Article: Two early Cretaceous spirulid coleoids of the north-western Caucasus: their shell ultrastructure and evolutionary implications
The rare phragmocone-bearing coleoids Adygeya adygensis gen. et sp. nov. and Naefia kabanovi sp. nov. are described from the Aptian of the north-western Caucasus (Russia). They are considered to be the earliest known members of the order Spirulida, belonging to two families, Adygeyidae fam. nov. and Groenlandibelidae, respectively. The siphuncular and shell wall ultrastructures of the coleoids were studied with SEM and compared with those of Recent Spirula.The two coleoids under consideration possess a longiconic shell, a comparatively wide and strongly expanded siphuncle with spherulitic-prismatic connecting rings, a shell wall as thin as the septa, a short final chamber and comparatively long camerae. Both genera lack rostra and a nacreous layer in the shell wall. They differ mainly in the position of the siphuncle and the shape of septal necks.Ultrastructural studies of the shell wall in Adygeya, Naefia and Spirula lead to the conclusions that in the lineage of Spirulida (1) the shell possesses the outer plate (sensu Appellof 1893) instead of a rostrum, and (2) the shell wall lacks the nacreous layer. On the basis of the high stability of shell wall structure in cephalopod evolution, it is assumed that these structural differences indicate an early divergence of Decabranchia and Belemnoidea, and that the taxa which had a rostrum and a nacreous layer in the shell wall can hardly be interpreted as precursors of the Spirulida lineage.