The Echinoid Directory

Contributed by Andrew Smith, January 2017

Wrightechinus Smith, 2016, p. 93

Diagnostic Features	Apical disc opening large; plating monocyclic with ocular plates slightly smaller than genital plates. Periproct covered by a tesselated pavement of plates, these plates angularly indenting into the apical disc ring. Ambulacral zones straight, trigeminate, plate compounding acrosaleniid in style at ambitus and adapically. Pore pairs uniserially arranged at ambitus and adapically but expanded to form phyllodes adorally. Ambulacral tubercles much smaller than interambulacral tubercles around the ambitus. All tubercles perforate and crenulate. Interambulacral plates with a single large primary tubercle. Surrounded by small, non-contiguous scrobicular tubercles.
Distribution	Middle Jurassic (Aalenian-Bajocian) of England and France.
Type	Acrosalenia lycetti Wright, 1851, by original designation.
Species Included	No other species are included.
Classification and/or Status	Euechinoidea; Carinacea; Calycina; Stem group Salenioida?
Remarks	Wrightechinus resembles acrosaleniids in having interambulacral tubercles which are perforated, strongly crenulated and surrounded by scrobicular tubercles, and which are much larger than the corresponding ambulacral tubercles. It also has trigeminate plating with plate-compounding in acrosaleniid style, short phyllodes and a large apical disc with periproctal plates indenting into the ring of ocular and genital plates. However, like phymosomatoids its apical disc is monocyclic, whereas all other acrosaleniids have a hemicyclic disc, with the anterior ocular plate exsert. It therefore holds a very interesting phylogenetic position, as it appears to occupy an intermediate position between the Salenioida and the Phymosomatoida. For the moment, it is included as the most primitive member of the stem group Salenioida. Smith, A. B. 2016. British Jurassic regular echinoids. Part 2 Carinacea. Monographs of the Palaeontographical Society (no. 646), pp. 69-176, pls 42-82.