Silurian oncoceridOctamerella(Cephalopoda) from Gotland, Sweden

Abstract

An actinosiphonate deposit with radial lamellae occurs in the siphuncle of several genera classified within the Oncocerida. Unique structural preservation of the connecting ring and the actinosiphonate deposit is here described in the oncocerid Octamerella sp. from the Silurian of Gotland, Sweden. The connecting ring of Octamerella is of the Nautilus type. It consists of an outer porous spherulitic-prismatic layer and an inner thin fibrous organic layer, the latter destroyed by diagenesis. The actinosiphonate deposit consists of two, structurally different, spherulitic-prismatic sublayers. The thin outer sublayer has a fragile porous structure and low mechanical strength, similar to the outer spherulitic-prismatic layer of the connecting ring. The inner spherulitic-prismatic sublayer that forms the actinosiphonate lamellae has a more rigid, less porous structure and higher mechanical strength than the outer sublayer. Each lamella is longitudinally traversed by a calcareous central plate that has a solid structure and consists of thin acicular crystallites that are orientated parallel to the long axis of the plate. The secretion of the actinosiphonate deposit probably began immediately after a new siphuncular segment was completed. The connecting ring together with the actinosiphonate deposit functioned as a reservoir and transport route for the cameral liquid improving the buoyancy regulation.

Characterization of two new superorders Nautilosiphonata and Calciosiphonata and a new order Cyrtocerinida of the subclass Nautiloidea; siphuncular structure in the Ordovician nautiloidBathmoceras(Cephalopoda)

“…The latter layer is typically destroyed by diagenesis. In addition to the Nautilus, this siphuncular-type is present in the orders Nautilida, Tarphycerida, Ellesmerocerida, Discocerida, Oncocerida, new order Cyrtocerinida and probably also in Ascocerida (Mutvei 2002a(Mutvei , 2011(Mutvei , 2013Mutvei & Stumbur 1971;Mutvei & Dunca 2011, Mutvei et al 2010Stumbur & Mutvei 1983;Hewitt & Stait 1985;Evans & King 1990;Kröger 2012;Kröger & Mutvei 2005;Kröger & Landing 2008;Kröger et al 2009aKröger et al , 2009bNielsen et al 2009). The orders Ellesmerocerida, Discosorida, oncocerida and probably also Ascocerida, are closely related and belong to the superorder Multiceratoidea that, in addition to the nautilus-type of the siphuncle, is characterized by short body chamber, more or less constricted aperture at terminal growth stage, and multiple muscle scars at the base of the body chamber.…”

“…5(D) -(G)). The original porosity in the spherulitic -prismatic layer cannot be reconstructed in detail, because the original aragonitic crystallites probably are covered by a layer of small calcium phosphate crystals that were precipitated diagenetically (Mutvei 2011 , fig. 7D).…”

“…from the Middle Ordovician of Estonia and it is compared with that in Bathmoceras sp. (Holm 1899) and that in the Silurian oncocerid nautiloid Octamerella (Mutvei 2011(Mutvei , 2013.…”

Gigantism, taphonomy and palaeoecology of Basiloceras, a new oncocerid genus from the Middle Devonian of the Tafilalt (Morocco)

“…We mostly use the terminology of the Treatise (Sweet 1964;Teichert 1964) with the addition of the term actinosiphonate lamellae in preference to actinosiphonate deposits to more clearly distinguish them from the endosiphuncular deposits of the Orthocerida or Actinocerida (see also Flower 1943Flower , 1964Crick and Teichert 1979;Mutvei 2011). Also note that height always refers to measurements in dorsoventral direction, width to measurements in lateral direction and length to measurements in longitudinal direction of the conch.…”

Spatial distribution of oncocerid cephalopods on a Middle Devonian bedding plane suggests semelparous life cycle

“…Phragmocone implosion is also unlikely because the Tafilalt Platform was most likely above the implosion depth of an oncocerid phragmocone 42,47 . Lastly, there are indications that oncocerids were more efficient than Nautilus in using their siphuncle for buoyancy control 49 . Thus, if the animals died near the sea floor (for example because of reproduction), their chambers were potentially already filled with water to a certain degree when they died.…”