The following taxa are based on fragments or spines and are too poorly known to be classified. They are omitted from the following keys and discussion:
- Anaulocidaris Zittel, 1879
- Aplocidaris Lambert & Thiery, 1909
- Batheritiaris Vadet, 2001
- Braunechinus Vadet, 2001
- Kieritiaris Vadet, 2001
- Thielicidaris Vadet, 2001
- Wissmannechinus Vadet, 2001
- Zardinitiaris Vadet, 2001
Kier, P.M. 1977. Triassic echinoids. Smithsonian Contributions to Paleobiology 30, 1-88.
Kier, P. M. 1984. Echinoids from the Triassic (St Cassian) of Italy, their lantern supports, and a revised phylogeny of Triassic echinoids. Smithsonian Contributions to Paleobiology 56, 1-41.
Smith, A. B. 1994. Triassic echinoids from Peru. Palaeontographica A 233, 177-202.