The Echinoid Directory

Classification

Two major clades of clypeasteroid have for long been recognized, these being the Clypeasterina and Laganina of Mortensen (1948). Mortensen based this classification on the nature of the lantern supports and the form of the miliary spines. Laganina have simple ambulacral plating in the petals, miliary spines ending in a crown or galdular sac, and internal lantern support structures comprising five fused auricles that are interradial in position. Clypeasterina on the other hand have demiplates in their petals, simple tips to miliary spines and ten unfused, adradially positioned auricles. Of these the fused auricles provide an excellent synapomorphy showing that Laganina are a derived clade, while the demiplates similarly suggest monophyly of the Clypeasterina.

Durham (1955) modified Mortensen`s classification by dividing Mortensen's Laganina into three suborders, the Laganina, for forms with normal basicoronal circlets and interambulacral zones terminating in a single plate, the Scutellina for forms with normal basicoronal circlets and interambulacral zones remaining biserial to the apex, and Rotulina for forms with the basicoronal circlet with entirely internal interambulacral elements and interambulacral zones terminating in a single plate. The single adapical interambulacral element is a synapomorphy uniting rotulines and laganines, while the basicoronal circlet structure shows rotulines to be derived with respect to laganines. There is no strong synapomorphy uniting all scutellines, and it is likely that some of the early members of this group will turn out to be members of the common stem group to all three.

Durham`s classification fails to capture the observed hierarchical structure and some subsequent workers (Philip 1965, Smith 1984, Wang 1984) have reverted to recognizing Mortensen's original bipartite subdivision, giving presidence to the oldest available name. Others (Nisiyama 1968, Mooi 1990) have followed Durham. The classification adopted here is as follows:-

Suborder Clypeasterina L. Agassiz, 1835
Suborder Scutellina Haekel, 1896
Infraorder Laganiformes Desor, 1847
Infraorder Scutelliformes Haekel, 1896

Cladogram
A checklist and detailed key to the genera of Clypeasteroida can be found in Mooi (1989), while Mooi (1990) provides the most thorough and detailed cladistic analysis of the groupings of clypeasteroid at this level.

Mortensen, T. 1948. A Monograph of the Echinoidea IV.2 Clypeasteroida. C. A. Reitzel, Copenhagen.

J. W. Durham 1955. Classification of clypeasteroid echinoids. University of California Publications in Geological Sciences 31(4), 73-198.

Mooi, R. 1989. Living and fossil genera of the Clypeasteroida (Echinoidea: Echinodermata): an illustrated key and annotated checklist. Smithsonian Contributions to Zoology 488, 1-51.

Mooi, R. 1990. Paedomorphosis, Aristotle's lantern, and the origin of the sand dollars (Echinodermata: Clypeasteroida). Paleobiology 16, 25-48.

Nisiyama, S. 1968. The echinoid fauna from Japan and adjacent regions. Part II. Palaeontological Society of Japan Special Papers 13, 1-491, pls 19-30.

Philip, G. M. 1965. Classification of echinoids. Journal of Paleontology 39, 45-62.

Smith, A. B. 1984 Echinoid palaeobiology George Allen & Unwin, London.

Wang, C.-C. 1984. New classification of Clypeasteroid echinoids. Proceedings of the Geological Society of China 27, 119-152.