The Echinoid Directory

Clypeasterines fall into two very clearly defined clades, each with its own synapomorphies. Mortensen (1948) and Durham (1955) distinguished these as the families Clypeasteridae L. Agassiz, 1835 and Arachnoididae Duncan, 1889. That these two families combined represent a monophyletic clade seems reasonably well established, based on the pseudocompound plating of the petals, and paired enclosed sphaeridia that separate them from other clypeasteroids. The lantern support arrangement also distinguishes them, but this is primitive, in that it consists of paired auricles of ambulacral origin, as in oligopygids and juvenile cassiduloids.

The Clypeasteridae, comprising the single genus Clypeaster, resemble the immediate outgroup to clypeasteroids, the oligopygids, in having a sunken infundibulum leading to the mouth. Their only derived character is the possession of five gonopores.

The Arachnoididae contains 8 genera which have been variously classified in the past. Except for the tiny paedomorphic forms (Fossulaster, Willungaster), the pores and tubercles are characteristically aligned in oblique rows forming `combed areas`, showing the group to be monophyletic. Philip & Foster (1971) erected the family Fossulasteridae for small forms with mostly juvenile features (e.g. petals reduced or absent, simple ambulacral plating). However, they also share the derived characteristic of having basicoronal interambulacral plates reduced and enclosed by adjacent ambulacral plates, and their interambulacral plating is often highly derived. The three genera placed in this group all share a very distinctive autapomorphy: their aboral surface is constructed of just 10 large triangular ambulacral plates - interambulacral plates are absent. The sister group to fossulasterids is probably Arachnoides, since both have interambulacral plates separated by three pairs of ambulacral plates on the oral surface. Of the remaining three genera Monostychia and Ammotrophus are very similar, having oral periprocts, while Fellaster is distinctly different in having bidentate pedicellariae and a distinct posterior channel from peristome to periproct.

The classification adopted here is:-

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.

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

Philip, G. & Foster, R. J. 1971. Marsupiate Tertiary echinoids from South-eastern Australia and their zoogeographic significance. Palaeontology 14, 666-695.

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.