The Echinoid Directory

Gymnodiadema de Loriol, 1884, p. 606

Diagnostic Features	Test large, globular or hemispherical. Apical disc small - test damaged so no details seen. Dicyclic in G. hessi with hydropores scattered over three or more genital plates Ambulacra straight, relatively narrow. Large primary tubercles adorally, with plating clearly compound, but elsewhere apparently absent; fine granulation only. Pore-pairs uniserial and relatively large; plating simple aborally. Large phyllodes developed adorally. Interambulacral plates covered in small uniform granules; no discernable primary tubercles on ambital and aboral plates. Below the ambitus a distinct double column of adradial tubercles is developed. Tubercles are non-crenulate and perforate. Peristome not seen. Spines and lantern unknown.
Distribution	Middle Jurassic (Callovian), Portugal.
Name gender	neuter
Type	Gymnodiadema choffati de Loriol, 1884. p. 606. Plastercast of holotype: Museum d\'Histoire Naturelle, Geneva 26022.
Species Included	G. choffati de Loriol, 1884; Callovian, Portugal. G. hessi Smith, 2011; Bajocian, Morocco
Classification and/or Status	Euechinoidea, Echinacea, stem group Arbacioida. Valid genus.
Remarks	Similar to Thierychinus in having large numbers of small tubercles on its interambulacral plates with primary tubercles confined to the oral surface. However, the ambulacra appear to be uniserial rather than forming a broad band aborally. de Loriol described the ambulacral plating as simple. However, adoral plating is definitely compound, since large primary ambulacral tubercles are present. Above the ambitus plating there are no primary tubercles and plate organisation simply cannot be determined. A second species, G. hessi, greatly clarifies the morphology of this genus - see Smith (2011). Smith, A. B. 2011. Gymnodiadema and the Jurassic roots of the Arbacioida (stirodont echinoids). Swiss Journal of Palaeontology 130, 155-171.