2 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. no v. and Naefia kabanovi sp. nov. are described from the Aptian of the n orth-western Caucasus (Russia). They are considered to be the earliest known members of the order Spirulida, belonging to two families, Adyg eyidae fam. nov. and Groenlandibelidae, respectively. The siphuncular and shell wall ultrastructures of the coleoids were studied with SEM a nd compared with those of Recent Spirula. The two coleoids under consi deration possess a longiconic shell, a comparatively wide and strongly expanded siphuncle with spherulitic-prismatic connecting rings, a she ll wall as thin as the septa, a short final chamber and comparatively long camerae. Both genera lack rostra and a nacreous layer in the shel l wall. They differ mainly in the position of the siphuncle and the sh ape of septal necks. Ultrastructural studies of the shell wall in Adyg eya, Naefia and Spirula lead to the conclusions that in the lineage of Spirulida (1) the shell possesses the outer plate (sensu Appellof 189 3) instead of a rostrum, and (2) the shell wall lacks the nacreous lay er. On the basis of the high stability of shell wall structure in ceph alopod evolution, it is assumed that these structural differences indi cate an early divergence of Decabranchia and Belemnoidea, and that the taxa which had a rostrum and a nacreous layer in the shell wall can h ardly be interpreted as precursors of the Spirulida lineage.