Contributed by Andrew Smith, October 2007
Lower Cretaceous (Valanginian), France.
Tetracidaris reynesi Cotteau, 1872, p. 165, by monotypy. Holotype: University of Lyon EM 12131.
|Classification and/or Status|
Tetracidaris is unique amongst all post-Palaeozoic echinoids in having more than two columns of interambulacral plates in each zone. That this is a derived condition is obvious from the fact that the test is initially constructed as all other cidaroids with just two columns of interambulacral plates in each zone. It is only after the first four plates in each column have formed that the interambulacra become quadriserial. Tetracidaris is identical in its pore-pair and ambulacral structure to Diplocidaris from which it is no doubt derived. Heterocidaris is superficially similar in having four subequal primary interambulacral tubercles abreast in interambulacral zones, but in Heterocidaris there are two primary tubercles to each plate, not four columns of plates. Tetracidaris further differs in having pore-pairs biserially offset.