New data on the morphology of the Maastrichtian pycnodonts (Gryphaeidae, Bivalvia) of Eastern Mangyshlak (Western Kazakhstan)

Abstract

Introduction: to this date, a lot of literature has been published on Late Cretaceous bivalves from the Caspian Sea and adjacent areas. However, the issue of the reliability of determining the taxonomic affiliation of the shells themselves remains stay relevant. Not only the preservation of the latter, but also the variability of their morphological features have repeatedly created significant difficulties for specialists who were compelled to introduce new taxa of a species and subspecies rank. Some difficulties also arise when researchers have to compare their shells with only a textual description, as is the case with the right flaps of some Late Cretaceous representatives of Pycnodonte. In this study, the author attempted to partially solve the above-described difficulties based on the material presented by two valves extracted from the Late Maastrichtian deposits in Western Kazakhstan by A.S. Alekseev.

Results: the article presents the results of the present study of the morphology of shells of some ostreids from the terminal layers of the Maastrichtian stage of Aksai ravine (Ustyurt Plateau, Mangyshlak Peninsula). Based on the studied morphological features, it was determined that only one of them belong to the species Pycnodonte vesicularis Lamarck, 1806. The geometric parameters of the drilling trace on the surface of the left leaf are described, followed by its comparison with those ones from Western and Central Europe.

Conclusion: in the course of the study, it was possible to reliably establish that only the right wing belongs to Pycnodonte vesicularis. As for the left one, according to a very satisfactory degree of preservation, the author confines himself to the definition Pycnodonte sp. Despite this, the hole on the surface of the last (left) flap allowed to restore to a certain extent the trophic relationship between the two classes of mollusks – bivalves and gastropods. It can be considered a reliable fact that within the Late Cretaceous waters of Mangyshlak, the role of predator was assigned to some small-sized representatives of gastropods. A similar phenomenon has already been noted by foreign authors on the Late Cretaceous fossil material of a number of countries in both the Northern and Southern hemispheres, which led some of them to create and improve their systems based on drilling traces in many ways similar to those from the Caspian region.