Improvement of the description of ostracods of the family TRACHYLEBERIDINAE

Abstract

Introduction. The definition of criteria for separating closely related species of representatives of the genus Cythereis is currently incomplete. Ostracods of this genus have wide intraspecific variability and high adaptability to changing living conditions. These factors make it difficult to use these ostracod species for regional stratigraphic correlation. In addition to the selection of criteria for species separation, there is a problem of the imperfection of the terminological apparatus for describing ostracods, which complicates the description of species and omits potentially important information for the separation of related species. Analysis of previous publications. Numerous works are devoted to the issue of the development of the terminological apparatus for describing ostracods, however, qualitative methods of description are inherent only to certain large taxa, where a separate method of description has been developed for each taxon. Ostracods of the genus Cythereis are widespread in the Cretaceous Tethys. Their wide variability is considered by various authors often without taking into account intraspecific variability and adaptation, which leads to the selection of subspecies or ignoring variability. Materials and methods. Numerous ostracods of the species Cythereis hirsuta Damotte&Grosdidier from the Late Cenomanian and Cythereis ornatissima (Reuss) from the Turonian of Ukraine were used in the work. Ostracods were selected from different deposits according to the conditions of origin. Results and discussion. The paper describes in detail the morphology of ostracods of the genus Cythereis with qualitative characteristics of morphological elements. The ontogenetic changes of the two species and the change in morphology depending on the conditions of existence are also described. The change in the morphology of juvenile forms depending on the conditions of existence is described separately. Conclusions. Based on the study and description of numerous paleontological materials, a number of refinements were proposed to the descriptive characteristics of the morphological elements of ostracod valves of members of the family TRACHYLEBERIDINAE Sylvester-Bradley, 1948 and the genus Cythereis in particular. The proposed clarifications regarding the morphology of the mesosculpture made it possible to describe in detail the ontogenetic changes of the studied species and their differences in adult stages depending on the facial conditions of existence. It should be noted that a detailed scheme for describing the morphology of mesosculpture is proposed for the first time in the literature. The paleontological description of numerous materials from the Middle Cretaceous of Ukraine allowed to establish the morphological elements that are of primary importance for the separation of related species within the genus. The analysis of these elements suggests that some of the similar species from the outcrops of Europe and Asia are actually other species. The question of their phylogenetic ties can be solved under the condition of studying ontogenesis and expanding the geography of research. Based on this, the validity of the names of the studied species from the territory of Ukraine is also contradictory to the actual comparison with the holotype collection.