Smith, A.B. 2011. Gymnodiadema and the Jurassic roots of the Arbacioida (stirodont echinoids). Swiss Journal of Palaeontology, 130/1, 155-171.

151801

10.1007/s13358-010-0014-z [view]

Smith, A. B.

2010

130(1), 155-171

Publication

The morphology of Gymnodiadema, until now incompletely documented from a single poorly preserved individual from the Callovian of Portugal, is clarified based on a new species from the Bajocian of Morocco. The new species has a plating and tuberculation pattern that is identical to that of the Cretaceous arbacioid Codiopsis, but has perforate tubercles. Gymnodiadema is accompanied in Morocco by the oldest species of Magnosia whose morphology is also described. Both Gymnodiadema and Magnosia are shown to retain their basicoronal interambulacral plates as adults, suggesting arbacioid affinities. The composition of arbacioids is reviewed and a phylogenetic analysis carried out of the better-known relevant taxa. Crown group arbacioids are shown to include the fossil taxa Noetlingaster and Arbia, and thus originated in the late Cretaceous. Like other echinoid groups, therefore, the expansion of arbacioids into deep-water settings occurred from the late Cretaceous onwards. Codiopsis, Gymnodiadema and Magnosia are identified as stem group members. The presence of two morphologically distinct clades of arbacioid in the Bajocian suggests that arbacioids must have split from their nearest living sister group much earlier, probably in the early Jurassic.

Echinoidea

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