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Development and calcification of the ammonitella shell

100%

Kulicki C., Doguzhaeva L. A.

Acta Palaeontologica Polonica

1994

tom 39

nr 1

17-44

Aconeceras trautscholdi ammonitellae mass occurring in the Aptian of Symbirsk, central Russia represent consecutive calcification stages of the primary organic shell wall. Already after the formation of the organic shell with proseptum, the first whorl and umbilical walls of the initial chamber were calcified, then the remaining part of the initial chamber, and finally the nacreous primary constriction was formed and the proseptum was calcified. The original mineral participating in calcification was aragonite, which formed primary prismatic layers. The ammonite embryonic shell was thus formed similarly to the archaeogastropod larval shell. This explains the microstructural distinction of the ammonitella and proseptum walls with respect to the rest of the ammonite shell.

Maastrichtian Ceratisepia and Mesozoic cuttlebone homeomorphs

88%

Hewitt R. A., Jagt J. W. M.

Acta Palaeontologica Polonica

1999

tom 44

nr 3

305-326

The phylogenetics of potential Mesozoic ancestors of cuttlefish of a restricted order Sepiida von Zittel, 1895 (superorder Decabrachia Boettger, 1952) is reviewed. Microstructural studies of Mesozoic homeomorphs of cuttlebones (Pearceiteuthis gen. n., Loligosepia, Trachyteuthis, Actinosepia) are consistent with their assignement to the superorder Octobrachia Fioroni, 1981. The discovery of an embryonic Ceratisepia shell in the upper Maastrichtian of the Netherlands, indicates that true Sepiida did have a pre-Cenozoic origin. Cretaceous decabrachs of the order Spirulida Stolley, 1919 do not show evidence of the dorso-anterior shell growth vectors seen in Cenozoic spirulids, sepiids and octobrachs. Separate origins of the Sepiida and Spirulida within Cretaceous diplobelinid belemnites is still the most attractive hypothesis. Ceratisepia vanknippenbergi sp. n. from the upper Maastrichtian of the Netherlands and Pearceiteuthis buyi gen. et sp. n. from the Callovian of England are described.

Ograniczanie wyników

2 Acta Palaeontologica Polonica

1 Doguzhaeva L. A.

1 Hewitt R. A.

1 Jagt J. W. M.

1 Kulicki C.

1 1999

1 1994

Wyniki wyszukiwania

Development and calcification of the ammonitella shell

Maastrichtian Ceratisepia and Mesozoic cuttlebone homeomorphs