Mironenko Aleksandr (2018) Microstructure of aptychi of Upper Jurassic (Upper Oxfordian) ammonites from Central Russia. Lethaia, 51(1), 75-85

Abstract

Microstructure of aptychi of Upper Jurassic (Upper Oxfordian) ammonites from Central Russia.

To date, bivalve calcitic plates which cover the outer chitinous lamella of the lower jaws of Jurassic and Cretaceous ammonoids (aptychi sensu stricto) have been classified into several morphotypes (form genera) based on shape, surface sculpture and internal microstructure. However, previous works on aptychi microstructure focused mainly on thick morphotypes (e.g. Laevaptychus and Lamellaptychus), whereas little was investigated for thin Praestriaptychi. In this study, the microstructure of Praestriaptychi of Upper Oxfordian Perisphinctes and of recently discovered aptychi of aspidoceratid microconch Mirosphinctes (both belonging to the superfamily Perisphinctoidea) is described. These aptychi are compared with Laevaptychi of Upper Oxfordian macroconchs Euaspidoceras (a dimorphic counterpart of Mirosphinctes). This study demonstrated that the aptychi of Perisphinctes and Mirosphinctes differ from each other and from Laevaptychi in their microstructure, number of calcitic layers and in growth patterns. The aptychi of both aspidoceratid dimorphs are similar in terms of growth pattern, but differ in microstructure and the number of calcitic layers. Neither aptychi of Perisphinctes nor of Mirosphinctes have a tubular layer with a honeycomb surface pattern, which is typical for Laevaptychus. This indicates that aptychi were extremely diverse and their microstructure varied significantly not only within the superfamily, but even within a dimorphic pair of aspidoceratid ammonites. A lack of a tubular layer in Praestriaptychus indicates that it developed independently in Lamellaptychus of Haploceratoidea and Laevaptychus of Aspidoceratidae.

Synopsis

The article is devoted to the microstructure of the aptychi of Upper Jurassic (Oxfordian) ammonites Perisphinctes and Mirosphinctes and their comparison with the aptychi of other ammonites. The microstructure of the aptychi of these taxa, as well as the microstructure of all thin Jurassic aptychi (which belong to the formal genus Praestriaptychus), has not been studied previously. There was an opinion that all Jurassic aptychi consist of three layers of calcite, in contrast to Cretaceous two-layered aptychi. In the article one can see that the studied Oxfordian aptychi also consist of two layers, and the microstructure of the aptychi of sexual dimorphs of the family Aspidoceratidae - Euaspidoceras [M] and Miroshinctes [m] - differ from each other.

Keywords

Ammonites, aptychi, Aspidoceratidae, Laevaptychus, microstructure, Praestriaptychus

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