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Siphuncular structure in Ordovician endocerid cephalopods

100%

Mutvei H.

Acta Palaeontologica Polonica

1997

tom 42

nr 3

375-390

Exceptionally well-preserved shells of the endocerids Dideroceras wahlenbergi (Foord, 1887), Anthoceras vaginatum (Schlotheim, 1820), and Suecoceras barrandei (Dewitz, 1880) from phosphatized Early and Middle Ordovician limestones of Northern Estonia were studied by means of SEM. The septal neck in these endocerids is composed of three, structurally different, aragonite layers: outer spherulitic-prismatic, nacreous, and inner prismatic. The connecting ring is a continuation of the spherulitic-prismatic layer of the septal neck. Its inner surface was probably covered by a thin glycoprotein (conchiolin) sheet. Structural differentiations in the spherulitic-prismatic layer of the connecting ring, such as a layering and 'eyelet', reported by previous writers, were not observed. These differentiations probably result from diagenesis. The siphuncular structure in endocerids agrees in detail with that in Recent Spirula and Nautilus. The conical endosiphuncular deposits (endocones) of endocerids show extensive intraspecific variation. Morphological and structural differences in these deposits should therefore be used with caution in generic and specific diagnoses.

Connecting ring structure and its significance for classification of the orthoceratid cephalopods

100%

Mutvei H.

Acta Palaeontologica Polonica

2002

tom 47

nr 1

The connecting ring in orthoceratids is composed of two calcified layers: an outer spherulitic−prismatic and an inner calcified−perforate. The spherulitic−prismatic layer is a direct continuation of that layer in the septal neck, whereas the calcified−perforate layer is a structurally modified continuation of the nacreous layer of the septal neck. The latter layer is traversed by numerous pores which are oriented either transversally to the siphuncular surface, or have a somewhat irregularly anastomosing course. The connecting ring structure is positively correlated to the dorsal position of the scars of the cephalic retractor muscles. A similar type of connecting ring and a dorsal postion of retractor muscle scars also occur in lituitids, previously assigned to tarphyceratids, and in baltoceratids, previously assigned to ellesmeroceratids. These two taxa are therefore included in the suborder Orthoceratina, which, together with the suborder Actinoceratina, are assigned to the order Orthoceratida Kuhn, 1940.

Ograniczanie wyników

2 Acta Palaeontologica Polonica

2 Mutvei H.

1 2002

1 1997

Wyniki wyszukiwania

Siphuncular structure in Ordovician endocerid cephalopods

Connecting ring structure and its significance for classification of the orthoceratid cephalopods