The dissertation is focused on uncovering patterns of evolution, gross morphological and microanatomical features of the postcranial skeleton of semi-aquatic (Protocetidae) and early fully aquatic cetaceans (Pelagiceti), as well as finding and clarifying phylogenetic relationships between them. The research was conducted on materials from the collections of scientific institutions and museums of Ukraine, Georgia, Germany, Peru, USA, and Sweden. A total of 40 specimens of cetaceans belonging to the families Protocetidae, Basilosauridae, groups of baleen (Mysticeti) and toothed (Odontoceti) whales, as well as taxa of unknown status, were analyzed. Research methods include bone measurements, three-dimensional surface scanning, data analysis using traditional and geometric morphometric methods, computer tomography of the internal structure, preparation of paleohistological thin sections and their examination in natural and polarized light. Analysis of morphometric data was performed using PAST 4.09, MorphoJ 1.07a and Microsoft Excel software. The phylogenetic analysis was perfprmed using the matrix supplemented with new characters and taxa in the TNT 1.1 software.
Based on the example of a representative of the early Basilosauridae, the cetacean GMTSNUK 2638, it is shown that the evolution of the flattening of the centra of the cervical vertebrae occurred in parallel in different groups of fully aquatic whales. A new hypothesis is proposed for evolutionary changes in the vertebral formula that is characteristic of early Pelagiceti. The lumbar region of these cetacean groups increased due to the inclusion of the last thoracic and three anterior sacral vertebrae, thus acquiring the following appearance: C7T12L10CA21. The body size of some cetaceans, known from fragmentary finds, was estimated; for early basilosaurids which lived since Lutetian to the beginning of the Priabonian (42 - 37 million years ago) (MUSM 1443, Basilotritus wardii) the body length did not exceed 6 meters, and gigantic basilosaurids (12 -13 m) arose in the Bartonian. Morphometric analysis of the humerus of the early Pelagiceti showed that the humeri of the early Mysticeti and Odontoceti are similar to each other and to the ancestral (basilosaurid) shape, but later in the course of evolutionary development they acquired different shapes due to the increase in the relative size of the epiphyses and the reduction of the deltapectoral crest. A comparative morphological analysis of several specimens of the tibia of Eocene cetaceans showed an intermediate degree of reduction with possible preservation of some functional load during aquatic locomotion in the early basilosaurid "Platyosphys" paulsonii and anatomical rearrangements that indicate an active role in swimming in NMNP CS26-28 (Pelagiceti incertae sedis). The study of the histological structure of sections of vertebrae and ribs of Basilotritus uncovered a difference in the structure of these bones: the external compact bone tissue of the vertebrae is demarcated by concentric channels, while the compact bone tissue of the ribs is divided into layers by lines of arrested growth. Numerous resorption cavities in various postcranial bones suggest the function of the skeleton as a source of nutrients during postnatal ontogeny. Phylogenetic analysis clarified the position on the tree of some taxa of early basilosaurids (Basilotritus sp., GMTSNUK 2638). These cetaceans are closely related to the large clade that includes the modern cetaceans, Neoceti, and their sister clade to which Kekenodontidae belong. This group shares a common ancestor with other basilosaurids, the split from which occurred immediately after Pelagiceti.
The scientific novelty of the study consists in obtaining a comprehensive view of the postcranial skeleton reconstructions of early cetaceans, which occurred during the transition to a fully aquatic lifestyle and were accompanied by arising of several morphotypes. The practical value of the study is in the fact that its results can be used to create new and improve existing methods for identifying fragmentary remains of early cetaceans, developing biomechanical models of various types of aquatic locomotion of secondary aquatic tetrapods. During phylogenetic analysis, new important anatomical features were discovered, which can be used in determining the taxonomic position of new specimens of cetaceans.
