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The larvae of aspidodiadematids are primitive in form, having two pairs of arms as opposed to the single pair that is the derived condition in Diadematidae. This and the rather peculiar and distinctive disc structure suggests that Aspidodiadematoida are sister group to other Diadematoida. The classification of Aspidodiadematidae is straight forward and hinges on the form of ambulacral plating. There are four genera. Gymnotiara has the plesiomorphic ambulacral arrangement with small ambulacral tubercles overlapping two elements and separated by a third simple element. Culozoma is identical but has primary tubercles on all adapical interambulacral plates. Aspidodiadema has ambulacral tubercles as large as interambulacral tubercles and has two of the three ambulacral elements reduced to demiplates. Plesiodiadema has minute ambulacral tubercles, confined to every third element, and simple plating throughout. Although Plesiodiadema has the simplest ambulacral arrangement, it is clearly derived from a trigeminate arrangement since every third plate carries a primary tubercle and sphaeridial pits are also found on every third element. Thus this arrangement cannot be seen as linking diadematoids to cidaroids as some earlier authorities suggested. Aspidodiadema and Plesiodiadema are presumably both derived from Gymnotiara. Gymnotiara, having the plesiomorphic ambulacral state, may therefore be non-monophyletic. |
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Family Aspidodiadematidae Duncan, 1889 Genus Gymnotiara Pomel, 1883 |