|Range||Lower Jurassic (Pliensbachian) to Recent, worldwide.|
|Remarks||Although all living irregular echinoids can be distinguished by the fact that their periproct opens in the posterior interambulacrum at some distance from the apical disc, this is not the case in some of the most primitive members back in the Jurassic.
Jesionek-Szymanska's (1963, 1968) pioneering work demonstrated that many of the more primitive Jurassic irregular echinoids still had their periproct surrounded by plates of the apical disc. In these cases however, the posterior genital plate is imperforate, lacking a gonopore. Other regular echinoids with a distinctly posterior periproct such as Milnia and Heterodiadema differ in having a posterior gonopore.
There remain, however, a small number of problematic echinoids that probably belong to the very base of the total group Irregularia that are not covered by this definition, including Loriolella and Jesioneckechinus.
Jesionek Szymnaska, W. 1963. Echinides irreguliers du Dogger de Pologne. Acta Palaeontologica Polonica 8, 293-414.
Jesionek Szymnaska, W. 1968. Irregular echinoids - an insufficiently known group. Lethaia 1, 50-65.
Saucede, T., Moois, R. & David, B. 2003. Combining embryology and palaeontology: origins of the anterior - posterior axis in echinoids. C. R. Palevol 2, 399-412.
Smith, A. B. & Anzalone, L. 2000. Loriolella, a key taxon for understanding the early evolution of irregular echinoids. Palaeontology 43, 303-324.