All living echinoids have a test with alternating ambulacral and interambulacral zones of plates, each zone constructed from just two columns of plates. This is a synapomorphy for the crown group of the Echinoidea, and is also true for all post-Palaeozoic echinoids, bar one, Tiarechinus. By contrast almost all Palaeozoic echinoids have a test which departs from this basic pattern; Miocidaris being the only Palaeozoic taxon to follow the crown-group pattern. Bothriocidarids have a test that is composed exclusively of ambulacral plates while various other groups have either large numbers of ambulacral or interambulacral plates in each zone. These groups fall into the stem group of the Echinoidea.
A preliminary phylogenetic analysis of Palaeozoic echinoids was published by Smith (1984). However, a more rigorous analysis is needed to provide a robust framework for ordering these various groups. Here the stem group is divided into a number of taxa which, in some cases, probably represent grades rather than monophyletic clades.
An account of the morphological diversification of Palaeozoic echinoids is given by Kier (1965), and Jackson (1912) remains the most exhaustive account of many of the taxa.
The following Palaeozoic taxa are indeterminate and are omitted from the following keys and discussion:
- Lanternarius Regnell, 1956
- Kongielechinus Jesionek-Szymanska, 1979
- Protocidaris Whidborne, 1898
- Rhoechinus Keeping, 1876
- Silurocidaris Regnell, 1956
- Tornquistellus Berg, 1899
Jackson, R. T. 1912 Boston Society for Natural History Memoirs 7, p. 296, pl. 24, fig. 4, pl. 25, figs 1,2.
Kier, P. M. 1965. Evolutionary trends in Paleozoic echinoids. Journal of Paleontology 39, 436-465, pls 55-60.
Kier, P. M. 1966. Noncidaroid Paleozoic echinoids. Pp. U298-312 in Moore, R. C. (ed.). Treatise on Invertebrate Paleontology. University of Kansas Press & Geological Society of America, Boulder.
Smith, A. B. 1984. Echinoid Palaeobiology. George Allen & Unwin, London