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2 Annales Societatis Geologorum Poloniae

2 2021

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Structure of the calcitic layer of the aptychus of the ammonite genus Peltoceras

Mironenko Aleksandr A.

Annales Societatis Geologorum Poloniae

2021

Vol. 91, No. 4

397--404

The aptychi of ammonites combined the functions of lower jaws and protective opercula. They consist of two parts: an inner organic layer and an outer calcitic lamella. In different evolutionary lineages of ammonites, the shape of aptychi, the sculpture of their surface and the microstructure of the calcitic layer vary greatly. However, the structure of the aptychi is not known for all evolutionary lineages of ammonites. Although numerous aptychi have been described for the Jurassic family Aspidoceratidae, almost all of them belong to only one evolutionary branch of this family – the Aspidoceratinae (sensu lato). For the second branch – the Peltoceratinae, only one aptychus had been described to date and the structure of its calcitic layer remained unknown. In this article, for the first time, the structure of the aptychus of the Peltoceratinae (upper Callovian Peltoceras) is described. The surface of this aptychus is covered with rough ribs and the calcitic part consists of only one layer of dense calcite. The thickness of the aptychus is much greater than that of the aptychi of supposed ancestors of the Peltoceratinae. The increase in the thickness of the aptychi in both the Aspidoceratinae and the Peltoceratinae, contemporaneously with the appearance of spines on their shells, is most likely related to increasing the protective function of the aptychi of these ammonites in the late Callovian.

Discovery of plywood structure in Sphenothallus from Gurovo Formation (Mississippian), Central Russia

Vinn Olev, Mironenko Aleksandr A.

Annales Societatis Geologorum Poloniae

2021

Vol. 91, No. 1

67--74

Sphenothallus specimens are reported for the first time from the Mississippian of Central Russia. All Sphenothallus specimens have a phosphatic composition and a characteristic laminar structure, which is best observable in the thickened lateral parts of a tube. Most of the lamellae in the tube wall are straight, but some have a wavy morphology and a few are so wrinkled that they form hollow “ribs”. The wrinkled lamellae presumably had an originally higher organic content than the straight lamellae. There are borings on the surfaces of some lamel-lae that are similar in morphology to the bioerosional traces in various hard, biomineral substrates. Lamellae in the inner parts of the tube wall are composed of fibres. The fibres are parallel to the surface of the tube wall and in successive laminae they differ in orientation by irregularly varying angles. It is possible that the plywood micro-structure in Sphenothallus was originally organic and was later phosphatized during fossilization. An alternative, but less likely explanation is that the plywood structure was originally mineralized and therefore is comparable to the phosphatic lamello-fibrillar structures of vertebrates.